Patent Application
20160381690
The present invention relates to a wireless communication system and, more particularly, to a method for allocating resource for device-to-device (D2D) communication on the basis of interference measured in a wireless communication system supporting D2D communication, and a device supporting the same.
Mobile communication systems have been developed to provide voice services, while guaranteeing user activity. Service coverage of mobile communication systems, however, has extended even to data services, as well as voice services, and currently, an explosive increase in traffic has resulted in shortage of resource and user demand for a high speed services, requiring advanced mobile communication systems.
The requirements of the next-generation mobile communication system may include supporting huge data traffic, a remarkable increase in the transfer rate of each user, the accommodation of a significantly increased number of connection devices, very low end-to-end latency, and high energy efficiency. To this end, various techniques, such as small cell enhancement, dual connectivity, massive Multiple Input Multiple Output (MIMO), in-band full duplex, non-orthogonal multiple access (NOMA), supporting super-wide band, and device networking, have been researched.
In order to support D2D communication in a cellular network, existing cellular communication and D2D communication share cellular resource, and thus, frequency efficiency and interference are required to be considered.
Therefore, an object of the present invention is to provide a method for allocating resource for D2D communication to maximize spectrum usage efficiency in accordance with re-use of frequency, while minimizing interference with cellular communication.
The technical problems solved by the present invention are not limited to the above technical problems and those skilled in the art may understand other technical problems from the following description.
According to an aspect of the present invention, there is provided a resource allocation method for allocating resource for device-to-device (D2D) communication in a wireless communication system supporting D2D communication, including: receiving, by a base station (BS), shared resource least interfered by a neighbor terminal and discovered by a D2D reception terminal and resource discovery time information from the D2D reception terminal; and synchronizing, by the BS, scheduling in order to allocate entirely or partially same resource to a cellular terminal for which the shared resource is scheduled and the D2D reception terminal at the resource discovery time.
According to another aspect of the present invention, there is provided a base station for allocating resource for device-to-device (D2D) communication in a wireless communication system supporting D2D communication, including: a radio frequency (RF) unit transmitting and receiving a wireless signal; and a processor, wherein the processor is configured to receive shared resource least interfered by a neighbor terminal and discovered by a D2D reception terminal and resource discovery time information from the D2D reception terminal, and synchronize scheduling in order to allocate entirely or partially same resource to a cellular terminal for which the shared resource is scheduled and the D2D reception terminal at the resource discovery time.
Preferably, in the synchronizing of scheduling, the entirely or partially same resource of resource allocated to the cellular terminal may be allocated to the D2D reception terminal at every scheduling period.
Preferably, in the synchronizing of scheduling, resource allocated to the cellular terminal may be fixed and the entirely or partially same resource of the fixed resource may be allocated to the D2D reception terminal.
Preferably, the method may further include: when the D2D reception terminal fails to receive a D2D signal from a D2D transmission terminal by more than a predetermined number of times, receiving, by the BS, the least interfered shared resource re-discovered by the D2D reception terminal and resource re-discovery time information from the D2D reception terminal.
Preferably, the method may further include: transmitting, by the BS, the synchronized scheduling information to the cellular terminal and the D2D terminal.
Preferably, the synchronized scheduling information may include the entirely or partially same resource allocation information or terminal identifier pairing information between the D2D terminal and the cellular terminal.
Preferably, the method may further include: buffering, by the BS, uplink scheduling information in the cellular terminal during a predetermined period of time.
According to another aspect of the present invention, there is provided a resource allocation method for allocating resource for device-to-device (D2D) communication in a wireless communication system supporting D2D communication, including: discovering, by a D2D reception terminal, a shared resource least interfered by a neighbor terminal; transmitting, by the D2D reception terminal, the shared resource information and resource discovery time information to a base station (BS); and receiving, by the D2D reception terminal, scheduling information from the BS, wherein scheduling is synchronized in order to allocate entirely or partially same resource to a cellular terminal for which the shared resource is scheduled and the D2D reception terminal at the resource discovery time.
According to another aspect of the present invention, there is provided a device-to-device (D2D) reception terminal for receiving allocated resource for device-to-device (D2D) communication in a wireless communication system supporting D2D communication, including: a radio frequency (RF) unit transmitting and receiving a wireless signal; and a processor, wherein the processor is configured to discover a shared resource least interfered by a neighbor terminal, transmit the shared resource information and resource discovery time information to a base station (BS), and receive scheduling information from the BS, and wherein scheduling is synchronized in order to allocate entirely or partially same resource to a cellular terminal for which the shared resource is scheduled and the D2D reception terminal at the resource discovery time.
Preferably, the entirely or partially same resource of resource allocated to the cellular terminal may be allocated to the D2D reception terminal at every scheduling period.
Preferably, resource allocated to the cellular terminal may be fixed and the entirely or partially same resource of the fixed resource may be allocated to the D2D reception terminal.
Preferably, the method may further include: when the D2D reception terminal fails to receive a D2D signal from a D2D transmission terminal by more than a predetermined number of times, the D2D reception terminal may re-discover a shared resource least interfered by a neighbor terminal.
Preferably, the D2D reception terminal may transmit the re-discovered shared resource information and the resource re-discovery time information to the BS.
According to embodiments of the present invention, since cellular resource and resource for D2D communication are shared on the basis of interference awareness by a D2D reception terminal, spectrum usage efficiency according to reuse of a frequency may be maximized, while minimizing interference with cellular communication.
The effects of the present invention are not limited to the above-described effects and other effects which are not described herein will become apparent to those skilled in the art from the following description.
The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the principle of the invention.
Some embodiments of the present invention are described in detail with reference to the accompanying drawings. A detailed description to be disclosed along with the accompanying drawings are intended to describe some exemplary embodiments of the present invention and are not intended to describe a sole embodiment of the present invention. The following detailed description includes more details in order to provide full understanding of the present invention. However, those skilled in the art will understand that the present invention may be implemented without such more details.
In some cases, in order to avoid that the concept of the present invention becomes vague, known structures and devices are omitted or may be shown in a block diagram form based on the core functions of each structure and device.
In this specification, a base station has the meaning of a terminal node of a network over which the base station directly communicates with a device. In this document, a specific operation that is described to be performed by a base station may be performed by an upper node of the base station according to circumstances. That is, it is evident that in a network including a plurality of network nodes including a base station, various operations performed for communication with a device may be performed by the base station or other network nodes other than the base station. The base station (BS) may be substituted with another term, such as a fixed station, a Node B, an eNB (evolved-NodeB), a Base Transceiver System (BTS), or an access point (AP). Furthermore, the device may be fixed or may have mobility and may be substituted with another term, such as User Equipment (UE), a Mobile Station (MS), a User Terminal (UT), a Mobile Subscriber Station (MSS), a Subscriber Station (SS), an Advanced Mobile Station (AMS), a Wireless Terminal (WT), a Machine-Type Communication (MTC) device, a Machine-to-Machine (M2M) device, or a Device-to-Device (D2D) device.
Hereinafter, downlink (DL) means communication from an eNB to UE, and uplink (UL) means communication from UE to an eNB. In DL, a transmitter may be part of an eNB, and a receiver may be part of UE. In UL, a transmitter may be part of UE, and a receiver may be part of an eNB.
Specific terms used in the following description have been provided to help understanding of the present invention, and the use of such specific terms may be changed in various forms without departing from the technical spirit of the present invention.
The following technologies may be used in a variety of wireless communication systems, such as Code Division Multiple Access (CDMA), Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), Orthogonal Frequency Division Multiple Access (OFDMA), Single Carrier Frequency Division Multiple Access (SC-FDMA), and Non-Orthogonal Multiple Access (NOMA). CDMA may be implemented using a radio technology, such as Universal Terrestrial Radio Access (UTRA) or CDMA2000. TDMA may be implemented using a radio technology, such as Global System for Mobile communications (GSM)/General Packet Radio Service (GPRS)/Enhanced Data rates for GSM Evolution (EDGE). OFDMA may be implemented using a radio technology, such as Institute of Electrical and Electronics Engineers (IEEE) 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, or Evolved UTRA (E-UTRA). UTRA is part of a Universal Mobile Telecommunications System (UMTS). 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) is part of an Evolved UMTS (E-UMTS) using evolved UMTS Terrestrial Radio Access (E-UTRA), and it adopts OFDMA in downlink and adopts SC-FDMA in uplink. LTE-Advanced (LTE-A) is the evolution of 3GPP LTE.
Embodiments of the present invention may be supported by the standard documents disclosed in at least one of IEEE 802, 3GPP, and 3GPP2, that is, radio access systems. That is, steps or portions that belong to the embodiments of the present invention and that are not described in order to clearly expose the technical spirit of the present invention may be supported by the documents.
Furthermore, all terms disclosed in this document may be described by the standard documents.
In order to more clarify a description, 3GPP LTE/LTE-A is chiefly described, but the technical characteristics of the present invention are not limited thereto.
General System to which the Present Invention may be Applied
3GPP LTE/LTE-A support a radio frame structure type 1 which may be applicable to Frequency Division Duplex (FDD) and a radio frame structure which may be applicable to Time Division Duplex (TDD).
One slot includes a plurality of Orthogonal Frequency Division Multiplexing (OFDM) symbols in the time domain and includes a plurality of Resource Blocks (RBs) in a frequency domain. In 3GPP LTE, OFDM symbols are used to represent one symbol period because OFDMA is used in downlink. An OFDM symbol may be called one SC-FDMA symbol or symbol period. An RB is a resource allocation unit and includes a plurality of contiguous subcarriers in one slot.
In the frame structure type 2 of a TDD system, an uplink-downlink configuration is a rule indicating whether uplink and downlink are allocated (or reserved) to all subframes. Table 1 shows the uplink-downlink configuration.
Referring to Table 1, in each subframe of the radio frame, “D” is indicative of a subframe for downlink transmission, “U” is indicative of a subframe for uplink transmission, and “S” is indicative of a special subframe including three types of a DwPTS, GP, and UpPTS. An uplink-downlink configuration may be classified into 7 types. The positions and/or number of downlink subframes, special subframes, and uplink subframe are different in each configuration.
A point of time at which a change is performed from downlink to uplink or a point of time at which a change is performed from uplink to downlink is called a switching point. The periodicity of the switching point means a cycle in which an uplink subframe and a downlink subframe are changed is identically repeated. Both 5 ms and 10 ms are supported in the periodicity of a switching point. If the periodicity of a switching point has a cycle of a 5 ms downlink-uplink switching point, the special subframe S is present in each half frame. If the periodicity of a switching point has a cycle of a 5 ms downlink-uplink switching point, the special subframe S is present in the first half frame only.
In all the configurations, 0 and 5 subframes and a DwPTS are used for only downlink transmission. An UpPTS and a subframe subsequent to a subframe are always used for uplink transmission. Such uplink-downlink configurations may be known to both an eNB and
UE as system information. An eNB may notify UE of a change of the uplink-downlink allocation state of a radio frame by transmitting only the index of uplink-downlink configuration information to the UE whenever the uplink-downlink configuration information is changed. Furthermore, configuration information is kind of downlink control information and may be transmitted through a Physical Downlink Control Channel (PDCCH) like other scheduling information. Configuration information may be transmitted to all UEs within a cell through a broadcast channel as broadcasting information.
The structure of a radio frame is only one example. The number of subcarriers included in a radio frame or the number of slots included in a subframe and the number of OFDM symbols included in a slot may be changed in various ways.
Referring to
Each element on the resource grid is referred to as a resource element, and one resource block (RB) includes 12×7 resource elements. The number of RBs NDL included in a downlink slot depends on a downlink transmission bandwidth.
The structure of an uplink slot may be the same as that of a downlink slot.
Referring to
A PCFICH is transmitted in the first OFDM symbol of a subframe and carries information about the number of OFDM symbols (i.e., the size of a control region) which is used to transmit control channels within the subframe. A PHICH is a response channel for uplink and carries an acknowledgement (ACK)/not-acknowledgement (NACK) signal for a Hybrid Automatic Repeat Request (HARQ). Control information transmitted in a PDCCH is called Downlink Control Information (DCI). DCI includes uplink resource allocation information, downlink resource allocation information, or an uplink transmission (Tx) power control command for a specific UE group.
A PDCCH may carry information about the resource allocation and transport format of a downlink shared channel (DL-SCH) (this is also called an “downlink grant”), resource allocation information about an uplink shared channel (UL-SCH) (this is also called a “uplink grant”), paging information on a PCH, system information on a DL-SCH, the resource allocation of a higher layer control message, such as a random access response transmitted on a PDSCH, a set of transmission power control commands for individual UE within specific UE group, and the activation of a Voice over Internet Protocol (VoIP), etc. A plurality of PDCCHs may be transmitted within the control region, and UE may monitor a plurality of PDCCHs. A PDCCH is transmitted on a single Control Channel Element (CCE) or an aggregation of some contiguous CCEs. A CCE is a logical allocation unit that is used to provide a PDCCH with a coding rate according to the state of a radio channel. A CCE corresponds to a plurality of resource element groups. The format of a PDCCH and the number of available bits of a PDCCH are determined by an association relationship between the number of CCEs and a coding rate provided by CCEs.
An eNB determines the format of a PDCCH based on DCI to be transmitted to UE and attaches a Cyclic Redundancy Check (CRC) to control information. A unique identifier (a Radio Network Temporary Identifier (RNTI)) is masked to the CRC depending on the owner or use of a PDCCH. If the PDCCH is a PDCCH for specific UE, an identifier unique to the UE, for example, a Cell-RNTI (C-RNTI) may be masked to the CRC. If the PDCCH is a PDCCH for a paging message, a paging indication identifier, for example, a Paging-RNTI (P-RNTI) may be masked to the CRC. If the PDCCH is a PDCCH for system information, more specifically, a System Information Block (SIB), a system information identifier, for example, a System Information-RNTI (SI-RNTI) may be masked to the CRC. A Random Access-RNTI (RA-RNTI) may be masked to the CRC in order to indicate a random access response which is a response to the transmission of a random access preamble by UE.
Referring to
A Resource Block (RB) pair is allocated to a PUCCH for one UE within a subframe. RBs belonging to an RB pair occupy different subcarriers in each of 2 slots. This is called that an RB pair allocated to a PUCCH is frequency-hopped in a slot boundary.
Physical Uplink Control Channel (PUCCH)
The Uplink Control Information (UCI) transmitted through a PUCCH can include Scheduling Request (SR), HARQ ACK/NACK information, and downlink channel measurement information as shown below.
SR (Scheduling Request): used for requesting uplink UL-SCH resources. SR is transmitted by On-Off Keying (OOK) scheme.
HARQ ACK/NACK: a signal responding to a downlink data packet on a PDSCH. This signal indicates whether a downlink data packet has successfully received or not. ACK/NACK 1 bit is transmitted in response to a single downlink codeword while ACK/NACK 2 bits are transmitted in response to two downlink codewords.
CSI (Channel State Information): feedback information about a downlink channel. CSI can include at least one of a Channel Quality Indicator (CQI), a Rank Indicator (RI), a Precoding Matrix Indicator (PMI), and a Precoding Type Indicator (PTI). For each subframe, 20 bits are used to represent the CSI.
HARQ ACK/NACK information may be generated depending on whether a downlink data packet on a PDSCH has been successfully decoded. In an existing wireless communication system, 1 bit is transmitted as ACK/NACK information with respect to the transmission of downlink single codeword, and 2 bits are transmission as ACK/NACK information with respect to the transmission of downlink 2 codewords.
Channel measurement information denotes feedback information related to a Multiple Input Multiple Output (MIMO) scheme and may include a Channel Quality Indicator (CQI), a Precoding Matrix Index (PMI), and a Rank Indicator (RI). Such channel measurement information may be commonly called a CQI.
In order to transmit a CQI, 20 bits may be used in each subframe.
A PUCCH may be modulated using a Binary Phase Shift Keying (BPSK) scheme and a Quadrature Phase Shift Keying (QPSK) scheme. Control information for a plurality of UEs may be transmitted through a PUCCH. If Code Division Multiplexing (CDM) is performed in order to distinguish the signals of UEs from each other, a Constant Amplitude Zero Autocorrelation (CAZAC) sequence of a length 12 is mostly used. The CAZAC sequence has a characteristic in that a constant size (amplitude) is maintained in a time domain and a frequency domain. Accordingly, the CAZAC sequence has a property suitable for increasing coverage by lowering the Peak-to-Average Power Ratio (PAPR) or Cubic Metric (CM) of UE. Furthermore, ACK/NACK information about downlink data transmission transmitted through a PUCCH is covered using an orthogonal sequence or an Orthogonal Cover (OC).
Furthermore, control information transmitted through a PUCCH may be distinguished from each other using a cyclically shifted sequence having a different Cyclic Shift (CS) value. The cyclically shifted sequence may be generated by cyclically shifting a base sequence by a specific CS amount. The specific CS amount is indicated by a CS index. The number of available CSs may be different depending on delay spread of a channel. A variety of types of sequences may be used as the base sequence, and the CAZAC sequence is an example of the sequences.
Furthermore, the amount of control information that may be transmitted by UE in one subframe may be determined depending on the number of SC-FDMA symbols which may be used to send the control information (i.e., SC-FDMA symbols other than SC-FDMA symbols which are used to send a Reference Signal (RS) for the coherent detection of a PUCCH).
In a 3GPP LTE system, a PUCCH is defined as a total of 7 different formats depending on control information that is transmitted, a modulation scheme, and the amount of control information. The attributes of Uplink Control Information (UCI) transmitted according to each PUCCH format may be summarized as in Table 2 below.
The PUCCH format 1 is used for SR-only transmission. In the case of SR-only transmission, a not-modulated waveform is applied. This is described in detail later.
The PUCCH format 1a or 1b is used to send HARQ ACK/NACK. If HARQ ACK/NACK is solely transmitted in a specific subframe, the PUCCH format 1a or 1b may be used. Alternatively, HARQ ACK/NACK and an SR may be transmitted in the same subframe using the PUCCH format 1a or 1b.
PUCCCH format 2 is used for transmission of CQI, and PUCCH format 2a or 2b is used for transmission of CQI and HARQ ACK/NACK. In the case of extended CP, PUCCH format 2 may be used for transmission of CQI and HARQ ACK/NACK.
PUCCH format 3 is used for carrying an encoded UCI of 48 bits. PUCCH format 3 can carry HARQ ACK/NACK about a plurality of serving cells, SR (if exists), and a CSI report about one serving cell.
In
The PUCCH format 2/2a/2b is described below. The PUCCH format 2/2a/2b is a control channel for transmitting channel measurement feedback (i.e., a CQI, a PMI, and an RI).
The report cycle of channel measurement feedback (hereinafter commonly called “CQI information”) and a frequency unit (or frequency resolution) to be measured may be controlled by an eNB. In a time domain, a periodic or aperiodic CQI report may be supported. The PUCCH format 2 may be used for a periodic report, and a PUSCH may be used for an aperiodic report. In the case of an aperiodic report, an eNB may instruct UE to carry an individual CQI report on a resource scheduled to transmit uplink data.
The SC-FDMA symbols 1 and 5 (i.e., the second and the sixth symbols) of the SC-FDMA symbols 0 to 6 of one slot are used to transmit a demodulation reference signal (DMRS), and the remaining SC-FDMA symbols of the SC-FDMA symbols 0 to 6 of the slot may be used to CQI information. Meanwhile, in the case of an extended CP, one SC-FDMA symbol (SC-FDMA symbol 3) is used for DMRS transmission.
In the PUCCH format 2/2a/2b, modulation by a CAZAC sequence is supported, and a QPSK-modulated symbol is multiplied by a CAZAC sequence of a length 12. A Cyclic Shift (CS) of the sequence is changed between a symbol and a slot. Orthogonal covering is used for a DMRS.
A reference signal (DMRS) is carried on 2 SC-FDMA symbols that belong to 7 SC-FDMA symbols included in one slot and that is spaced at 3 SC-FDMA symbols. CQI information is carried on the remaining 5 SC-FDMA symbols of the 7 SC-FDMA symbols. Two RSs are used in one slot in order to support high-speed UE. Furthermore, UEs are distinguished from each other using Cyclic Shift (CS) sequences. CQI information symbols are modulated into all SC-FDMA symbols and transferred. The SC-FDMA symbols consist of one sequence. That is, UE modulates a CQI using each sequence and sends the CQI.
The number of symbols which may be transmitted in one TTI is 10, and the modulation of CQI information is determined up to QPSK. If QPSK mapping is used for an SC-FDMA symbol, a CQI value of 10 bits may be carried on one slot because a CQI value of 2 bits may be carried on the SC-FDMA symbol. Accordingly, a CQI value having a maximum of 20 bits may be carried on one subframe. Frequency domain spread code is used to spread CQI information in a frequency domain.
A CAZAC sequence (e.g., ZC sequence) of a length 12 may be used as the frequency domain spread code. Control channels may be distinguished from each other by applying CAZAC sequences having different cyclic shift values. IFFT is performed on frequency domain-spread CQI information.
12 different UEs may be subjected to orthogonal multiplexing on the same PUCCH RB by 12 cyclic shifts having the same interval. In the case of a normal CP, a DMRS sequence on the SC-FDMA symbols 1 and 5 (on an SC-FDMA symbol 3 in the case of an extended CP) are similar to a CQI signal sequence on a frequency domain, but modulation, such as CQI information, is not applied to the DMRS sequence. UE may be semi-statically configured by higher layer signaling so that it periodically reports different CQI, PMI and RI Types on PUCCH resources indicated by PUCCH resource indices nPUCCH(1,{tilde over (p)}), nPUCCH(2,{tilde over (p)}), and nPUCCH(3,{tilde over (p)}), In this case, the PUCCH resource index nPUCCH(2,{tilde over (p)}) is information indicative of a PUCCH region that is used to transmit the PUCCH format 2/2a/2b and the value of a Cyclic Shift (CS) to be used.
Hereinafter, the PUCCH format 1a and 1b is described below.
In the PUCCH format 1a/1b, a symbol modulated using a BPSK or QPSK modulation scheme is multiplied by a CAZAC sequence of a length 12. For example, the results of a modulation symbol d(0) by a CAZAC sequence r(n)(n=0, 1, 2, . . . , N−1) of a length N become y(0), y(1), y(2), . . . , y(N−1). The symbols y(0), . . . , y(N−1) may be called a block of symbols. After the modulation symbol is multiplied by the CAZAC sequence, block-wise spread using an orthogonal sequence is applied.
A Hadamard sequence of a length 4 is used for common ACK/NACK information, and a Discrete Fourier Transform (DFT) sequence of a length 3 is used for shortened ACK/NACK information and a reference signal.
In the case of an extended CP, a Hadamard sequence of a length 2 is used in a reference signal.
A Reference Signal (RS) is carried on 3 contiguous SC-FDMA symbol that belong to 7 SC-FDMA symbols included in one slot and that are placed in a middle portion, and an ACK/NACK signal is carried on the remaining 4 SC-FDMA symbols of the 7 SC-FDMA symbols.
Meanwhile, in the case of an extended CP, an RS may be carried on 2 contiguous symbols placed in the middle of one slot. The number and positions of symbols used in an RS may be different depending on control channels, and the number and positions of symbols used in an ACK/NACK signal associated with the control channels may be changed depending on the number and positions of symbols used in the RS.
ACK information (not-scrambled state) of 1 bit and 2 bits may be represented as one HARQ ACK/NACK modulation symbol using respective BPSK and QPSK modulation schemes. Positive ACK (ACK) may be encoded as “1”, and negative ACK (NACK) may be encoded as “0”.
When a control signal is to be transmitted within an allocated bandwidth, two-dimensional spreading is applied in order to increase multiplexing capacity. That is, in order to increase the number of UEs or the number of control channels that may be multiplexed, frequency domain spreading and time domain spreading are used at the same time.
In order to spread an ACK/NACK signal in a frequency domain, a frequency domain sequence is used as a base sequence. A Zadoff-Chu (ZC) sequence which is one of CAZAC sequences, may be used as the frequency domain sequence. For example, by applying a different Cyclic Shift (CS) to a ZC sequence which is a base sequence, different UEs or different control channels may be multiplexed. The number of CS resources supported in a SC-FDMA symbol for PUCCH RBs for transmitting HARQ ACK/NACK is configured by a cell-specific upper layer signaling parameter ΔshiftPUCCH.
An ACK/NACK signal spread in a frequency domain is spread in a time domain using orthogonal spreading code. A Walsh-Hadamard sequence or DFT sequence may be used as the orthogonal spreading code. For example, an ACK/NACK signal may be spread for 4 symbols using an orthogonal sequence w0, w1, w2, or w3 of a length 4. Furthermore, an RS is also spread using an orthogonal sequence of a length 3 or length 2. This is called Orthogonal Covering (OC).
A plurality of UEs may be multiplexed using a Code Division Multiplexing (CDM) method using CS resources in a frequency domain and OC resources in a time domain, such as those described above. That is, ACK/NACK information and RSs of a large number of UEs may be multiplexed on the same PUCCH RB.
The number of spreading code supported for ACK/NACK information is restricted by the number of RS symbols with respect to such time domain spreading CDM. That is, the multiplexing capacity of an RS is smaller than the multiplexing capacity of ACK/NACK information because the number of SC-FDMA symbols for RS transmission is smaller than the number of SC-FDMA symbols for ACK/NACK information transmission.
For example, in the case of a normal CP, ACK/NACK information may be transmitted in 4 symbols. 3 pieces of orthogonal spreading code not 4 are used for ACK/NACK information. The reason for this is that only 3 pieces of orthogonal spreading code may be used for an RS because the number of symbols for RS transmission is limited to 3.
In case that 3 symbols of one slot may be used for RS transmission and 4 symbols of the slot may be used for ACK/NACK information transmission in a subframe of a normal CP, for example, if 6 Cyclic Shifts (CSs) may be used in a frequency domain and 3 Orthogonal Cover (OC) resources may be used in a time domain, HARQ ACK from a total of 18 different UEs may be multiplexed within one PUCCH RB. In case that 2 symbols of one slot are used for RS transmission and 4 symbols of one slot are used for ACK/NACK information transmission in a subframe of an extended CP, for example, if 6 CSs may be used in a frequency domain and 2 OC resources may be used in a time domain, HARQ ACK from a total of 12 different UEs may be multiplexed within one PUCCH RB.
The PUCCH format 1 is described below. A Scheduling Request (SR) is transmitted in such a way as to make a request or does not make a request that UE is scheduled. An SR channel reuses an ACK/NACK channel structure in the PUCCH format 1a/1b and consists of an On-Off Keying (OKK) method based on an ACK/NACK channel design. An RS is not transmitted in the SR channel. Accordingly, a sequence of a length 7 is used in the case of a normal CP, and a sequence of a length 6 is used in the case of an extended CP. Different cyclic shifts or orthogonal covers may be allocated to an SR and ACK/NACK. That is, in order to send a positive SR, UE sends HARQ ACK/NACK through a resource allocated for the SR. In order to send a negative SR, UE sends HARQ ACK/NACK through a resource allocated for ACK/NACK.
An enhanced-PUCCH (e-PUCCH) format is described below. An e-PUCCH may correspond to the PUCCH format 3 of an LTE-A system. A block spreading scheme may be applied to ACK/NACK transmission using the PUCCH format 3.
Unlike in the existing PUCCH format 1 series or 2 series, the block spreading scheme is a method of modulating control signal transmission using an SC-FDMA method. As shown in
In the example of
General Carrier Aggregation
A communication environment taken into consideration in embodiments of the present invention includes a multi-carrier support environment. That is, a multi-carrier system or Carrier Aggregation (CA) system that is used in an embodiment of the present invention refers to a system in which one or more Component Carriers (CCs) having a smaller bandwidth than a target bandwidth are aggregated and used when the target wideband is configured in order to support a wideband.
In an embodiment of the present invention, a multi-carrier means of an aggregation of carriers (or a carrier aggregation). In this case, an aggregation of carriers means both an aggregation between contiguous carriers and an aggregation between discontiguous (or non-contiguous) carriers. Furthermore, the number of CCs aggregated between downlink and uplink may be different. A case where the number of downlink CCs (hereinafter called “DL CCs”) and the number of uplink CCs (hereinafter called “UL CCs”) are the same is called a symmetric aggregation. A case where the number of DL CCs is different from the number of UL CCs is called an asymmetric aggregation. Such the term of a carrier aggregation may be replaced with terms, such as a carrier aggregation, bandwidth aggregation, or spectrum aggregation.
An object of a carrier aggregation configured by aggregating two or more component carriers is to support up to a 100 MHz bandwidth in an LTE-A system. When one or more carriers having a smaller bandwidth than a target bandwidth are aggregated, the bandwidth of the aggregated carriers may be restricted to a bandwidth which is used in an existing system in order to maintain backward compatibility with an existing IMT system. For example, in an existing 3GPP LTE system, {1.4, 3, 5, 10, 15, 20} MHz bandwidths may be supported. In a 3GPP LTE-advanced system (i.e., LTE-A), bandwidths greater than the bandwidth 20 MHz may be supported using only the bandwidths for a backward compatibility with existing systems. Furthermore, in a carrier aggregation system used in an embodiment of the present invention, new bandwidths may be defined regardless of the bandwidths used in the existing systems in order to support a carrier aggregation.
An LTE-A system uses the concept of a cell in order to manage radio resources.
The aforementioned carrier aggregation environment may also be called a multi-cell environment. A cell is defined as a combination of a pair of a downlink resource (DL CC) and an uplink resource (UL CC), but an uplink resource is not an essential element. Accordingly, a cell may consist of a downlink resource only or a downlink resource and an uplink resource. If specific UE has a single configured serving cell, it may have 1 DL CC and 1 UL CC. If specific UE has two or more configured serving cells, it has DL CCs corresponding to the number of cells, and the number of UL CCs may be the same as or smaller than the number of DL CCs.
In some embodiments, a DL CC and an UL CC may be configured in an opposite way. That is, if specific UE has a plurality of configured serving cells, a carrier aggregation environment in which the number of UL CCs is greater than the number of DL CCs may also be supported. That is, a carrier aggregation may be understood as being an aggregation of two or more cells having different carrier frequency (the center frequency of a cell). In this case, the “cell” should be distinguished from a “cell”, that is, a region commonly covered by an eNB.
A cell used in an LTE-A system includes a Primary Cell (PCell) and a Secondary Cell (SCell). A PCell and an SCell may be used as serving cells. In the case of UE which is in an RRC_CONNECTED state, but in which a carrier aggregation has not been configured or which does not support a carrier aggregation, only one serving cell configured as only a PCell is present. In contrast, in the case of UE which is in the RRC_CONNECTED state and in which a carrier aggregation has been configured, one or more serving cells may be present. A PCell and one or more SCells are included in each serving cell.
A serving cell (PCell and SCell) may be configured through an RRC parameter. PhysCellId is the physical layer identifier of a cell and has an integer value from 0 to 503. SCellIndex is a short identifier which is used to identify an SCell and has an integer value of 1 to 7. ServCellIndex is a short identifier which is used to identify a serving cell (PCell or SCell) and has an integer value of 0 to 7. The value 0 is applied to a PCell, and SCellIndex is previously assigned in order to apply it to an SCell. That is, in ServCellIndex, a cell having the smallest cell ID (or cell index) becomes a PCell.
A PCell means a cell operating on a primary frequency (or primary CC). A PCell may be used for UE to perform an initial connection establishment process or a connection re-establishment process and may refer to a cell indicated in a handover process. Furthermore, a PCell means a cell that belongs to serving cells configured in a carrier aggregation environment and that becomes the center of control-related communication. That is, UE may receive a PUCCH allocated only in its PCell and send the PUCCH and may use only the PCell to obtain system information or to change a monitoring procedure. An Evolved Universal Terrestrial Radio Access Network (E-UTRAN) may change only a PCell for a handover procedure using the RRC connection reconfiguration (RRCConnectionReconfiguration) message of a higher layer including mobility control information (mobilityControlInfo) for UE which supports a carrier aggregation environment.
An SCell may mean a cell operating on a secondary frequency (or secondary CC). Only one PCell is allocated to specific UE, and one or more SCells may be allocated to the specific UE. An SCell may be configured after RRC connection is established and may be used to provide additional radio resources. A PUCCH is not present in the remaining cells, that is, SCells that belong to serving cells configured in a carrier aggregation environment and that do not include a PCell. When adding an SCell to UE supporting a carrier aggregation environment, an E-UTRAN may provide all types of system information related to the operation of a related cell in the RRC_CONNECTED state through a dedicated signal. A change of system information may be controlled by releasing and adding a related SCell. In this case, the RRC connection reconfiguration (RRCConnectionReconfigutaion) message of a higher layer may be used. An E-UTRAN may send dedicated signaling having a different parameter for each UE instead of broadcasting within a related SCell.
After an initial security activation process is started, an E-UTRAN may configure a network including one or more SCells by adding to a PCell that is initially configured in a connection establishing process. In a carrier aggregation environment, a PCell and an SCell may operate respective component carriers. In the following embodiments, a Primary Component Carrier (PCC) may be used as the same meaning as a PCell, and a Secondary Component Carrier (SCC) may be used as the same meaning as an SCell.
If N DL CCs are managed in a specific cell, a network may allocate M (NM) DL CCs to UE. In this case, the UE may monitor only the M limited DL CCs and receive a DL signal. Furthermore, a network may give priority to L (L≦M≦N) DL CCs and allocate major DL CCs to UE. In this case, the UE must monitor the L DL CCs. Such a method may be applied to uplink transmission in the same manner.
A linkage between a carrier frequency (or DL CC) of a downlink resource and a carrier frequency (or UL CC) of an uplink resource may be indicated by a higher layer message, such as an RRC message, or system information. For example, a combination of DL resources and UL resources may be configured by a linkage defined by System Information Block Type2 (SIB2). Specifically, the linkage may mean a mapping relationship between a DL CC in which a PDCCH carrying an UL grant is transmitted and an UL CC in which the UL grant is used and may mean a mapping relationship between a DL CC (or UL CC) in which data for an HARQ is transmitted and an UL CC (or DL CC) in which an HARQ ACK/NACK signal is transmitted.
Cross-Carrier Scheduling
In a carrier aggregation system, there are two methods, that is, a self-scheduling method and a cross-carrier scheduling method form the point of view of scheduling for a carrier or a serving cell. Cross-carrier scheduling may also be called cross-component carrier scheduling or cross-cell scheduling.
Cross-carrier scheduling means that a PDCCH (DL grant) and a PDSCH are transmitted in different DL CCs or that a PUSCH transmitted according to a PDCCH (UL grant) transmitted in a DL CC is transmitted through an UL CC different from an UL CC that is linked to the DL CC through which the UL grant has been received.
Whether cross-carrier scheduling will be performed may be activated or deactivate in a UE-specific way, and each UE may be notified through high layer signaling (e.g., RRC signaling) semi-statically.
If cross-carrier scheduling is activated, there is a need for a Carrier Indicator Field (CIF) providing notification that a PDSCH/PUSCH indicated by a PDCCH is transmitted through which DL/UL CC. For example, a PDCCH may allocate a PDSCH resource or PUSCH resource to any one of a plurality of component carriers using a CIF. That is, if a PDCCH on a DL CC allocates a PDSCH or PUSCH resource to one of multi-aggregated DL/UL CCs, a CIF is configured. In this case, a DCI format of LTE-A Release-8 may be extended according to the CIF. In this case, the configured CIF may be fixed to a 3-bit field, and the position of the configured CIF may be fixed regardless of the size of the DCI format. Furthermore, a PDCCH structure (resource mapping based on the same coding and the same CCE) of LTE-A Release-8 may be reused.
In contrast, if a PDCCH on a DL CC allocates a PDSCH resource on the same DL CC or allocates a PUSCH resource on a single-linked UL CC, a CIF is not configured. In this case, the same PDCCH structure (resource mapping based on the same coding and the same CCE) and DCI format as those of LTE-A Release-8 may be used.
If cross-carrier scheduling is possible, UE needs to monitor a PDCCH for a plurality of pieces of DCI in the control region of a monitoring CC based on a transmission mode and/or bandwidth corresponding to each CC. Accordingly, there is a need for the configuration of a search space and PDCCH monitoring capable of supporting such monitoring.
In a carrier aggregation system, a UE DL CC set is indicative of a set of DL CCs scheduled so that UE receives a PDSCH. A UE UL CC set is indicative of a set of UL CCs scheduled so that UE transmits a PUSCH. Furthermore, a PDCCH monitoring set is indicative of a set of one or more DL CCs for performing PDCCH monitoring. A PDCCH monitoring set may be the same as a UE DL CC set or may be a subset of a UE DL CC set. A PDCCH monitoring set may include at least one of DL CCs within a UE DL CC set. Alternatively, a PDCCH monitoring set may be separately defined regardless of a UE DL CC set. DL CCs included in a PDCCH monitoring set may be configured so that self-scheduling for a linked UL CC is always possible. Such a UE DL CC set, UE UL CC set, and PDCCH monitoring set may be configured in a UE-specific, UE group-specific, or cell-specific way.
If cross-carrier scheduling is deactivated, it means that a PDCCH monitoring set is always the same as UE DL CC set. In this case, there is no indication, such as separate signaling for a PDCCH monitoring set. However, if cross-carrier scheduling is activated, a PDCCH monitoring set may be defined in a UE DL CC set. That is, in order to schedule a PDSCH or PUSCH for UE, an eNB transmits a PDCCH through a PDCCH monitoring set only.
General ACK/NACK Multiplexing Method
In a situation in which UE has to simultaneously send a plurality of ACK/NACKs corresponding to a plurality of data units received from an eNB, an ACK/NACK multiplexing method based on the selection of a PUCCH resource may be taken into consideration in order to maintain the single frequency characteristic of an ACK/NACK signal and to reduce ACK/NACK transmission power.
The content of ACK/NACK responses for a plurality of data units, together with ACK/NACK multiplexing, is identified by a combination of a PUCCH resource used in actual ACK/NACK transmission and the resource of QPSK modulation symbols.
For example, if one PUCCH resource sends 4 bits and a maximum of 4 data units are transmitted, ACK/NACK results may be identified in an eNB as in Table 3 below.
In Table 3, HARQ-ACK (i) is indicative of ACK/NACK results for an i-th data unit. In Table 3, discontinuous transmission (DTX) means that there is no data unit transmitted for a corresponding HARQ-ACK(i) or that UE does not detect a data unit corresponding to the HARQ-ACK(i).
In accordance with Table 3, a maximum of 4 PUCCH resources nPUCCH,0(1), nPUCCH,1(1), npUCCH,2(1), and nPUCCH,3(1) are present, and b(0), b(1) has 2 bits transmitted using a selected PUCCH.
For example, if UE successfully receives all of the 4 data units, the UE sends 2 bits (1, 1) using the PUCCH resource nPUCCH,1(1).
If UE fails in decoding in first and third data units and succeed in decoding in second and fourth data units, the UE sends bits (1, 0) using the PUCCH resource nPUCCH,3(1).
In the selection of an ACK/NACK channel, if at least one ACK is present, NACK and DTX are coupled. The reason for this is that all of ACK/NACK states are unable to be represented using a combination of a reserved PUCCH resource and a QPSK symbol. If ACK is not present, however, DTX is decoupled from NACK.
In this case, a PUCCH resource linked to a data unit corresponding to one clear NACK may be reserved in order to send a signal for a plurality of ACKs/NACKs.
PDCCH Validation for Semi-Persistent Scheduling
Semi-Persistent Scheduling (SPS) is a scheduling method for allocating resources to specific UE so that the resources continue to be maintained during a specific time interval.
If a specific amount of data is transmitted during a specific time as in a Voice over Internet Protocol (VoIP), the waste of control information can be reduced using the SPS method because the control information does not need to be transmitted at each data transmission interval for resource allocation. In a so-called SPS method, a time resource area in which resources may be allocated is first allocated to UE.
In this case, in the semi-persistent allocation method, a time resource area allocated to specific UE may be configured to have a cycle. Next, the allocation of time-frequency resources is completed by allocating a frequency resource area, if necessary. The allocation of a frequency resource area as described above may be called so-called activation. If the semi-persistent allocation method is used, resource allocation is maintained by one signaling during a specific period. Accordingly, signaling overhead can be reduced because resource allocation does not need to be repeatedly performed.
Thereafter, if resource allocation for the UE is not required, signaling for releasing the frequency resource allocation may be transmitted from an eNB to the UE. The release of the allocation of a frequency resource area as described above may be called deactivation.
In current LTE, for SPS for uplink and/or downlink, first, UE is notified of that SPS transmission/reception need to be performed in what subframes through Radio Resource Control (RRC) signaling. That is, a time resource of time-frequency resources allocated for SPS is first designated through RRC signaling. In order to notify the UE of available subframes, for example, the UE may be notified of the cycle and offset of a subframe. However, the UE does not immediately perform transmission/reception according to SPS although it has received RRC signaling because only the time resource area is allocated to the UE through RRC signaling. The allocation of the time-frequency resources is completed by allocating a frequency resource area, if necessary. The allocation of a frequency resource area as described above may be called activation, and the release of the allocation of a frequency resource area may be called deactivation.
Accordingly, the UE receives a PDCCH indicative of activation, allocates a frequency resource based on RB allocation information included in the received PDCCH, and starts to perform transmission/reception based on a subframe cycle and offset allocated through RRC signaling by applying a modulation scheme and coding rate according to Modulation and Coding Scheme (MCS) information.
Next, when receiving a PDCCH indicative of deactivation from an eNB, the UE stops the transmission/reception. When a PDCCH indicative of activation or reactivation is received after the transmission/reception is stopped, the UE resumes transmission/reception using a subframe cycle and offset allocated through RRC signaling using RBs and an MCS designated in the corresponding PDCCH. That is, the allocation of time resources is performed through RRC signaling, but the transmission/reception of actual signals may be performed after a PDCCH indicative of the activation and reactivation of SPS is received. The stop of signal transmission/reception is performed after a PDCCH indicative of the deactivation of SPS is received.
If the following conditions are all satisfied, the UE may validate a PDCCH including an SPS indication. First, CRC parity bits added for PDCCH payload need to be scrambled with an SPS C-RNTI. Second, a New Data Indicator (NDI) field needs to be set to 0. In this case, in the case of the DCI formats 2, 2A, 2B, and 2C, an NDI field is indicative of one of activated transport blocks.
Furthermore, the validation of each field used in the DCI format is completed when each field is set based on Table 4 and Table 5 below. When such a validation is completed, the UE recognizes the received DCI information as being valid SPS activation or deactivation (or release). In contrast, if the validation is not completed, the UE recognizes that non-matching CRC is included in a received DCI format.
Table 4 illustrates fields for PDCCH validation indicative of SPS activation.
Table 5 illustrates fields for PDCCH validation indicative of SPS deactivation (or release).
If a DCI format is indicative of SPS downlink scheduling activation, a TPC command value for a PUCCH field may be used an index indicative of 4 PUCCH resource values set by a higher layer.
PUCCH Piggybacking
In a 3GPP LTE system (=E-UTRA, Rel. 8), in the case of UL, in order to efficiently use the power amplifier of UE, a Peak-to-Average Power Ratio (PAPR) characteristic or Cubic Metric (CM) characteristic affecting performance of the power amplifier are set to maintain good single carrier transmission. That is, in the case of PUSCH transmission in an existing LTE system, the single carrier characteristic of data may be maintained through DFT-precoding. In the case of PUCCH transmission, a single carrier characteristic may be maintained by carrying information on a sequence having a single carrier characteristic and sending the information. However, if DFT-precoded data is discontiguously allocated based on a frequency axis, or a PUSCH and a PUCCH are transmitted at the same time, such a single carrier characteristic is not maintained. Accordingly, if PUSCH transmission is to be performed in the same subframe as that of PUCCH transmission as in
In a subframe in which a PUSCH is transmitted, a method of multiplexing Uplink Control Information (UCI) (a CQI/PMI, HARQ-ACK, an RI, etc.) with a PUSCH region is used because existing LTE UE is unable to send a PUCCH and a PUSCH at the same time as described above.
For example, if a Channel Quality Indicator (CQI) and/or a Precoding Matrix Indicator (PMI) are to be transmitted in a subframe allocated to send a PUSCH, UL-SCH data and the CQI/PMI may be multiplexed prior to DFT-spreading and may be transmitted along with control information and data. In this case, the UL-SCH data is subjected to rate matching by taking the CQI/PMI resources into consideration. Furthermore, a method of puncturing the UL-SCH data into control information, such as HARQ ACK, and an RI, and multiplexing the results with a PUSCH region is used.
Hereinafter, a signal processing process for an uplink shared channel (hereinafter called an “UL-SCH”) may be applied to one or more transport channels or control information types.
Referring to
CRC parity bits p0, p1, p2, p3, . . . , pL−1 are attached to the bits a0, a1, a2, a3, . . . , aA−1 of the transport block received from a higher layer at step S120. In this case, A is the size of the transport block, and L is the number of parity bits. The input bits to which the CRC parity bits have been attached are b0, b1, b2, b3, . . . , bB−1. In this case, B is indicative of the number of bits of the transport block including the CRC parity bits.
The input bits b0, b1, b2, b3, . . . , bB−1 are segmented into several Code Blocks (CBs) based on the TB size. A CRC is attached to the segmented several CBs at step S121. Bits after the segmentation of the CBs and the attachment of the CRC are cr0, cr1, cr2, cr3, . . . , cr(K
Next, channel coding is performed at step S122. Output bits after the channel coding are dr0(i), dr1(i), dr2(i), dr3(i), . . . , dr(D
Next, rate matching is performed at step S123. Bits after the rate matching are er0, er1, er2, er3, . . . , er(Er−1). In this case, r is a CB number (r=0, . . . , C−1), and C is a total number of CBs. E, is the number of bits of a r-th code block that has been subjected to rate matching.
Next, a concatenation between the CBs is performed again at step S124. Bits after the concatenation of the CBs are f0, f1, f2, f3, . . . , fG−1. In this case, G is a total number of coded bits for transmission. When control information is multiplexed with UL-SCH transmission, the number of bits used for control information transmission is not included.
Meanwhile, when control information is transmitted in a PUSCH, channel coding is independently performed on a CQI/PMI, an RI, and ACK/NACK, that is, the control information, at steps S126, S127, and S128. The pieces of control information have different coding rates because different coded symbols are allocated for the transmission of the control information.
In Time Division Duplex (TDD), ACK/NACK feedback mode supports two types of ACK/NACK bundling mode and ACK/NACK multiplexing mode by the configuration of a higher layer. For ACK/NACK bundling, ACK/NACK information bits include 1 bit or 2 bits. For ACK/NACK multiplexing, ACK/NACK information bits include 1 bit to 4 bits.
After the concatenation between the CBs at step S124, the multiplexing of the coded bits f0, f1, f2, f3, . . . , fG−1 of the UL-SCH data and the coded bits q0, q1, q2, q3, . . . , qN
Next, the multiplexed data and CQI/PMI and the separately channel-coded RI and ACK/NACK are subjected to channel interleaving, thereby generating an output signal at step S129.
Multi-Input Multi-Output (MIMO)
A MIMO technology does not use single transmission antenna and single reception antenna that have been commonly used so far, but uses a multi-transmission (Tx) antenna and a multi-reception (Rx) antenna. In other words, the MIMO technology is a technology for increasing a capacity or enhancing performance using multi-input/output antennas in the transmission end or reception end of a wireless communication system. Hereinafter, MIMO is called a “multi-input/output antenna.”
More specifically, the multi-input/output antenna technology does not depend on a single antenna path in order to receive a single total message and completes total data by collecting a plurality of data pieces received through several antennas. As a result, the multi-input/output antenna technology can increase a data transfer rate within a specific system range and can also increase a system range through a specific data transfer rate.
It is expected that an efficient multi-input/output antenna technology will be used because next-generation mobile communication requires a data transfer rate much higher than that of existing mobile communication. In such a situation, the MIMO communication technology is a next-generation mobile communication technology which may be widely used in mobile communication UE and a relay node and has been in the spotlight as a technology which may overcome a limit to the transfer rate of another mobile communication attributable to the expansion of data communication.
Meanwhile, the multi-input/output antenna (MIMO) technology of various transmission efficiency improvement technologies that are being developed has been most in the spotlight as a method capable of significantly improving a communication capacity and transmission/reception performance even without the allocation of additional frequencies or a power increase.
Referring to
R
i=min(NT, NR) [Equation 1]
That is, in an MIMO communication system using 4 transmission antennas and 4 reception antennas, for example, a quadruple transfer rate can be obtained theoretically compared to a single antenna system.
Such a multi-input/output antenna technology may be divided into a spatial diversity method for increasing transmission reliability using symbols passing through various channel paths and a spatial multiplexing method for improving a transfer rate by sending a plurality of data symbols at the same time using a plurality of transmission antennas. Furthermore, active research is being recently carried out on a method for properly obtaining the advantages of the two methods by combining the two methods.
Each of the methods is described in more detail below.
First, the spatial diversity method includes a space-time block code-series method and a space-time Trelis code-series method using a diversity gain and a coding gain at the same time. In general, the Trelis code-series method is better in terms of bit error rate improvement performance and the degree of a code generation freedom, whereas the space-time block code-series method has low operational complexity. Such a spatial diversity gain may correspond to an amount corresponding to the product (NT×NR) of the number of transmission antennas (NT) and the number of reception antennas (NR).
Second, the spatial multiplexing scheme is a method for sending different data streams in transmission antennas. In this case, in a receiver, mutual interference is generated between data transmitted by a transmitter at the same time. The receiver removes the interference using a proper signal processing scheme and receives the data. A noise removal method used in this case may include a Maximum Likelihood Detection (MLD) receiver, a Zero-Forcing (ZF) receiver, a Minimum Mean Square Error (MMSE) receiver, Diagonal-Bell Laboratories Layered Space-Time (D-BLAST), and Vertical-Bell Laboratories Layered Space-Time (V-BLAST). In particular, if a transmission end can be aware of channel information, a Singular Value Decomposition (SVD) method may be used.
Third, there is a method using a combination of a spatial diversity and spatial multiplexing. If only a spatial diversity gain is to be obtained, a performance improvement gain according to an increase of a diversity disparity is gradually saturated. If only a spatial multiplexing gain is used, transmission reliability in a radio channel is deteriorated. Methods for solving the problems and obtaining the two gains have been researched and may include a double space-time transmit diversity (double-STTD) method and a space-time bit interleaved coded modulation (STBICM).
In order to describe a communication method in a multi-input/output antenna system, such as that described above, in more detail, the communication method may be represented as follows through mathematical modeling.
First, as shown in
First, a transmission signal is described below. If the NT transmission antennas are present as described above, a maximum number of pieces of information which can be transmitted are NT, which may be represented using the following vector.
s=[s1, s2, . . . , sN
Meanwhile, transmission power may be different in each of pieces of transmission information s1, s2, . . . , sNT. In this case, if pieces of transmission power are P1, P2, . . . , PNT, transmission information having controlled transmission power may be represented using the following vector.
ŝ=[ŝ1, ŝ2, . . . , ŝN
Furthermore, ŝ may be represented as follows using the diagonal matrix
P of transmission power.
Meanwhile, the information vector ŝ having controlled transmission power is multiplied by a weight matrix W, thus forming NT transmission signals x1, x2, . . . , xNT that are actually transmitted. In this case, the weight matrix functions to properly distribute the transmission information to antennas according to a transport channel condition. The following may be represented using the transmission signals x1, x2, . . . , xNT.
In this case, wij denotes weight between an i-th transmission antenna and a j-th transmission information, and W is an expression of a matrix of the weight. Such a matrix W is called a weight matrix or precoding matrix.
Meanwhile, the transmission signal x, such as that described above, may be considered to be used in a case where a spatial diversity is used and a case where spatial multiplexing is used.
If spatial multiplexing is used, all the elements of the information vector s have different values because different signals are multiplexed and transmitted. In contrast, if the spatial diversity is used, all the elements of the information vector s have the same value because the same signals are transmitted through several channel paths.
A method of mixing spatial multiplexing and the spatial diversity may be taken into consideration. In other words, the same signals may be transmitted using the spatial diversity through 3 transmission antennas, for example, and the remaining different signals may be spatially multiplexed and transmitted.
If NR reception antennas are present, the reception signals y1, y2, . . . , yNR of the respective antennas are represented as follows using a vector y.
y=[y1, y2, . . . , yN
Meanwhile, if channels in a multi-input/output antenna communication system are modeled, the channels may be classified according to transmission/reception antenna indices. A channel passing through a reception antenna i from a transmission antenna j is represented as hij. In this case, it is to be noted that in order of the index of hij, the index of a reception antenna comes first and the index of a transmission antenna then comes.
Several channels may be grouped and expressed in a vector and matrix form. For example, a vector expression is described below.
As shown in
hiT=[hi1, hi2, . . . , hiN
Furthermore, if all channels from the NT transmission antenna to NR reception antennas are represented through a matrix expression, such as Equation 7, they may be represented as follows.
Meanwhile, Additive White Gaussian Noise (AWGN) is added to an actual channel after the actual channel experiences the channel matrix H. Accordingly, AWGN n1, n2, . . . , nNR added to the NR reception antennas, respectively, are represented using a vector as follows.
n=[n1, n2, . . . , nN
A transmission signal, a reception signal, a channel, and AWGN in a multi-input/output antenna communication system may be represented to have the following relationship through the modeling of the transmission signal, reception signal, channel, and AWGN, such as those described above.
Meanwhile, the number of rows and columns of the channel matrix H indicative of the state of channels is determined by the number of transmission/reception antennas. In the channel matrix H, as described above, the number of rows becomes equal to the number of reception antennas NR, and the number of columns becomes equal to the number of transmission antennas NT. That is, the channel matrix H becomes an NR×NT matrix.
In general, the rank of a matrix is defined as a minimum number of the number of independent rows or columns. Accordingly, the rank of the matrix is not greater than the number of rows or columns. As for figural style, for example, the rank H of the channel matrix H is limited as follows.
rank(H)≦min(NT, NR) [Equation 11]
Furthermore, if a matrix is subjected to Eigen value decomposition, a rank may be defined as the number of Eigen values that belong to Eigen values and that are not 0. Likewise, if a rank is subjected to Singular Value Decomposition (SVD), it may be defined as the number of singular values other than 0. Accordingly, the physical meaning of a rank in a channel matrix may be said to be a maximum number on which different information may be transmitted in a given channel.
In this specification, a “rank” for MIMO transmission indicates the number of paths through which signals may be independently transmitted at a specific point of time and a specific frequency resource. The “number of layers” indicates the number of signal streams transmitted through each path. In general, a rank has the same meaning as the number of layers unless otherwise described because a transmission end sends the number of layers corresponding to the number of ranks used in signal transmission.
Reference Signal (RS)
In a wireless communication system, a signal may be distorted during transmission because data is transmitted through a radio channel. In order for a reception end to accurately receive a distorted signal, the distortion of a received signal needs to be corrected using channel information. In order to detect channel information, a method of detecting channel information using the degree of the distortion of a signal transmission when signal is transmitted through a channel and a method of transmitting signal known to both the transmission side and the reception side is mostly used. The aforementioned signal is called a pilot signal or Reference Signal (RS).
When data is transmitted/received using a multi-input/output antenna, a channel state between a transmission antenna and a reception antenna needs to be detected in order to accurately receive a signal. Accordingly, each transmission antenna needs to have an individual reference signal.
A downlink reference signal includes a Common Reference Signal (CRS) shared by all UEs within one cell and a Dedicated Reference Signal (DRS) for specific UE. Information for demodulation and channel measurement may be provided using such reference signals.
The reception side (i.e., UE) measures a channel state based on a CRS and feeds indicators related to channel quality, such as a Channel Quality Indicator (CQI), a Precoding Matrix Index (PMI) and/or a Rank Indicator (RI), back to the transmission side (i.e., an eNB). The CRS is also called a cell-specific RS. In contrast, a reference signal related to the feedback of Channel State Information (CSI) may be defined as a CSI-RS.
The DRS may be transmitted through resource elements if data on a PDSCH needs to be demodulated. UE may receive information about whether a DRS is present through a higher layer, and the DRS is valid only if a corresponding PDSCH has been mapped. The DRS may also be called a UE-specific RS or demodulation RS (DMRS).
Referring to
A CRS is described in detail below. The CRS is used to estimate the channel of a physical antenna and is a reference signal which may be received by all UEs located in a cell in common. The CRS is distributed to the entire frequency bandwidth. Furthermore, the CRS may be used for Channel Quality Information (CQI) and data demodulation.
The CRS is defined in various formats depending on an antenna array on the transmission side (i.e., an eNB). In a 3GPP LTE system (e.g., release-8), various antenna arrays are supported, and the transmission side of a downlink signal has three types of antenna arrays, such as 3 single transmission antennas, 2 transmission antennas, and 4 transmission antennas. If an eNB uses a single transmission antenna, reference signals for a single antenna port are arrayed. If an eNB uses 2 transmission antennas, reference signals for 2 transmission antenna ports are arrayed using a Time Division Multiplexing (TDM) method and/or a Frequency Division Multiplexing (FDM) method. That is, different time resources and/or different frequency resources are allocated so that reference signals for 2 antenna ports are distinguished from each other.
Furthermore, if an eNB uses 4 transmission antennas, reference signals for 4 transmission antenna ports are arrayed using the TDM and/or FDM methods. Channel information measured by the reception side (i.e., UE) of a downlink signal may be used to demodulate data transmitted using a transmission method, such as single transmission antenna transmission, transmission diversity, closed-loop spatial multiplexing, open-loop spatial multiplexing, or an multi-User-multi-input/output (MIMO) antennas.
If a multi-input/output antenna is supported, when a reference signal is transmitted by a specific antenna port, the reference signal is transmitted in the positions of resource elements specified depending on the pattern of the reference signal and is not transmitted in the positions of resource elements specified for other antenna ports. That is, reference signals between different antennas do not overlap.
A rule for mapping a CRS to a resource block is defined as follows.
In Equation 12, k and l denote a subcarrier index and a symbol index, respectively, and p denotes an antenna port. NsymbDL denotes the number of OFDM symbols in one downlink slot, and NRRDL denotes the number of radio resources allocated to downlink. ns denotes a slot index, and NIDcell denotes a cell ID. mod denotes modulo operation. The position of a reference signal is different depending on a value vshift in a frequency domain. Since the value vshift depends on a cell ID, the position of a reference signal has various frequency shift values depending on a cell.
More specifically, in order to improve channel estimation performance through a CRS, the position of a CRS may be shifted in a frequency domain. For example, if reference signals are placed at an interval of 3 subcarriers, reference signals in one cell are allocated to a 3k-th subcarrier, and reference signals in the other cell are allocated to a (3k+1)-th subcarrier. From the point of view of a single antenna port, reference signals are arrayed at an interval of 6 resource elements in a frequency domain. Reference signals are spaced apart from reference signals allocated in other antenna ports at an interval of 3 resource elements.
In a time domain, reference signals are started from the symbol index 0 of each slot and are arrayed at a constant interval. A time interval is different defined depending on the length of a cyclic prefix. In the case of a normal cyclic prefix, reference signals are placed in the symbol indices 0 and 4 of a slot. In the case of an extended cyclic prefix, reference signals are placed in the symbol indices 0 and 3 of a slot. A reference signal for an antenna port that belongs to 2 antenna ports and that has a maximum value is defined within one OFDM symbol. Accordingly, in the case of 4 transmission antenna transmission, reference signals for RS antenna ports 0 and 1 are placed in the symbol indices 0 and 4 of a slot (i.e., symbol indices 0 and 3 in the case of an extended cyclic prefix), and reference signals for antenna ports 2 and 3 are placed in the symbol index 1 of the slot. The positions of reference signals for antenna ports 2 and 3 in a frequency domain are changed in a second slot.
A DRS is described in more detail below. The DRS is used to demodulate data. In multi-input/output antenna transmission, precoding weight used for specific UE is combined with a transport channel transmitted by each transmission antenna when the UE receives a reference signal and is used to estimate a corresponding channel without any change.
A 3GPP LTE system (e.g., release-8) supports a maximum of 4 transmission antennas and uses a DRS for rank 1 beamforming. The DRS for rank 1 beamforming also indicates a reference signal for an antenna port index 5.
A rule on which a DRS is mapped to a resource block is defined as follows. Equation 13 illustrates a normal cyclic prefix, and Equation 14 illustrates an extended cyclic prefix.
In Equations 12 to 14, k and p denote a subcarrier index and an antenna port, respectively. NRBDL, ns, and NIDcell denote the number of RBs allocated to downlink, the number of slot indices, and the number of cell IDs. The position of an RS is different depending on the value vshift from the point of view of a frequency domain.
In Equations 13 and 14, k and l denote a subcarrier index and a symbol index, respectively, and p denotes an antenna port. NscRB denotes the size of an RB in a frequency domain and is represented as the number of subcarriers. nPRB denotes the number of physical RBs. NRBDPSCH denotes the frequency bandwidth of an RB for PDSCH transmission. ns denotes the index of a slot, and NIDcell denotes the ID of a cell. mod denotes modulo operation. The position of a reference signal is different depending on the value vshift in a frequency domain. Since the value vshift depends on the ID of a cell, the position of a reference signal has various frequency shift values depending on a cell.
Sounding Reference Signal (SRS)
An SRS is mostly used in the measurement of channel quality in order to perform uplink frequency-selective scheduling and is not related to the transmission of uplink data and/or control information, but the present invention is not limited thereto. The SRS may be used for various other purposes for improving power control or various startup functions of UEs which have not been recently scheduled. The startup functions may include an initial Modulation and Coding Scheme (MCS), initial power control for data transmission, a timing advance, and frequency semi-selective scheduling, for example. In this case, the frequency semi-selective scheduling means selectively allocating a frequency resource to the first slot of a subframe and pseudo-randomly hopping to another frequency in the second slot of the subframe and allocating frequency resources.
Furthermore, the SRS may be used to measure downlink channel quality, assuming that a radio channel is reciprocal between uplink and downlink. Such an assumption is particularly valid when the same frequency spectrum is shared between uplink and downlink and in Time Division Duplex (TDD) systems separated in a time domain.
The subframes of an SRS transmitted by UE within a cell may be represented by a cell-specific broadcasting signal. A 4-bit cell-specific parameter “srsSubframeConfiguration” indicates 15 available subframe arrays in which an SRS may be transmitted though respective radio frames. In accordance with such arrays, the flexibility of control of SRS overhead can be provided according to a deployment scenario.
A sixteenth array completely turns off the switch of an SRS within a cell, which is mostly suitable for a serving cell which provides service to high-speed UEs.
Referring to
Each SRS symbol is generated based on a base sequence (i.e., a random sequence or a sequence set based on Zadoff-Ch (ZC)) regarding a given time unit and frequency bandwidth. All UEs within the same cell use the same base sequence. In this case, the transmissions of SRSs from a plurality of UEs within the same cell in the same frequency bandwidth and the same time are orthogonal to each other by different cyclic shifts of a base sequence and are distinguished from each other.
SRS sequences from different cells may be distinguished from each other because different base sequences are allocated to respective cells, but orthogonality between the different base sequences is not guaranteed.
Coordinated Multi-Point (CoMP) Transmission and Reception
In line with the demand of LTE-advanced, there has been proposed CoMP transmission in order to improve system performance. CoMP is also called co-MIMO, collaborative MIMO, or network MIMO. CoMP is expected to improve performance of UE located in a cell edge and to improve the average throughput of a cell (or sector).
In general, inter-cell interference deteriorates performance of UE located in a cell edge and the average cell (or sector) efficiency in a multi-cell environment in which a frequency reuse factor is 1. In order to reduce inter-cell interference, a simple passive method, such as Fractional Frequency Reuse (FFR), has been applied to an LTE system so that UE placed in a cell edge in an interference-limited environment has proper performance efficiency. However, instead of reducing the use of frequency resources per cell, a method of reusing inter-cell interference as a signal required to be received by UE or reducing inter-cell interference is more advantageous. In order to achieve the above object, a CoMP transmission method may be used.
A CoMP method applicable to downlink may be divided into a Joint Processing (JP) method and a Coordinated Scheduling/Beamforming (CS/CB) method.
In the JP method, data may be used in each point (ie, eNB) of a CoMP unit. The CoMP unit means a set of eNBs used in the CoMP method. The JP method may be subdivided into a joint transmission method and a dynamic cell selection method.
The joint transmission method is a method of transmitting, by a plurality of points, that is, some or all of the points of a CoMP unit, signals through a PDSCH at the same time. That is, data transmitted to one UE is transmitted from a plurality of transmission points at the same time. The quality of a signal transmitted to UE can be improved coherently or non-coherently and interference between the UE and another UE can be actively removed through such a joint transmission method.
The dynamic cell selection method is a method of sending a signal by one point of a CoMP unit through a PDSCH. That is, data transmitted to one UE on a specific time is transmitted from one point, but is not transmitted from another point within the CoMP unit to the UE. A point at which data is transmitted to UE may be dynamically selected.
In accordance with the CS/CB method, a CoMP unit performs beamforming in cooperation in order to send data to one UE. That is, data is transmitted to UE in a serving cell only, but user scheduling/beamforming may be determined through cooperation between a plurality of cells within a CoMP unit.
In some embodiments, CoMP reception means the reception of a signal transmitted by cooperation between a plurality of points that are geographically separated. A CoMP method which may be applied to uplink may be divided into a Joint Reception (JR) method and a Coordinated Scheduling/Beamforming (CS/CB) method.
The JR method is a method of receiving, by a plurality of points, that is, some or all of the points of a CoMP unit, a signal transmitted through a PDSCH. In the CS/CB method, a signal transmitted through a PDSCH is received only at one point, but user scheduling/beamforming may be determined through cooperation between a plurality of cells within a CoMP unit.
Relay Node (RN)
In a relay node, data transmitted/received between an eNB and UE is transferred through two different links (i.e., a backhaul link and an access link). An eNB may include a donor cell. A relay node is wirelessly connected to a radio access network through a donor cell.
In relation to the use of the bandwidth (or spectrum) of a relay node, a case where a backhaul link operates in the same frequency bandwidth as that of an access link is called an “in-band”, and a case where a backhaul link and an access link operate in different frequency bandwidths is called an “out-band.” In both the in-band and the out-band, UE (hereinafter called “legacy UE”) operating in accordance with an existing LTE system (e.g., release-8) needs to be able to access a donor cell.
A relay node may be divided into a transparent relay node and a non-transparent relay node depending on whether UE recognizes a relay node. The term “transparent” means whether UE communicates with a network through a relay node is not recognized. The term “non-transparent” means whether UE communicates with a network through a relay node is recognized.
In relation to control of a relay node, a relay node may be divided into a relay node formed as part of a donor cell and a relay node autonomously controlling a cell.
A relay node formed as part of a donor cell may have a relay node identity (relay ID), but does not have its own cell identity.
If at least part of Radio Resource Management (RRM) is controlled by an eNB belonging to a donor cell, it is called a relay node formed as part of a donor cell although the remaining parts of the RRM are placed in the relay node. Such a relay node may support legacy UE. For example, various types of smart repeaters, decode-and-forward relays, and second layer (L2) relay nodes and a Type-2 relay node correspond to such a relay node.
In the case of a relay node autonomously controlling a cell, the relay node controls one or a plurality of cells, and a unique physical layer cell identity is provided to each of the cells controlled by the relay node. Furthermore, the cells controlled by the relay node may use the same RRM mechanism. From a viewpoint of UE, there is no difference between access to a cell controlled by a relay node and access to a cell controlled by a common eNB. A cell controlled by such a relay node can support legacy UE. For example, a self-backhauling relay node, a third layer (L3) relay node, a Type-1 relay node, and a Type-1a relay node correspond to such a relay node.
The Type-1 relay node is an in-band relay node and controls a plurality of cells, and each of the plurality of cells is seen by UE as a separate cell different from a donor cell. Furthermore, the plurality of cells has different physical cell IDs (this is defined in LTE release-8), and the relay node may send its own synchronization channel and reference signal. In the case of one cell operation, UE directly may receive scheduling information and HARQ feedback from a relay node and send its own control channels (e.g., a Scheduling Request (SR), a CQI, and ACK/NACK) to the relay node. Furthermore, the Type-1 relay node is seen by legacy UE (i.e., UE operating in accordance with an LTE release-8 system) as a legacy eNB (i.e., an eNB operating in accordance with an LTE release-8 system). That is, the Type-1 relay node has backward compatibility. Meanwhile, the Type-1 relay node is seen by UEs operating in accordance with an LTE-A system as an eNB different from a legacy eNB, thereby being capable of providing improved performance.
The Type-1a relay node has the same characteristics as the Type-1 relay node except that it operates in an out-band. The operation of the Type-1a relay node may be configured so that an influence on a first layer (L1) operation is minimized.
The Type-2 relay node is an in-band relay node, and it does not have a separate physical cell ID and thus does not form a new cell. The Type-2 relay node is transparent to legacy UE, and the legacy UE does not recognize the presence of the Type-2 relay node. The Type-2 relay node may send a PDSCH, but does not send at least CRS and PDCCH.
In order to prevent a relay node from operating in in-band, some resources in a time-frequency domain may need to be reserved for a backhaul link and may be configured so that they are not used for an access link. This is called resource partitioning.
A known principle in resource partitioning in a relay node may be described as follows. Backhaul downlink and access downlink may be multiplexed according to a Time Division Multiplexing (TDM) method on one carrier frequency (i.e., only one of a backhaul downlink and an access downlink in a specific time is activated). Likewise, backhaul uplink and access uplink may be multiplexed according to a TDM method on one carrier frequency (i.e., only one of a backhaul uplink and an access uplink in a specific time is activated).
In backhaul link multiplexing in FDD, backhaul downlink transmission may be performed in a downlink frequency bandwidth, and the transmission of a backhaul uplink may be performed in an uplink frequency bandwidth. In backhaul link multiplexing in TDD, backhaul downlink transmission may be performed in a downlink subframe of an eNB and a relay node, and the transmission of a backhaul uplink may be performed in an uplink subframe of an eNB and a relay node.
In the case of an in-band relay node, for example, when the reception of a backhaul downlink from an eNB and the transmission of an access downlink to UE are performed in the same frequency bandwidth at the same time, signal interference may be generated in the reception end of a relay node due to a signal transmitted by the transmission end of the relay node. That is, signal interference or RF jamming may be generated in the RF front end of the relay node. Likewise, when the transmission of a backhaul uplink to an eNB and the reception of an access uplink from UE are performed in the same frequency bandwidth at the same time, signal interference may be generated.
Accordingly, in order for a relay node to send/receive signals in the same frequency bandwidth at the same time, a sufficient separation needs to be provided between a reception signal and a transmission signal (e.g., that the reception signal and the transmission signal need to be sufficiently separated geographically, such as that a transmission antenna and a reception antenna are installed on the ground and in the grave, respectively).
One method for solving such signal interference is to allow a relay node to operate in such a way as not to send a signal to UE while receiving a signal from a donor cell. That is, a gap is generated in transmission from the relay node to the UE, and the UE (including legacy UE) is configured to not expect any transmission from the relay node during the gap. Such a gap may be configured by configuring a Multicast Broadcast Single Frequency Network (MBSFN) subframe.
In
The second subframe using an MBSFN subframe is described in detail. The control region of the second subframe may be said to be a relay node non-hearing period. The relay node non-hearing interval means an interval in which a relay node does not receive a backhaul downlink signal, but sends an access downlink signal. The interval may be configured to have a 1, 2 or 3 OFDM length, such as that described above. A relay node performs access downlink transmission to UE in a relay node non-hearing interval, but may perform backhaul downlink reception from an eNB in the remaining region. In this case, time is taken for the relay node to switch from transmission mode to reception mode because the relay node is unable to perform transmission/reception in the same frequency bandwidth at the same time. Accordingly, a Guard Time (GP) needs to be configured so that the relay node switches to transmission/reception mode in the first some interval of a backhaul downlink reception region. Likewise, a guard time for enabling the relay node to switch to reception/transmission mode may be configured although the relay node operates in such a way as to receive a backhaul downlink from the eNB and to send an access downlink to the UE. The length of such a guard time may be set as a value in a time domain. For example, the length of the guard time may be set as a k (k1) time sample (Ts) value or may be set as one or more OFDM symbol length. Alternatively, relay node backhaul downlink subframes may be contiguously configured, or the guard time of the last part of a subframe may not be defined or configured according to a specific subframe timing alignment relationship. Such a guard time may be defined only in a frequency domain configured for backhaul downlink subframe transmission in order to maintain backward compatibility (if a guard time is configured in an access downlink interval, legacy UE cannot be supported). In a backhaul downlink reception interval other than the guard time, the relay node can receive a PDCCH and a PDSCH from the eNB. This may be represented by a relay-PDCCH (R-PDCCH) and a relay-PDSCH (R-PDSCH) in the meaning of a relay node-dedicated physical channel.
Channel State Information (CSI) Feedback
An MIMO method may be divided into an open-loop method and a closed-loop method. In the open-loop method, a transmission end performs MIMO transmission without the feedback of CSI from an MIMO reception end. In the closed-loop MIMO method, a transmission end receives CSI fed back by an MIMO reception end and performs MIMO transmission. In the closed-loop MIMO method, in order to obtain the multiplexing gain of an MIMO transmission antenna, each of a transmission end and a reception end may perform beamforming based on CSI. A transmission end (e.g., an eNB) may allocate an uplink control channel or an uplink shared channel to a reception end (e.g., UE) so that a reception end (e.g., UE) is able to feed CSI back.
The feedback CSI may include a Rank Indicator (RI), a Precoding Matrix Index (PMI), and a Channel Quality Indicator (CQI).
The RI is information about a channel rank. The channel of a rank means a maximum number of layers (or streams) in which different information may be transmitted through the same time-frequency resource. A rank value may be fed back in a longer cycle (i.e., less frequently) than a PMI and CQI because it is mostly determined by long term fading of a channel.
The PMI is information about a precoding matrix which is used in transmission from a transmission end and is a value into which the spatial characteristic of a channel is reflected. The term “precoding” means that a transmission layer is mapped to a transmission antenna, and a layer-antenna mapping relationship may be determined based on a precoding matrix. The PMI corresponds to the PMI of an eNB, which is preferred by UE based on a metric, such as a Signal-to-Interference plus Noise Ratio (SINR). In order to reduce feedback overhead of precoding information, a method of previously sharing, by a transmission end and a reception end, a codebook including several precoding matrices and feeding only an index indicative of a specific precoding matrix in the corresponding codebook back may be used.
The CQI is information indicative of the intensity of channel or quality of channel. The CQI may be represented as a predetermined MCS combination. That is, a CQI index that is fed back is indicative of a corresponding modulation scheme and coding rate. In general, the CQI is a value into which a reception SINR which may be obtained when an eNB configures a space channel using a PMI is reflected.
In a system (e.g., LTE-A system) supporting an extended antenna configuration, to obtain additional multi-user diversity using a multi-user-MIMO (MU-MIMO) method is taken into consideration. In the MU-MIMO method, an interference channel is present between UEs multiplexed in an antenna region. Accordingly, it is necessary to prevent interference from occurring in another UE if an eNB performs downlink transmission using CSI fed back by one UE of multiple users. Accordingly, in order for an MU-MIMO operation to be correctly performed, CSI having higher accuracy compared to a single user-MIMO (SU-MIMO) method needs to be fed back.
A new CSI feedback method using improved CSI including an existing RI, PMI, and CQI may be used so that more accurate CSI can be measured and reported as described above. For example, precoding information fed back by a reception end may be indicated by a combination of two PMIs. One (the first PMI) of the two PMIs has the attributes of a long term and/or a wideband and may be called W1. The other (the second PMI) of the two PMIs has the attributes of a short term and/or a sub-band and may be called W2. The final PMI may be determined by a combination (or function) of W1 and W2. For example, assuming that the final PMI is W, W=W1*W2 or W=W2*W1 may be defined.
In this case, the average characteristics of a channel in terms of the frequency and/or time are reflected in W1. In other words, W1 may be defined as CSI in which the characteristics of a long term channel in terms of time are reflected, the characteristics of a wideband channel in terms of frequency are reflected, or the characteristics of a long term channel in terms of time and a wideband channel in terms of frequency are incorporated. In order to simply represent such characteristics of W1, W1 is called CSI of long term-wideband attributes (or a long term wideband PMI).
A channel characteristic that is instantaneous compared to W1 is reflected in W2. In other words, W2 may be defined as CSI in which the characteristics of a short term channel in terms of time are reflected, the characteristics of a sub-band channel in terms of frequency are reflected, or the characteristics of a short term channel in terms of time and a sub-band channel in terms of frequency are reflected. In order to simply represent such characteristics of W2, W2 is called CSI of a short term-sub-band attributes (or a short term sub-band PMI).
In order for one final precoding matrix W to be determined based on information about 2 different attributes (e.g., W1 and W2) indicative of a channel state, it is necessary to configure a separate codebook including precoding matrices indicative of channel information about attributes (i.e., a first codebook for W1 and a second codebook for W2). The form of a codebook configured as described above may be called a hierarchical codebook. Furthermore, to determine a codebook to be finally used using the hierarchical codebook may be called hierarchical codebook transformation.
If such a codebook is used, channel feedback of higher accuracy compared to a case where a single codebook is used is made possible. Single cell MU-MIMO and/or multi-cell cooperation communication may be supported using channel feedback of higher accuracy as described above.
Enhanced PMI for MU-MIMO or CoMP
In a next-generation communication standard, such as LTE-A, there has been proposed transmission schemes, such as MU-MIMO and CoMP, in order to achieve a high transfer rate. In order to implement such improved transmission schemes, UE needs to feed more complicated and various CSI back to an eNB.
For example, in MU-MIMO, a CSI feedback method of uploading, by UE-A, the PMI (hereinafter called a “best companion PMI (BCPMI)”) of UE to be scheduled along with the UE-A, together with the desired PMI of the UE-A, when the UE-A selects a PMI is taken into consideration.
That is, when co-scheduled UE is used as a precoder in a precoding matrix codebook, it calculates a BCPMI that provides less interference to UE-A and additionally feeds the calculated BCPMI back to an eNB.
The eNB schedules the UE-A and another UE which prefers BCPM (Best Companion Precoding Matrix (BCPM) corresponding to a BCPMI) precoding using the information.
A BCPMI feedback method is divided into explicit feedback and implicit feedback depending on whether feedback payload is present or not.
First, there is an explicit feedback method having feedback payload.
In the explicit feedback method, UE-A determines a BCPMI within a precoding matrix codebook and feeds the BCPMI back to an eNB through a control channel. In one method, UE-A may select an interference signal precoding matrix that maximizes an estimated SINR within a codebook and feed the interference signal precoding matrix back as a BCPMI value.
An advantage of the explicit feedback method is to select a BCPMI more effective in removing interference and to send the selected BCPMI. The reason for this is that, assuming that each of all codewords within a codebook is one interference beam, UE determines a value most effective in removing interference to be a BCPMI by performing comparison on metrics, such as SINRs. A greater feedback payload size is required because candidate BCPMIs are increased as a codebook size is increased.
Second, there is an implicit feedback method not having feedback payload.
In the implicit feedback method, UE-A does not search a codebook for a codeword having the least interference and select the retrieved codebook as a BCPMI, but a corresponding BCPMI is statically determined once a desired PMI is determined. In this case, a BCPMI may include vectors orthogonal to the determined desired PMI.
The reason for this is that it is effective to reduce interference from an interference signal when desired PM is selected in directions other than the direction of a PM because the desired PM has been configured in the direction in which the channel gain of a channel H can be maximized in order to maximize a reception SINR. If the channel H is analyzed as a plurality of independent channels through Singular Value Decomposition (SVD), such a BCPMI decision method is further justified. A 4×4 channel H may be decomposed through SVD as in Equation 15 below.
In Equation 15, U,V is a unitary matrix. ui, vi, and λi are the 4×1 left singular vector, 4×1 right singular vector, and singular value of a channel H and are arranged in λi>λi+| in descending order. All channel gains which may be theoretically obtained if a beamforming matrix V is used in a transmission end and a beamforming matrix U″ is used in a reception end can be obtained without a loss.
In the case of a rank 1, optimal performance may be obtained from the point of view of an SNR because a channel gain |λ1|2 is obtained when a transmission beamforming vector v1 and a reception beamforming vector u1 are used. For example, it is advantage for UE-A to select a PM most similar to v1 in the case of a rank 1. If a desired PM is ideally matched up with v1, an interference signal can be perfectly removed without a loss of a desired signal by setting a reception beam as u1 and setting the transmission beam of the interference signal in a direction orthogonal to the PM. If there is some difference between a desired PM and v1 due to a quantization error, however, an interference signal may not be perfectly removed without a loss of a desired signal because the transmission beam of the interference signal set in the direction orthogonal to the PM is no longer the same as a beam orthogonal to v1, but it may help control the interference signal if the quantization error is small.
As an example of implicit feedback, if an LTE codebook is used, a BCPMI may be statically determined to be a vector index orthogonal to a PMI.
In this case, it has been assumed that the number of transmission antennas is 4 and UE which has fed the PMI back has a reception rank of 1, and 3 vectors orthogonal to a desired PMI are represented as 3 BCPMIs.
For example if a PMI is 3, a BCPMI is determined to be 0, 1, or 2. The PMI and the BCPMI are indicative of the indices of a 4×1 vector codeword within a codebook. An eNB considers the BCPMI set (BCPMI=0, 1, 2) to be a valid precoding index for removing interference and uses some of or the entire BCPMI set as the precoder of co-schedule UE.
An advantage of an implicit PMI is that there is no additional feedback overhead because a desired PMI and a BCPMI set are mapped in a 1:1 way. However, a BCPM dependent on desired PM may have an error in the direction of an optimal interference removal beam due to the quantization error of the desired PM (i.e., a precoding matrix corresponding to a PMI). If a quantization error is not present, all 3 BCPMs represent interference beams (ideal interference beams) for perfectly removing interference. If a quantization error is present, however, there is a difference between the beam of each of the 3 BCPMs and an ideal interference beam.
Furthermore, a difference between the ideal interference beams of the BCPMs is the same in average, but may be different on a specific moment. For example, if a desired PMI=3, it may be effective to remove an interference signal in order of BCPMIs 0, 1, and 2. In this case, there is a possibility that an eNB unaware of a relative error between the BCPMIs 0, 1, and 2 may determine the BCPMI 2 having the greatest error with an ideal interference beam to be the beam of an interference signal and may perform communication in the state in which strong interference is present between co-scheduled UEs.
General (Device-to-Device) D2D Communication
A device-to-device (D2D) communication technology refers to a scheme in which terminals geographically close to each other directly communicate with each other without passing through an infrastructure such as a base station (BS) (or an eNB). As the D2D communication technology, a technology largely using a unlicensed frequency band such as Wi-Fi direct or Bluetooth which has already been commercialized has been developed. However, in order to enhance frequency use efficiency of a cellular system, development of a D2D communication technology utilizing a licensed frequency band and standardization are ongoing.
In general, D2D communication is limitedly used as a term indicative of communication between things or thing intelligence communication. In an embodiment of the present invention, however, D2D communication may include all types of communication between a variety of types of devices having a communication function, such as smart phones and personal computers, in addition to simple devices having a communication function.
A scenario for D2D communication may be basically divided into (1) an out-of-coverage network, (2) a partial-coverage network, and (3) an in-coverage network depending on where UE1 and UE2 are placed within cell coverage (i.e., in-coverage) and out of cell coverage (i.e. out-of-coverage).
The in-coverage network may be divided into an in-coverage-single-cell and an in-coverage-multi-cell depending on the number of cells corresponding to coverage of an eNB.
The out-of-coverage network scenario means that D2D communication is performed between D2D UEs without control of an eNB.
From
The partial-coverage network scenario means that D2D communication is performed between D2D UE placed within network coverage and D2D UE placed out of the network coverage.
From
The in-coverage network scenario means that D2D UEs perform D2D communication through control of an eNB within network coverage.
In
D2D communication is described in more detail below.
D2D communication may be performed in the scenarios of
Sidelink transmission may be performed in an uplink spectrum in the case of FDD and may be performed in an uplink (or downlink) subframe in the case of TDD. Time Division Multiplexing (TDM) may be used for the multiplexing of sidelink transmission and uplink transmission.
Sidelink transmission and uplink transmission are not occurred at the same time. Sidelink transmission is not occurred in a sidelink subframe which partially or generally overlaps an uplink subframe or UpPTS used for uplink transmission. Furthermore, the transmission and reception of a sidelink are also not occurred at the same time.
The structure of an uplink physical resource may be identically used as the structure of a physical resource used for sidelink transmission. However, the last symbol of a sidelink subframe includes a guard period and is not used for sidelink transmission.
A sidelink subframe may include an extended Cyclic Prefix (CP) or a normal CP.
D2D communication may be basically divided into discovery, direct communication, and synchronization.
1) Discovery
D2D discovery may be applied within network coverage (including an inter-cell and an intra-cell). In inter-cell discovery, both synchronous and asynchronous cell deployments may be taken into consideration. D2D discovery may be used for various commercial purposes, such as advertising, issuing coupons, and finding friends, to UE within a proximity region.
If UE 1 has a role of sending a discovery message, the UE 1 sends a discovery message, and UE 2 receives the discovery message. The transmission and reception roles of the UE 1 and the UE 2 may be changed. Transmission from the UE 1 may be received by one or more UE(s), such as the UE 2.
The discovery message may include a single MAC PDU. In this case, the single MAC PDU may include a UE ID and an application ID.
A physical sidelink discovery channel (PSDCH) may be defined as a channel for sending the discovery message. The structure of a PUSCH may be reused as the structure of the PSDCH.
Two types Type 1 and Type 2 may be used as a resource allocation method for D2D discovery.
In the case of Type 1, an eNB may allocate a resource for sending a discovery message in a non-UE-specific way.
To be specific, a radio resource pool comprising a plurality of subframe sets and a plurality of resource block sets for transmitting and receiving a discovery message within a specific period (in what follows, ‘discovery period’) is allocated, and a discovery transmitting UE selects a specific resource within the radio resource pool in an arbitrary manner and transmits a discovery message.
The periodic discovery resource pool can be allocated for transmission of a discovery signal in a semi-static manner. The configuration information of a discovery resource pool for discovery transmission includes a discovery period, a subframe set which can be used for transmission of a discovery signal within the discovery period, and information about a resource block set. The configuration information of the discovery resource pool can be transmitted to the UE through upper layer signaling. In the case of an in-coverage UE, the discovery resource pool for discovery transmission is set up by an eNB and can be informed to the UE through RRC signaling (for example, System Information Block (SIB)).
The discovery resource pool allocated for discovery within one discovery period can be multiplexed to a time-frequency resource block of the same size through TDM and/or FDM scheme, where the time-frequency resource block of the same size can be called a ‘discovery resource’. A discovery resource can be set as one subframe unit and include two Physical Resource Blocks (PRBs) per slot in each subframe. One UE can use one discovery resource for transmission of a discovery MAC PDU.
Also, a UE can transmit a discovery signal repeatedly within a discovery period for transmission of one transport block. Transmission of a MAC PDU by one UE can be repeated (for example, four times) contiguously or non-contiguously within the discovery period (namely radio resource pool). The transmission times of a discovery signal for one transmission block can be transmitted to the UE through upper layer signaling.
UE may randomly select a first discovery resource in a discovery resource set which may be used for the repetitive transmission of an MAC PDU and may determine the remaining discovery resources in relation to the first discovery resource. For example, a specific pattern may be previously determined, and a next discovery resource may be determined according to the predetermined specific pattern depending on the position of a discovery resource first selected by UE. Alternatively, UE may randomly select each discovery resource within a discovery resource set which may be used for the repetitive transmission of an MAC PDU.
In the case of Type 2, a resource for discovery message transmission is allocated in a UE-specific way. Type 2 is subdivided into Type-2A and Type-2B. Type-2A is a method of allocating, by an eNB, a resource at the instance at which UE sends a discovery message within a discovery cycle, and Type-2B is a method of allocating resources semi-persistently.
In the case of Type-2B, RRC_CONNECTED UE requests an eNB to allocate a resource for the transmission of a D2D discovery message through RRC signaling. Furthermore, the eNB may allocate the resource through RRC signaling. When the UE transits to an RRC_IDLE state or when the eNB withdraws resource allocation through RRC signaling, the UE releases the most recently allocated transmission resource. As described above, in the case of Type-2B, a radio resource may be allocated through RRC signaling, and the activation/deactivation of an allocated radio resource may be determined by a PDCCH.
A radio resource pool for receiving a discovery message may be configured by an eNB, and UE may be notified of the configured radio resource pool through RRC signaling (e.g., a System Information Block (SIB)).
Discovery message reception UE monitors both the aforementioned discovery resource pools of Type 1 and Type 2 in order to receive a discovery message.
2) Direct Communication
The region to which D2D direct communication is applied includes a network coverage edge area (i.e., edge-of-coverage) in addition to inside and outside network coverage (i.e., in-coverage and out-of-coverage). D2D direct communication may be used for purposes, such as Public Safety (PS).
If UE 1 has a role of direct communication data transmission, the UE 1 sends direct communication data, and UE 2 receives the direct communication data. The transmission and reception roles of the UE 1 and the UE 2 may be changed. The direct communication transmission from the UE 1 may be received by one or more UE(s), such as the UE 2.
D2D discovery and D2D communication may be independently defined without being associated with each other. That is, in groupcast and broadcast direct communication, D2D discovery is not required. If D2D discovery and D2D direct communication are independently defined as described above, UEs do not need to perceive adjacent UE. In other words, in the case of groupcast and broadcast direct communication, all reception UEs within a group are not required to be adjacent to each other.
A physical sidelink shared channel (PSSCH) may be defined as a channel for sending D2D direct communication data. Furthermore, a physical sidelink control channel (PSCCH) may be defined as a channel for sending control information (e.g., Scheduling Assignment (SA), a transmission format for direct communication data transmission, etc) for D2D direct communication. The structure of a PUSCH may be reused as the structures of the PSSCH and the PSCCH.
Two types of mode 1 and mode 2 may be used as a resource allocation method for D2D direct communication.
Mode 1 refers to a method of scheduling, by an eNB, data for D2D direct communication by UE or a resource used for UE to send control information. Mode 1 is applied to in-coverage.
An eNB configures a resource pool for D2D direct communication. In this case, the resource pool for D2D communication may be divided into a control information pool and a D2D data pool. When an eNB schedules control information and a D2D data transmission resource within a pool configured for transmission D2D UE using a PDCCH or ePDCCH (enhanced PDCCH), the transmission D2D UE sends control information and D2D data using the allocated resource.
Transmission UE requests a transmission resource from an eNB. The eNB schedules a resource for sending control information and D2D direct communication data. That is, in the case of mode 1, the transmission UE needs to be in the RRC_CONNECTED state in order to perform D2D direct communication. The transmission UE sends a scheduling request to the eNB, and a Buffer Status Report (BSR) procedure is performed so that the eNB may determine the amount of resources requested by the transmission UE.
Reception UEs monitors a control information pool. When decoding control information related to reception UE, the reception UE may selectively decode D2D data transmission related to corresponding control information. The reception UE may not decode a D2D data pool based on a result of the decoding of the control information.
Mode 2 refers to a method of randomly selecting, by UE, a specific resource in a resource pool in order to send data or control information for D2D direct communication. Mode 2 is applied to out-of-coverage and/or edge-of-coverage.
In mode 2, a resource pool for sending control information and/or a resource pool for sending D2D direct communication data may be pre-configured or may be configured semi-statically. UE is supplied with a configured resource pool (time and frequency) and selects a resource for D2D communication transmission in the resource pool. That is, the UE may select a resource for control information transmission in a control information resource pool in order to send control information. Furthermore, the UE may select a resource in a data resource pool in order to send D2D direct communication data.
In D2D broadcast communication, control information is transmitted by broadcasting UE. Control information is explicitly and/or implicitly indicative of the position of a resource for data reception in relation to a physical channel (i.e., a PSSCH) on which D2D direct communication data is carried.
3) Synchronization
A D2D Synchronization Signal/sequence (D2DSS) can be used by a UE to obtain time-frequency synchronization. In particular, since the eNB is unable to control a UE located beyond network coverage, a new signal and procedure can be defined to establish synchronization among UEs. A D2D synchronization signal can be called a sidelink synchronization signal.
A UE transmitting a D2D synchronization signal periodically can be called a D2D synchronization source or a sidelink synchronization source. In case a D2D synchronization source is an eNB, the structure of a D2D synchronization signal being transmitted can be identical to that of PSS/SSS. In case the D2D synchronization source is not an eNB (for example, a UE or GNSS (Global Navigation Satellite System)), the structure of a D2D synchronization signal being transmitted can be newly defined.
The D2D synchronization signal is transmitted periodically with a period not shorter than 40 ms. Each UE can have a physical-layer D2D synchronization identity. The physical-layer D2D synchronization identifier may be called a physical-layer sidelink synchronization identity or simply a D2D synchronization identifier.
The D2D synchronization signal includes a D2D primary synchronization signal/sequence and a D2D secondary synchronization signal/sequence. These signals can be called a primary sidelink synchronization signal and a secondary sidelink synchronization signal, respectively.
Before transmitting a D2D synchronization signal, the UE may first search for a D2D synchronization source. If a D2D synchronization source is found, the UE can obtain time-frequency synchronization through a D2D synchronization signal received from the D2D synchronization source found. And the corresponding UE can transmit the D2D synchronization signal.
Also, a channel for the purpose of transferring essential information used in communication between terminals together with synchronization may be required, and such a channel having the purpose may be defined. The channel may be referred to as a physical D2D synchronization channel (PD2DSCH) or a physical sidelink broadcast channel (PSBCH).
In D2D communication, direct communication between two devices is described below as an example, for clarity, but the scope of the present invention is not limited thereto. The same principle described in an embodiment of the present invention may be applied to D2D communication between a plurality of two or more devices.
Method for Allocating D2D Resource Based on Interference Measurement
In a cellular system, when terminals close to each other perform D2D communication, a load of a base station may be distributed, and since a signal is transmitted within a relatively short distance, power consumption of terminals may be reduced and transmission delay (latency) may also be reduced. From the point of view of an overall system, frequency use efficiency may be improved as an existing cellular terminal (or cellular UE) and a D2D terminal (or D2D UE) share the same frequency and re-use the frequency spatially. In addition, D2D communication may be utilized for a relay between terminals, and effects of collecting information of a shop within a distance in the proximity of a location of a mobile user or goods, precise positioning in an indoor area, group communication between people within a distance close to each other, or creating a new proximity-based service such as a network game, a location-based advertisement, and the like, are anticipated.
A difference of cellular-based D2D communication to D2D communication in an unlicensed frequency band is that a BS may participate in controlling D2D communication including resource allocation.
Cellular-based D2D communication may be classified into three types depending on a degree of intervention of a BS as illustrated in
In the distributed D2D scheme, a D2D terminal recognizes peripheral interference and allocates resource by itself. In this scheme, the BS is free from a load; however, since resource allocation of a cellular user is dynamically changed, it is difficult for the D2D terminal to control resource allocation.
The centralized D2D scheme is a scheme in which a BS collects channel information or distance information of links and allocates D2D resource on the basis of the collected information. This scheme exhibits excellent performance but is disadvantageous in that a load of signal processing is large and it is difficult to apply to a situation in which a BS is not properly functioned such as in a disaster situation.
Thus, the present invention proposes an interference awareness-based D2D communication technique in a cellular uplink based on a hybrid D2D communication technology complementing shortcomings of the centralized D2D scheme and the distributed D2D scheme.
The present invention proposes a method for determining resource to obtain optimal communication performance by measuring a magnitude of interference produced by a cellular terminal for a D2D terminal to share cellular resource.
Among interference-aware techniques for measuring interference produced by a cellular terminal, spectrum sensing is a technique for sub-user (e.g., a D2D terminal) to determine whether a main user (e.g., a cellular terminal) uses a spectrum in a cognitive radio system. Energy detection, signal characteristics detection, and the like, are representative methods of spectrum sensing.
Referring to
In a case in which cellular terminals use the same transmission power and only path loss depending on a distance is considered, the least interference may be measured in a resource (i.e., a resource block (RB)) allocated to a cellular terminal farthest away from a D2D receiver. That is, in
Here, however, in an actual environment, each cellular terminal may have different transmission power and due to a shadow effect and an influence of fading, a cellular terminal corresponding to an optimal RB may not be the simply closest cellular terminal. However, unless a channel situation is changed, it may be most desirable to share a resource determined to have the least interference in an interference awareness process.
In the present invention, resource allocation regarding a cellular terminal may be changed according to a scheduling period (e.g., a subframe), causing a problem in which a resource discovered in a process of discovering a resource having the least interference by the D2D terminal (or D2D UE) may be meaningless. That is, due to a gap between a time at which the D2D Rx terminal (or D2D Rx UE) discovers a resource having the least interference and a time at which a D2D terminal (hereinafter, referred to as a “D2D Tx terminal (or D2D Tx UE)” transmitting a D2D signal (e.g., a D2D discovery message/signal, or control information or data for D2D direct communication) uses an allocated resource, the same resource may be allocated to different cellular terminals.
Also, since there is a gap between a time at which a cellular terminal is allocated an uplink resource from a BS and a time at which the cellular terminal transmits uplink, a resource discovered by the D2D terminal in a process of discovering a resource having the least interference may be meaningless.
Hereinafter, a relationship of uplink resource allocation (UL grant) by a BS and an uplink data (PUSCH) transmission timing of a terminal will be described.
Referring to
Meanwhile, in a TDD wireless frame structure, since downlink/uplink subframe configurations are different by uplink-downlink configuration (please refer to Table 1), and thus, a PUSCH and PHICH transmission time is set to be different according to uplink-downlink configurations, and a transmission time of the PUSCH and the PHICH may be configured to be different according to indices (or numbers) of subframes.
In the LTE/LTE-A system, an uplink/downlink timing relationship of a PUSCH, a preceding PDCCH (including UL grant), and a PHICH in which a downlink HARQ ACK/NACK corresponding to the PUSCH is transmitted is determined in advance.
Table 6 illustrates a transmission timing of the PDCCH and the PUSCH corresponding thereto by uplink/downlink configuration.
Referring to Table 6, in the case of the uplink-downlink configurations 1 to 6, when a terminal receives a UL grant (i.e., a PDCCH/EPDCCH having a DCI format 0/4) and/or a PHICH in nth downlink subframe from a BS, the terminal transmits a PUSCH in (n+k)th uplink subframe corresponding to the downlink subframe index in which the PDCCH/EPDCCH and/or the PHICH were transmitted. Here, the value k is as shown in Table 6.
In order to solve the problem, in the present invention, D2D and cellular resource allocation is performed through scheduling synchronized based on a cellular terminal which has been allocated a corresponding resource at the same time as a time at which an optimal resource (i.e., resource with the least interference) of a D2D terminal is discovered by the help of a BS.
Hereinafter, in this disclosure, “resource” refers to a resource (i.e., a PSDCH resource) for transmitting a discovery message/signal or a resource (i.e., a PSSCH resource) for transmitting data or control information for D2D direct communication. That is, the discovery message/signal transmission will be described on the assumption of type 2, and the D2D direct communication will be described on the assumption of mode 1. Thus, hereinafter, the discovery message/signal, and data/control information for D2D direct communication will be generally referred to as a “D2D signal”.
Also, in this disclosure, “resource” may refer to a resource specific to a terminal, or may also be interpreted as an aggregation of resource candidates that may be used by a corresponding terminal, i.e., a resource pool.
However, for the purposes of description, it is assumed that one resource block (RB) is allocated to a D2D terminal and a cellular terminal in one scheduling time unit (i.e., a scheduling period). However, the present invention is not limited thereto and a plurality of RBs may also be allocated in one time unit.
Meanwhile, it may be assumed that a time for a D2D reception terminal (D2D Rx terminal) to measure interference in a resource used by a cellular terminal (i.e., a time required for discovering a resource) and feedback delay for the D2D Rx terminal to report measurement results to a BS are basically the same as a channel quality indicator (CQI) measurement and feedback delay of the cellular terminal.
Thus, it may be said that it is possible to measure interference having accuracy of a CQI level and a change in a channel due to feedback delay is weak. According to 3GPP LTE/LTE-A standard, CQI feedback delay is an interval of four subframes, i.e., 4 ms. It is equal to a time for a terminal, which moves at a rate of 100 kilometers/hr, to move about 11 centimeters, from which a change in a channel due to feedback delay is anticipated to be insignificant.
Referring to
When the D2D Rx terminal receives a D2D signal, an uplink transmission of a cellular terminal may be received as interference. Thus, the D2D Rx terminal measures a signal-to-noise ratio (SNR) or a signal-to-interference plus noise ratio (SINR) in an uplink resource of the cellular terminal, and selects a cellular uplink resource on the basis of the measured interference. That is, the D2D Rx terminal selects the cellular uplink resource with the least interference.
The D2D Rx terminal reports shared resource information determined based on the interference awareness technology in step S2301 and resource discovery time to a BS (S2302).
The D2D Rx terminal may transmit one or more pieces of shared candidate resource information determined to be lower than a preset threshold value and a resource discovery time to the BS. Here, in a case in which the D2D Rx terminal transmits one or more pieces of shared candidate resource information, the D2D Rx terminal may transmit interference information measured for each of the shared candidate resources together to the BS.
Here, the resource information may be indicated by an RB index for specifying a corresponding resource, and the resource discovery time may be indicated by a radio frame index and/or a subframe index.
The BS synchronizes scheduling of the D2D terminal to the cellular terminal scheduled with the same resource (i.e., reported shared resource) at the time (i.e., reported resource discovery time) at which the D2D Rx terminal has performed resource discovery (S2303).
First, the BS checks a cellular terminal allocated a shared resource received from the D2D Rx terminal at the resource discovery time received from the D2D Rx terminal. For this checking process, the BS requires uplink scheduling information of a previous (sub)frame regarding the cellular terminal, and thus, the BS should store scheduling information during a certain time (for example, a predetermined period of time). As mentioned above, in the case of the FDD system, when an uplink resource is allocated in a subframe n, the cellular terminal transmits uplink data in n+4. Also, in the case of a TDD system, the cellular terminal transmits uplink data at a predetermined time from a point in time at which uplink resource was allocated (please refer to Table 6).
Also, feedback delay occurs in a time during which the D2D Rx terminal measures interference in a resource used by the cellular terminal (that is, a time required for resource discovery) and a time required for reporting the results measured by the D2D Rx terminal to the BS.
Thus, for example, during a time of the sum of interference measurement result feedback delay time+uplink data transmission time point delay time, the BS may store scheduling information of the cellular terminal.
Also, the BS synchronizes resource scheduling of the D2D terminal (D2D Tx terminal and/or D2D Rx terminal) and the corresponding cellular terminal and allocates resource to the D2D terminal and the cellular terminal.
Here, synchronization refers to pairing terminals using shared resource and continuously performing scheduling on the same resource.
The BS pairs the D2D terminal and the cellular terminal and perform resource allocation (scheduling). Pairing may be performed in such a manner that one or more D2D terminals and one or more cellular terminals share resource partially or entirely in terms of resource allocation.
The BS transmits synchronized scheduling information to the D2D Tx terminal and/or D2D Rx terminal (S2304).
Here, the BS may transmit scheduling information to the D2D Tx terminal and/or the D2D Rx terminal in a unicast manner. Also, the BS may broadcast scheduling information.
The BS may allocate resource, which is allocated to the cellular terminal, to the D2D Tx terminal and/or the D2D Rx terminal in the same manner, and may transmit the allocated resource information to the cellular terminal and the D2D terminal (i.e., the D2D Tx terminal and/or the D2D Rx terminal). In this case, the scheduling information may refer to resource allocation information.
Meanwhile, the BS may transmit information regarding pairing between the cellular terminal and the D2D terminal (i.e., the D2D Tx terminal and/or D2D Rx terminal) to the cellular terminal and the D2D terminal (i.e., the D2D Tx terminal and/or D2D Rx terminal). In this case, the scheduling information may refer to pairing information.
For example, the BS broadcasts pairing information regarding an identifier (e.g., a C-RNTI, or the like) of the cellular terminal and an identifier of the D2D terminal. The D2D terminal monitors a PDCCH transmitted from the BS and checks downlink control information (DCI) of the corresponding cellular terminal on the basis of the identifier of the cellular terminal paired to the D2D terminal to use uplink resource allocated to the corresponding cellular terminal, as resource for D2D signal transmission.
In this manner, the D2D terminal may continuously share the same resource with the cellular terminal, which is optimal and has least interference, during the resource discovery process.
Referring to
As discussed above, the D2D Rx terminal measures an SNR or an SINR in an uplink resource of the cellular terminal, and selects a cellular uplink resource on the basis of the measured interference. That is, the D2D Rx terminal selects the cellular uplink resource with the least interference.
The D2D Rx terminal reports determined shared resource information and a resource discovery time to the BS (S2402).
The D2D Rx terminal may transmit information regarding one or more shared candidate resources having interference determined to be lower than a preset threshold value and a resource discovery time to the BS. In this manner, when the information regarding one or more shared candidate resources is transmitted, the D2D Rx terminal may transmit interference information measured for each of the shared candidate resources to the BS together.
The BS synchronizes scheduling of the D2D terminal and the cellular terminal scheduled in the same resource (i.e., the reported shared resource) at a point in time at which resource discovery was performed (i.e., the reported resource discovery time) (S2403).
First, the BS checks a cellular terminal with the shared resource allocated thereto from the D2D Rx terminal in the resource discovery time received from the D2D Rx terminal. As mentioned above, for this checking process, the BS should store scheduling information of previous (sub)frame regarding the cellular terminal in a buffer. For example, the BS may store the scheduling information of the cellular terminal during a time of the sum of interference measurement result feedback delay time and uplink data transmission time point delay time.
Also, the BS allocates resource to the D2D terminal and the cellular terminal by synchronizing resource scheduling of the D2D terminal (D2D Tx terminal and/or D2D Rx terminal) and the corresponding cellular terminal.
The BS transmits the synchronized scheduling information to the cellular terminal and simultaneously transmits the synchronized scheduling information to the D2D Tx terminal and/or the D2D Rx terminal (S2404).
Here, the BS may transmit the scheduling information to the D2D Tx terminal and/or the D2D Rx terminal in a unicast manner. Also, the BS may broadcast the scheduling information.
As mentioned above, the BS may transmit resource information allocated to the cellular terminal or the information regarding pairing with the cellular terminal as scheduling information to the D2D terminal (i.e., D2D Tx terminal and/or D2D Rx terminal).
The D2D Tx terminal transmits a D2D signal in the resource shared with the cellular terminal to the D2D Rx terminal according to the scheduling information received from the BS (S2405).
When the D2D signal transmitted from the D2D Tx terminal is successfully decoded, the D2D Rx terminal transmits an acknowledge (ACK) signal to the D2D Tx terminal (S2406). However, when decoding of the D2D signal transmitted from the D2D Tx terminal fails, the D2D Rx terminal transmits a non-acknowledge (NACK) signal to the D2D Tx terminal (S2407).
When the NACK signal is received from the D2D Rx terminal, the D2D Tx terminal re-transmits the D2D signal (S2408).
However, when NACK is continuously generated during D2D communication, it has a high possibility that an interference situation has been changed due to movement of the terminal, and thus, the D2D Rx terminal performs resource re-discovery through interference awareness, to determine an optimal shared resource (S2410). The D2D Rx terminal re-transmits the determined shared resource information and a resource discovery time to the BS (S2411) so as to be allocated a resource from the BS in the same manner as described above.
Also, alternatively, the D2D Rx terminal may periodically perform resource re-discovery, regardless of whether NACK is generated, to allow the D2D terminal to share resource with an optimal cellular terminal.
Hereinafter, a method for allocating resource of a D2D terminal will be described in detail.
Specifically,
In
Also, for the purposes of description, it is assumed that a delay time between an uplink resource allocation time point and an uplink data transmission time point is ‘0’.
Referring to
At time T2, the D2D Rx terminal reports information regarding the UL RB 6 and the T1 time information to the BS. Upon checking the received information regarding the UL RB 6, the BS checks scheduling information of the previous T1 time to determine that a cellular terminal for which UL RB 6 was scheduled is UE 6.
At time T3, the BS simultaneously allocates UL RB 7, which is allocated to UE 6, to the D2D terminal in scheduling regarding cellular terminals. Similarly, the BS simultaneously allocates UL RB, which is allocated to UE 6, to the D2D terminal at every scheduling period afterwards.
The D2D terminal (D2D Tx terminal and D2D Rx terminal) checks scheduling information from the BS at time T4 and performs communication using UL RB 7 used by UE 6. Thereafter, the D2D terminal (D2D Tx terminal and D2D Rx terminal) checks scheduling information from the BS at every scheduling period and performs communication using the same UL RB 7 as the UL RB used by the UE 6.
Meanwhile, when NACK is continuously generated during D2D communication, there is a high possibility that an interference situation has been changed due to movement of a terminal, and thus, the D2D Rx terminal performs resource re-discovery through interference awareness.
The D2D Rx terminal performs optimal resource discovery through interference awareness in uplink resource (i.e., UL RB 1 to UL RB 7) at time T8 and selects UL RB3 with the smallest amount of interference.
Also, at time T9, the D2D Rx terminal reports information regarding UL RB 3 and T8 time information to the BS.
Upon checking the received information regarding UL RB 3, the BS checks scheduling information at a previous T8 time to determine that a cellular terminal for which the UL RB 3 was scheduled is UE 1.
At time T10, the BS simultaneously allocates UL RB 1, which is allocated to UE 1, to the D2D terminal in scheduling regarding cellular terminals. Thereafter, similarly, the BS simultaneously allocates UL RB, which is allocated to the UE 1, to the D2D terminal at every scheduling period.
The D2D terminal (D2D Tx terminal and D2D Rx terminal) checks every scheduling information from the BS from time T10 and performs communication using UL RB used by the UE 1.
In this manner, the dynamic synchronization scheduling scheme is a scheme of adjusting scheduling of the D2D terminal based on scheduling information of the cellular terminal.
Since the scheduling information of the cellular terminal is dynamically changed, scheduling of the D2D terminal should be changed accordingly and the D2D terminal should check resource allocation information at every scheduling period like the cellular terminal.
Specifically,
In
Also, for the purposes of description, it is assumed that a delay time between an uplink resource allocation time point and an uplink data transmission time point is ‘0’.
Referring to
At time T2, the D2D Rx terminal reports information regarding the UL RB 6 and the T1 time information to the BS.
Upon checking the received information regarding the UL RB 6, the BS checks scheduling information of the previous T1 time to determine that a cellular terminal for which UL RB 6 was scheduled is UE 6.
At time T3, the BS simultaneously allocates UL RB 7, which is allocated to UE 6, to the D2D terminal in scheduling regarding cellular terminals.
Thereafter, the BS fixes the scheduling information regarding UE 6 to the UL RB 7, thereby fixing resource scheduling of D2D to the UL RB 7 in the same manner. That is, after the D2D Rx terminal reports the optimal shared resource to the BS through interference awareness, the D2D Tx terminal and/or the D2D Rx terminal is continuously allocated the first allocated uplink resource.
Also, in this case, when NACK is continuously generated during D2D communication, there is a high possibility that an interference situation has been changed due to movement of a terminal, and thus, the D2D Rx terminal performs resource re-discovery through interference awareness. As a result, the D2D Rx terminal is allocated the same uplink resource until resource re-discovery is performed through interference awareness.
The D2D Rx terminal performs optimal resource discovery through interference awareness in uplink resource (i.e., UL RB 1 to UL RB 7) at time T8 and selects UL RB 1 with the smallest amount of interference.
Also, at time T9, the D2D Rx terminal reports information regarding UL RB3 and T8 time information to the BS.
Upon checking the received information regarding UL RB 3, the BS checks scheduling information at a previous T8 time to determine that a cellular terminal for which the UL RB 3 was scheduled is UE 1.
At time T10, the BS simultaneously allocates UL RB 1, which is allocated to UE 1, to the D2D terminal in scheduling regarding cellular terminals.
Thereafter, the BS fixes scheduling information regarding the UE 1 to the UL RB 1, thereby fixing resource scheduling of D2D to the UL RB 1 in the same manner. That is, after the D2D Rx terminal reports the optimal shared resource to the BS through interference awareness, the D2D Tx terminal and/or the D2D Rx terminal is continuously allocated the first allocated uplink resource.
The static synchronization scheduling is a scheme of fixing scheduling information of the cellular terminal on the basis of the resource discovery information of the D2D terminal. Cellular performance may be lowered due to the fixed scheduling of the cellular terminal, but since there is no need for the D2D terminal to check additional scheduling information until resource re-discovery after the first resource allocation, a load is reduced.
The D2D performance and performance of the cellular system according to the resource allocation scheme for D2D signal transmission proposed in the present invention were verified through a simulation. The following six layout options are defined to evaluate performance of the
D2D technology in the LTE-Advanced system.
1) Option 1: Downtown macro (500 m ISD (Inter-Site Distance))+1 RRH (Remote Radio Head) per cell/indoor hot zone)
2) Option 2: Downtown macro (500 m ISD)+1 dual stripe per cell)
3) Option 3: Downtown macro (500 m ISD) (every UE outdoor)
4) Option 4: Downtown macro (500 m ISD)+3 RRH/indoor hot zone per cell
5) Option 5: Downtown macro (1732 m ISD)
6) Option 6: Downtown macro (100 m ISD)
Table 7 illustrates parameters of a simulation.
Options 1, 2, 3, 4, and 6 use parameters defined in 3GPP case 1 and option 5 uses 3GPP case 3 parameters
In order to evaluate performance of the proposed resource allocation scheme, option 1 and option 3 were considered. In the option 1 environment, ⅔ of the entire terminals was disposed within a small cell and the other remaining ⅓ was uniformly disposed in a macro cell outside of the small cell. In the option a 3 environment, all the users within a macro cell were uniformly disposed. Path loss according to distances and mutually independent Rayleight fading were applied to all the communication links (cellular-BS, cellular-RRH, D2D Tx-D2D Rx) and interference links (CUE-D2D Rx, D2D Tx-BS, D2D Tx-RRH, D2D Tx-other D2D Rx). A path loss exponent was commonly set to 3.5.
Table 7 illustrates major parameter values applied to the simulation. The number of entire UEs within a cell was set to 100, and performance when the number of the cellular UEs (CUEs) is larger than the number of D2D UEs (i.e., CUEs was ⅔ and D2D UEs was ⅓) and performance when the number of CUEs was smaller than the number of D2D UEs (i.e., CUEs was ⅓ and D2D UE was ⅔) were compared. It was assumed that the number of CUEs and the number of RBs were the same and one RB was allocated to each CUE, and a target SNR value for determining transmission power of the CUE was fixed to 220 dB for the sake of convenience. A radius of a macro cell was set to 167 meters corresponding to one sector of 500 m ISD (Inter-Sector Distance).
Specifically,
Specifically,
The proposed method is a hybrid D2D communication technique based on interference awareness through cooperation between a BS and a D2D terminal to solve the problem arising in the distributed D2D and centralized D2D. Since the D2D terminal discovers resource by itself using the interference awareness technique to reduce a load of a BS, compared with the centralized D2D, and a problem due to a dynamic change in cellular scheduling in the distributed D2D is solved through synchronization scheduling between a cellular terminal and a D2D terminal with the aid of a BS. Thus, D2D communication may be smoothly performed through the present invention.
Device to Which Present Invention may be Applied
Referring to
The eNB 3010 includes a processor 3011, a memory 3012, and a radio frequency (RF) unit 3013. The processor 3011 implements functions, processes and/or methods proposed in
The UE 3020 includes a processor 3021, a memory 3022, and a radio frequency (RF) unit 3023. The processor 3021 implements functions, processes and/or methods proposed in
The memory 3012 or 3022 may be present within or outside of the processor 3011 or 3021 and may be connected to the processor 3011 or 3021 through various well known units. Also, the eNB 3010 and/or the UE 3020 may have a single antenna or multiple antennas.
The aforementioned embodiments are achieved by combination of structural elements and features of the present invention in a predetermined manner. Each of the structural elements or features should be considered selectively unless specified separately. Each of the structural elements or features may be carried out without being combined with other structural elements or features. Also, some structural elements and/or features may be combined with one another to constitute the embodiments of the present invention. The order of operations described in the embodiments of the present invention may be changed. Some structural elements or features of one embodiment may be included in another embodiment, or may be replaced with corresponding structural elements or features of another embodiment. Moreover, it will be apparent that some claims referring to specific claims may be combined with another claims referring to the other claims other than the specific claims to constitute the embodiment or add new claims by means of amendment after the application is filed.
An embodiment of the present invention may be implemented by various means, for example, hardware, firmware, software or a combination of them. In the case of implementations by hardware, an embodiment of the present invention may be implemented using one or more Application-Specific Integrated Circuits (ASICs), Digital Signal Processors (DSPs), Digital Signal Processing Devices (DSPDs), Programmable Logic Devices (PLDs), Field Programmable Gate Arrays (FPGAs), processors, controllers, microcontrollers and/or microprocessors.
In the case of implementations by firmware or software, an embodiment of the present invention may be implemented in the form of a module, procedure, or function for performing the aforementioned functions or operations. Software code may be stored in the memory and driven by the processor. The memory may be placed inside or outside the processor, and may exchange data with the processor through a variety of known means.
It is evident to those skilled in the art that the present invention may be materialized in other specific forms without departing from the essential characteristics of the present invention. Accordingly, the detailed description should not be construed as being limitative from all aspects, but should be construed as being illustrative. The scope of the present invention should be determined by reasonable analysis of the attached claims, and all changes within the equivalent range of the present invention are included in the scope of the present invention.
In the wireless communication system of this disclosure, an example of the resource allocation method in D2D communication applied to a 3GPP LTE/LTE-A system is described, but the resource allocation method may also be applied to various wireless communication systems other than the 3GPP LTE/LTE-A system.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/KR2015/002279 | 3/10/2015 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 61950795 | Mar 2014 | US |
