Patent Application
20210203392
The present disclosure relates to methods and devices for transmitting and receiving channel state information in wireless communication systems, and particularly, to methods and devices for transmitting and receiving channel state information in wireless communication systems using multiple antennas.
2. Description of the Related Art
In order to meet the demand for wireless data traffic soaring since the 4G communication system came to the market, there are ongoing efforts to develop enhanced 5G communication systems or pre-5G communication systems. For the reasons, the 5G communication system or pre-5G communication system is called the beyond 4G network communication system or post LTE system.
For higher data transmit rates, 5G communication systems are considered to be implemented on ultra high frequency bands (mmWave), such as, e.g., 60 GHz. To mitigate pathloss on the ultra high frequency band and increase the reach of radio waves, the following techniques are taken into account for the 5G communication system: beamforming, massive multi-input multi-output (MIMO), full dimensional MIMO (FD-MIMO), array antenna, analog beamforming, and large-scale antenna.
Also being developed are various technologies for the 5G communication system to have an enhanced network, such as evolved or advanced small cell, cloud radio access network (cloud RAN), ultra-dense network, device-to-device (D2D) communication, wireless backhaul, moving network, cooperative communication, coordinated multi-point (CoMP), and interference cancellation.
There are also other various schemes under development for the 5G system including, e.g., hybrid FSK and QAM modulation (FQAM) and sliding window superposition coding (SWSC), which are advanced coding modulation (ACM) schemes, and filter bank multi-carrier (FBMC), non-orthogonal multiple access (NOMA) and sparse code multiple access (SCMA), which are advanced access schemes.
As such, wireless communication systems are evolving to high-speed, high-quality wireless packet data communication systems to provide data services and multimedia services beyond the initial versions that have provided voice-centered services. To that end, various standardization organizations, such as the 3GPP or the IEEE, proceed to standardize evolved wireless communication systems adopting multicarrier-based multiple access schemes. As an example, a diversity of wireless communication standards including 3GPP long term evolution (LTE) and IEEE 802.16m, have been developed to support high-rate, high-quality wireless packet data transmission services based on multiple access schemes. LTE, 802.16m, or other existing evolved wireless communication systems are based on multi-carrier multiple access schemes and may employ various techniques, such as the multi-antenna technology, multiple input multiple output (MIMO), beamforming, adaptive modulation and coding (AMC), and channel sensitive scheduling.
The above-enumerated techniques enhance system capability by, e.g., concentrating transmit power coming from several antennas depending on, e.g., channel quality, adjusting the amount of data transmitted, or selectively transmitting data to the user with a relatively good channel quality to produce a better transmission efficiency. Such schemes mostly operate based on the channel state information between the base station (eNB or BS) and the terminal (user equipment (UE) or MS). Accordingly, it is needed to measure the channel state between the base station and the UE. LTE systems use channel status indication reference signals (CSI-RSs) as reference signals for measuring the channel state. Base station means a device that is located in a predetermined place to carry out downlink transmission and uplink reception. One base station may perform transmission and reception for a plurality of cells. In a wireless communication system, a plurality of base stations may be geographically dispersed and each base station may perform transmission and reception for a plurality of cells.
The reference signal is a signal used to assist in the demodulation and decoding of data symbols received by measuring the state of channel between the base station and the UE(s), such as channel strength or distortion, interference strength, or Gaussian noise, in the wireless mobile communication system. Another use of such a reference signal is to measure the wireless channel state. The receiver may determine the state of radio channel between itself and the transmitter by measuring the strength of reference signal transmitted from the transmitter at an agreed-on transmit power and received via the radio channel. The so-determined state of radio channel is used to determine the data rate for which the receiver is to send a request to the transmitter.
The evolved wireless mobile communication system standards such as 3GPP LTE (-A) or IEEE 802.16m primarily adopt multiple-access schemes using multiple subcarriers such as orthogonal frequency division multiplexing (multiple access)(A) (OFDM(A)). Wireless communication systems adopting multiple access schemes using multiple carriers make differences in channel estimation and measurement capabilities depending on how many symbols and subcarriers the reference signal they are to place in on the time and frequency. Further, the channel estimation and measurement capabilities are influenced by how much power is to be assigned to the reference signal as well. Accordingly, if more time, frequency, and power or other radio resources are allocated to the reference signal, the channel estimation and measurement capability may be enhanced, resultantly leading to demodulation and decoding performance for reception data symbols, as well as increased accuracy of channel state measurement.
However, wireless communication systems are typically assigned limited time, frequency, transmit power, or other radio resources for transmitting signals, and thus, assigning relatively many radio resources to the reference signals may relatively reduce the radio resources that may be allocated to data signals. For that reason, radio resources allocated to the reference signals should be properly determined considering the system throughput. In particular, a technical factor that matters in adopting MIMO which performs transmission and reception using multiple antennas is to assign and measure a reference signal.
In the MIMO schemes, e.g., the full dimension (FD)-MIMO scheme, the precoding matrix indicator (PMI) overhead that the UE should report increases as the codebook size goes up. In particular, periodic channel state reporting is subject to a limitation in the size of the physical uplink control channel (PUCCH) payload, requiring a method for reducing the PMI overhead to fit the PUCCH payload. As an example, existing periodic channel state reporting has adopted codebook subsampling that removes duplicate beam groups or reduces the co-phasing count for compensating for the phase differences between the antennas with different polarizations and beams that are selectable in order to shrink the codebook. However, the periodic channel state reporting that is supported by FD-MIMO bears a large PMI overhead over the existing, and adopting the existing method as it is may cause a significant performance loss.
Thus, according to the present disclosure, there are provided a method and device for efficiently transmitting and receiving periodic channel state reports in a wireless communication system supporting multiple antennas.
According to the present disclosure, there are also provided a method and device capable of reducing the overhead of channel state reporting in a wireless communication system supporting multiple antennas.
According to the present disclosure, there are also provided a periodic channel state reporting method and device capable of minimizing the performance loss considering the increased circuit board and PMI overhead in a wireless communication system supporting the FD-MIMO.
According to the present disclosure, there are also provided a periodic channel state reporting method and device using a codebook defined to be adopted for various antenna arrays in a wireless communication system supporting the FD-MIMO.
According to an embodiment of the present disclosure, a method for transmitting channel state information by a user equipment (UE) in a wireless communication system using multiple antennas comprises receiving first configuration information about at least one reference signal from a base station, receiving second configuration information for generating periodic channel state information based on measuring the at least one reference signal, receiving and measuring the at least one reference signal from the base station, based on the first configuration information, and generating the periodic channel state information including a first precoding matrix indicator (PMI) for a wideband based on the second configuration information including configuration information for channel state reporting on a subband and transmitting the periodic channel state information to the base station.
According to an embodiment of the present disclosure, a UE in a wireless communication system using multiple antennas comprises a transceiver configured to transmit and receive data and a controller configured to perform controller to receive first configuration information about at least one reference signal from a base station, receive second configuration information for generating periodic channel state information based on a result of measuring the at least one reference signal, receive and measure the at least one reference signal from the base station, based on the first configuration information, and generate the periodic channel state information including a first precoding matrix indicator (PMI) for a wideband based on the second configuration information including configuration information for channel state reporting on a subband and transmitting the periodic channel state information to the base station.
According to an embodiment of the present disclosure, a method for receiving feedback information by a base station in a wireless communication system using multiple antennas comprises transmitting first configuration information about at least one reference signal, transmitting second configuration information for receiving from a UE, as the feedback information, periodic channel state information based on a result of measuring the at least one reference signal, and receiving the channel state information including a first PMI for a wideband from the UE based on the first configuration information and the second configuration information, wherein the first PMI for the wideband is received from the UE based on the second configuration information including configuration information for channel state reporting on a subband.
According to an embodiment of the present disclosure, a base station in a wireless communication system using multiple antennas comprises a transceiver configured to transmit and receive data and a controller configured to perform control to transmit first configuration information about at least one reference signal, transmit second configuration information for receiving from a UE, as feedback information, periodic channel state information based on a result of measuring the at least one reference signal, and receive the channel state information including a first PMI for a wideband from the UE based on the first configuration information and the second configuration information, wherein the first PMI for the wideband is received from the UE based on the second configuration information including configuration information for channel state reporting on a subband.
The above and other aspects, features, and advantages of certain embodiments of the present disclosure will be more apparent from the following description taken in conjunction with the accompanying drawings, in which:
Hereinafter, embodiments of the present disclosure are described in detail with reference to the accompanying drawings. The same reference denotations may be used to refer to the same or similar elements throughout the specification and the drawings. When making the gist of the present disclosure unclear, the detailed description of known functions or configurations is skipped.
Other aspects, advantages, and core features of the present disclosure will be apparent to one of ordinary skill in the art from the following detailed description of embodiments of the present disclosure, taken into conjunction with the drawings.
Prior to going into the detailed description of the disclosure, it might be effective to define particular words and phrases as used herein. As used herein, the terms “include” and “comprise” and their derivatives may mean doing so without any limitations. As used herein, the term “or” may mean “and/or.” As used herein, the phrase “associated with” and “associated therewith” and their derivatives may mean “include,” “be included within,” “interconnect with,” “contain,” “be contained within,” “connect to or with,” “couple to or with,” “be communicable with,” “cooperate with,” “interleave,” “juxtapose,” “be proximate to, “be bound to or with, “have, or “have a property of.” As used herein, the term “controller” may mean any device, system, or part thereof controlling at least one operation. As used herein, the term “device” may be implemented in hardware, firmware, software, or some combinations of at least two thereof. It should be noted that functions, whatever particular controller is associated therewith, may be concentrated or distributed or implemented locally or remotely. It should be appreciated by one of ordinary skill in the art that the definitions of particular terms or phrases as used herein may be adopted for existing or future in many cases or even though not in most cases.
For a better understanding of the present disclosure, various feedback transmission schemes for channel status information (CSI) in FD-MIMO systems and LTE/LTE-A systems to which the present disclosure may be applicable are described prior to specific embodiments of the present disclosure.
The present disclosure may be applied not only to FD-MIMO systems but also, in the same or similar manner, to various wireless communication systems that transmit data using a few tens of, or more, antennas.
The LTE/LTE-A or other wireless communication systems utilize the MIMO technique in which transmission is performed using a plurality of transmit/receive antennas in order to increase system capability and data transmission rate. The MIMO technique makes use of a plurality of transmission/reception antennas to spatially separate and transmit a plurality of information streams. As such, spatially separating and transmitting a plurality of information streams is called spatial multiplexing. Generally, the number of information streams to which spatial multiplexing may be applied varies depending on the number of antennas of the transmitter and receiver. In general, the number of information streams to which spatial multiplexing may apply is referred to as the rank of the corresponding transmission. The MIMO technique supported by the LTE/LTE-A release 11 and its predecessors supports spatial multiplexing for the case where there are eight transmission antennas and eight reception antennas and supports up to rank-8. The FD-MIMO system to which the technology proposed according to embodiments of the present disclosure applies may be evolved over the existing LTE/LTE-A MIMO technology, supporting more than eight, e.g., 32 or more transmit antennas.
The FD-MIMO system refers to a wireless communication system that transmits data using a few tens or more transmit antennas.
Referring to
In
In order to effectively implement the FD-MIMO system, the UE needs to exactly measure the channel context and/or interference magnitude and transmit effective channel state information to the base station using the same. The base station receiving the channel state information determines UEs on which it performs transmission, at what data transmission rate to perform transmission, or the precoding to apply using the channel state information in connection with downlink transmission. Since the FD-MIMO system has more transmit antennas than the existing LTE/LTE-A system, applying the channel state information of the existing LTE/LTE-A system (the LTE/LTE-A system may simply be referred to as LTE system unless stated otherwise) to the FD-MIMO system may cause the uplink overhead issue that massive control information should be td on the uplink.
The wireless communication system has limited time, frequency, and power resources. Thus, if more resources are allocated to the reference signal, the resources allocable for traffic channel (data traffic channel) transmission may be reduced, thus resulting in a decrease on the absolute amount of data transmitted. In such case, the channel measurement and estimation capability may be enhanced, but since the absolute amount of data transmitted is reduced, the overall system capability may be rather lowered. Accordingly, a proper distribution is required between resources for the reference signal and resources for signals for traffic channel transmission in order to bring up with the optimal performance from a point of view of the overall system capability.
The radio resource shown in
The LTE system may transmit a plurality of different types of signals in the radio resource shown in
Cell specific RS (CRS) 230: a reference signal that is periodically transmitted for all the UEs belonging to one cell and that may be shared by a plurality of UEs.
Demodulation reference signal (DMRS) 235: a reference signal transmitted for a particular UE. This signal is transmitted only when data is transmitted to the corresponding UE. A DMRS may consist of a total of eight DMRS ports. In LTE/LTE-A, port 7 to port 14 correspond to DMRS ports, and the ports maintains orthogonality not to interfere with each other using code division multiplexing (CDM) or frequency division multiplexing (FDM).
Physical downlink shared channel (PDSCH) 240: a data channel transmitted on the downlink, used for a base station to transmit traffic to a UE, and transmitted via an RE where no reference signal is transmitted in the data region of
Channel status information reference signal (CSI-RS) 250: a reference signal transmitted for UEs belonging to one cell and used to measure the channel state. A plurality of CSI-RSs may be transmitted in one cell.
Other control channels (PHICH, PCFICH, and PDCCH) 245: provide control information necessary for the UE to receive the PDSCH or transmit the ACK/NACK to operate the hybrid automatic repeat and request (HARD) for uplink data transmission.
Besides the signals, the LTE-A system may set a muting so that CSI-RS transmitted from another base station may be received without interfering with the UEs in the cell. The muting may apply in the position where the CSI-RS may be transmitted. Generally, the UE may skip the corresponding radio resource, where the CSI-RS is transmitted, and may receive a traffic signal. The muting in the LTE-A system is also called zero-power CSI-RS. This is why the muting applies likewise to the resource positions of the CSI-RS and no transmit power is transmitted.
Referring to
Where the CSI-RS is transmitted over two antenna ports, the CSI-RS transmits the respective signals of the antenna ports in two REs connected together on the time axis, and the respective signals of the antenna ports are distinguished by orthogonal codes. Further, where the CSI-RS is transmitted for four antenna ports, two REs are added to the CSI-RS for two antenna ports, so that signals for the two antenna ports are further transmitted by the same method. The same also applies where the CSI-RS is transmitted for eight antenna ports.
Generally, the wireless communication system needs to transmit a reference signal to measure the state of the downlink channel. In the 3GPP LTE-advanced (LTE-A) system, the UE may measure the channel state between the UE and the base station using the CRS or CSI-RS that the base station transmits. Basically, some factors should be considered for the channel state, e.g., the quantity of interference on the downlink. The interference quantity on the downlink includes interference signals or thermal noise that is caused by the antennas in the neighbor base station and this is critical for the UE to determine the channel context of the downlink. As an example, where the base station using one transmit antenna transmits signals to the UE using one receive antenna, the UE determines per-symbol energy receivable on the downlink using the reference signal received from the base station and the amount of interference to be simultaneously received in the period where the symbol is received and determines the signal-to-noise ratio (e.g., Es/Io). The determined Es/Io is converted into a data transmission speed or its corresponding value and is notified to the base station in the form of a CQI, thereby enabling determination as to the data transmission speed at which the base station should perform data transmission to the UE on downlink.
In the LTE-A system, the UE feedbacks information about the channel state of the downlink to the base station so that it may be utilized for downlink scheduling by the base station. That is, the UE measures the reference signal transmitted from the base station on downlink and feedbacks the information extracted therefrom to the base station in a form as defined in the LTE-LTE-A standards. The information fed back by the UE in the LTE/LTE-A system includes three types of information (RI, PMI, and CQI) as follows:
1) Rank indicator (RI): the number of spatial layers that may be received in the current channel state by the UE
2) Precoder matrix indicator (PMI): an indicator for a precoding matrix favored by the UE in the current channel state.
3) Channel quality indicator (CQI): a maximum data rate at which the UE may perform reception in the current channel state. The CQI may be replaced with the signal-to-noise ratio (SINR), maximum error correction code rate and modulation scheme, or data efficiency per frequency which may be utilized similar to the maximum data rate.
The RI, PMI, and CQI are associated with one another and have meanings. Different precoding matrices as supported in the LTE/LTE-A system are defined per rank as an example. Accordingly, the PMI value Y when the RI is 1 and the PMI value Y when the RI is 2 are interpreted differently, for example. Further, it is assumed that when the UE determines the CQI, the PMI value, Y, that the UE has provided to the base station has also applied. That is, the UE feedbacking the RI_X, PMI_Y, and CQI_Z to the base station is the same as the UE notifying the base station that data may be received at the data transmission rate corresponding to the CQI_Z when the rank is the RI_X and the precoding is the PMI_Y. As such, the UE may assume the transmission scheme that the base station is to perform when calculating the CQI so that the optimized performance can be achieved when the base station actually performs transmission in the transmission scheme.
In the LTE/LTE-A system, the UE's periodic feedback information is set to one of the following four feedback modes (or reporting modes) depending on what information is contained:
1) Reporting mode 1-0: RI, wideband CQI (hereinafter, wCQI)
2) Reporting mode 1-1: RI, wCQI, PMI
3) Reporting mode 2-0: RI, wCQI, subband CQI (hereinafter, sCQI)
4) Reporting mode 2-1: RI, wCQI, sCQI, PMI
The feedback timing of each piece of information for the four feedback modes is determined by values transferred by a higher layer signal, such as Npd, NOFFSET,CQI, MR1, and NOFFSET,R1. Here, Npd and NOFFSET,CQI mean the period and offset value in the subframes for CQI/PMI reporting, and MR1 and NOFFSET,R1 means the period and relative offset value in the subframes for RI reporting. In feedback mode 1-0, the transmission period of wCQI is NPd, and the feedback timing is determined with the subframe offset of NOFFSET,CQI. Further, the transmission period of RI is NPd×MR1, and the subframe offset is NOFFSET,CQI+NOFFSET,R1
Feedback mode 1-1, although having the same feedback timing as feedback mode 1-0, differs in that the PMI together with the wCQI 310 is transmitted at the wCQI transmission timing as shown in
Meanwhile, in feedback mode 2-0, the feedback transmission period for the sCQI is Npd, and the offset value is NOFFSET,CQI, and the feedback transmission period for the wCQI is fix H×Npd, and the offset value is NOFFSET,CQI as is the offset value of the sCQI. Here, H=J*H+1, where K is a value transferred through the higher layer signal, and J is a value determined depending on the system bandwidth. For example, for 10 MHz systems, J may be defined as 3. At last, the wCQI is transmitted once for every H sCQI transmissions. The transmission of the RI is MR1×H×Npd, and the offset is NOFFSET,CQI+NOFFSET,R1.
Feedback mode 2-1, although having the same feedback timing as feedback mode 2-0, differs in that the PMI together with the wCQI 410 is transmitted at the wCQI transmission timing as shown in
In
Specifically, for eight CSI-RS antenna ports, feedback mode 1-1 is divided again into two submodes. In the first submode, the RI together with the first PMI information is transmitted, and the second PMI information is transmitted together with the wCQI. Here, the feedback period and offset for the wCQI and the second PMI are defined as Npd and NOFFSET,CQI, and the feedback period and offset for the RI and the first PMI are defined as MR1×Npd and NOFFSET,CQI+NOFFSET,R1. Here, assuming that the precoding matrix corresponding to the first PMI is W1 (or W1), and the precoding matrix corresponding to the second PMI is W2 (or W2), the UE and the base station share the information that the UE's favored precoding matrices have been determined as W1 and W2.
In the case of feedback mode 2-1 for eight CSI-RS antenna ports, the feedback of precoding type indicator (PTI) information is added. The PTI is fed back along with the RI, the period is MR1×H×Npd, and the offset is defined as NOFFSET,CQI+NOFFSET,R1. Where the PTI is 0, the first PMI, the second PMI, and the wCQI are all fed back, and the wCQI and the second PMI are together transmitted in the same timing, and the period is Npd, and the offset is given as NOFFSET,CQI. Further, the period of the first PMI is H′×Npd, and the offset is NOFFSET,CQI. Here, H′ is transferred through a higher layer signal. Further, where PTI is 1, PTI together with RI is transmitted, and wCQI and the second PMI are transmitted together, and sCQI is fed back at an additional separate timing. In this case, the first PMI is not transmitted. The period and offset of the PTI and RI are the same as those where the PTI is 0, and the period and offset of the sCQI are defined as Npd and NOFFSET,CQI. Further, the wCQI and the second PMI are fed back in the period of H×Npd and the offset of NOFFSET,CQI, and H is defined like the number of CSI-RS antenna ports is four.
The LTE/LTE-A system supports the UE's aperiodic feedback transmission as well as the periodic feedback transmission. When the base station desires to obtain aperiodic feedback information of a particular UE, the base station sets the aperiodic feedback indicator included in the downlink control information (DCI, downlink control information) for uplink data scheduling of the UE to perform particular aperiodic feedback and performs the uplink data scheduling of the UE. The corresponding UE, when receiving the indicator set to perform aperiodic feedback transmission in an nth subframe, includes the aperiodic feedback information upon data transmission in an n+kth subframe and performs uplink transmission. Here, k is a parameter defined in the 3GPP LTE release 11 standards, and this is 4 for frequency division duplexing (FDD) while defined as shown in Table 1 for time division duplexing (TDD). Table 1 below represents an example of k for each subframe number n in the TDD UL/DL configuration.
Where the aperiodic feedback transmission is set, the feedback information includes RI, PMI, and CQI like in the case of the periodic feedback transmission, and the RI and the PMI might not be fed back according to feedback settings. The CQI may include both the wCQI and the sCQI or only the wCQI information.
The LTE/LTE-A system provides codebook sampling functionality for periodic channel state reporting. In the LTE/LTE-A system, the periodic feedback transmission of the UE is transmitted to the base station through the PUCCH. Since the amount of information that may be transmitted once through PUCCH is limited, various feedback objects, such as RI, wCQI, sCQI, wPMI2, and sPMI2, may be transmitted through PUCCH by way of subsampling or two or more pieces of feedback information may be encoded together (joint encoding) and may be transmitted through PUCCH. As an example, when eight CSI-RS ports are configured by the base station, RI and PMI1 reported in submode 1 of PUCCH mode 1-1 may be joint-encoded as shown in Table 2. PMI1 constituted of 4 bits and RI constituted of 3 bits based on Table 2 are joint-encoded with a total of five bits. Submode 2 of PUCCH mode 1-1 joint encodes PMI1 consisting of four bits and PMI2 consisting of other four bits as shown in Table 3 into four bits in total. Since submode 2 has a higher subsampling level than submode 1 (e.g., four bits=>three bits subsampling in the case of submode 1, and eight bits=>four bits subsampling in the case of submode 2), it cannot report more precoding indexes. As such, subsampling means reducing the number of information bits and transmitting the information. As another example, where eight CSI-RS ports are configured by the base station, PMI2 reported in PUCCH mode 2-1 may be subsampled as shown in Table 4 below. Referring to Table 4, PMI2, when its associated RI is 1, is reported with four bits. However, in case the associated RI is two or more, differential CQI for the second codeword should be reported together, and thus it may be seen that PMI2 is subsampled with two bits and reported. The LTE/LTE-A system may apply subsampling or joint encoding to six types of periodic feedback in total including Tables 2, 3, and 4. Table 2 below represents an example of joint encoding on a first codebook index (ii) and the RI for submode 1 of PUCCH mode 1-1.
Table 3 below represents an example of joint encoding on the first and second codebook indexes (i1, i2) and the RI for submode 2 of PUCCH mode 1-1.
Table 4 represents an example of codebook subsampling for PUCCH mode 2-1.
As described above, in order to effectively implement the FD-MIMO system, the UE needs to exactly measure the channel state and interference magnitude and generate and report effective channel state information to the base station using the same. The base station receiving the channel state information determines UEs on which it performs transmission, at what data transmission rate to perform transmission, or the precoding to apply using the channel state information in connection with downlink transmission. The FD-MIMO system has many transmit antennas and considers a two-dimensional antenna array, and the shape of the antenna array actually applicable thereto may be very diversified. Accordingly, it is not appropriate to apply the method for transmitting and receiving channel state information for the LTE/LTE-A system, which is designed considering only a one-dimensional transmit antenna array having up to eight antennas, to the FD-MIMO system as it is. To optimize the FD-MIMO system, it is necessary to define a new codebook that is applicable to diverse antenna array shapes.
To that end, according to embodiments of the present disclosure described below, there are proposed schemes for reducing the complexity in channel state reporting corresponding to multiple dimensions, such as the vertical and horizontal dimension or the first and second dimensions, in periodic channel state reporting by allowing the PMI to be transmitted using the existing 2, 4, or 8 CSI-RS port codebook and some of the CSI-RSs configured for the plurality of CSI-RS ports to reduce the overhead and design complexity upon PMI reporting using a two-dimensional (2D) codebook in the FD-MIMO-based transmission and reception that is based in the LTE-A system.
Generally, where multiple transmit antennas are used as is the FD-MIMO system, CSI-RS s should be transmitted in proportion thereto. As an example, where the LTE/LTE-A system uses eight transmit antennas, the base station transmits the CSI-RS corresponding to eight ports to the UE and allows it to measure the channel state of the downlink. At this time, the base station may use a radio resource constituted of eight REs such as As and Bs in
CSI-RS transmission method 1: a method of performing transmission, with as many radio resources as the number of antennas assigned to the CSI-RS
CSI-RS transmission method 2: a method of performing transmission, with the CSI-RS divided into a plurality of dimensions.
Referring to
Reference number 310 in
H-CSI-RS port 0: is configured of a combination of antennas A0, A1, A2, and A3
H-CSI-RS port 1: is configured of a combination of antennas B0, B1, B2, and B3
H-CSI-RS port 2: is configured of a combination of antennas C0, C1, C2, and C3
H-CSI-RS port 3: is configured of a combination of antennas D0, D1, D2, and D3
H-CSI-RS port 4: is configured of a combination of antennas E0, E1, E2, and E3
H-CSI-RS port 5: is configured of a combination of antennas F0, F1, F2, and F3
H-CSI-RS port 6: is configured of a combination of antennas G0, G1, G2, and G3
H-CSI-RS port 7: is configured of a combination of antennas H0, H1, H2, and H3
In the above embodiment, combining a plurality of antennas into one CSI-RS port means antenna virtualization which may typically be achieved by a linear combination of the plurality of antennas. The V-CSI-RS for measuring the channel state of the vertical direction may be transmitted through the following four antenna ports as in reference number 330.
V-CSI-RS port 0: is configured of a combination of antennas A0, B0, C0, D0, E0, F0, G0, and H0
V-CSI-RS port 1: is configured of a combination of antennas A1, B1, C1, D1, E1, F1, G1, and H1
V-CSI-RS port 2: is configured of a combination of antennas A2, B2, C2, D2, E2, F2, G2, and H2
V-CSI-RS port 3: is configured of a combination of antennas A3, B3, C3, D3, E3, F3, G3, and H3
As such, where a plurality of antennas are arrayed in 2D, e.g., by M×N (vertical direction x horizontal direction), the channel state in the FD-MIMO system may be measured using the N horizontal CSI-RS ports and the M vertical CSI-RS ports. That is, where two CSI-RSs are used, the channel state may be grasped using M+N CSI-RS ports for MN transmit antennas. As such, grasping the information on more transmit antennas using fewer CSI-RS ports acts as a critical advantage in reducing CSI-RS overhead. In the above embodiment, MN=K CSI-RSs are used to grasp the channel state for the transmit antennas in the FD-MIMO system, and such approach may apply likewise where two CSI-RSs are used. Although the embodiment of the present disclosure described assumes the use of CSI-RS transmission method 1, the above approach may apply likewise where CSI-RS transmission method 2 is used.
As in the above example, the existing 2, 4, or 8 port CSI-RSs may be bundled up for the CSI-RS port to support multiple antennas. Such CSI-RS port supporting method may be varied depending on whether to use the non-precoded (NP) CSI-RS in which the CSI-RS is transmitted in the same scheme as the existing 2, 4, or 8 port CSI-RS or to use the beamformed (BF) CSI-RS in which the CSI-RS overhead has been reduced by applying beamforming to the antennas. To support the NP CSI-RS and the BF CSI-RS, the CSI-RS resources or CSI-RS port positions for existing 1, 2, 4, or 8 CSI-RS ports may be bundled up using the RRC fields shown in Tables 5 to 11. Although the information shown in Tables 5 to 11 is separately set forth for convenience purposes, the information shown in Tables 5 to 11 may also be appreciated as one connected RRC field. That is, Tables 5 to 11 represent examples of configurations of CSI-process information and CSI-RS information for NP CSI-RS and BF CSI-RS transmission.
The NP CSI-RS may support 12, 16, or more CSI-RS ports using the positions for the existing CSI-RS in one subframe and using the information (e.g., RRC field) of Tables 5 to 11. To that end, the field information is set forth as nzp-resourceConfigList-r13 in Tables 5 to 11, and the field information may be used to set the position for the CSI-RS. Further, in the BF CSI-RS, csi-RS-ConfigNZPldListExt-r13 and csi-IM-ConfigIdListExt-r13 may be used to bundle up individual CSI-RS resources which may differ in the number of CSI-RS ports, subframe, and codebook subset restriction to be used as the BF CSI-RS. To support 2D antennas in the NP CSI-RS, a new 2D codebook is required which may be varied depending on per-dimension antennas and oversampling factors and codebook settings.
The terms as used herein are defined as follows.
RI: a rank indicator that has been reported from the UE to the base station for the rank of channel obtained by simultaneously applying horizontal and vertical precodings to the 2D-CSI-RS or that has been determined by a predetermined rule
i1: a first PMI (i.e., corresponding to Wi) that the UE has notified the base station by obtaining the optimal precoding based on the channel obtained by applying 2D precoding to the 2D-CSI-RS. The first PMI may indicate a beam group selected in the horizontal and vertical directions.
i11: a beam group selected in the first dimension for the 2D-CSI-RS. This may be some bits of the first PMI bit payload (Wi bit payload).
i12: a beam group selected in the second dimension for the 2D-CSI-RS. This may be some bits of the first PMI bit payload (Wi bit payload).
i2: a second PMI (i.e., corresponding to W2) that the UE has notified the base station by obtaining the optimal precoding based on the channel obtained by applying 2D precoding to the 2D-CSI-RS. The second PMI may indicate co-phrasing necessary to correct the phase difference between antennas with different polarizations and the beam selected from the beam group selected in the horizontal and vertical directions.
CQI: corresponds to the data transmission rate that the UE may support as generated under the assumption that 2D precoding has simultaneously applied.
The structure of the 2D codebook may be represented as in Equation 1 below.
W
632 (W11⊗W12)W2=W1W2 [Equation 1]
Here, W11 and W12 indicate the PMIs related to some bits of the first PMI bit payload (W1 bit payload) and are respectively selected by i11 and i12. In this case, Equation 1 may be directly represented in the codebook and thus be shown or may indirectly be represented. W2 is also selected by i2 that indicates the second PMI as does Tables 12 and 13 below exemplify the rank1 2D codebook using such a 2D codebook structure, representing an example of the 2D codebook for 1-layer CSI reporting.
In Table 12 above, configuration parameters necessary for PMI reporting, N1, N2, O1, O2 config, may adopt those set forth in Tables 5 to 11, i.e., codebookConfigN1-r13, codebookConfigN2-r13, codebookOverSamplingRateConfig-O1-r13, codebookOverSamplingRateConfig-O2-r13, and codebookSubsetSelectionConfig-r13. The following embodiments of the present disclosure propose schemes for supporting periodic channel state reporting based on a plurality of CSI-RS ports and codebooks. N1 and N2 mean the numbers of antennas per dimension, and O1 and O2 mean oversampling factors per dimension.
In order to support periodic channel state information reporting based on the 2D codebook, existing subband channel state reporting may expand to wideband and subband reporting.
An analysis of the PMI bits of the 2D codebook reveals that, for the second PMI, i.e., i2(W2), reporting, the existing channel state reporting method is available with 4 bits or less. However, in the case of 810 related to some bits of the first PMI bit payload (W1 bit payload), the PMI bits increase, as follows, for the configuration parameters, N1, N2, O1, O2, and Config, supported for PMI reporting as shown in Tables 14 and 16. Tables 14 and 16 represent examples of PMI overhead of the 2D codebook.
In the case of PUCCH format 2 which is used for existing periodic channel state information reporting, 11 bits may be transmitted in the normal CP context. Further, in the case of the bits for transmission of W11/W12 which is the PMI related to some bits of the first PMI bit payload (W1 bit payload), the sum of the two (W11,W12) does not exceed 11 bits. Accordingly, according to an embodiment of the present disclosure, the amount of payload data required for periodic channel state information reporting may be reduced by applying the existing CSI reporting instance used in the subband described in connection with
Applying this method of the present disclosure enables no or minimum application of the subsampling, which may deteriorate performance, thereby allowing for periodic channel state information reporting. Accordingly, as in the embodiment of
In Table 17 above, k and periodicityFactor-r10 are fields for configuring the wideband PMI reporting period when PTI=1 or 0. periodicityFactor-r10 is intended for setting the wideband PMI(W1) reporting period when PTI=0 (wideband information reporting), and k is a field for determining the subband reporting period on the subband, after which the wideband information reporting is to be carried out.
The following are methods for setting the channel state reporting time using the method of the first embodiment.
Reporting time setting method 1: sets in the same manner as existing ones used on the subband.
Reporting time setting method 2: is a brand-new method.
Reporting time setting method 1 uses the same scheme as existing ones used on the subband. In this case, RI reporting may be represented as in Equation 2.
(10×nf+└nS/2┘−NOFFSET,CQI−NOFFSET,R1)mod(H·Npd·MR1)=0, H=J·K+1 [Equation 2]
In Equation 2, J is the number of bandwidth parts, and K is the same as k mentioned above and this is a value to allow the RI to be reported every k times the subband reporting period nf is the subframe number, and ns is the slot number. Such RI reporting time is set same on the wideband and subband, and a difference from the existing reporting scheme is that k and the bandwidth part value based on the downlink bandwidth may also be used for wideband reporting. In this case, the reporting time for the wideband first PMI may also be represented as in Equation 3 in the same way as the existing one used on the subband.
(10×nj+└nS/2┘−NOFFSET,CQI)mod(H′·Npd)=0, H′=periodicityFactor [Equation 3]
The wideband second PMI/wideband CQI reporting may be represented as in Equation 4.
(10×nf+└nS/2┘−NOFFSET,CQI)modNpd=0 [Equation 4]
Accordingly, where the fields mentioned in connection with
(10×nf+└nS/2┘−NOFFSET,CQI)mod(H·Npd)=0 [Equation 5]
Further, the reporting time for the subband CQI and subband second PMI may be represented as in Equation 6 below.
(10×nf+└nS/2┘−NOFFSET,CQI)modNpd=0 [Equation 6]
Reporting time setting method 2 may freely set reporting times unlike the existing method. Where wideband CSI reporting is supported using the first embodiment, the reporting time need not be associated with the number of bandwidth parts unlike in the existing scheme, ensuring more freedom. Accordingly, the reporting time may be defined by adopting new parameters for wideband first PMI and RI reporting. For description purposes, the values may be assumed as MRI and Md. The parameters may be referred to with various denotations, such as PRI, Pi1, Mw1, H, and K, and they, if supporting the same operations, may be the same despite different denotations. The parameters may be particular values pre-defined in the standards or may be set by RRC signaling. At this time, the following are methods for setting the RI reporting period.
RI reporting period setting method 1: sets as a multiple of the CQI period.
RI reporting period setting method 2: sets as a multiple of the wideband PMI period.
RI reporting period setting method 1 is a method of setting as a multiple of the CQI period.
In this case, the wideband PMI period is irrelevant to the RI period. In this case, the corresponding reporting time may be represented as in Equation 7.
(10×n1+└nS/2┘−NOFFSET,CQI−NOFFSET,RI)mod(Npd·NR1)=0 [Equation 7]
In Equation 7, Npd and NOFFSET,CQI mean the period and offset value in the subframes for CQi/PMI reporting, and MR1 and NOFFSET,R1 and means the period and relative offset value in the subframes for RI reporting.
As above, the RI reporting period and the offset are set based on the CQI reporting period, thereby allowing the RI reporting time to be set.
RI reporting period setting method 2 is a method of setting as a multiple of the wideband PMI period. In this case, the corresponding reporting time may be represented as in Equation 8.
(10×n1+└nS/2┘−NOFFSET,CQI−NOFFSET,R1)mod(Mi1·Npd·MR1)=0 [Equation 8]
In this case, the period of the wideband PMI is set as a multiple of the CQI reporting period, and the RI reporting period is set as a multiple of the same. Accordingly, the RI reporting period and offset may be set and the RI reporting time may be set. At this time, the reporting time of the wideband PMI is shown in Equation 9.
(10×n1+└nS/2┘−NOFFSET,CQI)mod(Mi1·Npd)=0 [Equation 9]
As set forth above, the reporting time of the wideband PMI is a multiple of the period, and the offset may not be considered. However, an additional offset may be considered in which case the offset should be reflected to the RI reporting time.
In the above method, since the subband channel state reporting already supports the corresponding structure, it may be proper to limit the above-described method to use only for the wideband channel state reporting. Accordingly, a method that may be considered is to set the same k and periodicity Factor as the existing ones for subband reporting while setting new parameters for wideband reporting. Further, the MM may be the same parameter as the MRI supported in the existing standards.
To support the above-described first embodiment, subband CSI reporting requires additional information to report the subband position, and thus, additional subsampling may be considered for reporting for W2. For such subsampling, new different subsampling may be designed and applied per Config or the existing method may be reused. Further, subsampling may be reused only for partial channel state reporting corresponding to some Configs, e.g., Configs. 2, 3, and 4. This is why Config1 has a relatively small i2 size and is less necessary to apply sub sampling to. Accordingly, in the instant embodiment, the existing codebook sub sampling may be reused, and to that end, examples of table information are shown in Tables 18 and 19.
Table 18 represents an example of codebook subsampling for eight transmit antennas, and Table 19 represents an example of codebook subsampling for four transmit antennas.
The second embodiment is directed to a method for supporting periodic channel state reporting using one of CSI-RS resources supported for the NP CSI-RS. As set forth in Tables 5 to 11, the NP CSI-RS may support 12, 16, or more CSI-RS ports using the positions for the existing CSI-RSs in one subframe. The corresponding field is represented as nzp-resourceConfigList-r13 which may be used to set the position for the CSI-RS. Such supporting method is a method for bounding up a plurality of CSI-RS ports to support 12, 16, or more similar CSI-RSs. At this time, such a method is also available as to support the periodic channel state reporting only on a particular CSI-RS resource by designating the CSI-RS resource so as to reduce the overhead of the periodic channel state reporting.
For aperiodic channel state reporting, the UE measures the channel on all the CSI-RS ports to determine the RI/PMI/CQI and transmits them to the base station. At this time, although the use of the method proposed in the first embodiment is advantageously able to support more precoding to enhance performance, it may increase the reporting instances, resulting in additional consumption of resources and period. Accordingly, it may support more than eight CSI-RS ports by using the channel state reporting and codebook supported in the existing 2, 4, and 8 CSI-RS ports. At this time, since the wideband uses smaller reporting instances, more reporting may be done in the context where the UE is in higher mobility, and the uplink resources may be consumed less and may thus be used for other uplink transmissions. Further, for the resources used for periodic channel state reporting, the CSI-RSs may be transmitted only through the corresponding resources, and power consumption may thus be reduced. Further, the number of codebooks supported in the 2, 4, or 8 CSI-RS ports is relatively small, and thus the UE becomes less complicated. At this time, the following methods are used to designate the corresponding CSI-RS resource.
CSI-RS resource designating method 1: uses the first resource set for the RRC in periodic channel state reporting
CSI-RS resource designating method 2: puts an additional field in the RRC field to set what resource is to be used in periodic channel state reporting.
CSI-RS resource designating method 1 is a method for indirectly setting the CSI-RS resource to be used in periodic channel state reporting where the base station sets the CSI-RS resource using the RRC field set forth in Tables 5 to 11. In this case, no additional overhead is needed, and thus, the UE may be subject to less complexity in implementation.
CSI-RS resource designating method 2 is a method for allowing the base station to directly set on what CSI-RS resource is to be used for periodic channel state reporting. CSI-RS resource designating method 2 may present different optimal CSI-RS positions depending on contexts, advantageously allowing for flexible use depending on contexts.
Further, the above method may lose accuracy where less CSI-RS ports are set. For example, where two CSI-RS ports are set, only four codebooks are supported for rank 1. Accordingly, in such case, the performance may encounter a problem.
In the case of four ports, the LTE system supports two codebooks. One supports only 16 PMIs, the enhanced codebook supports up to 256, and the corresponding is used using 16 first (wideband) PMIs and second (subband) PMIs. Supporting more PMIs delivers a better performance. Thus, having more CSI-RS ports allows for use of an enhanced codebook when the 4-port codebook is based. Accordingly, using the 4-port codebook using the method proposed in the second embodiment may support a higher performance when always using the enhanced codebook. In this case, to support the corresponding operation, the UE may ensure the use of always using the enhanced 4Tx codebook where 4 port channel state reporting is performed for 12 or 16 port CSI-RS channel state reporting.
The third embodiment is directed to a method of using the existing 4 or 8 Tx codebooks by combining the CSI-RS ports as set. As mentioned above, the 2Tx codebook may lose freedom or accuracy to support 12 or 16 CSI-RS ports or multiple similar CSI-RS ports. Accordingly, in such case, the performance may encounter a problem. However, supporting 12, 16, or more similar CSI-RS ports using the 2 port CSI-RSs enables the position of the CSI-RS port to be more flexibly set, advantageously allowing for various settings given the mobility and accuracy or the CSI-RS transmit power. Thus, such method, although able to secure enough flexibility and power, is always based on the 2Tx codebook in periodic channel state reporting even in the case of 12 or 16 CSI-RS ports. Therefore, in such case, it is also possible to combine and use the resources. Various methods as follow may be used in such combining.
Combining method 1: combines and uses with respect to the CSI-RS port index
Combining method 2: includes and uses until the number for reporting is met, starting with the first one
Combining method 3: sets the corresponding resource using the RRC field.
Combining method 1 above is a method for combining with respect to the CSI-RS port index. In the LTE standards, the CSI-RS ports may be indexed with port indexes 15, 16, . . . , 30. At this time, upon reporting the periodic channel state information using the 8Tx codebooks, the CSI-RS ports corresponding to 15, . . . , 22 may be used for periodic channel state reporting.
Combining method 2 above is a method for combining and using with respect to the resource index. As mentioned above, the CSI-RS position resource indexes are set to the fields 0, 1, . . . , 7, respectively. Accordingly, where 8 Tx codebooks are supported for the 2 port CSI-RS ports, four 2 CSI-RS ports should be supported. Accordingly, periodic channel state reporting is supported using the corresponding number 0, 1, 2, and 3 resources. To determine the index of the resource, such a method may also be available to use 0, 1, 2, . . . for nzp-resourceConfigList-r13 depending on preset order.
Combining method 3 is a method of setting the corresponding resource using the RRC field. At this time, the bitmap may also be one method. For example, where 16 CSI-RS ports are supported using two CSI-RS ports, eight CSI-RS resources are needed. Accordingly, a method available in this case is to use eight bitmaps. For example, assuming that 11001100 is set, a bundle into eight ports may be made using the numbers 0, 1, 4, and 5 CSI-RS resources, thereby able to report the channel state information.
As mentioned above in the above combining method, a need exists for a method for setting the number of CSI-RS ports corresponding to the codebooks. The following methods may be available as the above methods.
Port count setting method 1: directly sets in the RRC field
Port count setting method 2: sets with respect to the maximum number of CSI-RS ports supported in the corresponding CSI-RS ports
Port count setting method 3: sets using the number defined in the standards.
Port count setting method 1 is a method of directly setting in the RRC field. The use of this method renders it possible to directly set in the RRC field how many CSI-RS ports are to be set in the periodic channel state reporting. For example, a field, such as CSI-RS ports Periodic, may be left in the RRC field, and a number may be set in the corresponding field. The number set at this time may be 2, 4, or 8.
Port count setting method 2 is a method of setting depending on the maximum number of CSI-RS ports supported in the corresponding CSI-RS ports. For example, 16 ports may be set using eight 2 CSI-RS ports or two 8 CSI-RS ports. In such case, since the maximum number of individual CSI-RS resources settable is 8, although 2 CSI-RS ports are used, reporting should be made always using 8. Taking 12 ports as an example, they may be set using three 4 CSI-RS ports and six 2 CSI-RS ports. In this case, thus, even though it is for 2 CSI-RS ports, reporting may be rendered to be done based on 4 CSI-RS ports.
Further, in the third embodiment, like the second embodiment, the method of always applying the 4 CSI-RS ports may likewise apply to ensure the periodic channel state reporting performance.
The first to third embodiments have their own merits and demerits. Accordingly, it is also possible to support the corresponding periodic channel state reports as submodes. An example method is to support the second or third embodiment in the case of submode 1 and the first embodiment in the case of submode2. Such method enables a selection as to whether to allow the base station to use the channel state report while supporting more accurate precoding or to use a shorter period and lower power while supporting less accurate precoding.
The second embodiment and the third embodiment may be selectively applied depending on contexts. For example, it may be varied depending on whether the RE position of the CSI-RS to support multiple CSI-RS ports is eight ports or two ports. Where 16 ports are supported using two 8 port CSI-RSs and eight 2 port CSI-RSs, since one 8 port CSI-RS is enough to support eight ports, periodic channel state information may be generated using the same. Accordingly, in this case, the periodic channel state report may be generated using the second embodiment. However, in the case of supporting using eight 2 port CSI-RSs, the accuracy of the channel state information or the quantization level may be insufficient. Accordingly, in this case, aggregation may be performed with respect to the port index or resource index using the third embodiment so that the periodic channel state is reported using the 8Tx codebooks, addressing such problem. 12 ports may also be set using three 4 port CSI-RSs and six 2 port CSI-RSs in which case aggregation may be performed with respect to the port index or resource index among the six 2 port CSI-RSs, and the periodic channel state is reported using the 4Tx codebooks, addressing such problem.
Referring to
Referring to
Referring to
Further, the controller 1220 may measure at least one or more reference signals received via the transceiver 1210 and generate feedback information according to the feedback configuration information. The controller 1220 may control the transceiver 1210 to transmit the generated feedback information to the base station at a feedback timing based on the feedback configuration information. Further, the controller 1220 may receive a channel status indication-reference signal (CSI-RS) from the base station, generate feedback information based on the received CSI-RS, and transmit the generated feedback information to the base station. In this case, the controller 1220 may select each precoding matrix per antenna port group of the base station and further select an additional precoding matrix based on the relationship between the antenna port groups of the base station.
Further, the controller 1220 may receive a CSI-RS from the base station, generate feedback information based on the received CSI-RS, and transmit the generated feedback information to the base station. In this case, the controller 1220 may select one precoding matrix for all the antenna port groups of the base station. Further, the controller 1220 may receive feedback configuration information from the base station, receive a CSI-RS from the base station, generate feedback information based on the received feedback configuration information and CSI-RS, and transmit the generated feedback information to the base station. In this case, the controller 1220 may receive additional feedback configuration information based on the relationship between the antenna port groups and the feedback configuration information corresponding to each antenna port group of the base station.
Referring to
Further, although the resource allocator 1330 is shown to be included in the controller 1310, it is not necessarily limited thereto. The controller 1310 may control the transceiver 1320 to transmit the configuration information for each of at least one or more reference signals to the UE or generate at least one or more reference signals. Further, the controller 1310 may control the transceiver 1320 to transmit to the UE feedback configuration information to generate feedback information based on the measurement result. Further, the controller 1310 may control the transceiver 1320 to transmit the at least one or more reference signals to the UE and receive the feedback information transmitted from the UE at a feedback timing according to the feedback configuration information. Further, the controller 1310 may transmit the feedback configuration information to the UE, transmit the CSI-RS to the UE, and receive from the UE feedback information generated based on the feedback configuration information and the CSI-RS. In this case, the controller 1310 may transmit additional feedback configuration information based on the relationship between the antenna port groups and the feedback configuration information corresponding to each antenna port group of the base station. Further, the controller 1310 may transmit to the UE the beamformed CSI-RS based on the feedback information and receive from the UE the feedback information generated based on the CSI-RS. According to the above-described embodiments of the present disclosure, it may be possible to prevent allocation of excessive feedback resources in transmitting CSI-RSs from the base station using a number of two-dimensional antenna array transmit antennas and the increase in channel estimation complexity of the UE. The UE may effectively measure the channel of all of the many transmit antennas, configure the same in feedback information, and notify the same to the base station.
This application is a National Phase Entry of PCT International Application No. PCT/KR2016/013374, which was filed on Nov. 18, 2016 and claims priority to U.S. Provisional Patent Application Nos. 62/256,895, 62/256,738 and 62/257,409, which were filed Nov. 18, 2015, Nov. 18, 2015 and Nov. 19, 2015, respectively, the entire disclosure of each of these application is incorporated herein by reference.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/KR2016/013374 | 11/18/2016 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 62257409 | Nov 2015 | US | |
| 62256895 | Nov 2015 | US | |
| 62256738 | Nov 2015 | US |
