Patent Application
20210184904
The present disclosure relates to a base station, and a user equipment that perform communication using an orthogonal frequency division multiple access system, as well as a control circuit and storage medium therefor.
The internet-of-things (IoT) that connects devices such as industrial control devices, sensors, meters, automobiles in the Internet protocol is considered to serve as an important future business. In a radio system used for the IoT, there have been demands for reduction in delay of transmission performed by a user equipment (UE). In order to achieve such reduction in delay of transmission performed by the user equipment, a round-trip time (RTT) needs to be shortened. In order to shorten an RTT, a method of shortening a transmission time interval (TTI) can be considered. A TTI is also called a sub-frame. In long term evolution (LTE) in a Release 8 version developed by Third Generation Partnership Project (3GPP), a TTI having a length of 14 orthogonal frequency division multiplexing (OFDM) symbols is employed.
Non Patent Literature 1 (Non Patent Literature 1: P. Schulz, M. Matthe, H. Klessig, M. Simsek, G. Fettweis, J. Ansari, S. Ali Ashraf, B. Almeroth, J. Voigt, I. Riedel, A. Puschmann, A. Mitschele-Thiel, M. Muller, T. Elste, and M. Windisch, “Latency Critical IoT Applications in 5G: Perspective on the Design of Radio Interface and Network Architecture,” IEEE Commun. Mag., vol. 55, no. 2, pp. 70-78, Feb. 2017) teaches a radio system employing a TTI having a length of two OFDM symbols and a TTI having a length of seven OFDM symbols in a frequency division duplex (FDD) mode in a physical layer while employing only a TTI having a length of seven OFDM symbols in a time division duplex (TDD) mode in the physical layer, so as to reduce a delay in transmission. Shortening the TTI length can make a processing time shorter, which enables reduction in transmission delay in radio communication.
In LTE, however, both a frame structure with a TTI length constituted by 14 OFDM symbols in the Release 8 version and a frame structure with TTI lengths of two OFDM symbols and seven OFDM symbols need to be present. This has been problematic in that resource allocation including retransmission becomes complicated because physical channels with different RTTs are multiplexed on one and the same carrier frequency.
The present disclosure has been made in view of the above, and an object thereof is to provide a base station capable of reducing complicated resource allocation, and reducing transmission delay in radio communication.
In order to solve the above-mentioned problems and achieve the object, the present disclosure provides a base station in a radio communication system in which a radio frame including a plurality of sub-frames is transmitted and received between the base station and a user equipment using an orthogonal frequency division multiple access scheme, wherein the sub-frames are each constituted by three or less orthogonal frequency division multiplexing (OFDM) symbols in a time direction and a plurality of sub-carriers in a frequency direction, and the base station comprises: a first multiplexer to multiplex downlink control information in first OFDM symbol positions of the sub-frames in a frequency domain by allocating resources in such a manner that the resources are dispersed in the frequency direction in units of resource blocks, each of the resource blocks being constituted by a plurality of sub-carriers; a second multiplexer to multiplex user information in OFDM symbol positions other than the first OFDM symbol positions of the sub-frames in a frequency domain by allocating resources in such a manner that the resources are dispersed in a frequency direction in units of the resource blocks; and an inserter to insert a guard time before and after each of the first OFDM symbol positions.
A base station and a user equipment according to an embodiment of the present disclosure will be described in detail below with reference to the drawings.
The radio communication system 1 is an IoT system including an IoT device. Radio communication between the base station 10 and the user equipment 20 is performed in accordance with a radio specification dedicated to IoT transmission. Examples of conditions required for IoT transmission include long lifetime and low power consumption, low cost, wide coverage, and the like of the user equipment 20. The radio specification dedicated to IoT transmission is called a low power wide area network (LPWAN). Examples of the LPWAN using a frequency band for which no radio license is required include SIGFOX that is a radio specification produced by SIGFOX Company, and LoRaWAN supported by LoRa Alliance that is a non-profit organization. Examples of the LPWAN using a frequency band for which a radio license is required include the category Ml of 3GPP (enhanced machine type communications: eMTC), and narrow band (NB)-IoT.
IoT transmission is characterized in that the number of information bits is smaller and an information bit rate is lower than those in communication performed by an information terminal such as a typical smartphone. For this reason, narrowing the transmission bandwidth is effective for reducing power consumption of the user equipment 20. As the signal band is narrower, the clock frequency for modulation and demodulation processes of a baseband is lower, and the power consumption of the user equipment 20 can be reduced. In addition, as the signal band is narrower, transmission power density and reception power density per 1 Hz can be increased, and a distance that radio waves can travel, that is, a coverage area can thus be extended.
Therefore, in the radio specification dedicated to IoT transmission mentioned above by way of example, bands of 100 Hz, 125 kHz, 250 kHz, etc. are used. In this case, although the power consumption of the user equipment 20 can be reduced, decoding error occurs when the reception level is lowered because of frequency flat fading beinf suffered. In short, a frequency diversity effect can be obtained with a wideband signal, whereas a frequency diversity effect is less likely to be obtained with a narrowband signal. Furthermore, in a case of one-antenna configuration intended to reduce the power consumption of the user equipment 20 that is an IoT terminal, a space diversity effect cannot be obtained, either. For this reason, in the case of one-antenna configuration, frequency flat fading is likely to be suffered, and decoding error occurs when the reception level is lowered.
Control information on a downlink and an uplink of the second sub-frame 310 is multiplexed on a first OFDM symbol period. In addition, the control information on the downlink and uplink is dispersed and multiplexed in units of resource blocks over different resource blocks in one and the same OFDM symbol period. In the case of the second sub-frame 310, the user information is multiplexed on a second OFDM symbol period and a third OFDM symbol period. This is because frequency hopping within a sub-frame (TTI) is applied when the number of information symbols resulting from error-correcting coding is less than the number of resource elements in one resource block. Details of the frequency hopping will be described later. A cyclic prefix (CP) is added to each OFDM symbol. The base station 10 and the user equipment 20 only use either one of the first sub-frame 300 and the second sub-frame 310. Accordingly, only one kind of round trip time (RTT) is used, which results in a configuration in which complicated timing control is not needed.
The third multiplexing unit 202 multiplexes control information on the uplink into a resource block in which control information on the downlink at the first OFDM symbol position is not multiplexed. In the case of the first sub-frame 300, the fourth multiplexing unit 203 multiplexes information symbols of the user information in units of resource blocks at the second OFDM symbol position in the sub-frame allocated according to decoded information of the control information on the downlink. In other words, the fourth multiplexing unit 203 multiplexes the user information in the frequency domain at OFDM symbol positions other than the first OFDM symbol position in the sub-frame. In addition, in the case of the second sub-frame 310, the fourth multiplexing unit 203 multiplexes information symbols of the user information in units of resource blocks at the second and third OFDM symbol positions in the sub-frame allocated according to decoded information of the control information on the downlink.
The transmitting unit 100 and the transmitting unit 200 are implemented by a processing circuit that is an electronic circuit for carrying out several processes.
The processing circuit may be based on dedicated hardware, or may be a control circuit including a memory and a central processing unit (CPU) that executes a program stored in the memory. Note that the said memory corresponds to a nonvolatile or volatile semiconductor memory such as a random access memory (RAM), a read only memory (ROM) or a flash memory, a magnetic disk, or an optical disk, for example.
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A guard time (GT) is inserted before and after the first OFDM symbols of the first sub-frame 300 and the second sub-frame 310. The insertion of the guard times is performed by the inserting unit 103. The uplink control information and the downlink control information multiplexed in units of resource blocks are dispersed and multiplexed over the entire system band in units of resource blocks each constituted by NRB sub-carriers. Thus, the frequency diversity effect is obtained by multiplexing the uplink control information and the downlink control information on wideband signals in each OFDM symbol period, and a low BLER or BER can be achieved by virtue of the coding gain of the error-correcting coding. Note that, the present disclosure is directed to a configuration in which the uplink control information and the downlink control information multiplexed in units of resource blocks are dispersed and multiplexed over the entire system band in units of resource blocks each constituted by NRB sub-carriers, but the uplink control information and the downlink control information may be dispersed and multiplexed over a specified band without limitation to the entire system band. Because the uplink control information and the downlink control information are multiplexed in one and the same OFDM symbol period, the transmitting unit 100 and the transmitting unit 200 attenuate leakage power to a resource block other than a resource block on which the control information is to be multiplexed, by applying a band-pass filter (BPF) configured to only allow transmission control information to pass therethrough, to a signal obtained by performing processing of inverse fast Fourier transform (IFFT) on each piece of control information. In addition, a guard band is inserted between resource blocks on which the uplink control information and the downlink control information are multiplexed. The receiving unit 110 and the receiving unit 210 make received signals pass through BPF sets that only extract the uplink and downlink control information pieces, respectively, and then performs an FFT process on the resultant outputs of the sets.
The downlink control information is broadcast information (broadcast channel). Examples of the downlink control information include information on allocation of a downlink data channel, a modulation scheme, error-correcting coding rate symbol repetition factor (transport block size), information on a hybrid automatic repeat request (ARQ), Grant information on an uplink, and Ack or Nack information of a data channel on an uplink. A piece of control information for a downlink data channel and another piece of control information for an uplink data channel, transmitted by the base station 10, are subjected to error-correcting coding independently of each other, and mapped to symbols of bits after the error-correcting coding independently of each other, so that the respective pieces of control information can be separately demodulated and decoded.
On an uplink control channel, one sub-frame is accessed at the same time by two or more user equipments 20. The orthogonal code division multiple access (CDMA) is therefore used as a multiplexing method for a control channel of different user equipments 20 within one and the same sub-frame. This is because, although multiplexing in the OFDM system can also be used, a control channel per one user equipment 20 becomes a narrow band in the case of the OFDM system, and so the orthogonal CDMA multiplexing is more effective in view of the frequency diversity effect. Specifically, Walsh-Hadamard codes or the like can be applied. A reception timing difference between user equipments 20 having their respective different propagation delay times is absorbed in a guard interval using a CP. In addition, in the uplink, the user equipment 20 performs transmission with a transmission power that compensates for an average bus loss including distance attenuation estimated using a downlink reference signal (RS) and shadowing variation.
In the case where the first sub-frame 300 is used, the second multiplexing unit 102 allocates resource blocks on which the user information is to be multiplexed, to second OFDM symbol positions in units of sub-frames. In addition, in the case where the second sub-frame 310 is used, the second multiplexing unit 102 allocates resource blocks on which the user information is to be multiplexed, to second and third OFDM symbol positions in units of sub-frames. In addition, the user information is dispersed and multiplexed over the entire system band in units of resource blocks by the second multiplexing unit 102. The base station 10 performs scheduling for determining which common channel to be used for which user equipment 20. In LTE, dynamic scheduling is standardized as this scheduling method. In the present embodiment, the frequency diversity effect is obtained by dispersing and multiplexing on wideband signals instead of dynamic scheduling. It is noted that the dispersing and multiplexing on wideband signals refers to an operation in which intervals between resource blocks are made away from each other to be dispersed, and then multiplexed. Thus, in the present embodiment, control on complicated resource allocation caused by the dynamic scheduling in the base station 10 can be eliminated.
In the present embodiment, a method of resource allocation depending on the number of information bits is performed. Description is given now for a case of the second sub-frame 310 in which the number of information symbols resulting from error-correcting coding is smaller than the number of resource elements in one resource block of one OFDM symbol.
A case where the number of information symbols resulting from error-correcting coding is equal to or larger than one resource block will be described.
A case where the number of information symbols resulting from error-correcting coding cannot be multiplexed in one sub-frame period will be described.
The second multiplexing unit 102 and the fourth multiplexing unit 203 control the modulation scheme, the coding rate of the error-correcting coding, and a symbol repetition factor depending on a target BLER or packet loss rate (PLR) according to the IoT traffic. In URLLC that achieves highly reliable reception with a BLER of about 10−6 or lower, however, quadrature phase shift keying (QPSK) or 16 quadrature amplitude modulation (QAM) are basically considered as being applicable as the modulation scheme. In LTE, although the lowest coding rate of turbo codes is 1/3, application of low density parity check (LDPC) codes with a rate of 1/9 or lower is effective. After LDPC decoding, some decoding error remains in a range in which the BER is low. Concatenated codes of LDPC codes and BCH codes or Reed-Solomon codes are therefore effective.
At the base station 10, a method of allocating sub-frames in TDD is used.
The first sub-frame 300 and the second sub-frame 310 may be, in addition to sub-frames for uplink only or sub-frames for downlink only, sub-frames that are basically used for downlink but can also be subjected to resource allocation for uplink, or sub-frames that are basically used for uplink but can also be subjected to resource allocation for downlink. When no uplink transmission data is present, the base station 10 allocates the sub-frames that are basically used for uplink but can also be subjected to resource allocation for downlink, to multiplexing and transmission of downlink user information. In addition, when no downlink transmission data is present, the base station 10 allocates the sub-frames that are basically used for downlink but can also be subjected to resource allocation for uplink, to an uplink data channel.
The first multiplexing unit 101 multiplexes control information on sub-frame allocation indicating whether or not to allocate sub-frames for uplink only or downlink only in the normal mode set in advance in a resource space of downlink control information in the first OFDM symbol period of a leading sub-frame in each radio frame.
The demodulating and decoding unit 201 detects whether sub-frame allocation is in a normal mode that has been set in advance or in an irregular mode (step S2). If the sub-frame allocation is in the normal mode (step S2, Yes), the demodulating and decoding unit 201 demodulates and decodes only uplink control information in sub-frame positions corresponding to sub-frames allocated to the uplink data channel. (step S3). Thus, the demodulating and decoding unit 201 does not perform demodulating and decoding of downlink control information other than that in a sub-frame position corresponding to a sub-frame allocated to the downlink data channel. Similarly, the demodulating and decoding unit 201 does not perform demodulating and decoding uplink control information other than that in a sub-frame position corresponding to a sub-frame allocated to the uplink data channel. In contrast, in the case of the irregular mode (step S2, No), the demodulating and decoding unit 201 demodulates and decodes downlink and uplink control information multiplexed on all the sub-frames (step S4).
In the normal mode, the transmitting unit 200 of the user equipment 20 multiplexes and transmits uplink control information in sub-frame positions corresponding to predetermined time positions relative to sub-frame positions to which the uplink data channel can be allocated. In contrast, in the irregular mode, the transmitting unit 200 of the user equipment 20 also multiplexes and transmits uplink control information in sub-frame positions other than the sub-frames corresponding to predetermined time positions relative to sub-frame positions to which the uplink sub-frames can be allocated.
As described above, in the present embodiment, the transmitting unit 100 and the transmitting unit 200 have configurations that do not need complicated resource allocation by transmitting and receiving only either one of the first sub-frame 300 and the second sub-frame 310. In addition, by performing wideband transmission with a system band of about 20 MHz or higher, it is possible to obtain a sufficient frequency diversity effect based on distribution and multiplexing and frequency hopping on the wideband signals. Thus, the base station 10 can reduce a transmission delay in radio communication by preventing complicated resource allocation and obtaining a certain diversity effect.
A base station according to the present disclosure produces an advantageous effect of achieving a base station capable of preventing complicated resource allocation and reducing a transmission delay in radio communication.
The configurations presented in the embodiment above are examples of the present disclosure, and each of them can be combined with other publicly known techniques and partly omitted and/or modified without departing from the scope of the present disclosure.
This application is a continuation application of International Application PCT/JP2018/036656, filed on Oct. 1, 2018, and designating the U.S., the entire contents of which are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP2018/036656 | Oct 2018 | US |
| Child | 17190058 | US |
