Wireless communication method and distributed antenna system

Abstract

Provided is a wireless communication method in a distributed antenna system comprising a base station and a plurality of antennas for performing communication with one or more terminal stations by spatial multiplexing in accordance with control by the base station, each of the plurality of antennas including a plurality of sub-arrays. The base station allocates a plurality of sub-arrays of different antennas to each of one or more candidate terminal stations as the subject for communication by spatial multiplexing using two or more sub-arrays among the plurality of sub-arrays. The plurality of sub-arrays allocated to the candidate terminal station perform spatial multiplexing transmission with respect to the candidate terminal station.　

Description

[Technical Field]

[0001] The present invention relates to a wireless communication method. and Distributed Antenna System Mu Regarding. [Background technology]

[0002] Communication systems using 5G (Generation 5G) and the like use high frequency bands in the millimeter wave band. Future communication systems, such as 6G, which is an evolution of 5G, are expected to use even higher frequency bands than 5G, which can ensure wider bandwidth, in order to achieve even higher speeds and larger capacities. High frequency bands are known to have large propagation losses and tend to travel in a straight line, and distributed antenna systems are being considered to improve connectivity in covering communication areas (see, for example, Non-Patent Documents 1 and 2).

[0003] In a distributed antenna system, by using multiple distributed antennas to perform SU-MIMO (Single User Multiple-Input and Multiple-Output) or MU-MIMO (Multi-User MIMO), it is possible to improve frequency utilization efficiency and communication capacity and throughput.When performing MIMO communication, it is generally assumed that in order to reduce inter-stream interference, channel information (CSI: Channel State Information) between the distributed antennas and the terminal station (UE: User Equipment) is acquired, and precoding is performed on the downlink at the base station (BS) side, and postcoding is performed on the uplink.

[0004] In high-frequency band communication systems, it is expected that beamforming will be performed using a massive MIMO configuration with a large number of elements to ensure gain. On the other hand, from the perspective of equipment costs, the application of hybrid beamforming, which divides the antenna array into subarrays and uses both analog and digital beamforming, is being considered (see, for example, Non-Patent Document 3). [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] NTT Docomo, Inc., “Docomo 6G White Paper 3.0,” November 2021 [Non-patent document 2] T. Uchida, T. Iwakuni, T. Kita, T. Onisawa, T. Kishiyama, T. Suyama, T. Nagata, and T. Asai, "Study on High-Frequency Band Distributed Antenna Systems for the 6G Era," IEICE Technical Report RCS2020-148, pp. 73-78, December 2020. [Non-patent document 3] Suyama, Okuyama, Nonaka, and Asai, “Subarray Configuration for Hybrid BF of 100GHz Band Massive MIMO,” IEICE University, B-5-59, September 2021 Summary of the Invention [Problem to be solved by the invention]

[0006] When performing SU-MIMO in a distributed antenna system using sub-array antennas, if antenna allocation is performed based on the received power standard, the probability of multiple streams being allocated from the same distributed antenna increases. In this case, the spatial correlation between the base station and terminal stations becomes very high, resulting in a problem of reduced communication capacity.

[0007] In view of the above circumstances, an object of the present invention is to provide a technique that can improve communication capacity when spatial multiplexing transmission is performed in a distributed antenna system. [Means for solving the problem]

[0008] One aspect of the present invention is a wireless communication method in a distributed antenna system including a base station and a plurality of antennas that communicate with one or more terminal stations by spatial multiplexing under the control of the base station, wherein each of the plurality of antennas includes a plurality of subarrays, and the base station assigns a plurality of subarrays of different antennas to each of one or more candidate terminal stations that are to communicate by spatial multiplexing using two or more subarrays of the plurality of subarrays, and the plurality of subarrays assigned to the candidate terminal stations perform spatial multiplexing transmission to the assigned candidate terminal stations.

[0009] One aspect of the present invention is a distributed antenna system comprising a base station and a plurality of antennas that communicate with one or more terminal stations by spatial multiplexing under the control of the base station, wherein each of the plurality of antennas includes a plurality of subarrays, and the base station comprises an allocation unit that allocates a plurality of subarrays of different antennas to each of one or more candidate terminal stations that are to communicate by spatial multiplexing using two or more subarrays of the plurality of subarrays, and the plurality of subarrays allocated to the candidate terminal stations perform spatial multiplexing transmission to the allocated candidate terminal stations.

[0010] One aspect of the present invention is a wireless communication device comprising a base station and a plurality of antennas that communicate with one or more terminal stations by spatial multiplexing under the control of the base station, wherein each of the plurality of antennas includes a plurality of subarrays, the base station comprises an allocation unit that allocates a plurality of subarrays of different antennas to each of one or more candidate terminal stations that are to communicate by spatial multiplexing using two or more subarrays of the plurality of subarrays, and the plurality of subarrays allocated to the candidate terminal stations perform spatial multiplexing transmission to the allocated candidate terminal stations. [Effects of the Invention]

[0011] According to the present invention, it is possible to improve communication capacity in a distributed antenna system when spatial multiplexing transmission is performed. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a diagram illustrating an example of a distributed antenna system according to an embodiment of the present invention. [Figure 2] FIG. 2 is a diagram illustrating an example of the configuration of a base station according to the present embodiment. [Figure 3] 10 is a flowchart showing a processing flow of a base station in the present embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. (overview) In the distributed antenna system 100 of this embodiment, each of the distributed antennas is configured as a sub-array, and stream allocation to the same terminal station is controlled so as to allocate sub-arrays of different distributed antennas.

[0014] In a distributed antenna system using a subarray configuration in the high frequency band, there is a high possibility that spatial correlation will be high when multiple beams from the same antenna are assigned to the same terminal station. By taking advantage of the benefits of the distributed antenna system to reduce spatial correlation with very simple control, it is possible to improve user throughput. A specific configuration for realizing the above processing will be described below.

[0015] Fig. 1 is a diagram showing an example of a distributed antenna system 100 according to this embodiment. The distributed antenna system 100 includes a base station 10 and a plurality of antennas 20-1 to 20-4. The base station 10 and the plurality of antennas 20-1 to 20-4 are connected by optical transmission paths. Communication between the base station 10 and the plurality of antennas 20-1 to 20-4 is performed by, for example, RoF (Radio over Fiber). The base station 10 and the plurality of antennas 20-1 to 20-4 are one aspect of a wireless communication device.

[0016] In the example shown in Fig. 1, multiple antennas 20-1 to 20-4 are installed on the ceiling of a building BL, and communicate with multiple terminal stations 30-1 to 30-4 located within the building BL. For example, the multiple antennas 20-1 to 20-4 are arranged spaced apart from one another as shown in Fig. 1. Each antenna 20 includes multiple subarrays 21. Note that the numbers of antennas 20, terminal stations 30, and subarrays 21 are not limited to those shown in Fig. 1. In the following description, when there is no need to distinguish between the antennas 20-1 to 20-4, they will simply be referred to as antennas 20.

[0017] The base station 10 controls each of the distributed antennas 20 through centralized control. The base station 10 realizes communication using SU-MIMO and MU-MIMO by controlling each antenna 20. Specifically, the base station 10 performs SU-MIMO by simultaneously transmitting multiple streams from multiple antennas 20 to a single terminal station 30, and performs MU-MIMO by simultaneously transmitting multiple streams from multiple antennas 20 to multiple terminal stations 30.

[0018] The maximum number of streams that can be communicated simultaneously is determined by the maximum number of SU-MIMO layers and the maximum number of MU-MIMO layers. The base station 10 may use all of the distributed antennas 20 simultaneously for communication, or may use only some of the antennas 20.

[0019] Each antenna 20 communicates with a terminal station 30. Each antenna 20 is configured to include multiple subarrays 21. Each subarray 21 radiates radio waves under control of the base station 10. The antenna 20 communicates with the terminal station 30 that is the communication target by performing beamforming using multiple array elements to ensure gain in the high frequency band.

[0020] Each terminal station 30 has one or more antennas and communicates with each antenna 20. A terminal station 30 having multiple antennas communicates with the antenna 20 using SU-MIMO. The terminal station 30 may perform beamforming.

[0021] Here, the stream allocation of SU-MIMO performed by the base station 10 will be described using an example of communication (downlink) from the base station 10 to a terminal station 30. As shown in FIG. 1, configuring each antenna 20 as a subarray enables simultaneous analog beam transmission in multiple directions using multiple beams. Here, when a specific terminal station 30 performs SU-MIMO, the base station 10 allocates multiple subarrays 21 to perform simultaneous transmission of multiple streams. When a subarray 21 with high reception power is allocated based on a general reception power standard, the probability of allocating multiple subarrays 21 of the same antenna 20 to the same terminal station 30 increases.

[0022] However, there is a possibility that throughput will decrease due to MIMO transmission because the spatial correlation between the subarrays 21 of the same antenna 20 is very high and the spatial correlation between the antennas on the terminal station 30 side is also very high. Therefore, in the stream allocation control performed by the base station 10, even if the received power of multiple streams to the same terminal station 30 is low, multiple stream transmission can be realized by allocating subarrays 21 of different antennas 20, thereby making it possible to improve user throughput.

[0023] For example, when communicating with terminal station 30-1 using SU-MIMO, base station 10 selects one subarray of antenna 20-1 and a subarray of antenna 20 other than antenna 20-1 to communicate with terminal station 30-1 using SU-MIMO. In this way, base station 10 does not select two or more subarrays 21 of the same antenna 20 when communicating with one terminal station 30. This prevents selection of a subarray 21 with very high spatial correlation. As a result, although there is a possibility that the received power for multiple streams will be low, since a subarray 21 with very high spatial correlation will not be selected, it is possible to improve user throughput.

[0024] Fig. 2 is a diagram showing an example of the configuration of a base station 10 in this embodiment. Note that Fig. 2 only shows the configuration related to SU-MIMO stream allocation, which is a feature of the present invention. The base station 10 includes a terminal station extraction unit 11, an allocation unit 12, a precoding unit 13, and an optical-to-electrical conversion unit 14.

[0025] The terminal station extraction unit 11 extracts terminal stations 30 (hereinafter referred to as "candidate terminal stations") that are candidates for spatial multiplexing using MU-MIMO. The method of extracting candidate terminal stations may be any of the following: a method of selecting candidate terminal stations based on an index such as an RI (Rank Indicator) using various scheduling methods; a method of selecting candidate terminal stations based on a PF (Proportional fair) standard; a method of selecting candidate terminal stations based on received power; and a method of selecting terminal stations 30 that reduce interference between the terminal stations 30 based on their positional relationships, etc. Examples of methods of selecting candidate terminal stations based on received power include a method of checking the received power of each terminal station 30 and selecting terminal stations 30 in descending order of received power, and a method of selecting terminal stations 30 with similar received power.

[0026] The allocation unit 12 determines the allocation of subarrays 21 (hereinafter referred to as "candidate subarrays") that communicate with the candidate terminal stations and the number of SU-MIMO layers for each candidate terminal station extracted by the terminal station extraction unit 11. As described above, the allocation unit 12 allocates multiple subarrays 21 of different antennas 20 to each candidate terminal station. The allocation unit 12 may select, as the candidate subarray, a subarray 21 with a received power that meets the minimum receiving sensitivity, or may select a subarray 21 that satisfies a predetermined quality threshold.

[0027] The precoding unit 13 calculates a weight matrix W to be used for communication with each terminal station 30-n that communicates using MU-MIMO or SU-MIMO. The precoding unit 13 multiplies the calculated weight matrix W by a transmission signal to be transmitted to the terminal station 30-n. As the weight matrix W to be multiplied by the transmission signal to the terminal station 30 that communicates using SU-MIMO, the precoding unit 13 calculates a weight matrix for the terminal station 30 that communicates using SU-MIMO, and as the weight matrix W to be multiplied by the transmission signal to the terminal station 30 that communicates using MU-MIMO, the precoding unit 13 calculates a weight matrix using CSI between the antenna 20 and all terminal stations 30 that communicate using MU-MIMO, as in the conventional case.

[0028] The photoelectric conversion unit 14 converts each transmission signal multiplied by the weight matrix W by the precoding unit 13 into an optical signal and transmits it to the antenna 20 .

[0029] 3 is a flowchart showing the flow of processing in the base station 10 in this embodiment. Note that the processing shown in FIG. 3 describes processing in the case where spatial multiplexing transmission using SU-MIMO is performed. The terminal station extracting unit 11 extracts candidate terminal stations (step S101). In the following description, the number of candidate terminal stations extracted by the terminal station extracting unit 11 is referred to as the number of spatial multiplexing terminal stations.

[0030] The allocation unit 12 allocates a number to each candidate terminal station. For example, the allocation unit 12 allocates numbers to each candidate terminal station in order starting from 1 as the candidate terminal station number. The allocation unit 12 assigns 1 to the candidate terminal station number n (step S102).

[0031] For each candidate terminal station, the allocation unit 12 extracts a subarray 21 (hereinafter referred to as a "candidate subarray") that is a candidate to be allocated as a target for communicating with the candidate terminal station. The allocation unit 12 determines whether or not there is a candidate subarray for the nth candidate terminal station (step S103). Since n is 1 at the start of processing, for example, the allocation unit 12 determines whether or not there is a candidate subarray for the first candidate terminal station.

[0032] If the allocation unit 12 determines that there is no candidate sub-array for the n-th candidate terminal station (step S103-NO), it adds 1 to n (step S104). Next, the allocation unit 12 determines whether the value of n is greater than the number of spatial multiplexing terminal stations (step S105). If the allocation unit 12 determines that the value of n is equal to or less than the number of spatial multiplexing terminal stations (step S105-NO), it determines whether the number of allocated streams is less than the maximum number of total MIMO layers (step S106).

[0033] Here, the number of allocated streams is the number of streams allocated to the (n-1)th or lower candidate terminal stations. The maximum number of total MIMO layers is the number of streams that the base station 10 can use to simultaneously perform SU-MIMO and MU-MIMO. If the allocation unit 12 determines that the number of allocated streams is less than the maximum number of total MIMO layers (step S106-YES), it executes the process of step S103 again.

[0034] If the allocation unit 12 determines in the process of step S105 that the value of n is greater than the number of spatial multiplexing terminal stations (YES in step S105), or if the base station 10 determines in the process of step S106 that the number of allocated streams is greater than the maximum number of total MIMO layers (NO in step S106), the base station 10 performs spatial multiplexing transmission (step S107). Specifically, first, the precoding unit 13 calculates a weight matrix W to be used for communication with each terminal station 30-n that communicates using MU-MIMO or SU-MIMO. The precoding unit 13 multiplies the calculated weight matrix W by a transmission signal to be transmitted to the terminal station 30-n. The transmission signal multiplied by the weight matrix W is output to the photoelectric conversion unit 14. The photoelectric conversion unit 14 converts the transmission signal multiplied by the weight matrix W output from the precoding unit 13 into an optical signal, and transmits the optical signal to a candidate subarray of the terminal station 30-n, thereby performing spatial multiplexing transmission (step S108). The candidate subarray for the terminal station 30-n converts the optical signal output from the base station 10 into an electrical signal, and then converts it into a radio signal for spatial multiplexing transmission to the terminal station 30-n.

[0035] In the process of step S103, if the allocation unit 12 determines that there is a candidate subarray for the n-th candidate terminal station (step S103-YES), it selects one candidate subarray from among the candidate subarrays. The allocation unit 12 determines whether the selected candidate subarray is an antenna 20 different from the subarray 21 already allocated to the n-th candidate terminal station (step S108). The allocation unit 12 excludes subarrays 21 of the same antenna 20 by the process of step S108.

[0036] If the allocation unit 12 determines that the selected candidate subarray is the same antenna 20 as the subarray 21 already allocated to the n-th candidate terminal station (step S108-NO), it executes the process of step S103 again without allocating the selected candidate subarray to the n-th candidate terminal station. Note that the allocation unit 12 executes the process of step S103 for the n-th candidate terminal station, excluding the candidate subarray that has already been selected.

[0037] On the other hand, if the allocation unit 12 determines that the selected candidate subarray is a different antenna 20 from the subarray 21 already allocated to the n-th candidate terminal station (step S108-YES), it allocates the selected candidate subarray to the n-th candidate terminal station (step S109). Next, the allocation unit 12 determines whether the number of subarrays already allocated to the n-th candidate terminal station is less than the maximum number of SU-MIMO layers (step S110).

[0038] If the allocation unit 12 determines that the number of subarrays already allocated to the n-th candidate terminal station is less than the maximum number of SU-MIMO layers (step S110-YES), it performs the process of step S106.

[0039] In the process of step S110, if the allocation unit 12 determines that the number of subarrays already allocated to the n-th candidate terminal station is equal to or greater than the maximum number of SU-MIMO layers (step S110-NO), the allocation unit 12 executes the process of step S104.

[0040] As shown in Fig. 3, the base station 10 selects a subarray 21 to be assigned to a candidate terminal station from among the candidate subarrays, and if the selected subarray 21 is not the same antenna 20 as the subarray 21 already assigned, it additionally assigns the selected subarray 21. The base station 10 repeats the process shown in Fig. 3 until the maximum number of total MIMO layers of the base station 10 is satisfied or until there are no more candidate terminal stations. If there are no more candidate terminal stations and the maximum number of total MIMO layers is not satisfied, the assignment unit 12 may add a new candidate terminal station.

[0041] Before executing the process of step S103, the allocation unit 12 may execute the process of step S103 after excluding, from the candidate subarrays, subarrays 21 that belong to the same antenna 20 as the assigned subarrays 21.

[0042] According to the distributed antenna system 100 configured as described above, when spatial multiplexing transmission is performed in the distributed antenna system, it is possible to improve communication capacity by reducing spatial correlation between the base station and the terminal stations. Specifically, the base station 10 assigns subarrays 21 of different antennas 20 as subarrays 21 to be assigned to candidate terminal stations. While this may result in a decrease in the received power of multiple streams transmitted to the candidate terminal stations, it is possible to avoid selecting subarrays 21 that result in very high spatial correlation. This can improve user throughput. Furthermore, it is possible to reduce spatial correlation when performing SU-MIMO by simple control, such as by assigning subarrays 21 of different antennas 20 as subarrays 21 to be assigned to candidate terminal stations. Therefore, it is possible to reduce spatial correlation by simple control when performing SU-MIMO in a high-frequency band distributed antenna system using a subarray configuration, thereby improving user throughput.

[0043] A modification of the distributed antenna system 100 will now be described. (Variation 1) In the above embodiment, a configuration for a downlink from the base station 10 to the terminal station 30 is shown, but the above processing in the distributed antenna system 100 can also be applied to an uplink from the terminal station 30 to the base station 10. For example, the distributed antenna system 100 may perform SU-MIMO by simultaneously transmitting multiple streams from a single terminal station 30 to multiple antennas 20, and may perform MU-MIMO by simultaneously transmitting multiple streams from multiple terminal stations 30 to multiple antennas.

[0044] (Variation 2) In the above-described embodiment, the base station 10 and the multiple antennas 20 are connected via an optical transmission path. However, the base station 10 and the multiple antennas 20 may be connected via an electrical transmission path such as a coaxial cable. In such a configuration, communication between the base station 10 and the multiple antennas 20 is performed via electrical signals. Therefore, the base station 10 does not include an optical-to-electrical conversion unit 14, and the partial weight matrix W for each terminal station 30 is generated by the precoding unit 13. n Each transmission signal multiplied by is transmitted to the antenna 20 as an electrical signal.

[0045] (Variation 3) In the above-described embodiment, a configuration has been shown in which candidate terminal stations are extracted by scheduling, and then a subarray 21 is assigned to each of the extracted candidate terminal stations, but the order of terminal station selection (scheduling) and subarray 21 assignment (selection of terminal station connection antenna) may be reversed. Normally, it is assumed that a candidate terminal station is already connected to one of the subarrays 21. Therefore, first, the assignment unit 12 assigns a subarray 21 to each candidate terminal station, and then the terminal station extraction unit 11 extracts the candidate terminal station to which the subarray 21 has been assigned. For example, the assignment unit 12 may continue to assign or change the subarrays 21 that are already connected to candidate terminal stations.

[0046] Although an embodiment of the present invention has been described above in detail with reference to the drawings, the specific configuration is not limited to this embodiment, and includes designs within the scope of the gist of the present invention. [Industrial Applicability]

[0047] The present invention can be applied to a wireless communication system using MIMO. [Explanation of symbols]

[0048] 10... base station, 20... antenna, 30... terminal station, 11... terminal station extractor, 12... allocation unit, 13... precoding unit, 14... optical-electrical converter

Claims

1. A wireless communication method in a distributed antenna system including a base station and a plurality of antennas that communicate with one or more terminal stations by spatial multiplexing under control of the base station, each of the plurality of antennas includes a plurality of subarrays; the base station assigns a plurality of subarrays of different antennas to each of one or more candidate terminal stations that are to perform communication by spatial multiplexing using two or more subarrays among the plurality of subarrays; the base station extracts, from among a plurality of subarrays, a plurality of candidate subarrays to be assigned for each of the one or more candidate terminal stations, and assigns, from among the extracted plurality of candidate subarrays, candidate subarrays that belong to an antenna different from an already assigned subarray, by the number of streams to be spatially multiplexed, to each of the one or more candidate terminal stations; the base station selects, as the candidate subarray, a subarray having a reception power that satisfies a minimum reception sensitivity or a subarray that satisfies a predetermined quality threshold; a plurality of subarrays assigned to the candidate terminal stations perform spatial multiplexing transmission to the assigned candidate terminal stations; Wireless communication method.

2. A distributed antenna system comprising a base station and a plurality of antennas that communicate with one or more terminal stations by spatial multiplexing under control of the base station, each of the plurality of antennas includes a plurality of subarrays; the base station includes an allocation unit that allocates a plurality of subarrays of different antennas to each of one or more candidate terminal stations that are to perform communication by spatial multiplexing using two or more subarrays among the plurality of subarrays, the allocation unit extracts, from among a plurality of subarrays, a plurality of candidate subarrays to be allocated for each of the one or more candidate terminal stations, and allocates, from among the extracted plurality of candidate subarrays, candidate subarrays that belong to an antenna different from an already allocated subarray, for each of the one or more candidate terminal stations, by the number of streams to be spatially multiplexed; the allocation unit selects, as the candidate subarray, a subarray having a reception power that satisfies a minimum reception sensitivity or a subarray that satisfies a predetermined quality threshold; a plurality of subarrays assigned to the candidate terminal stations perform spatial multiplexing transmission to the assigned candidate terminal stations; Distributed Antenna System.

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