Communication device, method for receiving, and program

Abstract

To provide a communication device capable of previously determining an appropriate rank and weight in MIMO transmission on the basis of the reception probability of an interference wave predicted in advance.SOLUTION: A communication device 2 for receiving a radio signal transmitted by MIMO transmission from a transmitting communication device includes: a calculation unit 24 configured to calculate an expected value of channel capacity for each rank by using a reception probability of each interference wave and a channel capacity for each rank corresponding to the presence or absence of an interference wave; and a determination unit 25 configured to determine the rank having the largest minimum value of channel capacity according to the presence or absence of the interference wave and determine the weight for an event in which the channel capacity is minimum at that rank in a case where the reception probability of the interference wave is within a predetermined range from 50%, and determine the rank having the maximum expected value and determine the weight for an event having the highest probability at that rank in a case where the reception probability is outside the range. A reception unit 21 extracts a desired signal from the received radio signal by using the determined weight.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a communication device or the like that receives radio signals in MIMO transmission. [Background technology]

[0002] In current broadband wireless communications, MIMO (Multiple Input Multiple Output) transmission is widely used. Furthermore, rank adaptation in MIMO transmission is an important technique for performing robust transmission when interference waves arrive (see, for example, Non-Patent Document 1).

[0003] In unlicensed bands, each wireless system operates autonomously. In wireless communications (e.g., wireless LAN) using such random access controlled wireless channels, the hidden node problem may occur. Rank adaptation is also performed in environments where sporadic interference waves exist (see Non-Patent Document 2). [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] F. Bohagen, P. Orten, GE Oien, "Design of Optimal High-Rank Line-of-Sight MIMO Channels", IEEE Transactions on Wireless Communications, vol. 6, no. 4, pp. 1420-1425, April 2007, doi: 10.1109 / TWC.2007.348338 [Non-patent document 2] Yusuke Okumoto, Takayuki Matsuuro, Kazuto Yano, Toshikazu Sakano, "Performance evaluation on rank adaptation of MIMO transmission method using probabilistic interference arrival information assuming probability prediction error", IEICE Communications Express, Vol.13, No.2, pp. 30-34, December 2023, DOI: 10.23919 / comex.2023XBL0139 Summary of the Invention [Problem to be solved by the invention]

[0005] Even if suitable rank adaptation can be performed in an environment where sporadic interference waves are present, if weights for extracting a desired signal from a radio signal received via MIMO transmission are generated in real time each time a radio signal is received, the processing load for generating the weights in real time is large, and there is a problem in that the communication devices that can generate such weights are limited.

[0006] Therefore, it is conceivable to predict the occurrence of interference waves in advance based on the reception probability of interference waves and generate weights in advance. However, if the predicted occurrence of interference waves does not match the actual occurrence of interference waves, there is a problem that the channel capacity will be significantly reduced.

[0007] The present invention has been made to solve the above-mentioned problems, and aims to provide a communication device etc. that can determine an appropriate rank and weight in advance based on a predicted probability of receiving an interference wave. [Means for solving the problem]

[0008] In order to achieve the above object, a communication device according to one aspect of the present invention is a communication device that receives a radio signal transmitted by MIMO transmission from a transmitting communication device, and includes: a receiver that receives the radio signal via a plurality of antennas; a channel capacity acquisition unit that acquires, using the radio signal received by the receiver, a channel capacity for each rank depending on the presence or absence of one or more interference waves; a probability acquisition unit that acquires a reception probability for each of the one or more interference waves after a predetermined time has elapsed, using a reception history for each of the one or more interference waves received by the receiver; a calculation unit that calculates an expected value of the channel capacity for each rank, using the reception probability for each of the one or more interference waves acquired by the probability acquisition unit and the channel capacity acquired by the channel capacity acquisition unit; and a calculation unit that calculates an expected value of the channel capacity for each rank, using the reception probability for each of the one or more interference waves acquired by the probability acquisition unit and the channel capacity acquired by the channel capacity acquisition unit. a determination unit that, if the range is within the predetermined range, determines the rank that the transmitting communication device will use after a predetermined time has elapsed to be the rank with the largest minimum value of channel capacity depending on the presence or absence of one or more interference waves, and determines the weight that the receiving unit will use after the predetermined time has elapsed to be a weight corresponding to the event with the smallest channel capacity at the determined rank; if not, determines the rank that the transmitting communication device will use after the predetermined time has elapsed to be the rank with the largest expected value calculated by the calculation unit, and determines the weight that the receiving unit will use after the predetermined time has elapsed to be a weight corresponding to the event with the highest probability at the determined rank; and a transmission unit that transmits the rank determined by the determination unit to the transmitting communication device, and the receiving unit extracts a desired signal from wireless signals received via multiple antennas using the weight determined by the determination unit. With this configuration, it is possible to determine appropriate ranks and weights in advance based on the predicted probability of receiving interference waves, and to realize MIMO transmission that is closer to a situation where optimal weights are used depending on whether or not interference waves are present.

[0009] In the communication device according to an aspect of the present invention, the predetermined range may be a range of ±α% (where α is a positive real number less than 50). With this configuration, by setting α to an appropriate value, it is possible to realize MIMO transmission that is closer to a situation where an optimal weight is used depending on whether or not there is an interference wave.

[0010] In the communication device according to an aspect of the present invention, the determination unit may adaptively change α according to a reception result by the reception unit of the radio signal transmitted by MIMO transmission. With this configuration, the value of α can be adaptively adjusted in response to changes in the communication environment.

[0011] In the communication device according to an aspect of the present invention, the determination unit may change α over time. With this configuration, it is possible to change α to an appropriate value depending on the change in the communication environment over the course of a day, for example.

[0012] Furthermore, a receiving method according to one aspect of the present invention is a method for receiving a radio signal transmitted by MIMO transmission from a transmitting communication device, and includes the steps of: using the radio signal received via a plurality of antennas, acquiring a channel capacity for each rank depending on the presence or absence of one or more interference waves; using a reception history for each of the received one or more interference waves to acquire a reception probability after a predetermined time has elapsed for each of the one or more interference waves; calculating an expected value of the channel capacity for each rank using the acquired reception probability for each of the one or more interference waves and the acquired channel capacity; and when the acquired reception probability after a predetermined time has elapsed for at least any of the interference waves is within a predetermined range from 50%, the transmitting communication device determining the rank to be used after a predetermined time has elapsed to be the rank with the largest minimum value of channel capacity depending on the presence or absence of one or more interference waves, determining the weight to be used after a predetermined time has elapsed to be the weight according to the event in which the channel capacity at the determined rank is the smallest, and if this is not the case, determining the rank to be used by the transmitting communication device after a predetermined time has elapsed to be the rank with the largest calculated expected value, and determining the weight to be used after the predetermined time has elapsed to be the weight according to the event with the highest probability at the determined rank; transmitting the determined rank to the transmitting communication device; and extracting a desired signal from wireless signals received via multiple antennas using the determined weight. [Effects of the Invention]

[0013] According to a communication device or the like of one aspect of the present invention, it is possible to determine an appropriate rank and weight in advance based on a previously predicted probability of receiving an interference wave, and to realize MIMO transmission that is closer to a situation using an optimal weight depending on whether or not an interference wave is present. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a schematic diagram showing the configuration of a MIMO transmission system according to an embodiment of the present invention. [Figure 2] FIG. 2 is a block diagram showing the configuration of a communication device according to the embodiment. [Figure 3]A flowchart showing the operation of the communication device according to the embodiment. [Figure 4] FIG. 10 shows channel capacities for each rank and for each presence or absence of interference waves, and expected values ​​for each rank, according to the embodiment. [Figure 5] FIG. 10 shows an example of a channel capacity for each rank and for each presence or absence of interference waves, and an expected value for each rank, according to the embodiment. [Figure 6] FIG. 10 is a diagram for explaining switching of a method for determining ranks and weights in the embodiment. [Figure 7] 10 is a graph showing the relationship between the prediction error included in the reception probability of an interference wave and the ratio of the channel capacity to the optimal weight in the simulation results of the embodiment. [Figure 8] FIG. 2 shows an example of the configuration of a computer system according to the embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0015] A communication device and a receiving method according to the present invention will be described below using embodiments. In the following embodiments, components and steps with the same reference numerals are the same or equivalent, and repeated description may be omitted. The communication device according to this embodiment determines the rank and weight to be used in MIMO transmission after a predetermined time has elapsed, using different methods depending on whether the reception probability of at least one of the interference waves is within a predetermined range from 50%.

[0016] FIG. 1 is a schematic diagram showing the configuration of a MIMO transmission system 100 according to this embodiment, and FIG. 2 is a block diagram showing the configuration of a communication device 2 on the receiving side of MIMO transmission. The MIMO transmission system 100 according to this embodiment includes a communication device 1 on the transmitting side that transmits radio signals by MIMO transmission, and a communication device 2 on the receiving side that receives the radio signals transmitted by MIMO transmission from the communication device 1 on the transmitting side. Note that the transmitting side and the receiving side refer to the transmitting side and the receiving side in the MIMO transmission described in this embodiment. For example, a control signal may be transmitted from the communication device 2 to the communication device 1. Also, regarding MIMO transmission, communication may be performed in which the communication device 2 is the transmitting side and the communication device 1 is the receiving side. Also, the communication devices 1 and 2 are not particularly limited, and as an example, the communication device 1 may be an access point and the communication device 2 may be a terminal device. In this case, downlink MIMO rank adaptation is performed.

[0017] In this embodiment, a case will be mainly described in which communication devices 1 and 2 perform 4×4 single-user MIMO (SU-MIMO) transmission via four antennas 1a and 2a, respectively, and communication device 2 receives interference waves from two interference sources 3 and 4, but it goes without saying that the number of antennas used in MIMO transmission and the number of interference sources are not limited to these. Hereinafter, the interference waves received by communication device 2 from interference sources 3 and 4 will be referred to as interference waves A and B, respectively. Interference sources 3 and 4 may be, for example, communication devices that perform wireless communication, or may be devices that do not perform wireless communication, such as a microwave oven.

[0018] 2, the communication device 2 includes a receiving unit 21, a channel capacity obtaining unit 22, a probability obtaining unit 23, a calculating unit 24, a determining unit 25, and a transmitting unit 26. In this embodiment, as described above, the case where the communication device 2 has four antennas 2a will be mainly described.

[0019] The receiver 21 receives radio signals via the multiple antennas 2a. The receiver 21 receives MIMO transmission radio waves transmitted from the transmitting communication device 1, and may also receive interference waves A and B transmitted from interference sources 3 and 4. When the interference waves A and B are transmitted autonomously, four events occur depending on whether the receiver 21 receives the interference waves A and B, respectively. The first event is an event in which neither interference wave A nor B is received, the second event is an event in which the interference wave A is received but not the interference wave B, the third event is an event in which the interference wave A is not received but the interference wave B is received, and the fourth event is an event in which both interference waves A and B are received.

[0020] The receiver 21 may also identify, for example, the interference waves A and B. If the interference wave is a demodulatable signal, such as a signal transmitted from another wireless system, the receiver 21 may demodulate the received wireless signal and obtain the identifier (e.g., address) of the source included in the wireless signal to acquire information for identifying the interference wave. On the other hand, if the interference wave is a signal that cannot be demodulated, such as a signal transmitted from a device such as a microwave oven and is not a communication signal, the receiver 21 may acquire an array response vector, which is a set of the amplitude ratio and phase difference between the received signal received by one reference antenna 2a among the four antennas 2a and the received signals received by the other three antennas 2a, and use this as information for identifying the interference wave. Then, interference waves whose array response vectors match may be determined to be interference waves from the same interference source. Note that a match between the array response vectors may mean, for example, that the array response vectors match exactly or within a predetermined error range.

[0021] Furthermore, when two interference waves A and B are received simultaneously, it is likely that one of the interference waves will be stronger. In this case, an array response vector similar to the array response vector of the stronger interference wave will be acquired. Therefore, for example, if an interference wave with a similar array response vector but not the same as the array response vector of interference wave A is received, it may be determined that interference waves A and B have been received simultaneously. Furthermore, it is likely that the busy periods of multiple interference waves rarely coincide perfectly. Therefore, it is likely that there will be periods in which one interference wave is received before and after a period in which two interference waves are received simultaneously. For example, if a first period in which only interference wave A is received is followed by a second period in which both interference waves A and B are received, and then a third period in which only interference wave B is received, it can be inferred that both interference waves A and B were received during the second period based on the conditions before and after the second period. Therefore, in this case, the array response vectors of the received signals received during the second period may be the array response vectors corresponding to both interference waves A and B. In this way, the receiver 21 can identify whether or not one or more interference wave signals are included in the received signal. Note that the method for identifying interference waves described here is just an example, and it goes without saying that interference waves may be identified by other methods.

[0022] Furthermore, the receiver 21 may extract a desired signal from radio signals received via the multiple antennas 2a using the weights determined by the determiner 25. For example, when the determiner 25 determines weights for time t+D after a predetermined time has elapsed, the receiver 21 may extract a desired signal from the received radio signals using the determined weights at time t+D after the predetermined time has elapsed. D is a positive real number and is usually not a large value. The determination of weights by the determiner 25 will be described later. The weights may be, for example, MMSE (minimum mean square error) weights. This process of extracting a desired signal is similar to the process in conventional MIMO transmission, and a detailed description thereof will be omitted. The receiver 21 may extract a desired signal using weights acquired by the channel capacity acquirer 22, for example.

[0023] The channel capacity acquisition unit 22 acquires channel capacities for each rank according to the presence or absence of one or more interference waves, using the radio signals received by the receiving unit 21. As an example, the channel capacity acquisition unit 22 may calculate weights and acquire channel capacities using the weights. That is, the channel capacity acquisition unit 22 may acquire weights as well as channel capacities. It is preferable that the channel capacities are acquired for all combinations of the presence or absence of one or more interference waves. For example, when n interference waves are received by the receiving unit 21, 2 ranks according to the presence or absence of each of the n interference waves are acquired. n There will be events, and for each event, one channel capacity is obtained, so 2 nchannel capacities are obtained. n is an integer equal to or greater than 1. The channel capacities are obtained for each rank. The rank is the number of streams in MIMO transmission, and strictly speaking, is the smaller of the number of transmitting antennas and the number of receiving antennas used in MIMO transmission. However, in this embodiment, since it is desired to reduce the influence of interference waves, it is not possible to reduce the number of receiving antennas used in MIMO transmission. Therefore, in this embodiment, the rank is usually the number of antennas 1a used by the transmitting communication device 1 for MIMO transmission. Therefore, if the transmitting communication device 1 has M antennas 1a, the rank is an integer value between 1 and M. M is an integer equal to or greater than 2. For each rank, 2 n Since the channel capacities are acquired, the total number of channel capacities acquired by the channel capacity acquisition unit 22 is 2 n ×M. The detailed process of acquiring the channel capacity will be described later.

[0024] The probability acquisition unit 23 acquires a reception probability for each of the one or more interference waves after a predetermined time has elapsed, using a reception history for each of the one or more interference waves received by the receiving unit 21. The reception history for the interference wave may be a history of the busy / idle states of the interference wave. The reception history for each interference wave may be acquired by the probability acquisition unit 23, for example, and stored in a recording medium (not shown). The probability acquisition unit 23 preferably acquires a reception probability for the interference wave after a predetermined time has elapsed, which is a future time when the transmitting communication device 1 will perform communication. For example, if the current time is t, the probability acquisition unit 23 may acquire a reception probability for the interference wave at time t+D. In this case, time t+D is after the predetermined time has elapsed. The probability acquisition unit 23 acquires a reception probability for each interference wave. Therefore, if n interference waves exist, the probability acquisition unit 23 may acquire n reception probabilities at time t+D, each corresponding to one of the n interference waves. Note that for a method of acquiring a reception probability for a certain interference wave at a future time using the reception history of that interference wave, see, for example, the following document. Literature: K. Yano, N. Egashira, J. Webber, M. Usui, Y. Suzuki, "Achievable throughput of multiband wireless LAN using simultaneous transmission over multiple primary channels assisted by idle length prediction based on PNN", Proc. of ICAIIC 2019, pp. 22-27, Feb. 2019, doi: 10.1109 / ICAIIC.2019.8668975. Reference: JP 2019-087916 A

[0025] For example, when the receiver 21 receives a transmission notice transmitted from the transmitting communication device 1, the probability acquirer 23 may acquire the reception probability of each interference wave at the time when transmission is performed in response to the transmission notice. In this case, the time after a predetermined time has elapsed may be, for example, the time when transmission is performed in response to the transmission notice. Furthermore, when there is no transmission notice, the probability acquirer 23 may, for example, periodically acquire the reception probability of each interference wave. Note that after the reception probability is acquired, a rank is determined as described below, and it takes a predetermined time for the determination result to be fed back to the transmitting communication device 1. Therefore, it is preferable that the probability acquirer 23 acquires the reception probability of each interference wave after a predetermined time has elapsed, which is a future time that is at least the predetermined time from the current time.

[0026] The calculation unit 24 calculates the expected value of the channel capacity for each rank using the reception probability for each of one or more interference waves acquired by the probability acquisition unit 23 and the channel capacity acquired by the channel capacity acquisition unit 22. More specifically, for a certain rank, the calculation unit 24 may multiply the probability of an event corresponding to whether or not each interference wave is received by the probability acquisition unit 23 and the channel capacity, and calculate the sum of the multiplication results to calculate the expected value of the channel capacity. The calculation unit 24 may then calculate the expected value for all ranks. If the maximum rank is M, for example, M expected values ​​may be calculated. As described above, when the probability acquisition unit 23 acquires the reception probability for each of one or more interference waves after a predetermined time has elapsed, the expected value calculated using the reception probability also becomes the expected value after the predetermined time has elapsed. The calculation unit 24 may calculate this expected value only when the rank and weight are determined using the expected value method described below. Note that the detailed process of calculating the expected value of the channel capacity will be described later.

[0027] When the reception probability of at least one interference wave after a predetermined time has elapsed, acquired by the probability acquisition unit 23, is within a predetermined range of 50%, the determination unit 25 determines the rank to be used by the transmitting communication device 1 after the predetermined time has elapsed to be the rank with the largest minimum channel capacity depending on the presence or absence of one or more interference waves, and determines the weight to be used by the receiving unit 21 after the predetermined time has elapsed to be the weight depending on the event that results in the smallest channel capacity at the determined rank. Hereinafter, this determination of rank and weight may be referred to as the "Max-Min method." With this Max-Min method, the rank with the largest minimum channel capacity is determined regardless of the situation regarding the presence or absence of one or more interference waves, i.e., regardless of the event.

[0028] In the Max-Min method, the determiner 25 may determine the rank that the transmitting communication device 1 will use after a predetermined time has elapsed as the rank with the largest minimum value of channel capacity depending on the presence or absence of one or more interference waves, based on the channel capacity acquired for each rank and for each event depending on the presence or absence of one or more interference waves. More specifically, the determiner 25 may, for example, identify the smallest channel capacity among the channel capacities for each event for each rank, and determine the rank corresponding to the largest channel capacity among the identified minimum channel capacities for each rank as the rank that the transmitting communication device 1 will use after a predetermined time has elapsed. Furthermore, the determiner 25 may determine the weight of the event with the smallest channel capacity depending on the presence or absence of interference waves in the determined rank as the weight that the receiving unit 21 will use after a predetermined time has elapsed.

[0029] When the reception probability of at least one of the interference signals acquired by the probability acquisition unit 23 is not within a predetermined range of 50% or less, the determination unit 25 determines the rank that the transmitting communication device 1 will use after a predetermined time has elapsed to be the rank that has the largest expected value calculated by the calculation unit 24, and determines the weight that the receiving unit 21 will use after a predetermined time has elapsed to be the weight corresponding to the most probable event at the determined rank. Hereinafter, this determination of ranks and weights may be referred to as the "expected value method." The expected value used to determine the rank is, for example, the expected value calculated by the calculation unit 24 using the reception probability after a predetermined time has elapsed. In this expected value method, the determination unit 25 may identify the largest expected value among the calculated expected values ​​and determine the rank corresponding to the identified maximum expected value as the rank that the transmitting communication device 1 will use after a predetermined time has elapsed. Furthermore, the determination unit 25 may determine the weight of the most probable event at the determined rank, depending on whether or not there is an interference signal, as the weight that the receiving unit 21 will use after a predetermined time has elapsed.

[0030] The Max-Min method always determines weights taking into account reception of interference waves, so it can obtain stable channel capacity, but the channel capacity when no interference waves are received is lower than that of the expected value method. On the other hand, the expected value method selects the event with the highest occurrence probability from multiple events corresponding to the presence or absence of reception of one or more interference waves as a weight, so it can obtain high channel capacity when the selected event matches the actual event, but the channel capacity drops significantly when the selected event does not match the actual event. Therefore, the determiner 25 determines ranks and weights using a hybrid method in which the Max-Min method is used when the reception probability of at least one interference wave is within a predetermined range from 50%, i.e., when it is uncertain whether an interference wave is received, and the expected value method is used when the reception probability of the interference wave is close to 0% or 100%, i.e., when it is certain whether an interference wave is received.

[0031] The transmitter 26 transmits the rank determined by the determiner 25 to the transmitting communication device 1. In this way, the determined rank is passed to the transmitting communication device 1, and the transmitting communication device 1 transmits a radio signal by MIMO transmission after a predetermined time has elapsed based on the determined rank. The transmitter 26 may, for example, transmit the determined rank directly to the transmitting communication device 1, or may transmit it indirectly via another server or the like. The transmitter 26 may or may not include a transmitting device (for example, a modem, a network card, etc.) for transmitting. The transmitter 26 may be realized by hardware, or may be realized by software such as a driver that drives the transmitting device.

[0032] Here, an example of timing for obtaining the reception probability of an interference wave, determining the rank and weight, and using the determined rank and weight will be described. First, for example, when the current time is t, the probability obtaining unit 23 may obtain the reception probability for each interference wave at time t+D. Furthermore, the determining unit 25 may determine the rank and weight using the reception probability for each interference wave at time t+D. The rank and weight determined in this manner may be used for a predetermined period including time t+D. That is, during the predetermined period, the communication device 1 may transmit a wireless signal with the determined rank. Furthermore, during the predetermined period, the communication device 2 may extract a desired signal from the received wireless signal using the determined weight.

[0033] If the length of the predetermined period is τ, the predetermined period may be, for example, from time t+D to time t+D+τ, or from time t+D-τ to time t+D, or any other period whose length is τ and includes time t+D. In the communication device 2 according to this embodiment, the weights used in the predetermined period are already obtained at time t, so there is no need to generate the weights in real time. Furthermore, the probability obtaining unit 23 and the determination unit 25 may obtain the reception probability for each interference wave and determine the rank and weight, for example, for each time τ.

[0034] Although the description here has mainly been made on the case where the reception probability for each interference wave at time t+D represents the reception probability for each interference wave in a period of length τ that includes time t+D, the probability acquiring unit 23 may, for example, acquire the reception probability for each interference wave in a predetermined period of length τ that includes time t+D. In this case, for example, the reception probability of a certain interference wave may be the probability that the interference wave is received during at least a part of the predetermined period.

[0035] Next, the operation of the communication device 2 will be described with reference to the flowchart of FIG. (Step S101) The channel capacity acquisition unit 22 determines whether to acquire channel capacity. If the channel capacity is to be acquired, the process proceeds to step S102; if not, the process proceeds to step S103. For example, when a reception signal for an event for which the channel capacity has not been acquired is received, the channel capacity acquisition unit 22 may determine to acquire channel capacity according to that event. Furthermore, when a change in the event occurs, such as the occurrence of new interference waves, the channel capacity acquisition unit 22 may determine to acquire channel capacity according to that event after the change when a reception signal for an event for which the channel capacity has not been acquired is received.

[0036] (Step S102) The channel capacity acquisition unit 22 acquires, for each rank, a channel capacity according to the event at that time using the received wireless signal. The acquired channel capacity may be stored in a recording medium (not shown). Then, the process returns to step S101. Note that by repeating the processes of steps S101 and S102, a channel capacity for each rank according to the presence or absence of one or more interference waves may be acquired.

[0037] (Step S103) The determination unit 25 determines whether to determine a rank or weight. If a rank or the like is to be determined, the process proceeds to step S104; if not, the process returns to step S101. For example, the determination unit 25 may determine to determine a rank or the like when acquisition of channel capacities for all events has been completed and a transmission notice has been received from the transmitting communication device 1, or may repeatedly determine to determine a rank or the like. In the latter case, for example, the determination to determine a rank or the like may be periodically made.

[0038] (Step S104) The probability acquisition unit 23 acquires the reception probability of each interference wave after a predetermined time has elapsed. This predetermined time may be, for example, the time when transmission is performed in response to the transmission notice, or may be a time when a predetermined time has elapsed from the present time.

[0039] (Step S105) The decision unit 25 determines whether the reception probability of at least any of the interference waves after the predetermined time has elapsed, acquired by the probability acquisition unit 23, is within a predetermined range from 50%. If the reception probability of at least any of the interference waves after the predetermined time has elapsed is within the predetermined range from 50%, the process proceeds to step S106; if not, the process proceeds to step S108.

[0040] (Step S106) The determination unit 25 determines the rank by the Max-Min method. That is, the determination unit 25 determines the rank to be used by the transmitting communication device 1 after a predetermined time has elapsed to be the rank with the largest minimum value of the channel capacity depending on the presence or absence of one or more interference waves.

[0041] (Step S107) The determination unit 25 determines the weight using the Max-Min method. That is, the determination unit 25 determines the weight that the receiving unit 21 will use after a predetermined time has elapsed to be the weight corresponding to the event in which the channel capacity is minimum in the event in which there is an interference wave of 1 or more in the rank determined in step S106. Note that this weight may be one that was acquired by the channel capacity acquisition unit 22 when acquiring the channel capacity. Then, the process proceeds to step S111.

[0042] (Step S108) The calculation unit 24 calculates the expected value of the channel capacity for each rank using the channel capacity for each event depending on whether or not there is one or more interference waves, and the reception probability of each interference wave after a predetermined time has elapsed.

[0043] (Step S109) The determination unit 25 determines the rank by the expected value method. That is, the determination unit 25 determines the rank that the transmitting communication device 1 will use after a predetermined time has elapsed to be the rank that has the maximum expected value calculated by the calculation unit 24.

[0044] (Step S110) The determination unit 25 determines the weight using the expected value method. That is, the determination unit 25 determines the weight that the receiving unit 21 will use after a predetermined time has elapsed to be the weight corresponding to the most probable event depending on the presence or absence of interference waves in the rank determined in step S109. Note that this weight may be one that was acquired by the channel capacity acquisition unit 22 when acquiring the channel capacity. Then, the process proceeds to step S111.

[0045] (Step S111) The transmitter 26 transmits the determination result by the determiner 25 to the transmitting-side communication device 1. Then, the process returns to step S101. In response to the transmission of this determination result, the transmitting-side communication device 1 transmits a transmission signal by MIMO transmission using the determined rank after a predetermined time has elapsed.

[0046] Although not included in the flowchart of FIG. 3 , the receiver 21 may extract a desired signal from radio signals received via multiple antennas 2a using the weight determined by the determiner 25. For example, if the determiner 25 determines a weight for time t+D, the receiver 21 may use the weight for time t+D when extracting a desired signal from radio signals received during a period of length τ that includes time t+D. The receiver 21 may also perform a process of identifying each interfering wave by receiving the interfering wave. The probability acquirer 23 may record the reception history of each interfering wave using the identification result of the interfering wave. The order of the processes in the flowchart of FIG. 3 is merely an example, and the order of the steps may be changed as long as the same results are obtained. In the flowchart of FIG. 3 , the process may end due to a power-off or an interrupt to end the process.

[0047] Next, the acquisition of channel capacity and the calculation of expected values ​​using channel capacity and reception probability will be described in more detail. Here, as described above, a case where 4x4 SU-MIMO transmission is performed will be described. Therefore, M=4, and there are four ranks to be determined, 1 to 4. Also, a case where the receiver 21 receives two interference waves A and B from two interference sources 3 and 4 will be described. In this case, depending on the presence or absence of each of the interference waves A and B, two 2 There will be 16 events. An index k that identifies each event is set as follows: no interference (k=1), interference wave A only (k=2), interference wave B only (k=3), or both interference waves A and B (k=4). In the following description, an event identified by index k may also be referred to as "event k." As shown in FIG. 4, 16 channel capacities C m k where m and k are integers from 1 to 4. The channel capacity acquisition unit 22 acquires the 16 channel capacities. The channel capacity acquisition unit 22 may calculate the 16 channel capacities using, for example, the following equation. The following equation is the channel capacity when MMSE is applied, and is found from the SINR of the desired signal and the interference signal.

number

[0048] In equation (1), the vector W k opt,j is the transmission signal s of the j-th stream transmitted from the j-th antenna 1a of the transmitting communication device 1 in the event k. j The optimal weight for (t) is given by the following equation (2) as the Wiener solution. For more information, see the following literature. The superscript T denotes transpose, * denotes complex conjugate, and H denotes complex conjugate transpose. The vector h jThe following will be described later. Note that the vectors in this specification are column vectors. As is clear from equation (1), weights are used to calculate the channel capacity, so when the channel capacity is calculated, the weights have already been calculated. Literature: B. Widrow, PE Mantey, LJ Griffiths, BB Goode, "Adaptive antenna systems", Proceedings of the IEEE, vol. 55, no. 12, pp. 2143-2159, Dec. 1967, doi: 10.1109 / PROC.1967.6092.

number

[0049] In equation (2), the matrix R k xx is the correlation matrix of the received signal, and is given by the following equation for each event:

number

[0050] where P S is the transmission power of the desired wave, which is known. The desired wave is the radio wave that the communication device 2 is trying to receive, and is the radio signal transmitted from the transmitting communication device 1. P A , P B are the transmission powers of interference waves A and B, respectively. σ 2 is the noise power, which can be obtained by observation. Also, the matrix H D is the propagation path matrix of the desired wave, and the matrix h A , h B are the propagation path matrices of the interference waves A and B, respectively. The propagation path matrix H of the desired wave D may be obtained using a pilot signal included in the desired wave. For example, the pilot signal may be a pilot symbol inserted in the header of each block of the transmission signal transmitted from the communication device 1. Also, the propagation path matrix h A, h B may be obtained using the interference waves A and B, respectively. The matrix I is R ×N R is the identity matrix of N R is the number of antennas 2a that the receiving communication device 2 has, and is usually an integer equal to or greater than 2. A , P B respectively as matrix h A , h B In this case, in the above formula, P A =P B = 1. Furthermore, P S Also matrix H D In this case, in the above formula, P S = 1. However, if the transmission power is included in the channel matrix, the norm will not be 1.

[0051] Also, in equation (2), the vector r xs,j is the correlation vector between the transmitted signal and the received signal, and is expressed by the following equation: j is the matrix H D is a propagation path vector corresponding to the j-th antenna 1a on the transmitting side.

number

[0052] The channel capacity acquisition unit 22 acquires, for example, the propagation path matrices of the desired wave and the interference wave, and substitutes them into equations (3) to (7) to obtain the correlation matrix R of the received signal. k xx and the correlation vector r between the transmitted signal and the received signal xs,j and then substituting them into equation (2) gives the optimal weight vector W for MMSE. k opt,j Calculate the optimal weight vector W k opt,j Alternatively, the channel capacity may be calculated by substituting the propagation path vector corresponding to a specific transmitting antenna 1a in the propagation path matrix of the desired wave into equation (1).

[0053] Note that the matrix R k xx is the received signal vector x k Using (t), it can also be calculated as in the following equation (8): Vector x k (t) may be a vector having, as elements, the received signals of each antenna 2a received by the receiver 21 at time t, which is the event k. In the following equation, E[X] is the expected value of X. Therefore, the channel capacity acquisition unit 22 calculates the matrix R using, for example, the following equation (8): k xx may be calculated.

number

[0054] Also, the vector r xs,j Also, the received signal vector x k (t), and the jth stream transmitted from the jth antenna 1a on the transmitter side is the transmitted signal s j Using (t), the transmission signal s can be calculated as follows: j (t) may be, for example, a pilot signal inserted when the transmitting communication device 1 transmits a radio signal. Therefore, the channel capacity acquisition unit 22 calculates the vector r xs,j may be calculated.

number

[0055] In this way, the channel capacity acquisition unit 22 calculates, for example, the vector x k (t) and the transmitted signal s j (t) is used to find the optimal weight vector W for MMSE. k opt,j , and the propagation path matrix H is calculated using the pilot signal included in the desired wave. D By obtaining the propagation path vector h jThe channel capacity acquisition unit 22 may then use these to calculate the channel capacity. j For example, the calculated r xs,j may be calculated by substituting into equation (7).

[0056] As is clear from equation (8), the matrix R corresponding to each event k xx is calculated using the received signal for each event. Therefore, for example, the matrix R corresponding to event 1 (no interference wave) is 1 xx is calculated using the received signal when only the desired wave is received, and the matrix R corresponding to event 2 (only interference wave A) is 2 xx is calculated using the received signal when the desired wave and interference wave A are received. Using equation (9), the vector r xs,j The same applies when calculating

[0057] The channel capacity calculation method described here is merely an example, and it goes without saying that other methods may be used to calculate the channel capacity. Furthermore, it is preferable that the channel capacity acquisition unit 22 recalculates the channel capacity when an event changes. For example, when a new interference wave is received or when an existing interference wave is no longer received, it is preferable that the channel capacity be recalculated accordingly.

[0058] The reception probability (i.e., arrival probability) of the interference wave A at time t+D after a predetermined time has elapsed, acquired by the probability acquisition unit 23, is denoted as p A The reception probability of interference wave B is p B Then, the probability of an event without interference is (1-p A )(1-p B ) and the probability of only interference wave A arriving is p A (1-p B ), and the probability of only interference wave B arriving is (1-p A )p B The probability that both interference waves A and B arrive is pA p B Therefore, the calculation unit 24 calculates the expected value E of the rank m after a predetermined time has elapsed. m can be calculated as the sum of the products of the probabilities corresponding to each event and the channel capacity, as in the following equation: In this way, four expected values ​​E1 to E4 may be calculated using the following equations.

number

[0059] Although the explanation here has been given mainly on the case where the maximum rank is 4 and there are two interference waves, it is clear that even in other cases, it is possible to calculate the channel capacity for each rank and each event in a similar manner, obtain the reception probability of each interference wave, and use these to calculate the expected value of the channel capacity for each rank.

[0060] Next, the determination of rank and weight by the Max-Min method and the expected value method will be explained using a concrete example. In this concrete example, the reception probability p A = 60%, and the reception probability of interference wave B is p B = 20%, and the channel capacity (bps / Hz) and expected value of the channel capacity for each rank and each event are as shown in Figure 5. In the table of Figure 5, the probability of the event is written to the right of the index k of the event.

[0061] In the Max-Min method, of the minimum channel capacity of 4.49 bps / Hz for rank 4, 5.94 bps / Hz for rank 3, 13.43 bps / Hz for rank 2, and 11.39 bps / Hz for rank 1 in the table of Fig. 5, rank 2, which corresponds to the maximum channel capacity of 13.43 bps / Hz, is determined as the rank to be used by the transmitting communication device 1 for a period of length τ including time t+D. Also, the weight to be used by the receiving unit 21 for a period of length τ including time t+D is determined to be a weight corresponding to the event k=4 for which the channel capacity in the determined rank 2 is the smallest. The weight is, for example, W 4 opt,1 , W 4 opt,2 This becomes:

[0062] In the expected value method, in the table of Fig. 5, rank 3, which corresponds to the highest expected value of channel capacity, 16.09 bps / Hz, is determined as the rank to be used by the transmitting communication device 1 for a period of length τ including time t+D. Also, the weight to be used by the receiving unit 21 for a period of length τ including time t+D is determined as a weight corresponding to the event k=2 with the highest probability at the determined rank 3. The weight is, for example, W 2 opt,1 , W 2 opt,2 , W 2 opt,3 This becomes:

[0063] Next, a method for determining the rank and weight used by the determiner 25 will be described with reference to Fig. 6. For example, as shown in Fig. 6, the determiner 25 may determine the rank and weight using the Max-Min method when the reception probability of at least any interference wave after a predetermined time has elapsed is within the range of 50±α(%), i.e., when 50-α(%)<reception probability after a predetermined time has elapsed<50+α(%) for at least any interference wave, and may determine the rank and weight using the expected value method in other cases. That is, the above-mentioned predetermined range may be a range of ±α%, where α is a positive real number less than 50. Note that at least one of the above-mentioned inequality signs "<" may be an inequality sign with an equal sign "≦".

[0064] Next, a description will be given of the results of a simulation regarding reception of a radio signal by MIMO transmission according to this embodiment. In this simulation, it is assumed that there are two interference waves A and B in 4x4 SU-MIMO transmission. Furthermore, the propagation path from the transmitting communication device 1 to the receiving communication device 2 is a Nakagami-Rice fading propagation path. Furthermore, the spacing between the antennas 1a and 2a on the transmitting and receiving sides in the linear array antenna is a half wavelength. Furthermore, the K factor is set to 10 dB, and the SNR (P S / σ 2 ) was set to 30 dB, and the transmission power of the desired wave and the two interference waves A and B was all equal. The arrival angles of the direct waves of the desired wave, interference wave A, and interference wave B were set to 0 degrees, 50 degrees, and -20 degrees, respectively.

[0065] α, which defines a predetermined range from 50%, was set from 0% to 30% in 5% increments, and for each value of α, the ratio of channel capacity when using the optimal weight relative to the predicted error of the reception probability of the interference wave (horizontal axis in FIG. 7) to when using a pre-determined weight (vertical axis in FIG. 7) was calculated. The results are shown in FIG. 7. For example, in FIG. 5, the weight for rank 2, k=4 was determined by the Max-Min method, but in the case where both interference waves A and B were not actually received, the ratio on the vertical axis is 79.7% (=13.43 / 16.86), which is the value obtained by dividing the channel capacity for rank 2, k=4 by the channel capacity for rank 2, k=1. The reception probability of each interference wave A and B was changed independently from 0% to 100% in 5% increments. In addition, when the reception probability of interference wave A was p A and the reception probability of interference wave B is p B When the prediction error of the reception probability of the interference wave is ε, the reception probability of the interference waves A and B is p A -ε to p A + range up to p B -ε to p B The value was varied in the range of +ε.

[0066] 7, when the prediction error of the reception probability of the interference wave is not large, it can be seen that the value of α suitable for switching between the Max-Min method and the expected value method is around α=20%. Therefore, the determination unit 25 may perform the process of determining the rank and weight, for example, with α=20%. By determining the rank and weight in this way, it can be seen that, as shown in FIG. 7, when the prediction error of the reception probability of the interference wave is not large, it is possible to achieve MIMO transmission with a larger channel capacity than either the Max-Min method or the expected value method, that is, MIMO transmission that is closer to the situation using the optimal weight.

[0067] As described above, the communication device 2 according to the present embodiment switches the method for determining the rank and weight to be used after a predetermined time has elapsed depending on whether the reception probability of at least one of the acquired interference waves is within a predetermined range from 50%, thereby making it possible to determine more appropriate ranks and weights in advance. As a result, it becomes possible to extract a desired signal more appropriately depending on whether interference waves are present. Furthermore, because the weights determined in advance are used, even a communication device 2 with low processing power can extract a desired signal from a wireless signal received via MIMO transmission in real time.

[0068] In the present embodiment, the case where α defining the predetermined range from 50% is a fixed value has been mainly described, but this is not necessarily the case. For example, if the communication environment changes over time, it is considered that the suitable value of α also changes in accordance with the change in the communication environment. Therefore, the determination unit 25 may, for example, change α over time. In this case, the value of α may change over time in a predetermined period, such as one day or one week. This change in the value of α over time may be determined in advance, for example. When the value of α changes over a day, for example, a value of α for each time slot may be set in advance, and the determination unit 25 may determine the rank and weight using the value of α corresponding to the time slot included in the current time. When the value of α changes over a week, for example, a value of α for each day of the week may be set in advance, and the determination unit 25 may determine the rank and weight using the value of α corresponding to the day of the week included in the current time.

[0069] The value of α may also be adaptively changed. In this case, the determination unit 25 may adaptively change α according to, for example, the reception result by the receiver 21 of the wireless signal transmitted by MIMO transmission. In this case, for example, the value of α may be decreased when the wireless signal transmitted by MIMO transmission is successfully received, and the value of α may be increased when the reception is unsuccessful. For example, if a predetermined percentage (e.g., 80%) or more of the signal is received, it may be determined that the reception is successful. Also, for example, if a predetermined percentage (e.g., 50%) or more of the signal is unsuccessful, it may be determined that the reception is unsuccessful. In this way, the value of α can be adaptively changed according to changes in the communication environment, and it becomes possible to determine the rank and weight using a value of α that is suitable for the current communication environment. Note that when the value of α is adaptively changed, the range of change may be, for example, a range of positive real numbers less than 50, or a narrower range (e.g., a range from 10 to 40).

[0070] In addition, in this embodiment, the case where the channel capacity and the expected value are calculated for all ranks has been mainly described, but this is not necessarily the case. For example, if the maximum value of the rank is M and the number of interference waves is N, IF In this case, even if all interference waves arrive at the same time, the rank (MN IF ), it is considered that all of the interference waves can be suppressed. Therefore, the channel capacity and expected value are calculated based on the rank (MN IF ) may be performed only for the above.

[0071] Furthermore, in this embodiment, the case where SU-MIMO transmission is performed has been mainly described. However, for example, the rank adaptation and weight determination described in this embodiment may also be performed when multi-user MIMO (MU-MIMO) transmission is performed. Furthermore, in this embodiment, the case where radio signals are transmitted from communication device 1 to communication device 2 by MIMO transmission has been mainly described. However, radio signals may also be transmitted from communication device 2 to communication device 1 by MIMO transmission, and rank adaptation and weight determination according to this embodiment may be performed in that MIMO transmission. As an example, rank adaptation for uplink MIMO or advance weight determination at an access point may also be performed. In this case, for example, the rank adaptation and weight determination process described in this embodiment may also be performed in communication device 1.

[0072] Furthermore, in the above embodiments, each process or function may be realized by centralized processing by a single device or a single system, or may be realized by distributed processing by multiple devices or multiple systems.

[0073] Furthermore, in the above embodiment, when two or more components included in the communication device 2 have a communication device, an input device, etc., the two or more components may have a physically single device or may have separate devices.

[0074] Furthermore, in the above-described embodiments, each component may be configured by dedicated hardware, or a component that can be realized by software may be realized by executing a program. For example, each component may be realized by a program execution unit such as a CPU reading and executing a software program recorded on a recording medium such as a hard disk or semiconductor memory. During execution, the program execution unit may execute the program while accessing a storage unit or recording medium. The software that realizes the communication device 2 in the above-described embodiments is the following program. That is, this program is a program related to reception of a wireless signal transmitted by MIMO transmission from a transmitting communication device, and includes the steps of: using the wireless signal received via multiple antennas, acquiring a channel capacity for each rank depending on the presence or absence of one or more interference waves; using a reception history for each of the received one or more interference waves to acquire a reception probability after a predetermined time has elapsed for each of the one or more interference waves; calculating an expected value of the channel capacity for each rank using the acquired reception probability for each of the one or more interference waves and the acquired channel capacity; and calculating the expected value of the channel capacity for each rank when the acquired reception probability after a predetermined time has elapsed for at least any of the interference waves is within a predetermined range from 50%. the rank to be used by the transmitting communication device after a predetermined time has elapsed is the rank with the largest minimum value of channel capacity depending on the presence or absence of one or more interference waves, and the weight to be used after a predetermined time has elapsed is the weight corresponding to the event with the smallest channel capacity at the determined rank; and if this is not the case, the rank to be used by the transmitting communication device after a predetermined time has elapsed is the rank with the largest calculated expected value, and the weight to be used after the predetermined time has elapsed is the weight corresponding to the event with the highest probability at the determined rank; transmitting the determined rank to the transmitting communication device; and extracting a desired signal from wireless signals received via multiple antennas using the determined weight.

[0075] In the above program, the steps of transmitting information and receiving information do not include processing that can only be performed by hardware, such as processing performed by a modem or interface card in the transmission step.

[0076] This program may be executed by being downloaded from a server or the like, or by being read from a predetermined recording medium (for example, an optical disk such as a CD-ROM, a magnetic disk, or a semiconductor memory). This program may also be used as a program constituting a program product.

[0077] Furthermore, the computer that executes this program may be a single computer or multiple computers, and may perform centralized processing or distributed processing.

[0078] 8 is a diagram showing an example of a computer that executes the above program to realize the communication device 2 according to the above embodiment. The above embodiment can be realized by computer hardware and a computer program executed thereon. In FIG. 8, a computer system 900 includes, for example, a computer 901 including a disk drive 905, a keyboard 902, a mouse 903, a monitor 904, and multiple antennas 906.

[0079] In addition to a disk drive 905, the computer 901 includes an MPU (Micro Processing Unit) 911, a ROM 912 for storing programs such as a boot-up program, a RAM 913 connected to the MPU 911 for temporarily storing instructions for application programs and providing temporary storage space, a hard disk 914 for storing application programs, system programs, and data, a communication device 916, and a bus 915 for interconnecting the MPU 911, the ROM 912, etc. The computer 901 can transmit and receive wireless signals from multiple antennas 906 via the communication device 916.

[0080] A program that causes the computer system 900 to execute the functions of the communication device 2 according to the above embodiment may, for example, be stored on a disk 921 such as a CD-ROM or DVD-ROM, inserted into the disk drive 905, and transferred to the hard disk 914. Alternatively, the program may be transmitted to the computer 901 via a network (not shown) and stored on the hard disk 914. The program is loaded into RAM 913 when executed. The program may also be loaded directly from the disk 921 or the network. Alternatively, for example, a recording medium such as an SSD (Solid State Drive) may be used instead of the hard disk 914.

[0081] The program does not necessarily include an operating system (OS) or a third-party program that causes the computer 901 to execute the functions of the communication device 2 according to the above embodiment. The program may include only instructions that call appropriate functions or modules in a controlled manner to achieve a desired result. How the computer system 900 operates is well known, and a detailed description thereof will be omitted.

[0082] Furthermore, the above-described embodiments are merely examples for specifically implementing the present invention, and are not intended to limit the technical scope of the present invention. The technical scope of the present invention is defined by the claims, not by the description of the embodiments, and is intended to include modifications within the literal scope of the claims and within the scope of equivalent meanings. [Explanation of symbols]

[0083] 1, 2 Communication equipment 21 Receiving unit 22 Channel capacity acquisition unit 23 Probability Acquisition Section 24 Calculation section 25 Decision Section 26 Transmitter

Claims

1. A communication device that receives a radio signal transmitted by MIMO transmission from a transmitting communication device, a receiving unit that receives wireless signals via a plurality of antennas; a channel capacity acquisition unit that acquires a channel capacity for each rank according to the presence or absence of one or more interference waves using the wireless signal received by the receiving unit; a probability acquisition unit that acquires a reception probability after a predetermined time has elapsed for each of the one or more interference waves using a reception history for each of the one or more interference waves received by the receiving unit; a calculation unit that calculates an expected value of a channel capacity for each rank using the reception probability for each of the one or more interference waves acquired by the probability acquisition unit and the channel capacity acquired by the channel capacity acquisition unit; a determination unit that, when the reception probability of at least any interference wave after a predetermined time has elapsed, acquired by the probability acquisition unit, is within a predetermined range of 50%, determines the rank to be used by the transmitting communication device after the predetermined time has elapsed to be the rank with the largest minimum value of channel capacity depending on the presence or absence of one or more interference waves, and determines the weight to be used by the receiving unit after the predetermined time has elapsed to be a weight corresponding to the event with the smallest channel capacity in the determined rank, and otherwise determines the rank to be used by the transmitting communication device after the predetermined time has elapsed to be the rank with the largest expected value calculated by the calculation unit, and determines the weight to be used by the receiving unit after the predetermined time has elapsed to be a weight corresponding to the event with the highest probability in the determined rank; a transmitter that transmits the rank determined by the determiner to the transmitting communication device; The receiving unit extracts a desired signal from the radio signals received via the plurality of antennas using the weights determined by the determining unit.

2. 2. The communication device according to claim 1, wherein the predetermined range is a range of ±α% (where α is a positive real number less than 50).

3. The communication device according to claim 2 , wherein the determination unit adaptively changes the value of α in accordance with a reception result by the reception unit of a radio signal transmitted by MIMO transmission.

4. The communication device according to claim 2 , wherein the determination unit varies the value of α in a time series manner.

5. A method for receiving a radio signal transmitted by MIMO transmission from a transmitting communication device, comprising: obtaining a channel capacity for each rank according to the presence or absence of one or more interference waves using wireless signals received via a plurality of antennas; A step of obtaining a reception probability after a predetermined time has elapsed for each of the one or more interference waves using a reception history for each of the one or more interference waves received; calculating an expected value of channel capacity for each rank using the obtained reception probability for each of the one or more interference waves and the obtained channel capacity; a step of determining, when the reception probability of at least any of the acquired interference waves after a predetermined time has elapsed is within a predetermined range from 50%, the rank to be used by the transmitting communication device after the predetermined time has elapsed to be the rank with the largest minimum value of channel capacity depending on the presence or absence of one or more interference waves, and determining, as the weight to be used after the predetermined time has elapsed, a weight corresponding to the event with the smallest channel capacity in the determined rank; otherwise, determining, as the rank to be used by the transmitting communication device after the predetermined time has elapsed to be the rank with the largest calculated expected value, and determining, as the weight to be used after the predetermined time has elapsed, a weight corresponding to the event with the highest probability in the determined rank; transmitting the determined rank to the transmitting communication device; and extracting a desired signal from the radio signals received via the plurality of antennas using the determined weights.

6. A program for receiving a radio signal transmitted by MIMO transmission from a transmitting communication device, On the computer, obtaining a channel capacity for each rank according to the presence or absence of one or more interference waves using wireless signals received via a plurality of antennas; A step of obtaining a reception probability after a predetermined time has elapsed for each of the one or more interference waves using a reception history for each of the one or more interference waves received; calculating an expected value of channel capacity for each rank using the obtained reception probability for each of the one or more interference waves and the obtained channel capacity; a step of determining, when the reception probability of at least any of the acquired interference waves after a predetermined time has elapsed is within a predetermined range from 50%, the rank to be used by the transmitting communication device after the predetermined time has elapsed to be the rank with the largest minimum value of channel capacity depending on the presence or absence of one or more interference waves, and determining, as the weight to be used after the predetermined time has elapsed, a weight corresponding to the event with the smallest channel capacity in the determined rank; otherwise, determining, as the rank to be used by the transmitting communication device after the predetermined time has elapsed to be the rank with the largest calculated expected value, and determining, as the weight to be used after the predetermined time has elapsed, a weight corresponding to the event with the highest probability in the determined rank; transmitting the determined rank to the transmitting communication device; and extracting a desired signal from the wireless signals received via the plurality of antennas using the determined weights.

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