Patent Publication Number: US-2022224461-A1

Title: Method and apparatus for wireless communication using beam-forming in wireless communication system

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based on and claims priority under 35 U.S.C. § 119 to Korean Patent Application No. 10-2021-0003565, filed on Jan. 11, 2021, in the Korean Intellectual Property Office, the disclosure of which is incorporated by reference herein in its entirety. 
     TECHNICAL FIELD 
     The present disclosure relates generally to wireless communication and more particularly to wireless communication using beamforming techniques based on determining channel characteristics. 
     DISCUSSION OF RELATED ART 
     Beamforming may refer to a method of transmitting or receiving directional signals using a plurality of antennas. As an example, a base station may transmit a downlink signal to a terminal using a beamforming method. To determine the beam to be formed, the base station may assume that the radio channel between the uplink (terminal to base station) and the downlink (base station to terminal) is reciprocal, that is, a channel reciprocity condition exists. In this case, the base station may transmit the downlink signal using the beamforming method based on a downlink channel condition which is estimated from the uplink signal received from the terminal. However, due to various factors, the uplink and downlink channels may differ, whereby this technique may result in inaccurate downlink channel information, leading to suboptimal beamforming. 
     SUMMARY 
     The present disclosure provides a wireless communication method and a wireless communication device for transmitting a “sounding reference signal (SRS) switching signal” by utilizing techniques such as antenna selection or beam selection, to optimize downlink beam determination of a base station. 
     In one aspect, a method of operating a wireless communication device including a plurality of antennas according to an exemplary embodiment of the present disclosure includes determining an antenna subset including at least one of the plurality of antennas, transmitting an SRS switching signal to a base station through at least one antenna of the antenna subset, receiving a channel state information-reference signal (CSI-RS) transmitted from the base station through a first beam, selecting a precoding matrix indicator (PMI) based on the CSI-RS, transmitting the selected PMI to the base station, and receiving a signal transmitted from the base station through a second beam determined based on the SRS switching signal and the PMI. 
     In another aspect, a wireless communication device according to an exemplary embodiment of the present disclosure includes a plurality of antennas, a radio-frequency integrated circuit (RFIC) including a switching network connected to a plurality of antennas, a switching network configured to transmit a SRS switching signal to a base station through at least one antenna included in the antenna subset; and a processor configured to determine an antenna subset including at least one of the plurality of antennas, select a PMI to be provided to the base station based on a CSI-RS transmitted using a first beam from the base station, and process a signal transmitted from the base station using a second beam determined based on the SRS switching signal and the PMI. 
     In still another aspect, a method of operating a base station communicating with a wireless communication device including a plurality of antennas according to an exemplary embodiment of the present disclosure includes receiving a SRS switching signal transmitted using an antenna subset including at least one of the plurality of antennas, estimating uplink channel information based on the SRS switching signal, estimating downlink channel information based on the estimated uplink channel information, determining and forming a first beam based on the estimated downlink channel information. A CSI-RS is transmitted through the first beam, and a PMI is received from the wireless communication device. A second beam is determined and formed based on the received SRS switching signal and the received PMI, and a signal including data is transmitted through the second beam. 
     In yet another aspect, a method of operating a wireless communication device including a plurality of antennas involves determining an antenna subset including at least two but less than all of the plurality of antennas. An SRS switching signal including a sequence of SRSs is transmitted to a base station, where each of the SRSs is respectively transmitted through a different one of the at least two antennas of the antenna subset. A reference signal, transmitted through a first beam from the base station, is received, and a PMI is selected based on the reference signal. The selected PMI is transmitted to the base station. A signal is thereafter received from the base station, which was transmitted through a second beam determined based on the SRS switching signal and the PMI. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the inventive concept will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG. 1A  is a block diagram schematically illustrating a wireless communication system according to an exemplary embodiment of the present disclosure, and  FIG. 1B  is a diagram for explaining a radio channel between a wireless communication device and a base station of  FIG. 1A ; 
         FIG. 2  is a flowchart illustrating a method of operating a wireless communication device and a base station in a wireless communication system according to an exemplary embodiment of the present disclosure; 
         FIGS. 3A and 3B  are flowcharts illustrating respective examples of a method of transmitting a sounding reference signal (SRS) switching signal according to exemplary embodiments of the present disclosure; 
         FIG. 4  is a flowchart illustrating an example of a method of transmitting an SRS switching signal according to a sequential antenna selection method according to an exemplary embodiment of the present disclosure; 
         FIGS. 5A and 5B  are flowcharts illustrating examples of a method of transmitting an SRS switching signal according to an opportunistic antenna selection method according to exemplary embodiments of the present disclosure; 
         FIG. 6  is a flowchart illustrating an example of a method of transmitting an SRS switching signal according to an antenna spatial correlation-based selection method according to an exemplary embodiment of the present disclosure; 
         FIGS. 7A, 7B and 7C  are flowcharts illustrating examples of a method of transmitting an SRS switching signal according to an antenna selection method based on reinforcement learning according to exemplary embodiments of the present disclosure; 
         FIG. 8  is a flowchart illustrating an example of a method of tracking a final antenna subset according to an exemplary embodiment of the present disclosure; 
         FIGS. 9A and 9B  are flowcharts illustrating examples of a method of selecting a pre-coding matrix indicator (PMI) according to exemplary embodiments of the present disclosure; and 
         FIGS. 10A, 10B and 10C  are block diagrams illustrating structures of a wireless communication device according to exemplary embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the inventive concept will be described in detail with reference to the accompanying drawings. 
       FIG. 1A  is a block diagram schematically illustrating a wireless communication system  10  according to an exemplary embodiment of the present disclosure, and  FIG. 1B  is a diagram for explaining a radio channel between a wireless communication device  100  and a base station  110  of  FIG. 1A . 
     The wireless communication system  10  may refer to any system including a wireless communication device  100  and a base station  110 . For example, the wireless communication system  10  may be any one of a New Radio (NR) system, a 5th generation wireless (5G) system, a Long Term Evolution (LTE) system, an LTE-Advanced system, a Code Division Multiple (CDMA) system, a Global System for Mobile Communication (GSM) system, or a Wireless Local Area Network (WLAN) system. In the case of a CDMA system, this may be implemented in various CDMA versions such as wideband CDMA (WCDMA), time division synchronization CDMA (TD-SCDMA), cdma2000, and the like. Hereinafter, the wireless communication system  10  will be described with reference mainly to a 5G system and/or an LTE system, but it will be understood that exemplary embodiments of the present disclosure are not limited thereto. 
     The wireless communication network of the wireless communication system  10  may support multiple users to communicate by sharing available network resources. For example, in a wireless communication network, information may be provided with various multiple access methods such as Code Division Multiple Access (CDMA), Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), Orthogonal Frequency Division Multiple Access (OFDMA), Single Carrier Frequency Division Multiple Access (SC-FDMA), OFDM-FDMA, OFDM-TDMA, OFDM-CDMA, and the like. 
     A base station (BS)  110  may be part of the wireless communication system  10 . The BS  110  may typically be a fixed station communicating with multiple user equipments (UEs), but in other examples, user equipment can be configured to function as a base station. The BS  110  may communicate with another BS  112 , and may exchange data and control information by communicating with a UE and/or other “cells” (e.g., other BSs that each typically service a certain geographical area). For example, the BS may be referred to as a cell, a Node B, an evolved-Node B (eNB), a next generation Node B (gNB), a sector, a site, a base transceiver system (BTS), an Access Point (AP), a relay node, a remote radio head (RRH), a radio unit (RU), a small cell, and the like. In this specification, the term “BS” may comprehensively signify some areas or functions covered by a Base Station Controller (BSC) in CDMA, a Node-B in WCDMA, an eNB in LTE, a gNB or sector (site) in NR, and the like, and may cover all of various coverage areas such as a megacell, a macrocell, a microcell, a picocell, a femtocell and relay node, an RRH, an RU, and a small cell communication range. 
     The wireless communication device  100  (hereinafter, interchangeably just “device  100 ” for brevity) may be a UE in the wireless communication system  10 . A “UE” may be stationary or mobile, and may refer to various devices capable of transmitting and receiving data and/or control information by communicating with the BS. For example, a UE may be referred to as a terminal equipment, a mobile station (MS), a mobile terminal (MT), a user terminal (UT), a subscriber station (SS), a wireless device, a handheld device, and the like. 
     As shown in  FIG. 1A , the wireless communication system  10  may include a plurality of BSs, e.g.,  110  and  112 , and a system controller  120 . In other examples, the wireless communication system  10  may include one or more further cells, and a plurality of network entities. The BSs  110  and  112  may communicate with device  100  or another cell to transmit and receive data signals or control information. The wireless communication device  100  may communicate with the wireless communication system  10  and may also receive signals from a broadcast station  114 . Furthermore, the wireless communication device  100  may receive a signal from a satellite  130  of a Global Navigation Satellite System (GNSS). Device  100  may support radio technologies for various wireless communication. 
     Technical aspects of the present disclosure may be applied between communication entities forming an uplink channel and a downlink channel in the wireless communication system  10 . Hereinafter, device  100  and the BS  110  will be described as communication entities to which technical aspects of the present disclosure are applied. 
     A downlink channel  102  and an uplink channel  104  may be formed as a data connection path between device  100  and the BS  110 . It may be assumed that the state of the downlink channel  102  and the state of the uplink channel  104  are either the same (a reciprocity condition) or similar. When the downlink and uplink channels are similar, this may be referred to as a “calibratable reciprocity condition” in which a calibration may be performed to effectively achieve reciprocity between the uplink and downlink channels. Hereafter, whether or not calibration is performed, it may be assumed in the following description that reciprocity exists. (non-reciprocity condition). Reciprocity of the downlink channel  102  and the uplink channel  104  may exist in a time division duplex (TDD)-based wireless communication system in which the uplink and the downlink share the same frequency spectrum but uplink and downlink transmissions are separated in the time domain. Reciprocity may also be predicted or achieved via calibration in a frequency division duplex (FDD)-based wireless communication system in which the uplink and downlink use a different frequency spectrum. 
     The BS  110  may receive a Sounding Reference Signal (SRS) transmitted through at least one of a plurality of antennas included in device  100 . In embodiments of the present disclosure, the BS  110  typically receives a sequence of SRSs from at least two of device  100 &#39;s antennas (a subset of the antennas), where each antenna transmits one SRS of the sequence. This sequence of SRSs may be referred to as an “SRS switching signal”. For example, when device  100  includes a plurality of antennas, at least two of the antennas may be sequentially selected in a predetermined order and each may transmit an SRS, which may be received by the BS  110  and in some cases by cells. The BS  110  may estimate the uplink channel  104  for each antenna of device  100  and estimate downlink channel information using the estimated uplink channel assuming channel reciprocity. 
     However, even if a transmitter or a receiver is calibrated to satisfy channel reciprocity, channels through which uplink and downlink signals are transmitted and received may be different due to an implementation problem of device  100 . For example, if the number of transmit/receive antennas of the terminal is different from the number of transmit/receive RF chains, or if there is a restriction on the SRS resource allocated from the BS  110 , in conventional systems, the BS  110  is unable to obtain complete downlink channel information from the signal received from device  100 . 
     In addition, when the BS  110  limits SRS resources per device to support multi-user Multiple Input Multiple Output (MU-MIMO), device  100  is assigned a limited SRS resource, e.g., using less frequencies and/or time slots for channel measurements as compared to an unrestricted SRS resource situation. When SRS is transmitted in a conventional manner using limited resources, beamforming using downlink channel information obtained by the BS  110  may be suboptimal. 
     The wireless communication system  10  according to an exemplary embodiment of the present disclosure efficiently transmits the SRS using an antenna or beam selection method of device  100  and efficiently acquires a downlink channel, resulting in beamforming-based communication with improved performance. 
     Referring further to  FIG. 1B , device  100  may include m antennas  1  to m, and the BS  110  may include n antennas  1  to n. Device  100  and the BS  110  may perform mutual beamforming-based communication, Multi-Input and Multi-Output (MIMO)-based communication, and the like using respective antennas. Because the theoretical channel transmission capacity is increased through the configuration of  FIG. 1B , a transfer rate may be improved and frequency efficiency may be significantly improved. 
     The uplink channel h j  (1≤j≤m, j is an integer) corresponding to the j-th antenna of the wireless communication device  100  may include channels h 1,j , h 2,j , . . . , h n,j  corresponding to the respective n antennas of the BS  110 . The BS  110  may receive the SRS transmitted from the j-th antenna of device  100  and estimate the uplink channel h j  using the received SRS. The BS  110  may estimate a downlink channel from the uplink channel h j  assuming channel reciprocity, generate a downlink signal using the estimated downlink channel, and transmit a downlink signal to device  100  through at least one of the n antennas. 
     Descriptions of the uplink channel h j  corresponding to the j-th antenna of device  100  may be applied to uplink channels corresponding to other antennas of device  100 , and based on the above, technical concepts of the present disclosure will be described below. 
     It is noted, throughout the specification, the terms “antenna selection” and “beam selection” may be used interchangeably. Some technical concepts of the present disclosure will be described below mainly with the use of “antenna selection”. 
       FIG. 2  is a flowchart illustrating a method of operating device  100  and the BS  110  in a wireless communication system according to an exemplary embodiment of the present disclosure. In the method, the wireless communication system  10  may include device  100  and the BS  110 , and in operation S 210 , device  100  may transmit an “SRS switching signal” to the BS  110  using the SRS switching resource set by the BS  110 . (Device  100  may have learned of the SRS switching resource during a prior signal exchange with the BS  110 .) As noted earlier, an SRS switching signal includes a plurality of SRSs transmitted in a sequence, where each SRS is transmitted from a respective one of the device  100 &#39;s antennas. The “SRS switching resource” may be defined as set of uplink resource elements (REs) (e.g., set of frequencies and/or time slots for SRS transmissions) used when device  100  transmits an SRS signal for the purpose of obtaining channel information for downlink beamforming by the BS  110 . The BS  110  may allocate SRS switching resources to device  100 . 
     According to one embodiment, the available resources of the BS  110  are limited for the transmission of the SRS switching signal. In this scenario, there are less SRS switching resources allocated by the BS  110  for the SRS switching signal than the number of reception antennas of device  100 . In this case, only some antennas are used for transmission (“the first scenario”). For example, supposing there are 5 wireless communication devices in a cell covered by the BS  110  and that 4 antennas are included in each wireless communication device  100 , if there are 16 resources that may be allocated by the BS  110 , some of the 5 wireless communication devices cannot be allocated 4 SRS switching resources (assuming switching resources that are to be utilized simultaneously). 
     In another situation, due to a limitation in hardware implementation of the wireless communication device  100  when transmitting the SRS switching signal, there may be a scenario in which only some antennas are used for transmission (“the second scenario”). This corresponds to a case where the number of Tx radio frequency (RF) chains of device  100  is less than the number of reception antennas of  100 . For example, due to a limitation in hardware implementation of device  100 , the number of reception antennas is four but the number of transmission antennas may be limited to two. 
     In the following discussion, to facilitate understanding of concepts taught herein, an example is presented in which device  100  has four reception antennas. In other examples, the concepts are applied to a device  100  having more or fewer antennas. 
     Operations of antenna selection and antenna switching will be described below with further reference to  FIGS. 3A and 3B . 
     In operation S 220 , the BS  110  may estimate uplink channel information based on the received SRS switching signal. In operation S 230 , the BS  110  may estimate downlink channel information by performing an operation such as calibration from the estimated uplink channel information. Here, calibration refers to a series of processes in which uplink and downlink channels are corrected to satisfy reciprocity in an entire signal path, e.g., baseband on transmit to baseband on receive through an RF filter. The BS  110  may determine beamforming using the downlink channel information estimated in operation S 240 , and transmit a channel state information-reference signal (CSI-RS) using beamforming in operation  250 . Depending on which antenna(s) is selected, the CSI-RS transmitted by the BS  110  to device  100  may include different information. 
     Device  100  may select a pre-coding matrix indicator (PMI) based on the CSI-RS received in operation S 260 , and transmit the PMI selected in operation S 270  to the BS  110 . When selecting a PMI, device  100  may consider information on a selected antenna(s), and accordingly, may select an optimal PMI. 
     In operation S 280 , the BS  110  may determine the final downlink beamforming using information obtained from the SRS switching signal received in operation S 210  and the information obtained from the PMI received in operation S 270 , and transmit a downlink signal including data or the like through an antenna beam determined by the beamforming in operation S 290 . 
     The beam determined in operation S 280  may be referred to as a “final beam” hereinafter, and may be defined in the form of a matrix as follows. 
     
       
         
           
             
               
                 
                   x 
                   = 
                   
                     
                       F 
                       SRS 
                     
                     ⁢ 
                     
                       F 
                       PMI 
                     
                     ⁢ 
                     s 
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                   ] 
                 
               
             
           
         
       
     
     F SRS  (the number of antennas of the BS  110 ×the number of CSI-RS antenna ports) may be defined as a matrix for a beam based on the SRS switching signal received from device  100 , and F PMI  (the number of CSI-RS antenna ports×the number of data layers or data streams) may be defined as a matrix for a beam based on a PMI received from device  100 . s (the number of data layers or data streams×1) may be defined as a matrix for a downlink signal including data that the BS  110  is to transmit to device  100 . 
     The BS  110  may form a finally synthesized beam using all of the information obtained from the SRS switching signal and the PMI, and may perform beamforming with device  100 . F SRS  and F PMI  may be expressed as F SRS_PMI , which is a matrix for the finally synthesized beam, that is, in the form of one beam. x (the number of BS antennas×1) corresponds to the final signal transmitted from the BS  110  to device  100 . 
     According to the above operation method, device  100  adaptively transmits the SRS switching signal and adaptively transmits the PMI to optimize the acquisition of downlink channel information and the beamforming decision of the BS  110 . 
     Meanwhile, the CSI-RS corresponding to the downlink reference signal and the SRS corresponding to the uplink reference signal described with reference to  FIG. 2  are only examples of reference signals that may be applied in the method. Alternative reference signals may include a pilot signal transmitted by a BS in a downlink for channel estimation and a pilot signal transmitted in an uplink by a wireless communication device for channel estimation. 
       FIGS. 3A and 3B  are flowcharts illustrating examples of a method of transmitting an SRS switching signal according to exemplary embodiments of the present disclosure. 
       FIG. 3A  illustrates a process of determining an antenna subset to select an antenna for transmitting an SRS switching signal in operation S 210  of  FIG. 2 . 
     In operation S 310   a , a BS  110   a  may transmit a downlink reference signal to a wireless communication device  100   a  (hereafter, “device  100   a ”). Device  100   a  may estimate the downlink channel information from the received CSI-RS in operation S 320   a , and determine the antenna subset based on the estimated downlink channel information in operation S 330   a . The antenna subset may be determined based on at least one of a signal to interference plus noise ratio (SINR) of the wireless communication device, a transmission/reception characteristic of an antenna, and a linearity of a transmission power amplifier. In operation S 340   a , device  100   a  may transmit the SRS switching signal through at least one of antennas included in the determined antenna subset. 
     In an exemplary embodiment, a method of determining an antenna subset may include a method of sequentially determining the antenna subset and then switching the antenna subset according to the transmission period of the SRS switching signal, a method of aperiodic switching of the antenna subset after determining an antenna subset based on a gain value of/signal quality metric for a specific beam, a method of switching an antenna subset after determining an antenna subset according to spatial correlation between reception antennas of a wireless communication device, and a method of switching antenna subsets according to an update period after determining an antenna subset based on reinforcement learning. Detailed descriptions of this are provided below with reference to  FIG. 4 . 
     Referring to  FIG. 3B , a process of selecting a beam to transmit an SRS switching signal in operation S 210  of  FIG. 2  is illustrated. 
     In operation S 310   b , a BS  110   b  may transmit a downlink reference signal to a wireless communication device  100   b  (“device  100   b ”). Device  100   b  may estimate the downlink channel information from the received CSI-RS in operation S 320   b , and determine a beam for transmitting the SRS switching signal from each antenna based on the estimated downlink channel information in operation S 330   b . For instance,  100   b  selects a beam with good reception performance and spatial characteristics from a previously designed beam code-book using the estimated downlink channel information, or may be newly designed when there is no pre-designed beam codebook. In operation S 340   b , the SRS switching signal may be transmitted using the determined beam. 
     Table 1 below shows a beam codebook according to an embodiment of the present disclosure. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Beam#1 
                 Beam#2 
                 Beam#3 
                 Beam#4 
               
               
                   
               
             
            
               
                 Antenna#1 
                 A11 + B11i 
                 A12 + B12i 
                 A13 + B13i 
                 A14 + B14i  
               
               
                 Antenna#2 
                 A21 + B21i 
                 A22 + B22i 
                 A23 + B23i 
                 A24 + B24i  
               
               
                 Antenna#3 
                 A31 + B31i 
                 A32 + B32i 
                 A33 + B33i 
                 A34 + B34i  
               
               
                 Antenna#4 
                 A41 + B41i 
                 A42 + B42i 
                 B43 + B43i 
                 A44 + B44ii 
               
               
                   
               
            
           
         
       
     
     Referring to Table 1, in this example it is assumed there are four antennas of device  100   b  and four beams that may be formed from each antenna. For example, the beam codebook includes an index of a precoding matrix shared in advance in device  100   b  and the BS  110   b . Elements of each antenna for beam configuration may be expressed as arbitrary complex values. For example, the real component for the second beam of Antenna #4 is A42 and the imaginary component is B42. 
     Table 2 below shows a beam codebook according to an embodiment of the present disclosure. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                 Beam#1 
                 Beam#2 
                 Beam#3 
                 Beam#4 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Antenna#1 
                 1 
                 1 
                 1 
                 1 
               
               
                 Antenna#2 
                 1 
                 −1i 
                 −1 
                   1i 
               
               
                 Antenna#3 
                 1 
                 −1 
                 1 
                 −1 
               
               
                 Antenna#4 
                 1 
                   1i 
                 −1 
                 −1i 
               
               
                   
               
            
           
         
       
     
     Referring to Table 2, among the elements of the third antenna for configuring the beam, an element for the third beam may be composed of 1. 
       FIG. 4  is a flowchart illustrating an example of a method of transmitting an SRS switching signal according to a sequential antenna selection method according to an exemplary embodiment of the present disclosure. 
     The example of  FIG. 4  shows a method of transmitting an SRS switching signal according to a method of sequentially determining an antenna subset and then switching the antenna subset according to an SRS transmission period. This may be referred to as a “sequential antenna selection method”. 
     In  FIG. 4 , it is assumed that the number of reception antennas of device  100  is 4, and an SRS switching signal is transmitted to the BS  110  by using two transmission antennas that are less than the number of reception antennas. That is, it is assumed that the size of the antenna subset determined by device  100  is 2. For example, this correspond to a case where only two antennas are used when the BS  110  allocates two SRS switching resources to device  100  (first scenario), or due to a hardware implementation limitation of device  100 . 
     In an embodiment, when the indices of the first antenna, the second antenna, the third antenna, and the fourth antenna are {0, 1, 2, 3}, respectively, a subset consisting of a combination of two antennas for transmitting an SRS switching signal among the first to fourth antennas may be configured as follows. 
     Antenna subset: {0, 1} {0, 2} {0, 3} {1, 2} {1, 3} {2, 3} 
     In an exemplary embodiment, device  100  may sequentially determine each of the six possible antenna subsets in a predefined order. 
     Here, the fact that the antenna subset is sequentially determined, for example, means that the six possible antenna subsets are sequentially selected for each transmission period of the SRS switching signal among {0, 1} {0, 2} {0, 3} {1, 2} {1, 3}, and {2, 3}. 
     For example, in operation S 402 ,  100  may determine an antenna subset to transmit the SRS switching signal as {0, 1} in the first SRS switching transmission period. That is, device  100  may determine to transmit the SRS switching signal by using the first antenna and the second antenna among the four antennas. In some embodiments, device  100  may determine each of the antenna subsets in a different order than that described above. 
     In operation S 404 , after transmitting SRS_0, which is an SRS switching signal using the first antenna, and SRS_1, which is an SRS switching signal using a second antenna, the BS  110  may design a downlink beam F_SRS using SRS_0 and SRS_1 in operation S 406 . The BS  110  may transmit the CSI-RS through the F_SRS beam in operation S 408 . 
     Device  100  may select a PMI based on the CSI-RS received in operation S 410 , and feed back the PMI to the BS  110  in operation S 412 . The BS  110  may design the downlink beam F_PMI using the PMI received in operation S 414 , and transmit a downlink signal including data to device  100  by using the final beam determined by F_SRS and F_PMI in operation S 416 . 
     As an exemplary embodiment, operations S 402  to S 416  may be performed by device  100  for a time corresponding to the first SRS switching signal transmission period, and may be referred to as Cycle 1. In the second SRS switching signal transmission period corresponding to Cycle 2, the antenna subset {0, 2} is determined to transmit the SRS switching signal, and for example, in Cycle 4 corresponding to the fourth SRS switching signal transmission period, the antenna subset {1, 2} is determined to transmit SRS_1 and SRS_2 to the BS  110 . 
     In operation S 418 , device  100  may determine an antenna subset to transmit the SRS switching signal as {1, 2} in the fourth SRS switching transmission period after several SRS switching transmission periods are repeated. That is, device  100  may determine to transmit the SRS switching signal by using the second antenna and the third antenna among the four antennas. In some embodiments, device  100  may determine each of the antenna subsets in a different order than that described above. 
     In operation S 420 , after transmitting SRS_0, which is an SRS switching signal using the first antenna, and SRS_1, which is an SRS switching signal using a second antenna, the BS  110  may design a downlink beam F_SRS using SRS_0 and SRS_1 in operation S 422 . The BS  110  may transmit the CSI-RS through the F_SRS beam in operation S 424 . 
     Device  100  may select a PMI based on the CSI-RS received in operation S 426 , and feed back the PMI to the BS  110  in operation S 428 . The BS  110  may design the downlink beam F_PMI using the PMI received in operation S 430 , and transmit a downlink signal including data to the wireless communication device  100  by using the final beam determined by F_SRS and F_PMI in operation S 432 . 
     According to the method shown in  FIG. 4 , device  100  may monitor the “gain” of the final beam at every SRS switching signal transmission period and sequentially consider all possible antenna subsets to find an optimal antenna combination. Here, “beam gain” or just “gain” or “gain value” may be defined as a relative term (since the distance between the BS  110  and device  100  is typically unknown) for signal quality of the signal transmitted by an antenna beam transmitted from a base station and received by a wireless communication device (e.g.,  100 ). Hereafter, the terms “gain” and “signal quality” may be used interchangeably. Gain may be determined by the power of a signal received by device  100 , a signal to noise ratio (SNR) of the signal received by device  100 , a signal to noise and interference ratio (SINR) or the signal received by device  100 , and/or frequency efficiency. 
     According to embodiments of the present disclosure, an antenna subset finally selected as an optimal antenna combination may be referred to as a “final antenna subset”. 
     Note that in the above example, an antenna subset size of 2 was used to aid in understanding concepts taught herein. In other examples, more or fewer antennas may form an antenna subset. 
       FIGS. 5A and 5B  are flowcharts illustrating examples of a method of transmitting an SRS switching signal according to an opportunistic antenna selection method according to exemplary embodiments of the present disclosure. 
       FIG. 5A  illustrates a method of transmitting an SRS switching signal according to a method of aperiodic switching of the antenna subset after determining an antenna subset based on a gain value of a specific beam. This may be referred to as one of the “opportunistic antenna selection methods”. 
     In the opportunistic antenna selection method, a specific value corresponding to the gain value of the beam is preset as a threshold value B th , and when it is determined that the gain value of the final beam determined as an arbitrary antenna subset is less than the threshold value, regardless of the order, it may immediately decide to a different antenna subset. When it is determined that the gain value of the final beam determined as an arbitrary antenna subset is greater than the threshold value, the process for determining another antenna subset and transmitting the SRS switching signal may be stopped, and a corresponding antenna subset may be declared as an optimal antenna combination. That is, the corresponding antenna subset may be determined as the “final antenna subset”. 
     Meanwhile, after the final antenna subset is determined, device  100  may track an optimal antenna combination with an arbitrary tracking period. There may be a method of changing one antenna included in the previously determined final antenna subset, and there may be a method of changing one or more antennas. When the gain of the final beam is rapidly lowered, an antenna included in the previously determined final antenna subset may not be selected, or the final antenna subset itself may be replaced with another antenna subset. Tracking and a tracking period may be determined from an indicator related to a change in a radio channel. 
     Specifically, referring to  FIG. 5A , device  100  may determine an arbitrary antenna subset for transmitting the SRS switching signal in operation S 510   a , and after operation S 510   a , operations S 210  to S 290  described with reference to  FIG. 2  may be performed, and detailed information thereof will be omitted. 
     In operation S 520   a , device  100  may monitor the gain value of the final beam received in operation S 290 , as an exemplary embodiment. In operation S 530   a , device  100  may determine whether the gain value of the monitored final beam exceeds the threshold value B th . 
     In operation S 540   a , when the gain value of the final beam monitored by device  100  exceeds the threshold value B th , the antenna subset determined in operation S 510   a  may be determined as the final antenna subset. 
     On the other hand, when the gain value of the final beam monitored by device  100  is less than or equal to the threshold value B th , the process returns to operation S 510   a  of determining an antenna subset. For example, device  100  may return to operation S 510   a , select an antenna subset {2, 3}, and transmit the SRS switching signal using the third antenna and the fourth antenna. 
       FIG. 5B  illustrates a method of transmitting an SRS switching signal according to a method of aperiodic switching of the antenna subset after determining an antenna subset based on the values of channel gains corresponding to all antennas for a channel receiving CSI-RS. This may be referred to as one of the “opportunistic antenna selection methods”. 
     In an embodiment, when channel gains corresponding to all antennas for a channel receiving a CSI-RS have similar values, device  100  may determine the antenna subset determined in an operation akin to S 510   b  as the final antenna subset. The criterion for whether channel gains have similar values may be set differently according to embodiments. 
     On the other hand, when the channel gains corresponding to all antennas for the channel receiving the CSI-RS have a relatively large difference, device  100  may return to an operation akin to S 510   b  of determining the antenna subset and continuously attempt to transmit the SRS switching signal using the new antenna subset. 
     Device  100  may determine an arbitrary antenna subset for transmitting the SRS switching signal in operation S 510   b , and after operation S 510   b , operations S 210  to S 290  described with reference to  FIG. 2  may be performed (detailed descriptions thereof are omitted here). 
     In operation S 520   b , device  100  may monitor a gain value of a channel that receives a CSI-RS in operation S 290 , as an exemplary embodiment. In operation S 530   b , device  100  may determine whether the absolute value of the difference between the monitored channel gain values exceeds the threshold value C th . 
     When the absolute value of the difference between the gain values of the channel monitored in operation S 520   b  exceeds the threshold value C th , in operation S 540   b , device  100  may determine the final antenna subset as the antenna subset determined in operation S 510   b.    
     On the other hand, when the gain value of the final beam monitored by device  100  is less than or equal to the threshold value B th , the process returns to operation S 510   b  of determining an antenna subset. For example, device  100  may return to operation S 510   b , select an antenna subset {2, 3}, and thereafter, periodically or continuously transmit the SRS switching signal using the third antenna and the fourth antenna. 
     Meanwhile, after the final antenna subset is determined, device  100  may track an optimal antenna combination with an arbitrary tracking period. This may employ a method of changing one antenna included in the previously determined final antenna subset, or a method of changing one or more antennas. During tracking, when the gain of the final beam is rapidly lowered, an antenna included in the previously determined final antenna subset may not be selected, or the final antenna subset itself may be replaced with another antenna subset. Tracking and a tracking period may be determined from an indicator related to a change in a radio channel. 
       FIG. 6  is a flowchart illustrating an example of a method of transmitting an SRS switching signal according to an antenna spatial correlation-based selection method according to an exemplary embodiment of the present disclosure. 
     Specifically,  FIG. 6  illustrates a method of transmitting an SRS switching signal according to a method of switching an antenna subset (“Method  3 ”) after determining an antenna subset according to spatial correlation between reception antennas of a wireless communication device. Method  3  may be referred to as “an antenna spatial correlation-based selection method”. 
     Spatial correlation between antennas may be defined as an index indicating the degree of interference between antennas determined by factors such as a distance between antennas. For example, when the spatial correlation between antennas is low, different signals may be independently transmitted through the antennas. Accordingly, low spatial correlation between antennas is desirable for an antenna subset of an SRS switching signal according to embodiments of the present disclosure. A method of considering spatial correlation between antennas may have an objective of transmitting a signal through an antenna that guarantees a relatively independent channel by selecting a combination of antennas according to channel information. 
     Spatial correlation between antennas may be determined by a frequency band, a cell type covered by the BS, a separation distance between antennas, polarization, and the like. 
     According to an embodiment, antenna subsets may be arranged in an order of lowest to highest spatial correlation between reception antennas of device  100  and then sequentially determined as an antenna subset to transmit the SRS switching signal. 
     Alternatively, antenna subsets may be arranged in an order of highest to lowest spatial correlation between reception antennas of device  100  and then sequentially determined as an antenna subset to transmit the SRS switching signal. 
     In another embodiment, the antenna subset may be switched considering the arrangement of a plurality of antennas of device  100  without measuring spatial correlation. For example, antenna subsets may be arranged from the antenna combinations having the largest antenna separation distance values in the following order. 
     Antenna subset: {0, 3} {0, 2} {1, 3} {0, 1} {1, 2} {2, 3} 
     Referring to  FIG. 6 , in operation S 610 , device  100  may arrange antenna subsets in an order of low correlation between antennas. In operation S 620 , one of the antenna subsets listed in operation S 610  is sequentially selected and determined as the i-th antenna subset. After operation S 620 , operations S 210  to S 290  described with reference to  FIG. 2  may be performed (redundant descriptions thereof are omitted). 
     In operation S 630 , device  100  monitors the gain value of the final beam. For example, assuming that the number of antennas included in device  100  is 4 and the antenna subset is a combination of two antennas, there are a total of six possible antenna subsets, and operations S 620  to S 630  are repeated a total of six times. 
     When monitoring of the final beam gain value for all antenna subsets is complete, in operation S 640 , the antenna subset having the largest gain value may be determined as the final antenna subset. 
     Meanwhile, after the final antenna subset is determined, device  100  may track an optimal antenna combination with an arbitrary tracking period. This may employ a method of changing one antenna included in the previously determined final antenna subset, or a method of changing one or more antennas. During tracking, when the gain of the final beam is rapidly lowered, an antenna included in the previously determined final antenna subset may not be selected, or the final antenna subset itself may be replaced with another antenna subset. Tracking and a tracking period may be determined from an indicator related to a change in a radio channel. 
     Also, according to one embodiment, if there is one SRS switching resource allocated by the BS  110  for the SRS switching signal according to the “first scenario” (mentioned earlier), after determining the antenna subset based on the reception performance of the antenna, device  100  may switch the antenna subset to transmit the SRS switching signal. For example, antenna subsets may be sequentially determined after antennas having high antenna reception power are arranged in ascending order or antennas having small antenna reception power are arranged in ascending order. 
       FIGS. 7A to 7C  are flowcharts illustrating examples of a method of transmitting an SRS switching signal according to an antenna selection method based on reinforcement learning according to exemplary embodiments of the present disclosure. 
     Referring to  FIGS. 7A to 7C , after determining the antenna subset based on reinforcement learning according to the method of switching the antenna subset according to the update period, the SRS switching signal may be transmitted, which may be referred to as a “reinforcement learning-based antenna selection method”. 
     Reinforcement learning is a type of machine learning, and may be defined as a method in which an agent defined in a certain environment recognizes the current state and selects an action or action sequence that maximizes Reward among the selectable actions. The reinforcement learning may be performed by the reinforcement learning device  810   c  of device  100 , and the operation of the reinforcement learning device  810   c  will be described later with reference to  FIG. 8C . 
     The antenna selection method based on reinforcement learning includes a Q-learning-based antenna selection method and a Bandit learning-based antenna selection method according to an embodiment of the present disclosure. 
       FIG. 7A  is a flowchart illustrating a method of transmitting an SRS switching signal according to a Q-learning-based antenna selection method during reinforcement learning. 
     The Q function expressed as “(state, action)”, which is a pair of State and Action, may be defined as a function that may predict the expected value of the utility that executing a given action in a given state will provide. According to an embodiment, device  100  may select an antenna combination having a large Q value, that is, Q(S, A). 
     In an embodiment, the Action may be defined as an action of selecting an antenna combination for determining an antenna subset, and State, which means a state when a specific antenna is selected, may be defined as a parameter related to a downlink channel. The parameters related to the downlink channel may be determined by correlation in the time, space, and frequency domain, and the strength of the downlink signal. Reward may be defined as a reception performance index for a CSI-RS received from device  100  or a downlink signal including data received using a final beam. The reception performance for the downlink signal may be determined by a block error rate (BLER), frequency efficiency, and strength of the downlink signal. 
     Table 2 shows a Q-table according to an embodiment of the present disclosure. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                 A_1 
                 A_2 
                 . . . 
                 A_N 
               
               
                   
                   
               
             
            
               
                   
                 S_1 
                 Q(S_1, A_1) 
                 Q(S_1, A_2) 
                 . . . 
                 Q(S_1, A_N) 
               
               
                   
                 . . . 
                 . . . 
                 . . . 
                 . . . 
                 . . . 
               
               
                   
                 S_M 
                 Q(S_M, A_1) 
                 Q(S_M, A_2) 
                 . . . 
                 Q(S_M, A_N) 
               
               
                   
                   
               
            
           
         
       
     
     Referring to Table 2, for example, A_1 may mean a case where the antenna subset determined by device  100  is {0,1}. S_1 may mean a state corresponding to one of downlink-related parameter values that exist as many as M of possible cases due to various factors such as correlation in the time, space, frequency domain, and strength of a downlink signal. 
     Q(S_M, A_N) takes an action of selecting an antenna subset that includes the Nth antenna combination, and may mean a Q value in the case of a state corresponding to a parameter numbered by M based on the antenna combination. 
     In an exemplary embodiment, the update operation of Q may be expressed as follows. 
     
       
         
           
             
               
                 
                   
                     
                       Q 
                       new 
                     
                     ⁡ 
                     
                       ( 
                       
                         S 
                         , 
                         A 
                       
                       ) 
                     
                   
                   ← 
                   
                     
                       
                         Q 
                         old 
                       
                       ⁡ 
                       
                         ( 
                         
                           S 
                           , 
                           A 
                         
                         ) 
                       
                     
                     + 
                     
                       α 
                       ⁡ 
                       
                         [ 
                         
                           R 
                           + 
                           
                             γ 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               
                                 max 
                                 A 
                               
                               ⁢ 
                               
                                 Q 
                                 ⁡ 
                                 
                                   ( 
                                   
                                     
                                       S 
                                       ′ 
                                     
                                     , 
                                     A 
                                   
                                   ) 
                                 
                               
                             
                           
                           - 
                           
                             
                               Q 
                               old 
                             
                             ⁡ 
                             
                               ( 
                               
                                 S 
                                 , 
                                 A 
                               
                               ) 
                             
                           
                         
                         ] 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     2 
                   
                   ] 
                 
               
             
           
         
       
     
     α may be defined as a learning rate factor, and may have a value greater than 0 and less than or equal to 1. R corresponds to a Reward value, and γ is a discount factor, and may be defined as a value indicating how important a present reward is than a reward obtained in the future. Q(S′,A) may be defined as the optimal Q value expected in the future state S′. 
     Specifically, device  100  may initialize Q (S,A) to an arbitrary value in operation S 710   a , which is the first operation of the Q-learning-based algorithm. After initializing Q, the following processes for each episode are repeated: 
     In operation S 720   a , device  100  selects whether to take an action randomly with a probability of e or whether to take an action that satisfies 
     
       
         
           
             
               
                 
                   A 
                   = 
                   
                     
                       arg 
                       A 
                       max 
                     
                     ⁢ 
                     
                       Q 
                       ⁡ 
                       
                         ( 
                         
                           S 
                           , 
                           A 
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                     
                 
               
             
           
         
       
     
     with a probability of (1−e). In operation S 730   a , device  100  observes Reward according to the selected Action and the new state value S′, and updates the Q value using [Equation 2] in operation S 740   a , that is, calculate Q new (S,A). Device  100  determines whether the episode has ended in operation  750   a , and if device  100  determines that the episode has not ended, S′ is updated to S in operation S 760   a.    
     According to an embodiment, device  100  may select an antenna combination constituting an antenna subset based on Q, and after the antenna subset is determined, calculate State and Reward and updates Q to determine another antenna subset based on the updated Q. 
       FIG. 7B  is a flowchart illustrating a method of transmitting an SRS switching signal according to an antenna selection method based on an Upper Confidence Bound (UCB) algorithm during reinforcement learning. 
     The UCB algorithm may be defined as an algorithm that finds an upper limit value with a high probability of an expected reward at a specific time t, that is, an UCB value, from observation results during that time. For example, the UCB is updated for each action that selects an antenna combination included in the antenna subset, and device  100  may select an antenna subset including an antenna combination having a large UCB. This may be referred to as one of “Bandit learning-based antenna selection methods”. 
     In an exemplary embodiment, the UCB value may be expressed as follows. 
     
       
         
           
             
               
                 
                   
                     U 
                     ⁢ 
                     B 
                     ⁢ 
                     
                       
                         C 
                         k 
                       
                       ⁡ 
                       
                         ( 
                         t 
                         ) 
                       
                     
                   
                   = 
                   
                     { 
                     
                       
                         
                           
                             ∞ 
                             , 
                           
                         
                         
                           
                             
                               if 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               
                                 
                                   T 
                                   k 
                                 
                                 ⁡ 
                                 
                                   ( 
                                   
                                     t 
                                     - 
                                     1 
                                   
                                   ) 
                                 
                               
                             
                             &gt; 
                             0 
                           
                         
                       
                       
                         
                           
                             
                               
                                 
                                   
                                     μ 
                                     k 
                                   
                                   ^ 
                                 
                                 ⁡ 
                                 
                                   ( 
                                   
                                     t 
                                     - 
                                     1 
                                   
                                   ) 
                                 
                               
                               + 
                               
                                 
                                   
                                     2 
                                     ⁢ 
                                     ln 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     
                                       ( 
                                       
                                         1 
                                         δ 
                                       
                                       ) 
                                     
                                   
                                   
                                     
                                       T 
                                       k 
                                     
                                     ⁡ 
                                     
                                       ( 
                                       
                                         t 
                                         - 
                                         1 
                                       
                                       ) 
                                     
                                   
                                 
                               
                             
                             , 
                           
                         
                         
                           
                             
                               if 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               
                                 
                                   T 
                                   k 
                                 
                                 ⁡ 
                                 
                                   ( 
                                   
                                     t 
                                     - 
                                     1 
                                   
                                   ) 
                                 
                               
                             
                             &gt; 
                             0 
                           
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     3 
                   
                   ] 
                 
               
             
           
         
       
     
     In an exemplary embodiment, Reward and an empirical mean of Reward may be expressed as follows. 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           μ 
                           ^ 
                         
                         k 
                       
                       ⁡ 
                       
                         ( 
                         
                           t 
                           + 
                           1 
                         
                         ) 
                       
                     
                     = 
                     
                       
                         1 
                         
                           
                             T 
                             k 
                           
                           ⁡ 
                           
                             ( 
                             
                               t 
                               + 
                               1 
                             
                             ) 
                           
                         
                       
                       ⁢ 
                       
                         
                           ∑ 
                           
                             t 
                             = 
                             1 
                           
                           
                             
                               T 
                               k 
                             
                             ⁡ 
                             
                               ( 
                               
                                 t 
                                 + 
                                 1 
                               
                               ) 
                             
                           
                         
                         ⁢ 
                         
                           
                             X 
                             k 
                           
                           ⁡ 
                           
                             ( 
                             t 
                             ) 
                           
                         
                       
                     
                   
                   , 
                   
                     
                       where 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         
                           T 
                           k 
                         
                         ⁡ 
                         
                           ( 
                           
                             t 
                             + 
                             1 
                           
                           ) 
                         
                       
                     
                     = 
                     
                       
                         
                           T 
                           k 
                         
                         ⁡ 
                         
                           ( 
                           t 
                           ) 
                         
                       
                       + 
                       1 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     4 
                   
                   ] 
                 
               
             
           
         
       
     
     T k (t) may be defined as the number of times a corresponding antenna subset is selected up to time t, and S may be defined as a learning parameter. In addition, X k (t) may be defined as the observed Reward for the k-th Action at time t. In addition, {circumflex over (μ)} k (t) may be defined as an empirical mean of Reward accumulated up to time t. 
     For example, device  100  may calculate the UCB at time tin operation S 710   b , and may select a combination of antennas that maximizes the calculated UCB in operation S 720   b . In addition, device  100  may measure a CSI-RS channel by selecting one of the antenna combinations selected in operation S 730   b . Device  100  may calculate Reward and an empirical mean of Reward using [Equation 4] in operation S 740   b.    
     The UCB algorithm may operate in operations S 710   b  to S 740   b , and operations S 710   b  to S 740   b  may be repeated for an arbitrary number of times. 
       FIG. 7C  is a flowchart illustrating a method of transmitting an SRS switching signal according to a probability distribution-based antenna selection method during reinforcement learning. 
     As an embodiment, device  100  may determine an antenna subset by using a value representing a preference for selection of an antenna combination as a probability. This may be referred to as one of “Bandit learning-based antenna selection methods”. 
     The initial probability for the preference may be arbitrarily set as 
     
       
         
           
             
               
                 
                   p 
                   k 
                 
                 ⁡ 
                 
                   ( 
                   t 
                   ) 
                 
               
               = 
               
                 1 
                 K 
               
             
             , 
           
         
       
     
     where K may be defined as the number of antenna combinations, that is, the number of all possible antenna subsets. In operation S 710   c , device  100  may determine an antenna subset including an antenna combination selected according to the learned probability distribution. 
     As an exemplary embodiment, the probability for preference may be expressed as follows. 
     
       
         
           
             
               
                 
                   
                     
                       p 
                       k 
                     
                     ⁡ 
                     
                       ( 
                       t 
                       ) 
                     
                   
                   = 
                   
                     
                       
                         ( 
                         
                           1 
                           - 
                           γ 
                         
                         ) 
                       
                       ⁢ 
                       
                         
                           exp 
                           ⁡ 
                           
                             ( 
                             
                               ρ 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               
                                 
                                   
                                     S 
                                     ^ 
                                   
                                   k 
                                 
                                 ⁡ 
                                 
                                   ( 
                                   t 
                                   ) 
                                 
                               
                             
                             ) 
                           
                         
                         
                           
                             ∑ 
                             
                               j 
                               = 
                               1 
                             
                             K 
                           
                           ⁢ 
                           
                             exp 
                             ⁡ 
                             
                               ( 
                               
                                 
                                   
                                     S 
                                     ^ 
                                   
                                   j 
                                 
                                 ⁡ 
                                 
                                   ( 
                                   t 
                                   ) 
                                 
                               
                               ) 
                             
                           
                         
                       
                     
                     + 
                     
                       γ 
                       K 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     5 
                   
                   ] 
                 
               
             
           
         
       
     
     In operation S 720   c , device  100  may set a Positive Reward or Negative Reward {circumflex over (X)} k (t) according to an index indicating the performance of the CSI-RS channel allocated the selected antenna combination. For example, an index indicating the performance of the CSI-RS channel may include a signal to interference plus noise ratio (SINR). 
     In an exemplary embodiment, Reward may be calculated as follows. 
     
       
         
           
             
               
                 
                   
                     
                       
                         X 
                         ^ 
                       
                       k 
                     
                     ⁡ 
                     
                       ( 
                       t 
                       ) 
                     
                   
                   = 
                   
                     { 
                     
                       
                         
                           
                             
                               α 
                               
                                 
                                   p 
                                   k 
                                 
                                 ⁡ 
                                 
                                   ( 
                                   t 
                                   ) 
                                 
                               
                             
                             , 
                             
                                 
                             
                             ⁢ 
                             
                               
                                 if 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 SIN 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   R 
                                   k 
                                 
                               
                               ≥ 
                               τ 
                             
                           
                         
                       
                       
                         
                           
                             
                               
                                 - 
                                 β 
                               
                               
                                 1 
                                 - 
                                 
                                   
                                     p 
                                     k 
                                   
                                   ⁡ 
                                   
                                     ( 
                                     t 
                                     ) 
                                   
                                 
                               
                             
                             , 
                             
                                 
                             
                             ⁢ 
                             
                               
                                 if 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 SIN 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   R 
                                   k 
                                 
                               
                               &lt; 
                               τ 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     6 
                   
                   ] 
                 
               
             
           
         
       
     
     In operation S 720   c , device  100  may calculate Reward according to the performance of the CSI-RS channel using [Equation 6]. 
     In addition, optionally, device  100  may update Reward of the antenna combination having a high relevance in operation S 730   c  by introducing a weight w. For example, Reward may be updated by multiplying the (k−1)-th Reward and the (k+1)-th Reward by weights w 1  and w 2 , respectively. Next, device  100  updates the probability distribution value by accumulating Rewards for a predetermined time in operation S 740   c.    
     As an exemplary embodiment, the probability distribution of accumulated Rewards may be expressed as follows. 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           S 
                           ^ 
                         
                         k 
                       
                       ⁡ 
                       
                         ( 
                         t 
                         ) 
                       
                     
                     = 
                     
                       
                         ∑ 
                         
                           t 
                           = 
                           1 
                         
                         T 
                       
                       ⁢ 
                       
                         
                           
                             X 
                             ^ 
                           
                           k 
                         
                         ⁡ 
                         
                           ( 
                           t 
                           ) 
                         
                       
                     
                   
                   , 
                   
                       
                   
                   ⁢ 
                   
                     k 
                     = 
                     1 
                   
                   , 
                   … 
                   ⁢ 
                   
                       
                   
                   , 
                   
                       
                   
                   ⁢ 
                   K 
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     7 
                   
                   ] 
                 
               
             
           
         
       
     
     The probability distribution-based antenna selection method may operate in operations S 710   c  to S 740   c , and operations S 710   c  to S 740   c  may be repeated for an arbitrary number of times. 
     In the above-described plurality of reinforcement learning-based antenna selection schemes, Action may be applied by extending to an action of selecting an antenna combination for determining an antenna subset and selecting transmission power. For example, Action may be defined as follows. 
     Action_i: Selecting the first and third antennas &amp; selecting the transmission power of the first antenna as P_level4 and selecting the transmission power of the third antenna as P_level2 
     Action_j: Selecting the 0-th antenna and the third antenna &amp; Selecting the transmission power of the 0-th antenna as P_level1 and the transmission power of the third antenna as P_level2 
     For example, P_level2 may mean power corresponding to level2 set by device  100 . 
       FIG. 8  is a flowchart illustrating an example of a method of tracking a final antenna subset according to an exemplary embodiment of the present disclosure. 
     Referring to  FIG. 8 , a method of tracking a final antenna subset determined through the methods described with reference to  FIGS. 4 to 7C  is shown. 
     Through the embodiments of  FIGS. 4 to 7C , in operation S 810 , device  100  may determine a final antenna subset configured with an optimal antenna combination. After the final antenna subset is determined, in operation S 820 , device  100  may track the optimal antenna combination with an arbitrary tracking period, and in operation S 830 , one antenna included in the final antenna subset previously determined or one or more antennas may be changed according to the tracking result. 
     As an embodiment, when the gain of the final beam is rapidly lowered, an antenna included in the previously determined final antenna subset may not be selected, or the final antenna subset itself may be replaced with another antenna subset. For example, if the antenna subset size is 3, the antenna may be changed to one, changed to two, or changed to a different antenna subset with a different antenna subset size of 3. Tracking and a period are determined from an indicator related to a change in a radio channel, and the tracking period may be a value corresponding to several times the transmission period of the SRS switching signal. For example, the tracking period may be determined according to the Doppler characteristic, which is an index according to the time change of the radio channel, and when the Doppler transition value of the wireless communication device is large, the tracking period may be set short, and when the Doppler transition value is small, the tracking period may be set longer. 
       FIGS. 9A and 9B  are flowcharts illustrating examples of a method of selecting a pre-coding matrix indicator (PMI) according to exemplary embodiments of the present disclosure. 
     When  100   a  and  100   b  transmit the SRS using limited SRS resources and antennas due to the limitations of the scenarios 1 and 2 described above, beamforming using the downlink channel information obtained by the BS causes loss. Accordingly, the method of selecting a PMI for minimizing signal loss using downlink beamforming received by devices  100   a  and  100   b  may include a PMI selection method applying weights according to channels, a PMI selection method based on reinforcement learning, and the like. 
       FIG. 9A  is a flowchart showing a method of selecting a PMI to which a weight is applied according to an exemplary embodiment of the present disclosure. 
     Referring to  FIG. 9A , an example of a PMI selection method for minimizing redundancy of information received by device  100   a  and minimizing signal loss using beamforming is illustrated. Specifically, a PMI may be selected by applying different weights to channels corresponding to an antenna used for transmission of the SRS switching signal and an antenna not used for transmission of the SRS switching signal. 
     For example, in operation S 902   a , device  100   a  may determine an antenna subset to transmit the SRS switching signal as {0, 1} in the first SRS switching transmission period. In operation S 904   a , after transmitting SRS_0, which is an SRS switching signal using the first antenna, and SRS_1, which is an SRS switching signal using a second antenna, the BS  110  may design a downlink beam F_SRS using SRS_0 and SRS_1 in operation S 906   a . The BS  110   a  may transmit the CSI-RS through the F_SRS beam in operation S 908   a . Assuming that the channels used for CSI-RS reception are h0, h1, h2, and h3, in operation S 910   a , device  100   a  may set weights applied to each channel to w0, w1, w2, and w3. w0 to w3 may be determined according to selection and use when transmitting the SRS switching signal. For example, the weights w0 and w1 of the first and second antennas used for transmitting the SRS switching signal, and the weights w2 and w3 of the third and fourth antennas not used for transmitting the SRS switching signal may be set differently. In operation S 912   a , device  100   a  may select the PMI using information obtained by applying a weight to each channel [w0×h0, w1×h1, w2×h2, w3×h3], and feed back the selected PMI to the BS  110   a  in operation S 914   a.    
     According to an embodiment, the weight may be set to have a larger value as the gain of the channel corresponding to the antenna is smaller. Alternatively, the weights may be set so that the gains of channels corresponding to the weighted antennas are all the same. 
       FIG. 9B  is a flowchart showing a method of selecting a PMI based on reinforcement learning according to an exemplary embodiment of the present disclosure. 
     Referring to  FIG. 9B , as in the antenna combination selection method, a method of selecting a PMI based on reinforcement learning is illustrated. 
     According to an embodiment, in operation S 902   b , device  100   b  may determine an antenna subset to transmit the SRS switching signal as {0, 1} in the first SRS switching transmission period. In operation S 904   b , after transmitting SRS_0, which is an SRS switching signal using the first antenna, and SRS_1, which is an SRS switching signal using a second antenna, the BS  110   b  may design a downlink beam F_SRS using SRS_0 and SRS_1 in operation S 906   b . The BS  110   b  may transmit the CSI-RS through the F_SRS beam in operation S 908   b . In operation S 912   a , device  100  may select a PMI using reinforcement learning. Specifically, UCB-based among Q-learning and bandit learning, and gradient bandit learning-based among bandit learning PMI selection methods may all be applied. As an example, Action may be defined as an action that selects PMI, and State may be defined as a performance index for a channel in which a CSI-RS signal is received, and Reward may be defined as a performance index for a channel in which a downlink signal is received using a final beam. That is, device  100   b  may select a PMI that maximizes the “performance of the final beam reception channel” corresponding to Reward among selectable PMIs by recognizing State representing the performance of the channel used for receiving the CSI-RS signal. The wireless communication device  100   b  may feed back the selected PMI to the BS  110   b  in operation S 914   a.    
       FIGS. 10A to 10C  are block diagrams illustrating structures of a wireless communication device according to exemplary embodiments of the present disclosure. 
       FIG. 10A  is a block diagram showing structures of a wireless communication device based on an antenna selection method according to an exemplary embodiment of the present disclosure. 
     Referring to  FIG. 10A , device  100   a  may include first to m-th antennas  1  to m, a radio-frequency integrated circuit (RFIC)  1002   a , and a processor  1006   a . The RFIC  1002   a  may include a switching network  1004   a  and first to n-th RF chains. The RFIC  1002   a  may include a plurality of RF chains, and device  100   a  may include a plurality of RFICs. The switching network  1004   a  may be connected to the first to m-th antennas  1  to m. In some cases, only one RF chain may be in the RFIC  1002   a , or may be connected to each individual antenna. 
     The processor  1006   a  according to the exemplary embodiment of the present disclosure may identify the downlink reference signal from the SRS switching resource set by the BS  110   a . The processor  1006   a  may generate downlink channel information using the downlink reference signal, select an antenna combination including at least one of the first to m-th antennas  1  to m, determine an antenna subset including the corresponding antenna combination, and control the switching network  1004   a  based on the determined antenna subset. 
     The switching network  1004   a  according to an exemplary embodiment of the present disclosure may be connected to the processor  1006   a . Also, the switching network  1004   a  may select at least one of the antennas included in the antenna subset determined by the processor  1006   a . The SRS switching signal may be transmitted to the BS  110   a  through the selected antenna. 
     The processor  1006   a  according to an embodiment of the present disclosure may select a PMI based on the CSI-RS received from the BS  110   a  using the first beam, and may transmit the selected PMI to the BS. The BS  110   a  may transmit a downlink signal including data or the like using the final beam, and the processor  1006   a  may process the received downlink signal. The final beam may be determined by information obtained from the SRS switching signal and information obtained from the PMI. 
       FIG. 10B  is a block diagram showing structures of a wireless communication device based on a beam selection method according to an exemplary embodiment of the present disclosure. 
     Referring to  FIG. 10B , device  100   b  may include first to m-th antennas  1  to m, an RFIC  1002   b , and a processor  1006   b . The RFIC  1002   b  may include a beamformer  1004   b  and first to n-th RF chains. The RFIC  1002   b  may include a plurality of RF chains, and device  100   b  may include a plurality of RFICs. The beamformer  1004   b  may be connected to the first to m-th antennas  1  to m. In some cases, only one RF chain may be in the RFIC  1002   b , or may be connected to each individual antenna. 
     The processor  1006   b  according to the exemplary embodiment of the present disclosure may identify the downlink reference signal from the SRS switching resource set by the BS  110   b . The processor  1006   b  may generate downlink channel information using the downlink reference signal, and may select a beam using the previously shared beam codebook information. The processor  1006   b  may design an optimal beam when there is no pre-shared beam code book. The processor  1006   b  may design a new beam considering reception performance and spatial characteristics, and may control the beamformer  1004   b  based on the determined beam. 
     The beamformer  1004   b  according to an exemplary embodiment of the present disclosure may be connected to the processor  1006   b . In addition, the beamformer  1004   b  may form a beam according to beam information selected (or designed) by the processor  1006   b . The SRS switching signal may be transmitted to the BS  110   b  through the formed beam. 
     The processor  1006   b  according to an embodiment of the present disclosure may select a PMI based on the CSI-RS received from the BS  110   b  using the first beam, and may transmit the selected PMI to the BS. The BS  110   b  may transmit a downlink signal including data or the like using the final beam, and the processor  1006   b  may process the received downlink signal. The final beam may be determined by information obtained from the SRS switching signal and information obtained from the PMI. 
       FIG. 10C  is a block diagram showing structures of a wireless communication device based on a reinforcement learning method according to an exemplary embodiment of the present disclosure. 
     Referring to  FIG. 10C , a wireless communication device  100   c  may include first to m-th antennas  1  to m, an RFIC  1002   c , and a processor  1006   c . The processor  1006   c  may include a machine learning device  1010   c  for performing the reinforcement learning-based antenna selection method, the reinforcement learning-based beam selection method, or the reinforcement learning-based PMI selection method. 
     For example, the machine learning device  1010   c  may observe State, select an Action, and calculate Reward and Q in order to perform the Q-learning-based antenna selection method of  FIG. 7A . That is, the machine learning device  910   c  may perform the operations for reinforcement learning disclosed in  FIGS. 7A to 7C and 8B , and detailed descriptions thereof will be omitted. 
     As an example, in the Q-learning-based antenna selection method of  FIG. 7A , the machine learning device  910   c  may calculate Q(S,A) having the largest value in a given state using the Q-table in [Table 2], and select an antenna combination corresponding to the corresponding Q value based on the learned information. 
     While the inventive concept has been particularly shown and described with reference to embodiments thereof, it will be understood that various changes in form and details may be made therein without departing from the spirit and scope of the following claims and their equivalents.