Patent Publication Number: US-2023155787-A1

Title: Communication apparatus, method of controlling communication apparatus, and non-transitory computer-readable storage medium

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 17/133,713, filed on Dec. 24, 2020, which claims the benefit of and priority to Japanese Patent Application No. 2020-002435, filed Jan. 9, 2020, each of which is hereby incorporated by reference herein in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a wireless LAN communication technology. 
     Description of the Related Art 
     An IEEE 802.11 series standard is known as a communication standard related to a wireless LAN (local area network). The IEEE 802.11ax standard uses OFDMA to achieve high peak throughput as well as improve communication speeds in congested situations. Note that OFDMA is an abbreviation for Orthogonal Frequency-Division Multiple Access. 
     After having been a SG (study group) called an IEEE 802.11 EHT (Extreme or Extremely High Throughput), an 802.11be TG (task group) is now active in a subsequent standard to the IEEE 802.11ax standard for further improving throughput. 
     A multi-AP coordination configuration in which a plurality of APs (access points) coordinate and operate is being considered as one of the measures for achieving the goal of this TG which is to improve throughput. In order to operate this multi-AP coordination configuration efficiently, it is common to perform a sounding procedure between an access point (AP) and a terminal (STA). A sounding procedure is a procedure for an STA to receive an NDP (Null Data Packet) as a “sounding packet” from a plurality of APs and to transmit a feedback packet which includes channel state information (CSI) to each AP (Japanese Patent Laid-Open No. 2016-526856). 
     However, in a conventional sounding procedure, a procedure for a plurality of APs to cooperate and operate to transmit a sounding frame has not been made clear. Also, in the conventional technique, in relation to time-related overhead of the sounding procedure and a heavy load on the STA, it has been proposed, mostly, to reduce the amount of CSI information (reduce the amount of feedback) transmitted by the STA. However, reducing the amount of CSI information makes the estimation of a wireless medium inaccurate, and especially, in the multi-AP coordination configuration, effects of that inaccuracy such as inefficiency in medium usage and slowdown in speed can be significant. 
     SUMMARY OF THE INVENTION 
     The present disclosure in consideration of the foregoing problems provides an efficient sounding procedure in a multi-AP coordination configuration. 
     According to one aspect of the present invention, there is provided a communication apparatus operable to act as a master-AP in a multi-AP (access point) coordination configuration that supports an IEEE802.11 series standard, the apparatus comprises: a selection unit configured to select a sounding method which is a method for transmitting a sounding packet for receiving a CSI report as feedback from a terminal apparatus in accordance with a CSI (channel state information) calculation capability in the terminal apparatus that is connected to the communication apparatus. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments (with reference to the attached drawings). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    illustrates an example of a system configuration in an embodiment. 
         FIG.  2    illustrates an example of a hardware configuration of an AP in an embodiment. 
         FIG.  3 A  illustrates an example of an AP functional configuration in an embodiment. 
         FIG.  3 B  illustrates an example of an STA functional configuration in an embodiment. 
         FIG.  4    is a flowchart of processing that is executed when a connection with an STA is started. 
         FIG.  5    is a flowchart of processing that is executed when data to be transmitted to an STA is generated. 
         FIG.  6    is a flowchart of processing for selecting a sounding method. 
         FIG.  7    illustrates a configuration of a MAC frame. 
         FIG.  8    indicates a configuration of a trigger frame (TF). 
         FIG.  9    illustrates a configuration of a CSI report field. 
         FIG.  10    indicate configurations of a MIMO control field. 
         FIG.  11    is a sequence diagram “from a connection between the AP and the STA until a selection of the sounding method”. 
         FIG.  12    is a sequence diagram “in a case of simultaneous NDP transmission (no BFRP TF)”. 
         FIG.  13    is a sequence diagram “in a case of simultaneous NDP transmission (with BFRP TF). 
         FIG.  14    is a sequence diagram “in a case of sequential NDP transmission”. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, embodiments will be described in detail with reference to the attached drawings. Note, the following embodiments are not intended to limit the scope of the claimed invention. Multiple features are described in the embodiments, but limitation is not made to an invention that requires all such features, and multiple such features may be combined as appropriate. Furthermore, in the attached drawings, the same reference numerals are given to the same or similar configurations, and redundant description thereof is omitted. 
     System Configuration 
       FIG.  1    illustrates an example of a system configuration in this embodiment. An AP  102  and an AP  105  are access points comprising a function (multi-AP coordination function) that can achieve a multi-AP coordination configuration. A multi-AP coordination function is a function for achieving higher-speed or more stable communication in relation to a connected terminal than when there is one AP by coordination with the other APs. Here, a stable state is a state of an arbitrary combination of a good signal-noise ratio, low interference, low latency, and low jitter, for example. Note that there are various methods in techniques for achieving such a stable state. For example, a JTX (Joint Transmission) that uses a D-MIMO (Distributed Multiple Input Multiple Output); null steering; coordinated OFDMA; and fractional coordinated OFDMA can be given. The AP  102  and the AP  105 , in a case where they are not executing the multi-AP coordination function, manage only a network of a BSS (Basic Service Set)  101  and a network of a BSS  104 , respectively. 
     The AP  102  and the AP  105  are connected by a network (backhaul)  100 . The network  100  is a communication means for connecting a BSS (Basic Service Set) and another network to each other when an AP constructs a DS (Distributions System). The network  100  is achieved by wired communication such as Ethernet (registered trademark) and telephone line or wireless communication such as LTE (Long-Term Evolution) and WiMAX (Worldwide Interoperability for Microwave Access). Further, the network  100  may be a wireless LAN conforming to the IEEE 802.11 series standard. In such a case, the network  100  may be the same or different from a wireless channel that is used between an AP and an STA. 
     An STA  103  and an STA  106  are wireless LAN terminals. These STAs are capable of data communication with a plurality of APs. Data communication includes communication in the above-described sounding procedure. Specifically, reception of an NDPA (NDP Announcement) and an NDP, transmission of a frame including a CSI Report field, and reception of an NFRP TF (NDP Feedback Report Poll Trigger Frame). 
     Configuration of Communication Apparatuses (AP, STA) 
     In  FIG.  2   , an example of a hardware configuration of APs (AP  102 , AP  105 ) in the present embodiment is illustrated. An AP has, as an example of its hardware configuration, a storage unit  201 , a control unit  202 , a functional unit  203 , an input unit  204 , an output unit  205 , a communication unit  206 , and an antenna  207 . The storage unit  201  is configured by a memory such as a ROM or a RAM, and stores various information such as a program for performing various operations which will be described later, communication parameters for wireless communication, and the like. Note that as the storage unit  201 , a storage medium such as a floppy disk, a hard disk, an optical disk, a magneto-optical disk, a CD-ROM, a CD-R, a magnetic tape, a nonvolatile memory card, a DVD, and the like may be used in addition to a memory such as a ROM or a RAM. The storage unit  201  may include a plurality of memories and the like. 
     The control unit  202  is configured by one or more processors such as CPUs and MPUs, an ASIC (application specific integrated circuit), a DSP (digital signal processor), an FPGA (field-programmable gate array), and the like, for example. Here CPU is an acronym for Central Processing Unit and MPU is an acronym for Micro Processing Unit. The control unit  202  controls the AP by executing a program that is stored in the storage unit  201 . Note that a configuration may be taken such that the control unit  202  controls the AP by a program and an OS (Operating System) that are stored in the storage unit  201  cooperating. Also, a configuration may be taken such that the control unit  202  is made up of a plurality of processors such as a multi-core system and controls the AP. The control unit  202  may control the functional unit  203  to execute a predetermined process such as an AP function, imaging, printing, and projection. The functional unit  203  is hardware for the AP to execute predetermined processing. 
     The input unit  204  accepts various operations from a user. The output unit  205  performs various kinds of outputs for the user. Here, an output by the output unit  205  includes at least one of a display on a screen, a sound output by a speaker, a vibration output, and the like. Note that both the input unit  204  and the output unit  205  may be implemented by a single module, such as a touch panel. 
     The communication unit  206  performs control of wireless communication conforming to the IEEE 802.11 series standard, control of wireless communication conforming to Wi-Fi (registered trademark), and control of IP (Internet Protocol) communication. Further, the communication unit  206  controls the antenna  207  to transmit and receive wireless signals for wireless communication. The antenna  207  supports communication for the multi-AP coordination configuration. For example, in the AP, a D-MIMO (Distributed Multiple Input Multiple Output) transmission for a JTX (Joint Transmission) is possible. There may be plural antennas  207 , although only one is denoted for simplicity in the figure. Generally, the number of antennas  207  (elements) is a number according to the number of streams. Also, a frequency band that is supported by the antenna  207 , in addition to 2.4 and 5 GHz bands, is a 6 GHz band that is scheduled to be introduced from 802.11ax. 
     Note that the STAs (STA  103 , STA  106 ) also have the same hardware configuration as the AP illustrated in  FIG.  2   . In such a case, the control unit  202  can execute predetermined processing such as an STA function by controlling the functional unit  203 . 
     In  FIG.  3 A , an example of a functional configuration of the AP is illustrated. As an example of the functional configuration of the AP, the AP has a communication control unit  301 , a CSI processing capability setting unit  302 , a storage management unit  303 , a UI (user interface) control unit  304 , a multi-AP configuration control unit  305 , a sounding method decision unit  306 , and a sounding processing unit  307 . The communication control unit  301  performs control for performing transmission/reception of a wireless signal to and from another wireless LAN apparatus (for example, another AP or STA) via the communication unit  206 . The communication control unit  301  executes wireless LAN communication control such as reception of a wireless frame from another wireless LAN apparatus, frame generation, and frame transmission in accordance with the IEEE 802.11 standard series. The CSI processing capability setting unit  302  sets (the value of) the CSI processing capability of the STA based on information that is exchanged with the STA and the like. The storage management unit  303  performs storage control/management that is related to the storage unit  201 . The UI control unit  304  delivers to each configuration element a control signal according to an input operation to the input unit  204  by the user (not shown). The multi-AP configuration control unit  305  executes a multi-AP configuration function by selecting a multi-AP configuration method in accordance with the presence of a peripheral AP, the capability of the peripheral AP, and a connection state between the peripheral AP and the STA. The sounding method decision unit  306  decides whether to perform sounding processing by either an explicit method or an implicit method which will be described below. The sounding processing unit  307  performs sounding processing in accordance with a method that is decided by the sounding method decision unit  306 . In the present embodiment, sounding processing is performed based on the CSI processing capability of the STA. 
     In  FIG.  3 B , an example of a functional configuration of the STA is illustrated. The STA, as an example of its functional configuration, has a communication control unit  311 , a UI control unit  312 , a CSI processing capability decision unit  313 , and a CSI calculation unit  314 . The communication control unit  311  and the UI control unit  312  are the same as the communication control unit  301  and the UI control unit  304 . The CSI processing capability decision unit  313  decides the CSI processing capability of the STA itself. The CSI calculation unit  314  performs CSI calculation processing. 
     Processing Flow 
     Next, a processing flow of the AP having the above configuration will be described with reference to the figures. The processing indicated below may be started when a BSS is started or at a desired timing during BSS operation by the AP. 
     Connection Processing Between AP and STA 
       FIG.  4    is a flowchart of processing that is executed when a connection between the AP  102  and the STA  103  is started. In step S 401 , the communication control unit  301  of the AP  102 , when connecting with the STA  103 , exchanges capability information and operation information therewith. This is achieved by exchanging a Management frame that includes an information element (IE). The information element has been newly defined in response to the progress in IEEE802.11 standardization. For example, it is an HT Capability element in IEEE 802.11n, a VHT Capability element in IEEE 802.11ac, an HE Capabilities element in IEEE 802.11ax, and an EHT Capabilities element in IEEE 802.11be. Note that HT is an acronym for High Throughput, VHT is an acronym for Very High Throughput, HE is an acronym for High Efficiency, and EHT is an acronym for Extremely High Throughput. Also, a Management frame is a MAC (Medium Access Control) frame such as a Beacon, a Probe Request/Response, an Association Request/Response, and an Authentication Request/Response. The multi-AP coordination function is a function that is introduced by the IEEE 802.11be standard. Accordingly, capability information that is related to the function is included in an EHT Capabilities element. 
     In  FIG.  7   , a MAC frame configuration that is specified by the IEEE802.11 standard is indicated. In a MAC frame  700 , various fields are included. In Frame Control  701 , subfields  721  to  731  are included. In the information element (IE) of a Frame Body  710 , subfields  741  to  747  are included. In an Address1  703 , an Address2  704 , an Address3  705 , an Address  707 , addresses such as a BSSID, a transmission source, and a destination are set in accordance with a MAC frame type (Type  722 ). Also, in the MAC frame  700 , a Duration/ID  702 , Sequence Control  706 , QoS Control  708 , and an FCS (Frame Check Sequence)  711  are included; however, detailed description will be omitted. 
     HT Control  709  is specified as follows by the IEEE 802.11ax standard. That is, when the left-most bit is 0, the HT Control  709  is for HT (IEEE 802.11n); when the left-most 2 bits are 10, the HT Control  709  is for VHT (IEEE 802.11ac); and when the left-most 2 bits are 11, the HT Control  709  is for HE (IEEE 802.11ax). Note that the definition of the HT Control  709  for EHT (IEEE 802.11be) is unspecified. 
     In the Frame Control  701 , a Protocol Version  721  is a 2-bit number which indicates a protocol version, and in a case of an IEEE 802.11 frame, is “0”. The Type  722  is a 2-bit number which indicates the type of the frame and indicates one of Management, Control, or Data. The Subtype  723  is a 1-bit number which indicates the subtype of the frame and indicates one of Management, Control, or Data. To DS  724  indicates that the destination of the frame is a DS (Distribution System). Also, in the Frame Control  701 , From DS  725 , More Fragment  726 , Retry  727 , Power Management  728 , More Data  729 , Protected Frame  730 , and +HTC  731  are included; however, detailed description will be omitted. 
     In the Frame Body  710 , the subfields  741  to  747  are the configuration of the EHT Capabilities element for IEEE 802.11be. In an Element ID  741 , a value that is related to the EHT of IEEE 802.11be follows a value of the HE Capabilities element of IEEE 802.11ax and is 255. A Length  742  is the length of an information element. In an Element ID Extension  743 , an EHT Capabilities element that is related to capability information or an EHT Operation element that is related to operation information is defined. EHT MAC Capabilities Information  744 , EHT PHY Capabilities Information  745 , a Supported EHT-MCS And NSS Set  746 , and PPE (Physical layer Packet Extension) Thresholds  747  are the same configuration as in a case of IEEE 802.11ax (the HE Capabilities element). 
     In the present embodiment, a subfield (CSI processing capability subfield) that indicates a capability (CSI processing capability/CSI calculation capability) for calculating the CSI by the STA in the EHT MAC Capabilities Information  744  is defined. The CSI processing capability may be decided by the CSI processing capability decision unit  313  in the STA. The first example that is related to the CSI processing capability subfield is a 1-bit number that indicates whether the CSI processing capability is “high” or “low”. The second example that is related to the CSI processing capability subfield is a 1-bit number that indicates whether it is “capable” or “not capable of responding within an SIFS (Short Inter Frame Space) to a plurality of sounding packets”. The third example that is related to the CSI processing capability subfield is a time conversion value that is calculated from an equation consisting of “the number of transmission/reception antennas” and “the number of streams” of the AP and the STA and a “desired coefficient”. In such a case, the present invention is not limited to a 1-bit number. That is, it may be two or more values. The fourth example that is related to the CSI processing capability subfield is a value that is the same as the expression format of the processing capability of the CPU. In such a case, the present invention is not limited to a 1-bit number. 
     Note that a means for notifying the CSI processing capability is not limited to the above-described CSI processing capability subfield. As another example, a configuration may be taken so that the HE Capabilities element is used instead of the EHT Capabilities element. In such a case, a capability expression value will be a 1-bit number. This restriction of a 1-bit number is due to the fact that what is Reserved in the 48 bit HE Capabilities element of the IEEE 802.11ax standard is 1 bit. 
     Yet another example for notifying the CSI processing capability is a method of newly defining an Action frame of the IEEE802.11 standard and then using it. In such a case, the expression of the capability of information to be notified can be a desired number of bits. 
     Returning to the description of  FIG.  4   , in step S 401 , the communication control unit  301  of the AP  102  exchanges capability information and the like with the STA  103 , and then in step S 402 , the CSI processing capability setting unit  302  of the AP  102  determines whether the CSI processing capability of the STA  103  is obtained. If the CSI processing capability of the STA  103  is obtained (Yes in step S 402 ), the processing proceeds to step S 403 . In step S 403 , the storage management unit  303  sets to an STA management table a value that indicates the obtained CSI processing capability. Here, the STA management table is a region that holds the capability information and the state of the STA that are to be necessary when the AP manages the BSS (Basic Service Set) and is present within the storage unit  201 . If the CSI processing capability of the STA  103  is not obtained in step S 402  (No in step S 402 ), the processing proceeds to step S 404 . In step S 404 , the CSI processing capability setting unit  302  of the AP  102  determines whether the MinTrigProcTime of the STA  103  is obtained. In a case where the MinTrigProcTime of the STA  103  is obtained (Yes in step S 404 ), the processing proceeds to step S 405 . The determination in step S 404  is intended to confirm that an index to be in place of a value that indicates the CSI processing capability itself is obtained. 
     Here, a value that is indicated by MinTrigProcTime will be described. This value corresponds to the value of a Trigger Frame MAC Padding Duration of the HE MAC Capabilities Information of IEEE 802.11ax. This subfield is a 2-bit number, and 0/1/2 respectively correspond to 0 (zero) μsec/8 μsec/16 μsec of MinTrigProcTime. Note that the name MinTrigProcTime originates from “a minimum value of preparation time for performing a transmission using an RU (Resource Unit) that is assigned by a Trigger Frame (TF)”. 
     In  FIG.  8   , a configuration of a trigger frame (TF) is illustrated. A trigger frame is a frame to be newly introduced from IEEE802.11ax and is a frame for indicating an activation timing necessary for a plurality of STAs (users) to simultaneously transmit a frame, and the AP wireless channel information for using the frame, and the like. In Trigger Frame  800  of  FIG.  8   , various fields are included. Frame Control  801  is a common field in the IEEE802.11 series and in the present embodiment, a value that indicates that the field is a trigger frame of IEEE 802.11ax is entered. Common Info  805  indicates information that is common across the plurality of STAs (terminals) which are the destinations of this trigger frame and includes subfields  811  to  813 . Per User Info  806  indicates individual information that is related to the destination of this trigger frame. Padding  807  is something for providing a temporal postponement to a group of STAs that received this trigger frame. The AP decides from the MinTrigProcTime of each STA the temporal postponement (that is, the length of the padding  807 ). Generally, the length (value) of the padding that corresponds to the maximum value of the MinTrigProcTime among the MinTrigProcTimes of a group of STAs to be the destination of the trigger frame is used. Also, in the Trigger Frame  800 , a Duration  802 , an RA (Receiver Address)  803 , a TA (Transmitter Address)  804 , and a FCS (Frame Check Sequence)  808  are included; however, detailed description is omitted. 
     In the Common Info  805 , a value (Trigger Type subfield value) that is indicated by a Trigger Type  811  and description that corresponds to the value are indicated in a table at the bottom of  FIG.  8   . For example, in a case where the Trigger Type  811  is 7, the Trigger Type  811  indicates an NFRP (NDP Feedback Report Poll) TF. Also, in the Common Info  805 , a Length  812  and a Trigger Type Dependent  813  are included; however, detailed description is omitted. 
     Once again, returning to the description of  FIG.  4   , in step S 405 , the CSI processing capability setting unit  302  of the AP  102  sets to the value of the CSI processing capability the value to which the MinTrigProcTime is multiplied by a predetermined coefficient, and then the processing proceeds to step S 403 . Note that, a configuration may be taken so as to subtract a predetermined constant in place of multiplying (multiplication) the MinTrigProcTime by a predetermined coefficient. In any case, the MinTrigProcTime is handled as something whose CSI capability is higher the smaller its value. 
     In step S 404 , in a case where MinTrigProcTime of the STA  103  is not obtained (No in step S 404 ), the processing proceeds to step S 406 . In step S 406 , the CSI processing capability setting unit  302  of the AP  102  sets to a default value the value of the CSI processing capability. For example, this default value is a value that in a case of a 1-bit number, means “low” or “unable to respond within an SIFS to a plurality of sounding packets”. As described above, the reason why the CSI processing capability of the STA whose CSI processing capability is unknown is estimated to be low is for reliably executing irrespective of the capability of the STA the processing to be necessary thereafter. 
     Note that in a case where the STA  103  does not notify the CSI processing capability to the AP  102  in step S 401 , an inquiry for the CSI processing capability may be made to the STA  103  from the AP  102  in step S 402 . 
     Data Generation to STA 
       FIG.  5    is a flowchart of processing that is executed when data to be transmitted to the STA  103  is generated in the AP  102 . The data is data that is addressed to the STA  103  from the STA  106 , data that is addressed to the STA  103  from an STA (not shown) within the BSS, data that is addressed to the STA  103  from a terminal on the network  100 , data that is addressed to the STA  103  from the AP  102 , and the like. 
     In step S 501 , the communication control unit  301  of the AP  102  determines whether to transmit the generated data by a multi-AP coordination configuration. In a case where data is transmitted by the multi-AP coordination configuration (Yes in step S 501 ), the processing proceeds to step S 502 . In step S 502 , the multi-AP configuration control unit  305  of the AP  102  executes the multi-AP coordination function with an AP in the vicinity. Next, in step S 503 , the AP  102 , as a result of the execution of the multi-AP coordination function, determines whether to operate as a Master-AP or a Slave-AP of the multi-AP coordination configuration. Here, a Master-AP is an AP that manages the multi-AP coordination configuration, and a Slave-AP is an AP that operates under the control of the Master-AP. In a case where the AP  102  operates as the Master-AP (Yes in step S 503 ), the processing proceeds to step S 507 . The processing in step S 507  will be described later using  FIG.  6   . In a case where the AP  102  operates as the Slave-AP (No in step S 503 ), the processing proceeds to step S 506 . 
     The case of No in step S 503  in the present embodiment corresponds to a case where the AP  102  is the Slave-AP and the AP  105  is the Master-AP. In step S 506 , the communication control unit  301  of the AP  102  transmits the CSI processing capability of the STA  103  in relation to the Master-AP (that is, the AP  105 ). In the following step S 508 , the sounding processing unit  307  of the AP  102  performs the sounding processing of the Slave-AP. 
     In step S 501 , in a case where data is not transmitted by the multi-AP coordination configuration, the processing proceeds to step S 505 , and the AP  102  performs Single-AP transmission processing. Regarding this Single-AP transmission processing, there is no characteristic operation of the present invention; therefore, description will be omitted. 
     Note that in the present embodiment, a case where the AP  102  that has performed the connection processing with the STA  103  is the Master-AP is assumed; however, the AP  102  may execute a multi-AP coordination configuration function in relation to the STA  106  that has performed the connection processing with the Slave-AP (AP  105 ). In such a case, the AP  102  will perform processing for receiving information to be transmitted by the processing in step S 506  from the AP  105  following the processing in step S 503 . 
     Selection of Sounding Method by Master-AP 
     Next, processing in step S 507  in  FIG.  5    will be described with reference to  FIG.  6   . In step S 601 , the sounding method decision unit  306  of the AP  102  performs a selection of whether to perform the sounding processing by an explicit method or an implicit method. The selection can be performed based on the capability information that is exchanged to and from the STA  103 , the input information for the input unit  204  by the user, a predetermined setting, and the like. 
     Here, the explicit method and the implicit method will be described. In the explicit method, first, the AP (beamformer) transmits the NDP (Null Data Packet) as a sounding packet. The STA (beamformee) that receives the NDP calculates the CSI from the NDP and then feeds back the CSI to the AP. By this, the AP estimates a reception condition in the STA of a packet that it transmits. 
     In the implicit method, first, the STA transmits the NDP as a sounding packet, and the AP that received the NDP estimates the situation of the STA from the reception state of the packet. As described above, in the implicit method, the STA only needs to transmit the NDP at an assigned timing and the processing load in the STA is lesser in comparison to the explicit method for not needing the processing for calculating the CSI. Accordingly, in a case where the implicit method is selected in step S 601  (No in step S 601 ), the problem of an increase in the processing load in the STA does not occur. Therefore, description of the processing of the implicit method (step S 607 ) will be omitted. 
     In a case where the explicit method is selected in step S 601  (Yes in step S 601 ), the processing proceeds to step S 602 . In step S 602 , the following processing branches in accordance with the value of the CSI processing capability of the STA  103 . Note that in a case where the value registered in the STA management table in the processing in step S 403  in  FIG.  4    is a 1-bit number, the value is made to correspond to “high” or “low”. That is, in a case where the value of the CSI processing capability is indicated by two values which express the two levels “high” and “low”, the processing branches to step S 603  or S 606 . In a case where the resolution of the registered value is greater than 1 bit, the value is made to correspond to “high”, “medium”, or “low”. That is, in a case where the value of the CSI calculation capability is indicated by three values which express the three levels “high”, “medium”, and “low”, the processing branches to step S 603 , step S 604 , or step S 606 . 
     In a case where the CSI processing capability is “high”, the processing proceeds to step S 603 . In step S 603 , the AP  102  performs “simultaneous NDP transmission (No BFRP TF (BeamForming Report Poll Trigger Frame)) processing”. In a case where the CSI processing capability is “medium”, the processing proceeds to step S 604 . In step S 604 , the AP  102  decides the length (time) of the padding in BFRP TF. This decision processing is processing in which the larger the CSI processing capability is in relation to the CSI processing capability that is classified in the same “medium” level, the shorter the padding time for CSI. Next, in step S 605 , the AP  102  performs “simultaneous NDP transmission (with BFRP TF) processing”. In a case where the CSI processing capability is “low”, the processing proceeds to step S 606 . In step S 606 , the AP  102  performs “sequential NDP transmission processing”. The details of the processing of steps S 603 , S 605 , and S 606  will be described later using  FIGS.  12  to  14   , respectively. After the sounding processing (one of steps S 603 , S 605 , and S 606 ), the data transmission processing is performed (step S 608 ). 
     Calculation of CSI 
     Next, a CSI report field in a CSI frame to be generated (calculated) and then transmitted by the STA will be described with reference to  FIG.  9   .  FIG.  9    illustrates a configuration of a CSI report field. A CSI frame is an Action frame whose category is an HT or an Action No Ack frame in the IEEE802.11 standard, for example. This CSI frame is something for notifying Channel State Information to a beamformer. Note that an Action frame is a frame whose Type  722  holds a value 00 and whose Subtype  723  holds a value  1101  in  FIG.  7   . 
     In  FIG.  9   , an SNR in receive chain 1  901  is an 8-bit number and is a signal-to-noise ratio (SN) of a receive chain of the STA that transmits a CSI report. Here, a receive chain is an object that performs the necessary signal processing that is related to the received data. In this signal processing, filtering, amplification, downconversion, and sampling are included. A CSI Matrix for carrier  902  is a matrix that is derived from a CHAN_MAT of an RXVECTOR. Here, the RXVECTOR is a set of parameters that are related to the reception of a physical layer of an 802.11 frame, and the CHAN_MAT is one of those parameters. By this CHAN_MAT, whether that frame includes CSI matrices or beamforming feedback matrices is indicated. Note that,  FIG.  9    is a configuration of the CSI report field in a case where the operation frequency band is 20 MHz; however, in a case where the operation frequency band is 40 MHz, the CSI Matrix for carrier  902  is “from −58 to −2” and “from 2 to 58”. Further, Nb, Nc, and Nr in  FIG.  9    are values that are assigned by the MIMO control field. Here, the MIMO control field is used for exchanging channel state information or for managing the transmission of beamforming feedback information. 
     A configuration  10   a  and a configuration  10   b  in  FIG.  10    are respectively configurations of a MIMO control field before IEEE 802.11ax and of IEEE 802.11ax. Note that, the MIMO control field of IEEE 802.11ax is assumed to be used also in IEEE 802.11be. Here, the Nb is a number defined by a Coefficient Size field  1003  of the MIMO control field. The Nc is a number of columns in the CSI Matrix defined by an Nc Index field  1001  of the MIMO control field. The Nr is a number of rows in the CSI Matrix defined by an Nr Index field  1002  of the MIMO control field. 
     Incidentally, a plurality of APs simultaneously transmitting a sounding packet (NDP) by the multi-AP coordination configuration, for the receiving side, corresponds to an increase in antennas of the transmitting side. Here, the computation amount of CSI matrices increases in accordance with the number of transmission antennas; therefore, in the simultaneous NDP transmission, the computation amount of CSI matrices increases in comparison to a method in which each NDP is sequentially transmitted. Accordingly, in a case of executing a simultaneous NDP transmission method, in order to perform CSI report feedback that is related to an NDP within the SIFS time, the processing capability per unit of time in the STA must be higher than the processing capability per unit of time that is related to the sequential method. 
     Considering this point, in the present embodiment, as in the branch in step S 602  in  FIG.  6   , the AP  102  changes the sounding processing in accordance with the processing capability of the STA  103 . By this, the CSI report feedback that is related to all NDPs is caused to be completed in a period based on a predetermined time (for example, SIFS). 
     Sequence Diagram 
     Next, the operation sequence of the AP  102  and the STA  103  will be described with reference to  FIGS.  11  to  14   .  FIG.  11    is a sequence diagram “from a connection between the AP and the STA until a selection of the sounding method”. In F 1101 , connection processing is performed between the AP  102  and the STA  103  (step S 401 ). This connection processing corresponds to the exchange of a Management frame such as a Probe Request/Response, an Association Request/Response, and an Authentication Request/Response frame. In F 1102 , the AP  102  confirms whether the CSI processing capability of the STA  103  is obtained (step S 402 ). In F 1103 , the AP  102  decides the CSI processing capability of the STA  103 . In this processing, a calculation of the CSI processing capability from the MinTrigProcTime, a request for the CSI processing capability in a case where it has not been obtained, a setting of the CSI processing capability to a default value, and the like are included (step S 404  to S 406 ). In F 1104 , the AP  102  registers the CSI processing capability in the STA management table (step S 403 ). In F 1105 , the AP  102  recognizes that data addressed to the STA  103  is generated. In F 1106 , the AP  102  determines whether to construct the multi-AP coordination configuration and then transmit data (step S 501 ). In F 1107 , the multi-AP coordination function is executed between the AP  102  and the AP  105  and the multi-AP coordination configuration is constructed (step S 502 ). In the present example, it is assumed that the AP  102  is the Master-AP. In F 1108 , the AP  102  starts an operation as the Master-AP. In F 1109 , the AP  105  starts an operation as the Slave-AP. In F 1110 , the AP  102  that is the Master-AP performs a determination for selecting the explicit method or the implicit method as the sounding method (step S 601 ). In the present example, it is assumed that the explicit method is selected. In F 1111 , the AP  102  confirms the CSI processing capability of the STA  103  (step S 602 ). This is performed by referencing the STA management table. In F 1112 , the AP  102  selects the sounding method by the CSI processing capability of the STA  103  (step S 602 ). 
     Next, the operation sequence of the AP  102  and the STA  103  in each sounding method selected in F 1112  will be described with reference to  FIG.  12    to  FIG.  14   . 
     Simultaneous NDP Transmission (No BFRP TF) 
       FIG.  12    is a sequence in which the CSI processing capability of the STA  103  is determined to be “high” (“high” in step S 602 ) and the sounding method that is selected in F 1112  in  FIG.  11    is “a case of a simultaneous NDP transmission (No BFRP TF)” (step S 603 ). In F 1201 , a sounding TF is transmitted to the AP  105  from the AP  102 . This sounding TF is a frame that is used by the Master-AP and the Slave-AP for simultaneously transmitting a NDP. A sounding TF is transmitted to the Slave-AP from the Master-AP. This sounding TF does not exist in the IEEE 802.11ax standard but can be achieved by a frame of the same format as the Trigger Frame  800  in  FIG.  8   . In such a case, the value of the Trigger Type  811  is any one of “8” to “15” that is reserved. In F 1202 , the AP  102  transmits an NDPA (NDP Announcement) to the STA  103 . This NDPA includes an STA Info List of terminals that are to receive an NDP. This individual STA Info List is configured by a 12-bit format Association ID (AID  12 ) and a Feedback Type that indicates whether the user is an SU (Single User) or an MU (Multi User), and an Nc Index. In F 1203 , the AP  105  transmits the NDPA to the STA  103 . Here, F 1202  and F 1203  are performed after the SIFS has elapsed from F 1201 . In F 1204 , the AP  102  transmits an NDP to the STA  103 . In F 1205 , the AP  105  transmits an NDP to the STA  103 . Here, F 1204  and F 1205  are performed after the SIFS has elapsed from F 1202  (F 1203 ). 
     In F 1206 , the CSI calculation unit  314  of the STA  103  calculates the CSI. In F 1207 , the STA  103  transmits a frame including a CSI report field  900  in  FIG.  9    to the AP  102 . In F 1208 , the AP  102  performs sharing of the CSI to and from the AP  105 . Note that, this sharing may be performed when sharing data for the STA  103  between the AP  102  and the AP  105  or immediately prior to a transmission of data to the STA  103 . Note that a configuration may be taken so as to perform the transmission in F 1202  without performing the transmission in F 1201  or F 1203 . In such a case, the AP  105  can control an address and a timing for the NDP transmission in F 1205  in accordance with the content of a sounding TF in F 1201  from the AP  102 . 
     Simultaneous NDP Transmission (No BFRP TF) 
       FIG.  13    is a sequence in which the CSI processing capability of the STA  103  is determined to be “medium” (“medium” in step S 602 ) and the sounding method that is selected in F 1112  in  FIG.  11    is “a case of a simultaneous NDP transmission (with BFRP TF)” (step S 604 , step S 605 ). The processing from F 1201  to F 1205  are the same as in the case of “simultaneous NDP transmission (no BFRP TF)” in  FIG.  12   . 
     The difference from  FIG.  12    is that the AP  102  transmits a BFRP TF (BeamForming Report Poll Trigger Frame) to the STA  103  in F 1301 . This BFRP TF can be achieved by a frame of the same format as the Trigger Frame  800  in  FIG.  8   . In such a case, the value of the Trigger Type  811  is “7”. Further, the AP  102  decides a padding for CSI (step S 604 ). As described above, the AP  102  decides the padding time for CSI so that the larger the CSI processing capability is in relation to the CSI processing capability that is classified in the same “medium” level, the shorter the padding time for CSI. Note that this means that the number of bits of the padding  807  in  FIG.  8    will further increase. The padding  807  is data whose bit sequence is all 1. The STA (terminal) that is analyzing the structure of the trigger frame indicated in  FIG.  8    can determine that the Per User Info  806  is ended in accordance with the data whose bit sequence is all 1. This determination is by the IEEE 802.11ax standard. 
     Further, the STA can allocate the calculation resource of the control unit  202  to another processing from the analysis of the trigger frame by this end determination. Note that the CSI calculation by the STA  103  in F 1302  in  FIG.  13    starting in the middle of a padding for CSI is a schematic representation of that. Here, in this sequence diagram, time elapses in a direction from the top to the bottom. Also, the length of F 1302  in a longitudinal direction being longer than the length of F 1206  in a longitudinal direction indicates that the CSI calculation capability in the STA  103  is lower. 
     By such a configuration of the BFRP TF, the STA  103  in F 1207  can perform a CSI transmission after the SIFS elapses from the end of the padding  807  including a padding for CSI. The processing in F 1208  is the same as in a case of “simultaneous NDP transmission (no BFRP TF)” in  FIG.  12   . 
     Note that in  FIG.  13   , the STA  103  starts a CSI calculation in F 1302  after receiving the BFRP TF in F 1301 . The flow of this processing is an example and a configuration may be taken such that the STA  103  starts a CSI calculation after receiving an NDP in F 1204  and F 1205 , temporarily interrupt the calculation during the reception of BFRP TF in F 1301 , and then resume the calculation after determining that the Per User Info  806  is ended. 
     Sequential NDP Transmission 
       FIG.  14    is a sequence in which the CSI processing capability of the STA  103  is determined to be “low” (“low” in step S 602 ) and the sounding method that is selected in F 1112  in  FIG.  11    is “a case of a sequential NDP transmission” (step S 606 ). The processing in F 1201  is the same as in a case of “simultaneous NDP transmission (no BFRP TF)” in  FIG.  12   . In F 1401 , the AP  102  transmits an NDPA (NDP Announcement) to the STA  103 . In F 1402 , the AP  105  transmits the NDPA to the STA  103 . In F 1403 , the AP  102  transmits the NDPA to the STA  103 . In F 1404 , the STA  103  calculates and then holds the CSI. In F 1405 , the AP  105  transmits an NDP to the STA  103 . As described above, “in a case of a sequential NDP transmission”, the AP  102  (Master-AP) and the AP  105  (Slave-AP) transmit an NDP to the STA  103  at their respective timings (F 1403 , F 1405 ). 
     In F 1406 , the STA  103  calculates and then holds the CSI. In F 1407 , the AP  102  transmits the BFRP TF of the STA  103 . In this BFRP TF, the padding for CSI that is included in the BFRP TF that is transmitted by the AP  102  in F 1301  in  FIG.  13    is not added. The reason is that it is assumed that in the STA  103 , a CSI calculation that is related to one NDP is possible within the SIFS time. The processing from F 1207  and F 1208  are the same as in the case of “simultaneous NDP transmission (no BFRP TF)” in  FIG.  12   . 
     Note that the above embodiment is something to be applied in all multi-AP coordination configurations; however, a configuration may be taken so as to limit the use case to a case of null steering or a JTX (Joint Transmission) that uses a D-MIMO (Distributed Multiple Input Multiple Output). 
     Also, in any of the cases in  FIG.  12    to  FIG.  14   , data transmission to the STA  103  is performed by control of the AP  102  after the AP  102  and the AP  105  have received the CSI report (step S 608 ). 
     As described above, each AP can obtain the CSI from the STA at a desired timing without reducing the amount of information by performing the sounding processing by a method decided in accordance with the CSI processing capability of the STA. By this, it becomes possible to achieve high-speed/high-efficiency/stable wireless communication by the multi-AP coordination configuration. Also, by executing sounding without reducing the CSI, it becomes possible to achieve improvement in usage efficiency of a wireless medium and system-wide and individual communication speed and stability which are the aims of the IEEE802.11be standard. 
     Other Embodiments 
     Embodiment(s) of the present invention can also be realized by a computer of a system or apparatus that reads out and executes computer executable instructions (e.g., one or more programs) recorded on a storage medium (which may also be referred to more fully as ‘non-transitory computer-readable storage medium’) to perform the functions of one or more of the above-described embodiment(s) and/or that includes one or more circuits (e.g., application specific integrated circuit (ASIC)) for performing the functions of one or more of the above-described embodiment(s), and by a method performed by the computer of the system or apparatus by, for example, reading out and executing the computer executable instructions from the storage medium to perform the functions of one or more of the above-described embodiment(s) and/or controlling the one or more circuits to perform the functions of one or more of the above-described embodiment(s).The computer may comprise one or more processors (e.g., central processing unit (CPU), micro processing unit (MPU)) and may include a network of separate computers or separate processors to read out and execute the computer executable instructions. The computer executable instructions may be provided to the computer, for example, from a network or the storage medium. The storage medium may include, for example, one or more of a hard disk, a random-access memory (RAM), a read only memory (ROM), a storage of distributed computing systems, an optical disk (such as a compact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)™), a flash memory device, a memory card, and the like. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2020-002435, filed Jan. 9, 2020, which is hereby incorporated by reference herein in its entirety.