Patent Publication Number: US-2023144318-A1

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

Description:
BACKGROUND OF THE INVENTION 
     Cross-Reference to Related Applications 
     This application is a Continuation Application of U.S. Pat. Application No. 17/547,683, filed Dec. 10, 2021, which is a Continuation Application of U.S. Pat. Application No. 16/838,146, filed Apr. 2, 2020, which claims the benefit of Japanese Patent Application No. 2019-075030, filed Apr. 10, 2019, the entire disclosures of which are hereby incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a communication control technique in a wireless LAN. 
     DESCRIPTION OF THE RELATED ART 
     Standards of the IEEE 802.11 series are known as communication standards related to a wireless LAN (Wireless Local Area Network). An 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. At present, in order to further improve throughput, a Study Group called IEEE 802.11 EHT (Extreme (or Extremely) High Throughput) has been launched in the IEEE as a successor standard to the IEEE 802.11ax standard. As a throughput improvement measure aimed at by IEEE 802.11 EHT, a Multi-AP Coordination configuration in which a plurality of APs (access points) operate by coordinating with each other has been studied. In the multi-AP coordination configuration, since a plurality of APs operate by coordinating with each other, it is possible to perform communication with a connected wireless LAN terminal where the communication is higher speed or stabler than in the case of a single AP. There are a plurality of technical methods for realizing a multi-AP coordination configuration. 
     Distributed MIMO (Multiple-Input Multiple-Output) (D-MIMO) communication has also been proposed in a wireless LAN environment that conforms to standard of the IEEE 802.11 series (US-2018-263045). D-MIMO is a technique in which a plurality of APs communicate with one wireless LAN terminal at the same timings on the same frequency channel, in which high-speed communication can be realized by multiplexed usage of space. 
     As described above, there are a plurality of technical methods for realizing a multi-AP coordination configuration. However, it has so far not been proposed how to decide a technical method for realizing a multi-AP coordination configuration in accordance with, for example, a communication condition between communication apparatuses that communicate in the multi-AP coordination configuration. 
     SUMMARY OF THE INVENTION 
     In view of the above problems, the present disclosure provides a technique for appropriately deciding a technical method for realizing a multi-AP coordination configuration. 
     According to one aspect of the present invention, there is provided a communication apparatus which comprises: a first determination unit configured to determine whether or not the communication apparatus and one or more other communication apparatuses capable of performing coordinated communication by coordinating with the communication apparatus to use a predetermined frequency band can obtain channel state information from one or more communication partner apparatuses that perform the coordinated communication; and a selection unit configured to select a communication method for performing the coordinated communication from among a plurality of communication methods based on a result of the determination by the first determination unit. 
     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    shows an example of a network configuration. 
         FIG.  2    shows an example of a hardware configuration of an AP. 
         FIG.  3    shows an example of a functional configuration of the AP. 
         FIG.  4    is a flow chart of processing executed by an AP. 
         FIG.  5    shows schematic diagrams for describing configurations according to some multi-AP coordination methods. 
         FIG.  6    is a sequence chart for describing channel state confirmation and a null steering configuration. 
         FIG.  7    is a sequence chart for describing a JTX (D-MIMO) configuration. 
     
    
    
     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 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. 
     Network Configuration 
       FIG.  1    shows a configuration example of a wireless communication network according to the present embodiment. A BSS (Basic Service Set) 1 (BSS1) network managed by an AP (Access Point)  102  is indicated by a two-dot line circle  101 . A BSS2 network managed by an AP  105  is indicated by a dashed-line circle  104 . STAs (stations)  103  and  106 , which are wireless LAN terminals, can maintain connection relationships with a plurality of APs. The APs  102  and  105  and the STAs  103  and  106  are devices (EHT devices) conforming to the IEEE 802.11 EHT (Extreme (or Extremely) High Throughput) standard. The AP  102  and the AP  105  correspond to a multi-AP coordination operation (have a multi-AP coordination function). Between APs having the multi-AP coordination function, one AP can perform coordinated communication by coordinating using a predetermined frequency band with other APs. As a result, higher speed or more stable communication can be realized with respect to a connected wireless LAN terminal than in the case of one AP. Here, a stable state is any combination of a good signal-to-noise ratio, low interference, low delay, and low jitter. As a technical method for realizing the multi-AP coordination function (a multi-AP coordination method), there are various methods as will be described later. 
     A backhaul  100  is a communication means by which a plurality of APs managing networks of differing BSS communicate with each other. The backhaul  100  may be configured by a wired system such as Ethernet (registered trademark) or a telephone line, or may be configured by a wireless system such as LTE (Long-Term Evolution) or WiMAX (Worldwide Interoperability for Microwave Access). Alternatively, the backhaul  100  may be configured by a wireless LAN of a standard of the IEEE 802.11 series. When the backhaul  100  is configured by such a wireless LAN, this wireless LAN may be the same as or different from wireless channels used between the APs  102  and  105  and the STAs  103  and  106 . 
     It should be noted that the configuration of the wireless communication network shown in  FIG.  1    is merely an example for illustrative purposes, and, for example, a network including a large number of EHT devices and legacy devices (communication apparatuses that comply with an IEEE 802.11a/b/g/n/ax standard) in a wider area may be configured. Further, there is no limitation to the arrangement of each communication apparatus shown in  FIG.  1   , and the following discussion can be applied to the positional relationship of various communication apparatuses. 
     AP Configuration 
       FIG.  2    is a block diagram showing a hardware configuration of the AP  102 . The AP  105  has a similar hardware configuration to that of the AP  102 . The AP  102  includes, as an example of its hardware configuration, a storage unit  201 , a control unit  202 , a function unit  203 , an input unit  204 , an output unit  205 , a communication unit  206 , and one or more antennas  207 . 
     The storage unit  201  is configured by a memory such as a ROM or a RAM, and stores a program for performing various operations to be described later, and various information such as communication parameters for wireless communication. 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, CD-R, a magnetic tape, a nonvolatile memory card, a DVD, or 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 or the like. 
     The control unit  202  is configured by, for example, a processor such as a CPU or and MPU, an ASIC (application specific integrated circuits), a DSP (digital signal processor), an FPGA (field programmable gate array) or the like. Here CPU is an acronym for Central Processing Unit and MPU is an acronym for Micro Processing Unit. The AP is controlled by executing a program stored in the storage unit  201 . The control unit  202  may control the AP  102  in accordance with cooperation between a program stored in the storage unit  201  and an OS (Operating System). In addition, the control unit  202  may include a plurality of processors such as multi-cores to control the AP  102 . The control unit  202  controls the function unit  203  to execute predetermined processing such as imaging, printing, and projection. The function unit  203  is hardware for the AP  102  to execute predetermined processing (which can include imaging, printing, projection, and the like). 
     The input unit  204  accepts various operations from a user. The output unit  205  performs various outputs to a user. Here, output by the output unit  205  includes at least one of display on a screen, sound output by a speaker, vibration output, and the like. It should be noted that both the input unit  204  and the output unit  205  may be realized by one module as in a touch panel. 
     The communication unit  206  performs control of wireless communication conforming to a standard of the IEEE 802.11 series, control of wireless communication conforming to Wi-Fi (registered trademark), and control of IP (Internet Protocol) communication. Further, the communication unit  206  controls the one or more antennas  207  to transmit and receive wireless signals for wireless communication. The one or more antennas  207  can also be configured to enable D-MIMO (Distributed Multiple-Input Multiple-Output) communication as described below. 
       FIG.  3    is a block diagram showing an example of a functional configuration of the AP  102 . The AP  105  has a similar functional configuration to that of the AP  102 . The AP  102  includes, as an example of its functional configuration, a wireless LAN control unit  301 , a UI control unit  302 , a selection unit  303 , a Single-AP configuration control unit  304 , a JTX configuration control unit  305 , a null steering configuration control unit  306 , a Coordinated OFDMA configuration control unit  307 , a Fractional Coordinated OFDMA configuration control unit  308 , and a schedule adjustment configuration control unit  309 . 
     The wireless LAN control unit  301  is configured by including a circuit for transmitting and receiving wireless signals to and from another wireless LAN apparatus (for example, another AP or STA) and a program for controlling the circuit. The wireless LAN control unit  301  performs wireless LAN communication control, such as frame generation and frame transmission, reception of a wireless frame from another wireless LAN apparatus, and the like, in accordance with an IEEE 802.11 standard series. The wireless LAN control unit  301  also has a function of analyzing received wireless frames and determining whether or not a predetermined condition is satisfied based on information included in the wireless frames. 
     The UI control unit  302  receives an operation with respect the input unit  204  ( FIG.  2   ) by a user (not shown) of the AP  102 , and performs control for conveying a control signal corresponding to the operation to respective components and control for output (including display) with respect to the output unit  205  ( FIG.  2   ). 
     The selection unit  303  selects (decides) a communication method in accordance with the AP  102  in response to a result of the analysis/determination by the wireless LAN control unit  301 . Details of the operation of the selection unit  303  will be described later with reference to  FIG.  4   . When a Single-AP method is selected by the selection unit  303 , the Single-AP configuration control unit  304  performs communication control for realizing a configuration according to the method. When a JTX (Joint Transmission) method is selected by the selection unit  303 , the JTX configuration control unit  305  performs communication control for realizing a configuration according to the method. When a null steering method is selected by the selection unit  303 , the null steering configuration control unit  306  performs communication control for realizing a configuring according to this method. When a Coordinated OFDMA method is selected by the selection unit  303 , the Coordinated OFDMA configuration control unit  307  performs communication control for realizing a configuration according to the method. When a Fractional Coordinated OFDMA method is selected by the selection unit  303 , the Fractional Coordinated OFDMA configuration control unit  308  performs communication control for realizing the configuration according to the method. When a schedule adjustment method is selected by the selection unit  303 , the schedule adjustment configuration control unit  309  performs communication control for realizing a configuration according to this method. The operation of the Single-AP configuration control unit  304 , the JTX configuration control unit  305 , the null steering configuration control unit  306 , the Coordinated OFDMA configuration control unit  307 , the Fractional Coordinated OFDMA configuration control unit  308 , and the schedule adjustment configuration control unit  309 , which are function units, will be described later. 
     STA Configuration 
     The hardware configuration of the STAs  103  and  106 , which are communication partner apparatuses of the APs  102  and  106 , can be a similar configuration to the hardware configuration of the AP  102  ( FIG.  2   ). That is, the STAs  103  and  106  can each be configured to include a storage unit  201 , a control unit  202 , a function unit  203 , an input unit  204 , an output unit  205 , a communication unit  206 , and one or more antennas  207 . Although the functional configuration of the STAs  103  and  106  is not illustrated, it can be configured so that communication is possible by a configuration according to a communication method selected by the AP  102 . 
     Processing Flow 
     Next, a flow of processing executed by the AP configured as described above will be described.  FIG.  4    shows a flow chart of a communication method selection process executed by the AP  102 . The processing shown in  FIG.  4    can be performed at a timing when the AP  102  establishes a BSS1 network (circle  101  in  FIG.  1   ) or at any time during BSS1 network operation. The flow chart shown in  FIG.  4    can be realized by the control unit  202  of the AP  102  executing a control program stored in the storage unit  201 , executing calculation and processing of information, and executing control of respective hardware. 
     In step S 401 , the wireless LAN control unit  301  of the AP  102  performs confirmation and searches for whether there is an AP (a peripheral AP) that is present in the periphery of the AP  102 . Here, a peripheral AP refers to an AP capable of wired or wireless communication with the AP  102 . A procedure for confirmation and searching include a passive procedure for receiving a Beacon frame in wireless communication or receiving a frame by broadcast/multicast in wired communication, and an active procedure for transmitting an inquiry frame. In the case of the network configuration of  FIG.  1   , the AP  102  discovers the AP  105  as a peripheral AP as a result of the confirmation and the search. 
     If a peripheral AP cannot be discovered (No in step S 401 ), the process proceeds to step S 403 . In step S 403 , the selection unit  303  selects the Single-AP method, and ends the process. A Single-AP configuration is a configuration where, in a case where an AP operates and manages a BSS network, the control of another AP is not required. When the Single-AP configuration is selected, the Single-AP configuration control unit  304  of the AP  102  performs control for independently managing the BSS1 network (circle  101  in  FIG.  1   ) without control by the AP  105 . 
     If it was possible to discover a peripheral AP (Yes in step S 401 ), the process proceeds to step S 402 . In step S 402 , the wireless LAN control unit  301   determines whether or not the discovered peripheral AP supports multi-AP coordination operation (whether or not the discovered peripheral AP has a multi-AP coordination function). The wireless LAN control unit  301  can determine whether or not the peripheral AP supports multi-AP coordination operation based on an information element regarding capabilities or the like included in a frame received at the time of the operation for searching for the peripheral AP in step S 401 . As described above, in the case of the network configuration of  FIG.  1   , the AP  102  and the AP  105  support multi-AP coordination operation. If the peripheral AP does not support multi-AP coordination operation (NO in step S 402 ), the process proceeds to step S 403 , and the selection unit  303  selects the Single-AP method, and ends the process. 
     If the peripheral AP supports multi-AP coordination operation (Yes in step S 402 ), the process proceeds to step S 404 . In step S 404 , the wireless LAN control unit  301  determines whether or not the self AP (AP  102 ) and the peripheral AP can obtain CSI (Channel State Information) from all the STAs connected to these APs. In the case of the network configuration of  FIG.  1   , it is determined whether the AP  102  can obtain channel information with respect to the STA  106  in addition to with respect to the STA  103 , and whether the AP  105  can obtain channel information with respect to the STA  103  in addition to with respect to the STA  106 . The CSI can be obtained by a sounding procedure using an NDP (Null Data Packet) or a beamforming procedure. In a CSI Report field of an IEEE 802.11 standard, SNR (Signal-to-Noise Ratio) and CSI Matrix are defined as information that can be obtained by this process. 
     Here, an example of a condition under which CSI can be obtained will be described. The first condition is that a certain AP and all STAs connected to this AP and the peripheral AP are within a radio wave reach range. Note that in addition to merely a radio wave arriving, configuration may be taken to add a condition that an RSSI (Received Signal Strength Indicator) or a CQI (Channel Quality Indicator) is equal to or higher than a predetermined value. The second condition is that the STA can transmit data to another peripheral AP in a state where the STA is under the management of one AP. Here, the term “under the management” means that an Association (connection) of a standard of the IEEE 802.11 series is established. Thus, the function of establishing connections with a plurality of APs at the same time is called a Multi-AP Association function. Generally, the AP grasps whether or not an STA supports Multi-AP Association by exchanging an Information element relating to capabilities when the AP connects to the STA. Further, APs that support the multi-AP coordination operation can exchange therebetween information as to whether each terminal supports Multi-AP Association. A condition under which CSI can be obtained is not limited to the above first condition and/or the second condition, and other conditions may be used. For example, there is a condition that Implicit beamforming can be used. Implicit beamforming refers to beamforming without sounding by an NDP (treated as a predetermined channel state). Implicit beamforming can be made usable, for example, when a predetermined setting is made by a user via the input unit  204  ( FIG.  2   ) or when an RSSI for communication with the communication partner apparatus is good. If the condition for using Implicit beamforming is met (if Implicit beamforming is available), the AP can process subsequent transmissions as if the AP was able to obtain a channel state having a predetermined value without sounding in accordance with an NDP. 
     If it is determined in step S 404  that the CSIs of all the STAs can be obtained (Yes in step S 404 ), the process proceeds to step S 405 , otherwise (No in step S 404 ), the process proceeds to step S 408 . In the case of yes in step S 404 , a communication method that uses CSI is to be selected, and in the case of no in step S 404 , a communication method that does not use CSI is to be selected. 
     In step S 405 , the wireless LAN control unit  301  confirms the connection state between the AP  102  and the peripheral AP, and determines whether or not a high-speed connection is possible. Here, the state in which high-speed connection is possible means a state in which communication between the AP and the peripheral AP is possible independently without affecting communication between the AP and the STA. A first example of satisfying this state is that the backhaul  100  for communication between the AP and the peripheral AP is wired communication such as Ethernet (registered trademark) or xDSL (some kind of Digital Subscriber Line), or public wireless communication such as LTE (Long-Term Evolution) or WiMAX (Worldwide Interoperability for Microwave Access). A second example of satisfying this state is that the AP and the peripheral AP support Multi-band or Multi-channel (are connected via Multi-band or Multi-channel). Here, supporting Multi-band means being able to simultaneously communicate in a plurality of operating frequency bands of a wireless LAN. Thus, for example, communication using a 2.4 GHz band between the AP and the STA and using a 5 GHz band or a 6 GHz band between the AP and the peripheral AP is included. Here, supporting Multi-channel means being able to simultaneously communicate in a plurality of frequency channels of a wireless LAN. Therefore, for example, in the single band of the 5 GHz band, between the AP and the STA includes communication using four channels of W52 (36 ch, 40 ch, 44 ch, 48 ch), and between the AP and the peripheral AP includes communication using 11 channels of W56 (100 ch, 104 ch, ..., 140 ch). 
     If it is determined in step S 405  that a high-speed connection is possible (Yes in step S 405 ), the process proceeds to step S 406 . In step S 406 , the selection unit  303  selects the JTX (Joint Transmission) method as the multi-AP coordination configuration method, and ends the processing. If it is not determined in step S 405  that a high-speed connection is possible (No in step S 405 ), the process proceeds to step S 407 . In step S 407 , the selection unit  303  selects the null steering method as the multi-AP coordination configuration method, and ends the processing. It should be noted that null steering is sometimes referred to as a zero point forming of a beam. 
     If it is determined in step S 404  that it is not possible to obtain the CSI of every STA (No in step S 404 ), the processing proceeds to step S 408 . In step S 408 , the wireless LAN control unit  301  determines whether or not it is possible to adjust a communication schedule using a predetermined frequency band in time division. The wireless LAN control unit  301  can determine whether or not the communication schedule can be adjusted based on the information element regarding the capabilities exchanged with the peripheral AP. In addition, in the system, whether or not each AP can adjust the communication schedule may be set in advance. If it is determined that adjustment of the communication schedule is possible (Yes in step S 408 ), the processing proceeds to step S 409 . In step S 409 , the selection unit  303  selects the schedule adjustment method as the multi-AP coordination configuration method, and ends the processing. 
     If it is determined in step S 408  that adjustment of the communication schedule is not possible (No in step S 408 ), the process proceeds to step S 410 . In step S 410 , the wireless LAN control unit  301  determines whether or not there is an interference limited STA (terminal). Here, an interference limited STA refers to an STA that is affected by communication from an AP to which the STA itself is not connected (also belongs to a network of another BBS that overlaps with a network of the BBS to which the STA itself belongs). In the case of the network configuration of  FIG.  1   , both the STA  103  and the STA  106  are interference limited STAs. An unaffected STA is referred to as a Non-Interference limited STA. For example, in  FIG.  1   , if the STA  106  is outside the BBS1 network (circle  101 ) and inside the BBS2 network (circle  104 ), the STA  106  becomes a non-interference limited STA. The wireless LAN control unit  301  can perform the determination of step S 410  by receiving reports from the respective STAs that the position information and the strength/quality of a received frame are greater than or equal to a predetermined level. For example, when each STA receives, at strength/quality greater than or equal to the predetermined level, a frame from an AP (a network of another BSS) different from the AP to which the STA is connected (a network of a BSS to which the STA belongs), the STA can report the information to the AP to which the STA is connected. Further, it is assumed that the information is shared among the APs. The wireless LAN control unit  301  receives the information to thereby be able to perform the determination of step S 410 . An example of strength/quality is RSSI (Received Signal Strength Indicator). 
     If it is determined that there is an interference limited STA (Yes in step S 410 ), the process proceeds to step S 411 . In step S 411 , the selection unit  303  selects the Coordinated OFDMA method as the multi-AP coordination configuration method, and ends the process. If it is determined in step S 410  that there is no interference limited STA (No in step S 410 ), the process proceeds to step S 412 . In step S 412 , the selection unit  303  selects the Fractional Coordinated OFDMA method as the multi-AP coordination configuration method, and ends the processing. 
     Next, a configuration according to the multi-AP coordination method selected by the selection unit  303  will be described.  FIG.  5    shows schematic diagrams for describing configurations according to some multi-AP coordination method, where an configuration  5   a  shows a Coordinated OFDMA configuration (step S 411 ), An configuration  5   b  shows a Fractional Coordinated OFDMA configuration (step S 412 ), and an configuration  5   c  shows a schedule adjustment configuration (step S 409 ). An configuration  5   d  will be described later. Note that, as a notation common to the configurations  5   a  to  5   d , the horizontal axis represents time and the vertical axis represents frequency. Here, the absolute section length of the time axis and the granularity/unit of time can vary depending on the use case of the multi-AP coordination configuration. For example, it may be set to be in milliseconds, seconds, or more numbers or units related to human sensitivities or operations, from the microseconds of a TU (Time Unit) of IEEE 802.11. The granularity and units of the frequency axis are, for example, RUs (Resource Unit) which are units of a frequency band of communication in OFDMA defined by IEEE 802.11ax. However, depending on the capabilities of the APs and the STAs, the granularity and units may be bands (frequency bands) that can be used in Multi-band communication or channels that can be used in Multi-channel communication. Solid line rectangles indicate the time axis/frequency region used by the AP  102  (BSS1 network), and dot-dash line rectangles indicate the time axis/frequency region used by the AP  105  (BSS2 network). 
     The configuration  5   a  shows a Coordinated OFDMA configuration. This configuration is realized by the Coordinated OFDMA configuration control unit  307 . In this configuration, RUs are clearly separated between a plurality of BSSs (AP and STA). That is, the RUs used by the AP  102 , the STA  103 , the AP  105 , and the STA  106  in communication do not overlap. Edge STA in  FIG.  5 A  is synonymous with interference limited STA. 
     The configuration  5   b  shows a Fractional Coordinated OFDMA configuration. This configuration is realized by the Fractional Coordinated OFDMA configuration control unit  308 . In this configuration, some or all of the RUs used between a plurality of BSSs (AP and STA) may overlap. In the drawings, two rectangles are shown staggered for the sake of clarity, but they may be completely overlapped with each other. Thus, Fractional means that the frequencies are not completely divided, i.e. that they are used in places (intermittently) as in OFDMA. In addition, “center STA” of  FIG.  5 B  is a synonym for a non-interference limited STA. 
     The configuration  5   c  shows a schedule adjustment configuration. This configuration is realized by the schedule adjustment configuration control unit  309 . In this configuration, the times of communication for the BSS1 network (AP  102  and the STA  103 ) and the BSS2 network (AP  105  and the STA  106 ) are clearly separated. That is, the BSS1 network and the BSS2 network use the same frequency band in time division. As described above, units for TU1 (Time Unit) and TU2 in the drawing may be any amount of time as long as they can be adjusted between APs. 
     According to the communication according to the configurations of  FIGS.  5 A to  5 C , the time resource or the frequency resource (resource unit, band, channel) can be effectively used in accordance with coordination between the plurality of APs and the STA, which leads to efficient use of the wireless medium. 
       FIG.  6    is a sequence diagram for describing null steering configuration (step S 407 ). This configuration is realized by the null steering configuration control unit  306 . In this method, firstly each AP obtains the CSI of communication with the STA. F 601  is a procedure of confirming the channel state between the AP  102  and the STA  103 . As described above, as an example, the AP transmits an NDP, the STA estimates the channel state in response to reception of the NDP, and feeds back channel information as the CSI. Similarly, channel state confirmation procedures are executed for the AP  105  and the STA  103  at F 602 , for the AP  102  and the STA  106  at F 603 , and for the AP  105  and the STA  106  at F 604 . At F 605 , respective channel state confirmation information are exchanged and shared between the AP  102  and the AP  105 . This information exchange also makes it possible for a plurality of APs to perform DL (downlink) MU (multi-user) operations from the AP to a plurality of STAs at the same time. Further, in the obtainment of the CSI, the state of transmission from an STA to an AP may also be confirmed. This information is used when performing an UP (uplink) MU (multi-user) operation from a plurality of STAs to an AP, as a multi-AP coordination operation. 
     Note that null steering configuration is sometimes referred to as Coordinated BF (Beam Forming) or Coordinated BF and Nulling because it directs a null point of the beam of beamforming. 
     Next, at F 606 , a null steering TF (Trigger Frame) is sent from the AP  102  to the AP  105 . The TF is for measuring a timing the next transmission operation. Here, it is assumed that the negotiation of which of the AP  102  and the AP  105  transmits the TF is performed according to the procedure for F 605 . At F 607  after the passage of an SIFS (Short Inter Frame Space) or after the passage of another predetermined amount of time after F 606 , a data frame addressed to the STA  103  is transmitted from the AP  102 . Since this data frame is a null point of the beam for the STA  106 , the data frame does not affect the reception operation of the STA  106 . Similarly, a data frame addressed to the STA  106  is transmitted from the AP  105  at F 608 . This data frame has a null point for the beam at the STA  103 , and thus does not affect the reception operation of the STA  103 . 
     By communication according to this null steering configuration, the time resource or the frequency resource (resource unit, band, channel) can be used without division in accordance with coordination between the plurality of APs and the STA, which leads to efficient use of the wireless medium. 
       FIG.  7    is a sequence diagram for describing a JTX configuration (step S 406 ). Here, the JTX configuration refers to a configuration in which a plurality of APs transmit data to one STA. This configuration is realized by the JTX configuration control unit  305 . One wireless technology for realizing JTX is D-MIMO. Here, D-MIMO is an abbreviation of Distributed MIMO or Distributed MU MIMO. D-MIMO is a technique in which a plurality of access points communicate with one STA at the same time on the same channel (or OFDMA RU: Resource Unit) and realizes high-speed communication through multiplexed usage of space. 
     In the D-MIMO method, firstly each AP obtains the CSI of communication with the STA. This is similar to the processing for F 601  to F 604  of  FIG.  6   , and description thereof is omitted. Next, at F 701 , a procedure for deciding a master AP is executed between the AP  102  and the AP  105 . A master AP is an AP that controls D-MIMO operation and is also referred to as an M-AP (master access point). An S-AP (slave access point) executes JTX under the control of the M-AP. In the present example, it is assumed that the AP  102  is decided as the master AP, and the AP  102  starts operation as the master AP at F 702 . Also, at F 703 , the AP  105  starts operation as the slave AP. 
     At F 704 , data is shared between the AP  102  and the AP  105  through the backhaul  100 . By such communication via the backhaul, “communication between the APs” and “communication between the AP and the STA” can be performed in parallel. At F 705 , the AP  102  sends a JTX TF (Trigger Frame) to the AP  105 . This JTX TF is for designating that transmission by the AP  105  be activated and a timing for when to do so. At F 706 , the AP  102  transmits data to the STA  106 , and at F 707 , the AP  105  transmits data to the STA  106 , where timings for F 706  and F 707  are synchronized. Similarly, the AP  102  transmits data to the STA  103  at F 708 , and the AP  102  transmits data to the STA  106  at F 709 , where timings for F 708  and F 709  are synchronized. Such synchronization is realized by a provision that transmission be performed after the passage of an SIFS (Short Inter Frame Space) of JTX TF. The specification of the transmission timing synchronization may include in the JTX TF an absolute amount of time or a relative time other than an SIFS. According to communication according to the JTX configuration, effective multiplex communication can be performed in one frequency band in a plurality of APs and STAs, and high-speed communication (high throughput communication) can be performed. 
     Configuration may be taken to transmit the same data at F 706  and F 708  or at F 707  and F 709 . By coordinated operation of a plurality of APs in this manner, although the processing may be redundant, an effect of improving the reliability and stability of communication will arise. 
     First Variation 
     As an example of JTX, there is a case where one STA is a target of coordinated operation of a plurality of APs. The configuration shown in  FIG.  1    corresponds to a case where the AP  102  and the AP  105  communicate with the STA  106  by D-MIMO. Referring to  FIG.  7   , at F 710 , the AP  102  sends to the AP  105  data that is actually to be transmitted by JTX. At F 711 , the AP  102  sends a JTX TF (Trigger Frame) to the AP  105 . At F 712 , the AP  105  transmits data to the STA  106 . At F 713 , the AP  102  transmits data to the STA  106 . Here, the timings of F 712  and F 713  are synchronized for similar reasons as described above. 
     Note that in this variation, the AP  102  does not need a channel state with respect to the STA  103 . Therefore, configuration may be taken such that the determination in step S 404  of  FIG.  4    is based only on a condition relating to the AP  102  and the STA  106 . Also, this variation does not exclude a connection between the AP  102  and the STA  103 . Since the AP  102  is coordinated with the AP  105 , it can communicate with the STA  103  at a timing when the AP  105  does not transmit. This operation may be performed by Single-AP configuration control unit  304 . In addition, configuration may be taken to transmit respectively the same data at F 712  and F 713 . By coordinated operation of a plurality of APs in this manner, although it is redundant, gives rise to an effect of improving the reliability and stability of communication. 
     Second Variation 
     TPC (Transmission Power Control) may be used in combination with the schedule adjustment configuration (step S 409 ) described in the above embodiment. The configuration  5   d  in  FIG.  5    illustrates a schematic diagram of a schedule adjustment method with which transmission output control is used in combination. The solid line rectangles in the diagram are the AP  102 /BSS1 and the dashed-dotted lines are the AP  105 /BSS2 areas to be used, respectively. The filled areas indicates that TPC is present. In the configuration  5   d , the AP  102  that manages the BSS1 network sets the transmission power to a normal value in the TU1 section, and sets it to a value for limiting the transmission power in accordance with TPC in the TU2 section. The AP  105  that manages the BSS2 network sets the transmission power to a value limited by the TPC in the TU1 section, and sets the transmission power to a normal value in the TU2 section. Here, this transmission power control may be realized by applying a spatial reuse technique introduced from IEEE 802.11ax. 
     As described above, according to the embodiment described above, there is a contribution to the improvement of the use efficiency of the wireless medium, and the communication speed and the stability of the entire system and the individual communication apparatuses. 
     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 a ‘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. 2019-075030, filed Apr. 10, 2019, which is hereby incorporated by reference herein in its entirety.