Patent Publication Number: US-11647455-B2

Title: Communication apparatus and communication method for establishing protection areas for downlink communications

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 16/966,039, filed Jul. 30, 2020, which is based on PCT filing PCT/JP2018/045318, filed. Dec. 10, 2018, now U.S. Pat. No. 11,272,444, which claims priority to JP 2018-019807, filed Feb. 7, 2018, the entire contents of each are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The technology disclosed in the present specification relates to a communication apparatus and a communication method that avoid mutual interference between networks. 
     BACKGROUND ART 
     Recent years have seen a plurality of wireless base stations (access points: APs) for home use sold in a bundle as wireless LAN (Local Area Network) systems typified by IEEE802.11 have become pervasive, resulting, in some cases, in installation of a plurality of APs in homes. Although a single AP is not enough to deliver radio waves to every corner of the house, installing a plurality of APs allows for wireless connection from any place in the house. That is, a terminal needs only to connect to the nearest AP, thus holding promise for reducing the transmission distance between transmission and reception and contributing to improved frequency utilization efficiency. In a densely populated network environment such as that in which a plurality of APs coexists in a single space, however, operation of each network independently of other networks leads to increased mutual interference. 
     For example, a proposal has been made regarding a wireless communication system in which a master base station collects traffic information and channel information of terminals in a cooperative BSS (Basic Service Set) group and assigns wireless resources on the basis of traffic information and channel information of each terminal in such a manner as to provide an improved communication quality index to each cooperative BSS group (refer, for example, to PTL 1). 
     Also, in order to reduce interference in a network environment including a plurality of BSSs developed with high density, a proposal has been made regarding a system for a highly efficient wireless (HEW) access point (AP) cooperative protocol that allows for cooperation of one or a plurality of dimensions including time, frequency, space, and power between an AP and STAs (refer, for example, to PTL 2). 
     For example, it is possible to reduce mutual interference by coordinating schedules or controlling transmission power through cooperation between a plurality of APs installed in a home or a plurality of BSSs. However, it is difficult to reduce mutual interference between APs that are unable to cooperate with each other such as those installed in different homes. 
     CITATION LIST 
     Patent Literature 
     
         
         [PTL 1] 
         Japanese Patent Laid-Open No. 2016-19239 
         [PTL 1] 
         Japanese Patent Laid-Open No. 2016-537905 
       
    
     SUMMARY 
     Technical Problem 
     It is an object of the technology disclosed in the present specification to provide a communication apparatus and a communication method that avoid mutual interference between networks. 
     Solution to Problem 
     The technology disclosed in the present specification has been devised in consideration of the above problem, and a first aspect thereof is a communication apparatus that includes a communication section and a control section. The communication section sends and receives wireless frames. The control section controls transmission and reception of frames. The control section controls transmission, to one or more target stations and with a transmission parameter specified for each target station, of a trigger frame that induces transmission of a transmission refrainment frame. 
     The control section determines, as a transmission parameter for each target station, transmission power that allows the transmission refrainment frame to reach an interference source. The control section selects a transmission destination of a data frame as a target station and causes the trigger frame to be sent ahead of transmission of the data frame. 
     Also, the communication apparatus functions as a base station. The control section selects one or more subordinate terminals to which a downlink signal is to be sent as target stations and causes the trigger frame to be sent ahead of the transmission of the downlink signal. 
     Also, a second aspect of the technology disclosed in the present specification is a communication method that includes a step of selecting a target station to which to request transmission of a transmission refrainment frame, a step of determining a transmission parameter regarding the transmission refrainment frame for each target station, and a step of sending, to the target station, a trigger frame that induces the transmission of a transmission refrainment frame with the determined transmission parameter. 
     Also, a third aspect of the technology disclosed in the present specification is a communication apparatus that includes a communication section and a control section. The communication section sends and receives wireless frames. The control section controls transmission and reception of frames. The control section causes, in response to reception of a trigger frame that induces transmission of a transmission refrainment frame, the transmission refrainment frame to be sent with a transmission parameter specified by the trigger frame. 
     The control section causes a transmission refrainment frame to be sent with transmission power specified as a transmission parameter or in such a manner as to achieve a reach range specified as a transmission parameter. 
     Also, the communication apparatus functions as a subordinate terminal of a base station, and the control section causes a transmission refrainment frame to be sent in response to the trigger frame received from the base station. 
     A fourth aspect of the technology disclosed in the present specification is a communication method that includes a step of receiving a trigger frame that induces transmission of a transmission refrainment frame and a step of sending a transmission refrainment frame with a transmission parameter specified by the trigger frame. 
     Advantageous Effects of Invention 
     The technology disclosed in the present specification can provide a communication apparatus and a communication method that streamlines downlink communication by avoiding mutual interference between networks. 
     It should be noted that the effect described in the present specification is merely illustrative and that effects of the present invention are not limited thereto. Also, the present invention may bring about further additional effects in addition to the above effect. 
     Still other objects, features, and advantages of the technology disclosed in the present specification will become apparent from detailed description based on embodiments described later and attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a diagram illustrating a communication environment to which the technology disclosed in the present specification is applied. 
         FIG.  2    is a diagram illustrating an example of a communication sequence for carrying out downlink communication in an AP_B network. 
         FIG.  3    is a diagram illustrating an example of a protection area determined by the AP_B in the network topology illustrated in  FIG.  1   . 
         FIG.  4    is a flowchart illustrating an example of a processing procedure for a base station (AP) to send a trigger frame to its subordinate terminal (STA) ahead of transmission of a downlink signal. 
         FIG.  5    is a flowchart illustrating an example of a processing procedure for sending a transmission refrainment frame in response to a trigger frame received from the base station (AP) to which the terminal (STA) is connected. 
         FIG.  6    is a diagram illustrating a configuration example of a communication apparatus  600 . 
     
    
    
     DESCRIPTION OF EMBODIMENT 
     A detailed description will be given below of an embodiment of the technology disclosed in the present specification with reference to drawings. 
     As wireless LAN systems typified by IEEE802.11 have become pervasive, a plurality of wireless base stations may be sold in a bundle and a plurality of APs may be installed in homes. While reducing the transmission distance between transmission and reception and contributing to improved frequency utilization efficiency, the coexistence of a plurality of APs in a single space as described above raises concern over possible increase in mutual interference between networks. 
     For example, it is possible to reduce mutual interference by coordinating schedules or controlling transmission power through cooperation between a plurality of APs installed in a home or a plurality of BSSs. However, it is difficult to reduce mutual interference between APs that are unable to cooperate with each other such as those installed in different homes. Increased mutual interference leads to reduced communication efficiency and frequency utilization efficiency. 
     Also, in an infrastructure communication mode in which one or more terminals (STAtions: STAs) operate under an AP, transmission demand is generally higher in downlink communication than in uplink communication. 
     For this reason, the present specification proposes below a wireless communication technology that contributes to improved frequency utilization efficiency by streamlining downlink communication having higher transmission demand in a densely populated network environment. The wireless communication technology disclosed in the present specification is suitably applicable not only to a communication environment where networks cooperating with each other are densely populated but also to a communication environment where networks unable to cooperate with each other coexist, thus streamlining downlink communication having higher transmission demand and contributing to improved frequency utilization efficiency. 
     Here, a communication environment (multi-AP environment) where a plurality of APs exists as illustrated in  FIG.  1    is considered. A plurality of APs, namely, AP_A, AP_B, AP_X, and AP_Y, coexists in the communication environment illustrated, and the networks (BSSs) established by the respective APs partially overlap with each other. 
     Of the plurality of these APs, the AP_A and the AP_B cooperate with each other. For example, the AP_A and the AP_B have been sold in a bundle and installed in a home. Therefore, the AP_A network and the AP_B network can avoid mutual interference by coordinating transmission schedules and controlling the transmission power. That is, the AP_A network does not constitute an interference source for the AP_B network. 
     Also, the AP_X and the AP_Y do not cooperate with the AP_B. For example, the AP_X and the AP_X are not products sold in a bundle with the AP_B. For example, it is imaginable that radio waves output from the AP_X and the AP_Y installed in the house next door penetrate through walls and reach the AP_B network. For this reason, the AP_X and AP_Y networks constitute interference sources for the AP_B network. 
     Here, three terminals STA_B 1 , STA_B 2 , and STA_B 3 , are connected to the AP_B. Also, a terminal STA_A is connected to the AP_A that cooperates with the AP_B. In the example illustrated, the STA_A is located near the STA_B 1 , a subordinate terminal of the AP_B. The STA_A and the STA_B 1  are located within their communication ranges, and therefore, mutual interference can take place. However, when the AP_B wishes to communicate with the STA_B 1 , its subordinate terminal, the AP_B can avoid interference by coordinating schedules with the AP_A. It should be noted that when the AP_A wishes to communicate with the STA_A, its subordinate terminal, the AP_A can similarly avoid interference by coordinating schedules with the neighboring cooperative network. 
     On the other hand, terminals STA_X and STA_Y are connected, respectively, to the AP_X and the AP_Y that constitute interference sources for the AP_B network. In the example illustrated, the STA_X is located near the terminal STA_B 2 , a subordinate terminal of the AP_B, and the STA_Y is located near the terminal STA_B 3 , a subordinate terminal of the AP_B. For this reason, the STA_B 2  and the STA_X are located within their communication ranges, and therefore, mutual interference can take place, and the STA_B 3  and the STA_Y are located within their communication ranges, and therefore, mutual interference can take place. 
     The case where the AP_B wishes to send a downlink signal to each of the subordinate terminals STA_B 1 , STA_B 2 , and STA  3  is now investigated. 
     The STA_A and the STA_B 1  are located within their communication ranges, and therefore, mutual interference can take place. However, it is possible to avoid mutual interference through schedule coordination between the AP_A and the AP_B that cooperate with each other. When the AP_B wishes to send a downlink signal to the STA_B 1 , its subordinate terminal, it is possible to avoid interference through schedule coordination between the two networks such that the AP_A refrains from communicating with the STA_A, its subordinate terminal. Specifically, the AP_B sends a downlink signal to the STA_B 1  at a timing when the STA_A does not send any signal on the basis of schedule information within the AP_A network. Alternatively, the AP_A schedules a timing for permitting transmission from the STA_A at a timing when the AP_B does not send any downlink signal to the STA_B 1  on the basis of schedule coordination within the AP_B network. 
     On the other hand, the STA_B 2  and the STA_X are located within their communication ranges, and therefore, mutual interference can take place, and the STA_B 3  and the STA_Y are located within their communication ranges, and therefore, mutual interference can take place. Also, the AP_B neither cooperates with the AP_X nor with the AP_Y, and therefore it is impossible to avoid mutual interference by coordinating schedules. 
     For example, an access control sequence based on RTS/CTS (Request to Send/Clear to Send) is employed in IEEE802.11 system and so on. A data sending terminal a sending terminal sends an RTS signal before transmission, and a receiving terminal returns a CTS signal upon receipt of the RTS signal. Then, other terminal that receives at least either an RTS or CTS signal avoids interference by refraining from a transmission action only for the duration of an NAV (Network Allocation Vector: transmission disabled period) included in these signals. 
     In contrast, the present embodiment has introduced a mechanism that sends, to terminals connected to non-cooperative other networks (networks that constitute interference sources), a transmission refrainment frame for causing the terminals that constitute interference sources to refrain from sending a signal before initiating data transmission, thus reducing mutual interference between networks for higher efficiency of downlink communication and contributing to improved frequency utilization efficiency. 
       FIG.  2    illustrates an example of a communication sequence for the AP_B to carry out downlink communication in the network topology illustrated in  FIG.  1   . 
     It is assumed here that the AP_B wishes to send a downlink signal to each of the subordinate terminals STA_B 1 , STA_B 2 , and STA_B 3 . In this case, before the AP_B begins to send a downlink signal, it is necessary to cause the STA_X and the STA_Y that constitute interference sources for each of the STA_B 2  and the STA_B 3  to refrain from sending signals. 
     Each of the subordinate terminals of the AP_B, namely, STA_B 1 , STA_B 2 , and STA_B 3 , performs an interference signal measurement (Measurement). Each of the terminals STA_B 1 , STA_B 2 , and STA_B 3  may perform an interference signal measurement on a regular basis or on the basis of an instruction from the connected AP_B. Each of the terminals STA_B 1 , STA_B 2 , and STA_B 3  acquires, through measurement, information regarding an interference status such as identification information of the terminal constituting an interference source and information regarding interference power. Terminal identification information includes, for example, MAC (Media Access Control) address and identification information added to a physical header. 
     The AP_B receives an interference status report (Measurement Report) frame, an interference signal measurement result, from its subordinate terminals STA_B 1 , STA_B 2 , and STA_B 3  ahead of the downlink signal transmission. The AP_B can acquire, on the basis of the report frame received from the terminal STA_B 1 , the fact that the STA_B 1  is located within a range where a signal can be received from the subordinate terminal STA_A of the cooperative AP_A, interference power received by the STA_B 2  from the nearby interference source STA_X, and interference power received by the STA_B 3  from the nearby interference source STA_Y. Each of the terminals STA_B 1 , STA_B 2 , and STA_B 3  may measure reception power of interference signals. Alternatively, each interference signal may include transmission power. On the assumption that all terminals send signals with uniform transmission power, it is possible to estimate the distance to the interference source on the basis of reception power. A measurement result report frame may report a combination of transmission and reception power of an interference signal or an estimated distance to the interference source. 
     Also, the AP_B receives, from the cooperative AP_A, an information frame regarding traffic scheduling (Schedulling Announcement) as a radio wave utilization status. This scheduling information also includes information regarding a transmission timing of the STA_A, the subordinate terminal of the AP_A. It should be noted that it is assumed that, although not illustrated, the AP_B also sends a Schedulling Announcement frame to the AP_A. 
     Then, the AP_B determines an area to be reached by a transmission refrainment frame, i.e., a protection area (Protection Area), on the basis of two types of information, namely, a Measurement Report received from each of the subordinate terminals STA_B 1 , STA_B 2 , and STA_B 3  and a Schedulling Announcement received from the cooperative AP_A. The protection area is determined by the location of the terminal that sends a transmission refrainment frame and the transmission power of the transmission refrainment frame sent therefrom (reach range). 
       FIG.  3    illustrates an example of a protection area determined by the AP_B in the network topology illustrated in  FIG.  1   . 
     The AP_B finds out, on the basis of the Measurement Report received from the STA_B 2 , that the interference source STA_X is located relatively far from the STA_B 2 . For this reason, the AP_B sets a relatively small protection area denoted by reference number  301  for the STA_B 2 . 
     Also, the AP_B finds out, on the basis of the Measurement Report received from the STA_B 3 , that the interference source STA_Y is located relatively close to the STA_B 3 . For this reason, the AP_B sets a relatively large protection area denoted by reference number  302  for the STA_B 3 . 
     Also, the AP_B knows, on the basis of the Schedulling Announcement received from the AP_A, the timing when the STA_A receives a signal from the AP_A (in other words, the timing when the STA_A does not send any signal). Therefore, the AP_B can avoid interference between the STA_B 1  and the STA_A by coinciding its downlink signal with that timing. Therefore, the AP_B does not set any protection area for the STA_B 1 . 
     A description will follow of the communication sequence for the AP_B to carry out downlink communication with reference to  FIG.  2    again. 
     Thereafter, the AP_B sends, to the terminals STA_B 2  and STA_B 3  for which protection areas have been set, a trigger frame (Trigger Frame) that induces the transmission of a transmission refrainment frame ahead of the transmission of a downlink signal. A trigger frame specifies a terminal to which to send a transmission refrainment frame and transmission power that allows the terminal to establish an appropriate protection area (in other words, transmission power for achieving a desired reach range of the transmission refrainment frame in question). It should be noted that the trigger frame itself is stipulated in IEEE802.11ax as a frame for enabling uplink channel access. 
     Each of the terminals STA_B 2  and STA_B 3  returns, in response to a trigger frame received from the connected AP_B, a trigger response (Trigger Response) to the AP_B with the transmission power specified by the trigger frame. It should be noted that the term “trigger response” here corresponds, for example, to a trigger-based PPDU (PLCP (Physical Layer Convergence Protocol) Protocol Data Unit). 
     The trigger response sent from the terminal STA_B 2  also reaches the interference source STA_X as a transmission refrainment frame, and the trigger response sent from the terminal STA_B 3  also reaches the interference source STA_Y as a transmission refrainment frame (not illustrated in  FIG.  2   ). 
     Each of the terminals STA_X and STA_Y that has received a transmission refrainment frame refrains from performing a transmission action for a predetermined time period (e.g., NAV specified by the transmission refrainment frame). 
     When a trigger response is received from each of the terminals STA_B 2  and STA_B 3  to which a trigger frame has been sent, the AP_B sends a downlink signal to each of the subordinate terminals STA_B 1 , STA_B 2 , and STA_B 3  (Downlink Tx). 
     During the transmission period of the downlink signal, each of the interference sources STA_X and STA_Y refrains from performing a transmission action, thus allowing for mutual interference to be avoided between the networks and ensuring successful transmission of the downlink signal. Also, the need to impose refraining from transmission over an excessive range can be eliminated as the terminals STA_B 2  and STA_B 3  set the protection areas  301  and  302  of appropriate sizes by sending transmission refrainment frames with appropriate transmission power. This streamlines downlink communication with higher transmission demand and contributes to improved frequency utilization efficiency. 
       FIG.  4    illustrates, in a flowchart form, an example of a processing procedure for a communication apparatus that functions as a base station (AP) to send a trigger frame to its subordinate terminal (STA) ahead of transmission of a downlink signal. It should be noted, however, that it is assumed that the base station has received not only a report (Measurement Report) frame regarding an interference signal measurement result from each of the subordinate terminals but also a scheduling information (Schedulling Announcement) frame from the cooperative base station in the surrounding area before carrying out the processing procedure illustrated. 
     When a downlink signal (downlink data) to be sent occurs (step S 401 ), the base station selects target stations to which to send a trigger frame (in other words, terminals to which to send a transmission refrainment frame) and sets a transmission parameter for each target station. 
     Here, 1 is substituted into a variable i first assuming that a downlink signal is sent to N subordinate terminals (step S 402 ). 
     The base station checks whether the ith transmission destination terminal has an interference source (step S 403 ). 
     Here, if the ith transmission destination terminal does not have any interference source (No in step S 403 ), there is no possibility that the terminal will receive interference during reception of a downlink signal, thus eliminating the need to send a transmission refrainment frame. For this reason, the base station removes the ith transmission destination terminal from the target stations, increments i by 1 (step S 408 ), and proceeds to the selection of target stations and the setting of a transmission parameter for the (i+1)th transmission destination terminal. 
     In the case where the ith transmission destination terminal has an interference source (Yes in step S 403 ), the base station further checks whether the interference source of the ith transmission destination terminal is connected to a non-cooperative base station in the surrounding area (step S 404 ). 
     In the case where the ith transmission destination terminal is not connected to a non-cooperative base station in the surrounding area (No in step S 404 ), in other words, in the case where the ith transmission destination terminal is connected to a cooperative base station in the surrounding area, the base station can avoid interference of a downlink signal to the ith transmission destination terminal through schedule coordination with this base station in the surrounding area, thus eliminating the need to send a transmission refrainment frame. For example, the base station can avoid interference by scheduling a downlink signal transmission timing to the ith transmission destination terminal in such a manner as not to coincide with the transmission timing of this interference source. In the case where an interference source is connected to the cooperative base station in the surrounding area, the interference source becomes no longer an interference source through schedule coordination. For this reason, the base station removes the ith transmission destination terminal from the target stations, increments i by 1 (step S 408 ), and proceeds to the process of selecting target stations for the (i+1)th transmission destination terminal and setting a transmission parameter. 
     On the other hand, in the case where the interference source of the ith transmission destination terminal is connected to a non-cooperative base station in the surrounding area (Yes in step S 404 ), the base station selects the ith transmission destination terminal as a target station to which to send a trigger frame. Then, the base station sets transmission power for establishing an appropriate protection area as a transmission parameter of a transmission refrainment frame on the basis of the interference status (e.g., interference power information) notified from the ith transmission destination terminal (step S 405 ). 
     In the case where i is smaller than N, that is, in the case where there is still a terminal that has yet to be processed as a transmission destination of a downlink signal (No in step S 406 ), the base station increments i by 1 (step S 408 ) and proceeds to the process of selecting target stations for the (i+1)th transmission destination terminal and setting a transmission parameter. 
     Then, when i reaches N, that is, when the selection of target stations and the setting of a transmission parameter are complete for all transmission destination terminals to which to send downlink signals (Yes in step S 406 ), the base station sends a trigger frame that specifies transmission parameters for all the selected target stations (step S 407 ) and terminates the present process. 
       FIG.  5    illustrates, in a flowchart form, an example of a processing procedure for a communication apparatus that functions as a terminal (STA) to send a transmission refrainment frame in response to a trigger frame received from the connected base station (AP). It should be noted that it is assumed that the terminal has carried out a report (Measurement Report) frame regarding an interference signal measurement result to the connected base station before the illustrated processing procedure is initiated. 
     When a trigger frame including its own address is received from the base station (or inducing the transmission of a transmission refrainment frame to itself) while on standby (Yes in step S 501 ), the terminal checks whether a transmission parameter of a transmission refrainment frame is specified by the trigger frame (step S 502 ). 
     In the case where a transmission parameter of a transmission refrainment frame such as transmission power is specified by the trigger frame (Yes in step S 502 ), the terminal sets the specified transmission parameter (step S 503 ). Also, in the case where no transmission parameter is specified by the trigger frame, the terminal sets a default transmission parameter (step S 504 ). 
     Then, the terminal sends a transmission refrainment frame with the set transmission parameter (step S 505 ) and terminates the present process. 
       FIG.  6    illustrates a configuration example of a communication apparatus  600  to which the technology disclosed in the present specification is applicable. The communication apparatus  600  can function both as a base station (AP) and a terminal (STA), for example, in the network topology illustrated in  FIG.  1   . 
     The communication apparatus  600  includes a data processing section  601 , a control section  602 , a communication section  603 , and a power supply section  604 . Also, the communication section  603  further includes a modulation/demodulation section  611 , a signal processing section  612 , a wireless interface (IF) section  614 , and an amplification section  615 . An antenna  616  is connected to the amplification section  615 . It should be noted that one or more sets of the wireless interface section  614 , the amplification section  615 , and the antenna  616  may be included as components. Also, the function of the amplification section  615  is included, in some cases, in the wireless interface section  614 . 
     The data processing section  601  generates, at the time of transmission when data is input from a protocol&#39;s upper layer (not illustrated), a packet for wireless transmission from the data, performs processes such as adding a header and an error detection code for media access control (MAC), and provides processed data to the modulation/demodulation section  611  in the communication section  603 . Conversely, at the time of reception when input is received from the modulation/demodulation section  611 , the data processing section  601  performs processes such as analyzing a MAC header, detecting a packet error, and performing a reordering process and provides processed data to its own protocol&#39;s upper layer. 
     The control section  602  controls exchange of information between the different sections of the communication apparatus  600 . Also, the control section  602  handles parameter setting in the modulation section  611  and the signal processing section  612  and packet scheduling in the data processing section  601 . Also, the control section  602  handles parameter setting and controls transmission power in the wireless interface section  614  and the amplification section  615 . 
     In the case where the communication apparatus  600  functions as a base station, the control section  602  performs processes for improving the frequency utilization efficiency of the system as a whole while at the same time avoiding mutual interference at the time of transmission of a downlink signal. 
     Specifically, the control section  602  determines, ahead of the transmission of a downlink signal to one or more subordinate terminals, a transmission refrainment area for each terminal on the basis of the interference status of each terminal to which the downlink signal is to be sent and the radio wave utilization status of the cooperative base station in the surrounding area and controls the operation of the communication apparatus  600  to send a trigger frame that induces the transmission of a transmission refrainment frame with the transmission power specified for each terminal such that the transmission refrainment frame reaches into the transmission refrainment area. 
     Also, in the case where the communication apparatus  600  functions as a terminal, the control section  602  controls, in response to the trigger frame received from the connected base station, the operation of the communication apparatus  600  to send a transmission refrainment frame with the transmission power specified by the trigger frame. 
     The modulation/demodulation section  611  encodes, interleaves, and modulates, at the time of signal transmission, the input data from the data processing section  601  on the basis of a coding and modulation scheme set by the control section  602 , generates a data symbol stream, and provides the stream to the signal processing section  612 . Also, the modulation/demodulation section  611  performs, at the time of signal reception, processes opposite to those performed at the time of transmission on the input from the signal processing section  612  and provides the received data to the data processing section  601  or the control section  602 . 
     The signal processing section  612  performs, at the time of signal transmission, signal processing on the input from the modulation/demodulation section  611  and provides one or more transmission symbol streams acquired to the respective wireless interface sections  614 . Also, the signal processing section  612  performs, at the time of signal reception, signal processing on received symbol streams input from the respective wireless interface sections  614  and provides the stream to the modulation/demodulation section  611 . 
     It should be noted that the signal processing section  612  performs, as necessary, spatial processing including spatial multiplexing of a plurality of streams at the time of signal transmission and spatial decomposition of a plurality of streams of a received signal at the time of signal reception. For this reason, the signal processing section  612  calculates, from preamble portions and training signal portions of input signals from the respective wireless interface sections  614 , complex channel gain information of a propagation path for use for spatial processing. 
     The wireless interface section  614  converts the input from the signal processing section  612  into an analog signal at the time of signal transmission, performs filtering and upconversion into a carrier frequency, and sends out the signal to the antenna  616  or the amplification section  615 . Also, the wireless interface section  614  performs, at the time of signal reception, opposite processes on the input from the antenna  616  or the amplification section  615  and provides the data to the signal processing section  612 . 
     The amplification section  615  amplifies, at the time of signal transmission, the analog signal input from the wireless interface section  614  to a predetermined power level and sends the signal to the antenna  616 . Also, the amplification section  615  amplifies, at the time of signal reception, the signal input from the antenna  616  to a predetermined power level with low noise and outputs the signal to the wireless interface section  614 . Also, at least either the function of this amplification section  615  at the time of transmission or reception is included, in some cases, in the wireless interface section  614 . 
     The power supply section  604  includes a battery power supply or a stationary power supply and supplies power to the respective sections of the communication apparatus  600 . 
     INDUSTRIAL APPLICABILITY 
     A detailed description has been given above of the technology disclosed in the present specification with reference to specific embodiments. However, it is apparent that a person skilled in the art can modify or substitute the embodiments in question without departing from the gist of the technology disclosed in the present specification. 
     The technology disclosed in the present specification is applicable, for example, to wireless networks based on the IEEE802.11 standard. It should be noted, however, that the technology disclosed in the present specification is not limited in scope of application to specific communication standards. It is possible to streamline downlink communication with higher transmission demand and contribute to improved frequency utilization efficiency by applying the technology disclosed in the present specification to a densely populated network environment. 
     In short, the technology disclosed in the present specification has been described by way of illustration, and the description of the present specification should not be interpreted restrictively. In order to evaluate the gist of the technology disclosed in the present specification, the claims should be taken into consideration. 
     It should be noted that the technology disclosed in the preset specification can have the following configurations: 
     (1) A communication apparatus including: 
     a communication section adapted to send and receive wireless frames; and 
     a control section adapted to control transmission and reception of frames, in which 
     the control section controls transmission, to one or more target stations and with a transmission parameter specified for each target station, of a trigger frame that induces transmission of a transmission refrainment frame. 
     (2) The communication apparatus of feature (1), in which 
     the control section determines, as a transmission parameter, transmission power of a transmission refrainment frame for each target station. 
     (3) The communication apparatus of feature (2), in which 
     the control section determines transmission power of the transmission refrainment frame for each target station on the basis of a status of interference received by the target station. 
     (4) The communication apparatus of feature (2) or (3), in which 
     the control section determines, for each target station, transmission power that allows the transmission refrainment frame to reach an interference source. 
     (5) The communication apparatus of any one of features (1) to (4), in which 
     the control section selects a transmission destination of a data frame as a target station and causes the trigger frame to be sent ahead of transmission of the data frame. 
     (6) The communication apparatus of any one of features (1) to (5), in which 
     the communication apparatus functions as a base station, and 
     the control section selects, as target stations, one or more subordinate terminals to which a downlink signal is to be sent and controls the transmission of the trigger frame. 
     (7) The communication apparatus of feature (6), in which 
     the control section causes the trigger frame to be sent ahead of transmission of the downlink signal. 
     (8) The communication apparatus of feature (6) or (7), in which 
     the control section controls the transmission of the trigger frame on the basis of a status of interference of a terminal to which the downlink signal is to be sent and a radio wave utilization status of a base station in a surrounding area. 
     (9) The communication apparatus of feature (6) or (7), in which 
     the control section controls the transmission of the trigger frame on the basis of a radio wave utilization status of a base station in a surrounding area of a terminal to which the downlink signal is to be sent. 
     (10) The communication apparatus of feature (9), in which 
     the control section does not select, as a target station to which the trigger frame is to be sent, a terminal that receives interference only from an interference source connected to a base station in a surrounding area whose radio wave utilization status can be found out. 
     (11) The communication apparatus of any one of features (6) to (10), in which 
     the control section determines, as a transmission parameter for each terminal, a target station, transmission power that allows the transmission refrainment frame to reach an interference source. 
     (12) A communication method including: 
     a step of selecting a target station to which to request transmission of a transmission refrainment frame; 
     a step of determining a transmission parameter regarding the transmission refrainment frame for each target station; and 
     a step of sending, to the target station, a trigger frame that induces the transmission of a transmission refrainment frame with the determined transmission parameter. 
     (13) A communication apparatus including: 
     a communication section adapted to send and receive wireless frames; and 
     a control section adapted to control transmission and reception of frames, in which 
     the control section causes, in response to reception of a trigger frame that induces transmission of a transmission refrainment frame, the transmission refrainment frame to be sent with a transmission parameter specified by the trigger frame. 
     (14) The communication apparatus of feature (13), in which 
     the control section causes a transmission refrainment frame to be sent with transmission power specified as a transmission parameter or in such a manner as to achieve a reach range specified as a transmission parameter. 
     (15) The communication apparatus of feature (13) or (14), in which 
     the communication apparatus functions as a subordinate terminal of a base station, and 
     the control section sends a transmission refrainment frame in response to the trigger frame received from the base station. 
     (16) The communication apparatus of feature (15), in which 
     the control section controls notification of an interference status to the base station. 
     (17) The communication apparatus of feature (16), in which 
     the control section causes identification information of an interference source and information regarding interference power to be notified to the base station as an interference status. 
     (18) The communication apparatus of any one of features (15) to (17), in which 
     the communication apparatus receives the trigger frame sent ahead of a downlink signal from the base station. 
     (19) The communication apparatus of any one of features (15) to (18), in which the communication apparatus receives a downlink signal from the base station after having sent a transmission refrainment frame in response to the trigger frame. 
     (20) The communication apparatus of any one of features (15) to (19), in which 
     the communication apparatus does not send any transmission refrainment frame in a case where interference is received only from an interference source connected to a base station in a surrounding area whose radio wave utilization status can be found out. 
     (21) A communication method including: 
     a step of receiving a trigger frame that induces transmission of a transmission refrainment frame; and 
     a step of sending a transmission refrainment frame with a transmission parameter specified by the trigger frame. 
     REFERENCE SIGNS LIST 
     
         
           600  Communication apparatus 
           601  Data processing section 
           602  Control section 
           603  Communication section 
           604  Power supply section 
           611  Modulation/demodulation section 
           612  Signal processing section 
           614  Wireless interface section 
           615  Amplification section 
           616  Antenna