Patent Publication Number: US-2018041911-A1

Title: Control apparatus, wireless communication system, and control method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2016-152106, filed on Aug. 2, 2016, the entire contents of which are incorporated herein by reference 
     FIELD 
     The embodiments discussed herein are related to a control apparatus, a wireless communication system, and a control method. 
     BACKGROUND 
     Currently, a terminal (or a user) can use not only a wireless access method such as long term evolution (LTE), but also a wireless access method such as a wireless local area network (wireless LAN). The wireless LAN is a wireless access method whose specification was examined by, for example, Institute of Electrical and Electronic Engineers (IEEE). Examples of the wireless LAN include IEEE 802.11ac (or Very High Throughput (VHT)), IEEE 802.11n, and the like. 
     In a wireless LAN system, a Carrier Sense Multiple Access/Collision Detection (CSMA/CA) communication method may be used. The CSMA/CA communication method is, for example, a communication method in which a carrier sense method and a collision avoidance algorithm are combined. For example, in the CSMA/CA communication method, when the received power value of a certain channel (or frequency) is equal to or greater than a carrier sense threshold, an access point (AP) waits for transmission. Otherwise, the AP starts transmission. 
     For example, in an environment where APs are disposed at high density, such as stations and underground malls, the AP detects many signals transmitted from other APs compared to other environments. In this case, when the received power value of a signal detected by the AP is equal to or greater than the carrier sense threshold, the AP waits for transmission. Therefore, in the environment in which the APs are disposed in this manner, there is a high possibility that the AP receives a signal which is equal to or greater than the carrier sense threshold, compared to other environments, such that the opportunity of signal transmission in the AP decreases, and the throughput may decrease. Such a phenomenon may sometimes be referred to as, for example, an exposed terminal problem. 
     Examples of a technique relating to a wireless LAN are as follows. That is, there is a wireless communication system in which simultaneous communication is performed by updating carrier sense level values of two base stations, only in a case where any base station in a pair of the two base stations is capable of setting a carrier sense level value to a value larger than the RSSI value received from the opposite station. In this case, in the wireless communication system, when only one of two base stations can raise the carrier sense level value, the level value is kept unchanged without being updated. 
     Japanese Laid-open Patent Publication No. 2015-167286 is an example of the related art. 
     According to this technique, it is said that throughput by carrier sense control can be improved in environments of dense wireless LANs. 
     SUMMARY 
     According to an aspect of the invention, a control apparatus includes a memory, and a processor coupled to the memory and configured to receive a first received power value for a first signal received from a first base station and a second received power value for a second signal received from a second base station adjacent to the first base station in a first terminal under control of the first base station, from the first terminal, through the first or second base station, calculate a threshold for each pair of the one or more first terminals and the one or more second base stations, based on the received first and second received power values, and transmit the calculated threshold to the first base station. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram illustrating a configuration example of a wireless communication system; 
         FIG. 2  is a diagram illustrating a configuration example of a central control station device; 
         FIG. 3  is a diagram illustrating a configuration example of an AP; 
         FIG. 4  is a diagram illustrating a configuration example of a terminal; 
         FIG. 5A  is a diagram illustrating an example of a relationship between a received power value and a carrier sense threshold, and  FIG. 5B  is a diagram illustrating an example of a relationship between received power values of adjacent APs; 
         FIG. 6  is a diagram illustrating a configuration example of the wireless communication system; 
         FIG. 7  is a flowchart illustrating an operation example; 
         FIG. 8A  and  FIG. 8B  are diagrams illustrating examples of carrier sense thresholds; 
         FIG. 9  is a flowchart illustrating an operation example; 
         FIG. 10  is a flowchart illustrating an operation example; 
         FIG. 11A  and  FIG. 11B  are diagrams illustrating examples of the carrier sense thresholds; 
         FIG. 12  is a flowchart illustrating an operation example; 
         FIG. 13  is a diagram illustrating an example of a relationship between S and I; 
         FIG. 14  is a diagram illustrating a configuration example of a wireless communication system; 
         FIG. 15A  is a diagram illustrating a hardware configuration example of a central control station device, and  FIG. 15B  is a diagram illustrating a hardware configuration example of an AP; and 
         FIG. 16  is a diagram illustrating a hardware configuration example of the terminal. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In the technique of updating carrier sense level values of two base stations, for example, when only one of the two base stations can raise the carrier sense level value, the level value is kept unchanged without being updated. Therefore, according to the technique, even if there is one terminal that does not satisfy a desired communication quality among a plurality of terminals under control of one base station, even if the other terminals satisfy the desired communication quality, the terminals under control of the one base station do not perform simultaneous communication. In this case, opportunities for simultaneous communication decrease, and this leads to a decrease in throughput of the whole system. 
     Accordingly, a disclosure is to provide a control apparatus, a wireless communication system, and a control method that improve throughput of the whole system. 
     Hereinafter, embodiments will be described. The following embodiments are not intended to limit the disclosed technique. Respective embodiments can be combined appropriately as long as the processing contents do not contradict each other. 
     In addition, the terms and technical contents described in the specification as the standard related to communication such as IEEE may be appropriately used for the terms and technical contents described herein. 
     First Embodiment 
     Configuration Example of Wireless Network System 
       FIG. 1  is a diagram illustrating a configuration example of a wireless communication system  10  in a first embodiment. The wireless communication system  10  includes a central control station device (hereinafter, may be referred to as a “central control station”)  100 , access points (hereinafter, may be referred to as an “AP”)  200 - 1  and  200 - 2 , terminal devices (hereinafter, may be referred to as “terminals”)  300 - 1  to  300 - 3 . 
     In the first embodiment, each of the APs  200 - 1  and  200 - 2  performs wireless communication with the terminal  300 - 1  to  300 - 3  through a wireless LAN method. As the wireless LAN method, for example, IEEE 802. 11 series, such as IEEE 802.11ac and IEEE 802.11n may be used. In this case, the APs  200 - 1  and  200 - 2  and the terminals  300 - 1  to  300 - 3  respectively perform wireless communication using a carrier sensing method. As the carrier sensing method, for example, a CSMA/CD method, a CSMA/Collision Avoidance (CA) method, or the like may be used. In the first embodiment, the CSMA/CD method will be described. 
       FIG. 5A  illustrates an example of a relationship between a carrier sense threshold and a received power value. In the carrier sense method, for example, each of the APs  200 - 1  and  200 - 2  waits for transmission when the received power value for the use frequency is equal to or greater than the carrier sense threshold (“B” in  FIG. 5A ). On the other hand, each of the APs  200 - 1  and  200 - 2  starts transmission when the received power value is less than the carrier sense threshold (“A” in  FIG. 5A ). In the first embodiment, each of the APs  200 - 1  and  200 - 2  performs wireless communication with the terminals  300 - 1  to  300 - 3  in a downlink (DL) direction using such a carrier sense method. For example, the DL direction is a communication direction from the AP  200  to the terminal  300 , and a UL direction is a communication direction from the terminal  300  to the AP  200 . 
     Returning to  FIG. 1 , the central control station  100  collects various types of information acquired by the APs  200 - 1  and  200 - 2  during wireless communication with the subordinate terminals  300 - 1  to  300 - 3 , from each of the APs  200 - 1  and  200 - 2 . Examples of various types of information include the received power value of a signal received from the AP  200 - 1  and the received power value of a signal received from the AP  200 - 2 , in the terminal  300 - 1 . Such received power values are measured in the terminal  300 - 1 , and transmitted to the central control station  100  through the AP  200 - 1  or the AP  200 - 2 . The central control station  100  calculates a carrier sense threshold based on these pieces of information collected from the respective APs  200 - 1  and  200 - 2 , and transmits the calculated carrier sense threshold to the respective APs  200 - 1  and  200 - 2 . A method by which the central control station  100  calculates the carrier sense threshold will be described in the operation example. 
     The APs  200 - 1  and  200 - 2  are, for example, base stations (or wireless communication apparatuses) capable of wireless communication with the terminals  300 - 1  to  300 - 3  using a wireless LAN. Each of the APs  200 - 1  and  200 - 2  provides various services such as a call service and a Web browsing service, in a service available range (or a communication coverage) of each station. 
     In the first embodiment, the two APs  200 - 1  and  200 - 2  are adjacent to each other. Being adjacent (or adjacent APs) means, for example, a relationship in which the two communication coverages of the two APs  200 - 1  and  200 - 2  overlap and the two APs  200 - 1  and  200 - 2  are disposed in the overlapping coverage. Since the two APs  200 - 1  and  200 - 2  are adjacent to each other, the AP  200 - 1  can receive the signal transmitted from the AP  200 - 2  and the AP  200 - 2  can also receive the signal transmitted from the AP  200 - 1 . 
     The terminals  300 - 1  to  300 - 3  are, for example, wireless communication device capable of wireless communication with the respective APs  200 - 1  and  200 - 2 . Examples of the terminal  300 - 1  to  300 - 3  are feature phones, smartphones, tablet terminals, personal computers, game devices, or the like. 
     The example of  FIG. 1  represents an example in which the terminals  300 - 1  and  300 - 3  perform wireless communication with the AP  200 - 1  and the terminal  300 - 2  performs wireless communication with the AP  200 - 2 . In this case, the terminals  300 - 1  to  300 - 3  performing wireless communication with each of the APs  200 - 1  and  200 - 3  may be referred to as, for example, “a terminal under control” or “a subordinate terminal”. In the example of  FIG. 1 , the subordinate terminals of the AP  200 - 1  are the terminals  300 - 1  and  300 - 3 , and the subordinate terminals of the AP  200 - 2  are the terminal  300 - 2 . 
     The example of  FIG. 1  represents an example in which there are two APs  200 - 1  and  200 - 2 , but three or more APs may be used. Further, the number of terminals  300 - 1  to  300 - 3  under control of the respective APs  200 - 1  and  200 - 2  may be one or plural. 
     In the following description, the APs  200 - 1  and  200 - 2  may be referred to as the AP  200  and the terminals  300 - 1  to  300 - 3  may be referred to as the terminal  300  in some cases, unless otherwise noted. 
     Next, the configuration examples of the central control station  100 , the AP  200 , and the terminal  300  will be described. 
     Configuration Example of Central Control Station 
       FIG. 2  is a diagram illustrating a configuration example of the central control station  100 . The central control station  100  includes a reception unit  110 , an adjacent AP identification information generation unit  120 , a carrier sense threshold calculation unit  130 , and a transmission unit  140 . 
     The reception unit  110  receives the information transmitted from each AP  200 . The transmitted information includes the received power value when the subordinate terminal  300  of the AP  200  receives the signal transmitted from the AP  200 , the received power value when the subordinate terminal  300  of the AP  200 - 1  receives the signal transmitted from the adjacent AP  200 - 2 , power between adjacent APs, or the like. The received power value between adjacent APs includes the received power value when the AP  200 - 1  receives a signal transmitted from the adjacent AP  200 - 2 , and the received power value when the AP  200 - 2  receives a signal transmitted from the adjacent AP  200 - 1 . The reception unit  110  outputs the received power values to the carrier sense threshold calculation unit  130  and outputs the power between adjacent APs to the adjacent AP identification information generation unit  120 . 
     The adjacent AP identification information generation unit  120  extracts the identification information of the adjacent AP included in the adjacent AP power and outputs the extracted identification information of the adjacent AP to the carrier sense threshold calculation unit  130  and the transmission unit  140 . 
     The carrier sense threshold calculation unit  130  calculate a carrier sense threshold for each pair of a subordinate terminal  300  of the AP  200 - 1  and an adjacent AP  200 - 2 , as a carrier sense threshold for the AP  200 - 1 , based on the received power value collected from each AP  200  or the like.  FIG. 8A  illustrates an example of a carrier sense threshold for the AP  200 - 1 , and  FIG. 8B  illustrates an example of a carrier sense threshold for the AP  200 - 2 . As illustrated in  FIG. 8A  and  FIG. 8B , the carrier sense thresholds have, for example, different thresholds for each subordinate terminal  300  and for each adjacent AP  200 . Details of the calculation will be described in the operation example. Returning to  FIG. 2 , the carrier sense threshold calculation unit  130  outputs the calculated carrier sense threshold to the transmission unit  140 . 
     The transmission unit  140  transmits the carrier sense threshold and the adjacent AP identification information to each AP  200 . 
     Configuration Example of AP 
       FIG. 3  is a diagram illustrating a configuration example of the AP  200 . The AP  200  includes a control station-side reception unit  210 , a carrier sense threshold storage unit  220 , a wireless reception unit  230 , a dynamic carrier sense operation unit  240 , a wireless transmission unit  250 , and a control station-side transmission unit  260 . 
     The control station-side reception unit  210  receives the carrier sense threshold and the like transmitted from the central control station  100 . The control station-side reception unit  210  stores the received carrier sense threshold and the like in the carrier sense threshold storage unit  220 . 
     The carrier sense threshold storage unit  220  is, for example, a memory, or the like, and stores a carrier sense threshold for each pair of the subordinate terminal  300  and the adjacent AP  200 .  FIG. 8A  and  FIG. 8B  above represent examples of the carrier sense threshold stored in the carrier sense threshold storage unit  220 . 
     Returning to  FIG. 3 , the wireless reception unit  230  receives wireless signals transmitted from the subordinate terminal  300  and the adjacent AP  200 , and extracts packet data from the wireless signal. The wireless reception unit  230  extracts, from the packet data, the received power value measured in the terminal  300 , the power between adjacent APs measured by the adjacent AP  200 , packet information, or the like. The wireless reception unit  230  outputs the received power value and the like to the control station-side transmission unit  260 , and outputs packet information and the like to the dynamic carrier sense operation unit  240 . 
     Further, the wireless reception unit  230  may measure the received power value of the signal transmitted from the adjacent AP  200 , and transmit it to the control station-side transmission unit  260  as the power between the adjacent APs or transmit it to the adjacent power AP  200 . In this case, the wireless reception unit  230  may generate adjacent AP identification information indicating APs between which the power is measured, for the measured power between adjacent APs, and transmit the power between adjacent APs to the central control station  100  and the adjacent AP. When receiving the power between adjacent APs measured in the adjacent AP  200 - 2 , the wireless reception unit  230  may transmit the received power between adjacent APs together with the adjacent AP identification information to the central control station  100 . 
     The dynamic carrier sense operation unit  240  reads the carrier sense threshold stored in the carrier sense threshold storage unit  220 , and acquires the received power value of the packet data received by the wireless reception unit  230 . Then, the dynamic carrier sense operation unit  240  determines transmission availability as to whether the AP  200  is to perform transmission, based on the carrier sense threshold and the received power value. The dynamic carrier sense operation unit  240  outputs the transmission availability (or a carrier sense result) to the wireless transmission unit  250 . The operation of the dynamic carrier sense operation unit  240  will be described in an operation example. 
     The wireless transmission unit  250  converts the packet data into a wireless signal according to the transmission availability of the dynamic carrier sense operation unit  240 , and transmits the wireless signal to the terminal  300  under control. 
     The control station-side transmission unit  260  transmits the received power value and the like received from the wireless reception unit  230  to the central control station  100 . 
     Configuration Example of Terminal 
       FIG. 4  is a diagram illustrating a configuration example of the terminal  300 . The terminal  300  includes a wireless reception unit  310 , an information acquisition unit  320 , and a wireless transmission unit  330 . 
     The wireless reception unit  310  receives a wireless signal transmitted from the AP  200 , and extracts packet data and the like from the received wireless signal. The wireless reception unit  310  outputs the extracted packet data to the information acquisition unit  320 . In addition, the wireless reception unit  310  measures the received power value based on the received wireless signal. In this case, the wireless reception unit  310  is able to measure the received power value of the wireless signal transmitted from the AP  200 - 1  under control which it performs wireless communication, or measure the received power value of the wireless signal transmitted from the AP  200 - 2  adjacent to the AP  200 - 1 . The wireless reception unit  310  outputs the acquired received power value to the information acquisition unit  320 . 
     The information acquisition unit  320  receives the received power value measured in the wireless reception unit  310 , identifies an AP  200  for which the received power value is, based on the packet information or the like received from the wireless reception unit  310 , and adds AP identification information to each received power value. The information acquisition unit  320  outputs the received power value and the AP identification information to the wireless transmission unit  330 . 
     The wireless transmission unit  330  generates a data packet for the received power value and the AP identification information received from the information acquisition unit  320 , converts the data packet into a wireless signal, and transmits the converted wireless signal to the AP  200 - 1 . 
     Operation Example 
     Next, an operation example will be described.  FIG. 6  illustrates a configuration example of the wireless communication system  10  used in the operation example. The example of the wireless communication system  10  illustrated in  FIG. 6  represents an example in which an AP  200 - 3  is further provided and performs wireless communication with the terminal  300 - 4 . 
       FIG. 7  is a flowchart illustrating an operation example of calculating a carrier sense threshold. The carrier operation example illustrated in  FIG. 7  is performed by, for example, the carrier sense threshold calculation unit  130  of the central control station  100 . The flowchart of  FIG. 7  will be described by appropriately using  FIG. 6 . 
     The flowchart illustrated in  FIG. 7  is divided into two blocks: simultaneous communication performance evaluation of an adjacent AP (S 12  to S 14 , and S 22  to S 23 ) and simultaneous communication performance evaluation of a subordinate terminal (S 15  to S 19 ). 
     In the simultaneous communication performance evaluation of the adjacent AP, the central control station  100  evaluates, for example, whether or not the adjacent AP  200 - 2  and the AP  200 - 1  can perform simultaneous communication while the AP  200 - 1  does not interfere in the subordinate terminal  300  of the adjacent AP  200 - 2 . On the other hand, in the simultaneous communication performance evaluation of a subordinate terminal, when it is evaluated that simultaneous communication is possible, the central control station  100  sets, for example, a carrier sense threshold for each adjacent AP  200 - 2 , with respect to each subordinate terminal  300  of the AP  200 - 1 . Each process will be described below. 
     Upon starting the process (S 10 ), the central control station  100  performs the following process for each AP(i) (S 11 ). For example, the carrier sense threshold calculation unit  130  sets i=1 and performs the following process for the AP  200 - 1  (=AP( 1 )). 
     Next, the central control station  100  performs the simultaneous communication performance evaluation of an adjacent AP (S 12  to S 14 , and S 22  to S 23 ). In the example of  FIG. 6 , the AP  200 - 2  and the AP  200 - 1  are adjacent to each other and the AP  200 - 3  and the AP  200 - 3  are adjacent to each other, but the AP  200 - 1  and the AP  200 - 3  are not adjacent to each other. The central control station  100  performs the simultaneous communication performance evaluation for the adjacent AP  200 - 2 . 
     That is, the central control station  100  sets j=2, and performs the following process for the adjacent AP  200 - 2  (=AP( 2 )). 
     Next, the central control station  100  determines whether or not a received signal strength indicator (RSSI) i, j  is greater than the minimum NG power value (S 13 ). The RSSI i, j  represents, for example, a received power value between adjacent APs, and represents a received power value received from the AP(j) by the AP(i). For example, in the example of  FIG. 6 , the carrier sense threshold calculation unit  130  performs a process by comparing the received power value received from the adjacent AP  200 - 2  by the AP  200 - 1  with the minimum NG power value. At this stage, since the minimum NG power value is not calculated, the carrier sense threshold calculation unit  130  shifts the process to S 14 . 
     In S 14 , the central control station  100  determines whether there is a terminal  300  whose a signal to interference plus noise ratio (SINR) is less than a desired value by simultaneous communication among subordinate terminals of the adjacent AP(j). For example, the carrier sense threshold calculation unit  130  calculates the SINR for the subordinate terminal  300 - 2  of the adjacent AP  200 - 2 , with the received power value of the signal received from the AP  200 - 2  as S, the received power value of the signal received from the AP  200 - 1  as I, and the noise power of the terminal  300 - 2  as N. In this case, the SINR represents, for example, a ratio of the received power value (S) for the AP  200 - 2  to the received power value (I) for the AP  200 - 1 , in the terminal  300 - 2 . The carrier sense threshold calculation unit  130  can calculate an SINR based on the received power values collected from the respective APs  200 - 1  and  200 - 2 . 
     In this process (S 14 ), when the SINRs of all the terminals  300 - 2  under control of the adjacent AP  200 - 2  are equal to or greater than a desired value (or when the desired value is satisfied) (no in S 14 ), it is determined that the AP  200 - 1  and the adjacent AP  200 - 2  can perform simultaneous communication, and the process proceeds to simultaneous performance evaluation of subordinate terminals. 
     On the other hand, if the SINR of even one of all the subordinate terminals  300 - 2  of the adjacent AP  200 - 2  does not satisfy the desired value (yes in S 14 ), it is determined that the adjacent AP  200 - 2  and the AP  200 - 1  may not perform simultaneous communication, and the process of S 22  to S 23  is performed. 
     In the example of  FIG. 6 , it is assumed that the terminal  300  under control of the adjacent AP  200 - 2  is only the terminal  300 - 2 , and the SINR of the terminal  300 - 2  is equal to or greater than the desired value. Therefore, the carrier sense threshold calculation unit  130  determines that all the terminals  300 - 2  under control of the adjacent AP  200 - 2  can perform simultaneous communication with the AP  200 - 1  (no in S 14 ), and shifts the process to the simultaneous communication performance evaluation of a subordinate terminal. 
     Returning to  FIG. 7 , next, the central control station  100  performs the following process for each terminal  300  under control of the AP( 1 ) (S 15 ). For example, the carrier sense threshold calculation unit  130  sets k=1 so as to process all the terminals  300 - 1  and  300 - 3  under control of the AP  200 - 1 , and performs the following process sequentially from the terminal  300 - 1 . 
     Next, the central control station  100  determines whether or not the SINR of the terminal  300 - 1  (STA  1 ) during simultaneous communication is larger than the desired value (or whether or not the desired value is satisfied) (S 16 ). For example, the carrier sense threshold calculation unit  130  calculates the SINR, with the received power value of the AP  200 - 1  as S and the received power value of the adjacent AP  200 - 2  as I. In this case, the SINR represents, for example, a ratio of the received power value (S) for the AP  200 - 1  to the received power value (I) for the AP  200 - 2 , in the terminal  300 - 1 . For example, the carrier sense threshold calculation unit  130  determines whether or not the terminal  300 - 1  under control of the AP  200 - 1  can perform simultaneous communication with the adjacent AP  200 - 2 . In addition, the “desired value” in S 16  may be the same value as or may be a different value from the “desired value” in S 14 . 
     When the SINR of the terminal  300 - 1  is greater than the desired value (or when the desired value is satisfied) (yes in S 16 ), the central control station  100  calculates the carrier sense threshold for the adjacent AP(j) of the terminal  300 - 1  to a value larger than RSSI i, j  (S 17 ). 
     For example, it is assumed that the SINR of the terminal  300 - 1  is larger than the desired value. That is, even if signals from the AP  200 - 1  and the adjacent AP  200 - 2  are received at the same time, it is assumed the terminal  300 - 1  can normally receive the signal from the AP  200 - 1  without an interference of the signal from the adjacent AP  200 - 2 . In this case, the carrier sense threshold calculation unit  130  sets the carrier sense threshold for the adjacent AP  200 - 2  of the terminal  300 - 1  to a value larger than RSSI 1, 2 . In this case, similar to “A” in  FIG. 5A , even if the AP  200 - 1  receives signals from the adjacent AP  200 - 2 , the received power value is less than the carrier sense threshold, and can communicate with the terminal  300 - 1  under control. 
     Next, the central control station  100  repeats the above-described process for all the terminals  300  under control of the AP  200 ( i ) (a loop from S 15  to S 19 ). 
     In the example of  FIG. 6 , the carrier sense threshold calculation unit  130  compares the SINR and the desired value, with respect to the subordinate terminal  300 - 3  of the AP  200 - 1  (S 16 ). In this case, it is assumed that the SINR (S is the AP  200 - 1  and I is the adjacent AP  200 - 2 ) of the subordinate terminal  300 - 3  is equal to or less than the desired value. That is, even if signals from the AP  200 - 1  and the adjacent AP  200 - 2  are received at the same time, it is assumed that the terminal  300 - 2  may not normally receive the signal from the AP  200 - 1  due to the interference of the signal from the adjacent AP  200 - 2 . In this case, the carrier sense threshold calculation unit  130  sets the carrier sense threshold for the adjacent AP  200 - 2  of the terminal  300 - 2  to a value less than RSSI 1, 2 . In this case, similar to “B” in  FIG. 5A , if the AP  200 - 1  receives signals from the adjacent AP  200 - 2 , the carrier sense threshold is less than the received power value of the signal. Therefore, the AP  200 - 1  may not communicate while the adjacent AP  200 - 2  is communicating, and simultaneous communication becomes impossible. 
       FIG. 8A  illustrates examples of a carrier sense threshold for the AP  200 - 1 . The example of  FIG. 8A  represents an example in which there are two subordinate terminals  300 - 1  and  300 - 3 , but if there are three subordinate terminals, a carrier sense threshold is set for each subordinate terminal. If there are a plurality of adjacent APs  200 , the subordinate terminal  300  sets a carrier sense threshold for each adjacent AP  200 . 
     Returning to  FIG. 7 , if the simultaneous communication performance evaluation of a subordinate terminal for all terminals  300  under control of the AP( 1 ) is performed, the central control station  100  determines whether or not the adjacent AP loop for the AP( 1 ) is completed (S 20 ). In the example of  FIG. 6 , since the adjacent AP of the AP  200 - 1  is only the AP  200 - 2 , the process for all adjacent APs  200 - 3  of the AP  200 - 1  is completed. 
     Upon completion of the process for all adjacent APs to the AP( 1 ), the central control station  100  confirms whether the process is performed for all the APs  200  (S 21 ). In the example of  FIG. 6 , the central control station  100  completes the process for the AP  200 - 1 , and thereafter performs the process for the AP  200 - 2 . 
     When the process is not completed for all the APs  200 , the central control station  100  increments “i” to shift the process to S 11 , and repeats the above-described process (a loop from S 11  to S 21 ). 
     In the example of  FIG. 6 , the central control station  100  performs the following process for the AP  200 - 2  by setting i=2 (S 11 ). 
     Next, the central control station  100  performs simultaneous communication performance evaluation of the adjacent APs, for the adjacent AP  200 - 1  of the AP  200 - 2  by setting j=1. The central control station  100  determines whether or not the received power value SSI 2, 1  from the adjacent AP  200 - 1  of the AP  200 - 2  is larger than the minimum NG power value (S 13 ), but the minimum NG power value is not set, such that the process is proceeds to S 14 . 
     In S 14 , the central control station  100  determines whether or not there is a terminal whose the SINR is less than the desired value when the adjacent AP  200 - 1  and the AP  200 - 2  perform simultaneous communication, for the subordinate terminals  300 - 1  and  300 - 3  of the adjacent AP  200 - 1  (S 14 ). 
     As in the above case, it is assumed that the SINR (S is the received power value for the AP  200 - 1 , and I is the received power value for the AP  200 - 2 ) of the terminal  300 - 1  is larger than the desired value, and the SINR of the terminal  300 - 3  is less than the desired value. 
     In this case, since all the terminals  300 - 1  and  300 - 3  under control of the adjacent AP  200 - 1  do not satisfy the desired value (yes in S 14 ), the central control station  100  updates (or sets) the minimum NG power value (S 22 ). For example, the carrier sense threshold calculation unit  130  detects that the SINR of the terminal  300 - 1  under control of the adjacent AP  200 - 1  is less than the desired value (yes in S 14 ), and sets the received power value RSSI 2, 1  for the AP  200 - 1  in the AP  200 - 2  as the minimum NG power value (S 22 ). 
     Next, the central control station  100  sets the carrier sense threshold for the adjacent AP  200 - 1  of all terminals  300 - 2  under control of the AP  200 - 2  to a value less than the received power value RSSI 2, 1  (S 23 ). For example, the carrier sense threshold calculation unit  130  sets the carrier sense threshold so that the AP  200 - 2  does not communicate with the terminal  300 - 2  when the AP  200 - 1  is communicating. In the example of  FIG. 5A , assuming that the received power value from the adjacent AP  200 - 1  in the AP  200 - 2  is “B”, the carrier sense threshold can be set such that the AP  200 - 1  and the AP  200 - 2  do not perform simultaneous communication, by lowering the carrier sense threshold than “B”. Since the terminal  300 - 3  under control of the adjacent AP  200 - 1  may not normally receive the signal from the adjacent AP  200 - 1  due to the interference of the signal from the AP  200 - 2  even when performing simultaneous communication. In order to protect the terminal  300 - 3  from simultaneous communication, the carrier sense threshold is set such that simultaneous communication with the AP  200 - 2  is not performed. 
       FIG. 8B  illustrates examples of a carrier sense threshold for the AP  200 - 2 . With respect to the subordinate terminal  300 - 2  (STA  2 ), the carrier sense threshold is set so that simultaneous communication with the adjacent AP  200 - 1  (AP  1 ) is not performed. 
     Referring to  FIG. 7 , if a carrier sense threshold for the adjacent AP  200 - 1  of the terminal  300 - 3  under control of the AP  200 - 2  is set (S 23 ), the central control station  100  performs simultaneous communication performance evaluation of the adjacent AP for the next adjacent AP  200 - 3  (j=3) (S 20 , S 12 ). 
     In this case, the central control station  100  determines whether or not the received power value RSSI 2, 3  for the adjacent AP  200 - 3  of the AP  200 - 2  is larger than the minimum NG power value (=received power value RSSI 2, 1 ) (S 13 ). In the example of  FIG. 6 , it is assumed that received power value RSSI 2, 3  for the AP  200 - 3  is larger than the received power value RSSI 2, 1  for the AP  200 - 1 , in the AP  200 - 2 . 
     Accordingly, the central control station  100  determines “yes” in S 13 , and sets the carrier sense threshold for the adjacent AP  200 - 3  of all the terminals  300 - 2  under control of the AP  200 - 2  to a value less than RSSI 2, 3  (S 23 ). That is, the carrier sense threshold calculation unit  130  sets the carrier sense threshold so that the AP  200 - 2  does not communicate with the terminal  300 - 2  when the AP  200 - 3  is communicating. 
     That is, the central control station  100  sets the received power value RSSI 2, 1  from the adjacent AP  200 - 1 , which is not allowed to perform simultaneous communication, as the minimum NG power value, in order to protect the adjacent AP  200 - 1  which is not allowed to perform simultaneous communication. The central control station  100  does not allow the AP  200 - 2  to perform simultaneous communication with the adjacent AP  200 - 3  having the received power value between APs larger than the minimum NG power value so as to protect the terminal  300 - 3  of the adjacent AP  200 - 1  which is not allowed to perform simultaneous communication.  FIG. 8B  illustrates an example of a carrier sense threshold for the adjacent AP  200 - 3  of the terminal  300 - 2  under control of the AP  200 - 2 . 
     Returning to  FIG. 7 , when the central control station  100  ends the process for all the adjacent APs  200 - 1  and  200 - 3  of the AP  200 - 2  (S 20 ), the central control station  100  repeats the above-described process for the AP  200 - 3  (a loop of S 21  and S 11 ). A description of the AP  200 - 3  is omitted. 
     As described above, the central control station  100  transmits the calculated carrier sense threshold to each AP  200 . Each AP  200  performs carrier sense using the received carrier sense threshold. An operation example in each AP  200  will be described below. 
     Operation Example of AP 
       FIG. 9  is a flowchart illustrating an operation example in the AP  200 . Each process illustrated in  FIG. 9  is performed in, for example, the dynamic carrier sense operation unit  240  of the AP  200 . It is assumed that the AP  200  stores the carrier sense threshold received from the central control station  100  in the carrier sense threshold storage unit  220 . 
     Upon starting the process (S 30 ), the AP  200  receives the signal transmitted from the adjacent AP or the subordinate terminal  300 , and determines whether or not the preamble signal can be detected from the received signal during the carrier sense period (S 31 ). For example, the wireless reception unit  230  determines this process depending on whether or not the preamble signal can be detected from the received wireless signal. 
     When detecting the preamble (yes in S 31 ), the AP  200  extracts a basic service set (BSS) color included in the preamble signal. The BSS color is, for example, identification information for identifying the AP  200 . For example, the wireless reception unit  230  extracts the BSS color and outputs the BSS color to the dynamic carrier sense operation unit  240 . 
     Next, the AP  200  determines whether the BSS color is the BSS color of the AP  200  (S 33 ). 
     When the extracted BSS color is the BSS color of its own station (yes in S 33 ), the AP  200  determines that transmission is impossible (S 37 ). In this case, since the AP  200  does not perform transmission when the AP  200  receives the wireless signal of its own AP  200  station from the subordinate terminal  300  and the AP  200  performs reception, the AP  200  determines that transmission is impossible. For example, if it is checked that the BSS color received from the wireless reception unit  230  is the BSS color of its own AP  200 , the dynamic carrier sense operation unit  240  determines that transmission is impossible and notifies the wireless transmission unit  250  that transmission is impossible. The wireless transmission unit  250  does not perform the transmission process. 
     On the other hand, when the extracted BSS color is not the BSS color of its own station (NO in S 33 ), the AP  200  reads the threshold for each adjacent AP of each subordinate terminal, from the carrier sense threshold storage unit  220  (S 34 ). In this case, for example, the wireless signal received by the AP  200 - 1  is not the wireless signal addressed to the own AP  200 - 1  but the wireless signal transmitted by the adjacent AP  200 - 2 . 
     For example, the dynamic carrier sense operation unit  240  reads the carrier sense threshold from the carrier sense threshold storage unit  220  in the following manner. That is, when the packet data addressed to the subordinate terminal  300 - 1  or the subordinate terminal  300 - 2  is stored in a buffer or the like, the dynamic carrier sense operation unit  240  reads the packet data from the buffer. The dynamic carrier sense operation unit  240  reads the destination of the packet data. In addition, the dynamic carrier sense operation unit  240  determines adjacent APs between which the BSS color of the received signal is, based on a table or the like stored in the buffer. The dynamic carrier sense operation unit  240  sets the destination of the packet data stored in the buffer as “subordinate terminal”, and the adjacent AP determined from the BSS color as “adjacent AP”, and reads the carrier sense threshold corresponding to “subordinate terminal” and “adjacent AP” from the carrier sense threshold storage unit  220 . 
     By using the read carrier sense threshold, when the received power value of the preamble is larger than the carrier sense threshold (yes in S 35 ), the AP  200  determines that transmission is impossible (S 37 ); and when the received power value is equal to or less than the carrier sense threshold (no in S 35 ), the AP  200  determines that transmission is possible (S 36 ). For example, the dynamic carrier sense operation unit  240  compares the extracted carrier sense threshold and the received power value received from the wireless reception unit  230  to determine whether transmission is possible or not, and outputs the determined result to the wireless transmission unit  250 . According to the determined result, the wireless transmission unit  250  can transmit or not transmit a wireless signal. 
     On the other hand, when the preamble may not be detected (no in S 31 ), the AP  200  determines the signal as a wireless LAN signal, for example, a Bluetooth (registered trademark) signal, and compares the received power value and the energy detection threshold (S 38 ). The energy detection threshold is, for example, a threshold defined in IEEE802. 11ac and the like and is a threshold for avoiding a collision with a signal of another wireless system. When the received power value of the received signal is larger than the energy detection threshold (yes in S 38 ), the AP  200  determines that transmission is impossible (S 37 ); and when the received power value is equal to or less than the energy detection threshold (no in S 38 ), the AP  200  determines that transmission is possible (S 36 ). For example, the dynamic carrier sense operation unit  240  reads the energy detection threshold stored in the memory and compares it with the received power value received from the wireless reception unit  230 , thereby determining whether transmission is possible or not. 
     As described above, in the first embodiment, the central control station  100  sets a carrier sense threshold for the AP  200 , for each pair of the terminal  300  under control of the AP  200 - 1  and the adjacent AP  200 - 2  adjacent to the AP  200 . That is, the central control station  100  sets the carrier sense threshold for the subordinate terminal  300 - 2  of the adjacent AP  200 - 2 , which interferes in the terminal  300  under control of the AP  200 - 1  by the simultaneous communication, such that the simultaneous communication with the AP  200  is not performed (S 14  and S 23  in  FIG. 7 ). On the other hand, when the simultaneous communication with the adjacent AP  200 - 2  does not interfere, the central control station  100  sets the carrier sense thresholds with respect to the terminals  300 - 1  and  300 - 3  under control of the AP  200 - 1  and the adjacent AP, for each pair of the terminals  300 - 1  and  300 - 3  and the adjacent AP (S 16  to S 18  in  FIG. 7 ). 
     Accordingly, even when simultaneous communication with the adjacent AP  200 - 2  is not performed, the AP  200 - 1  can also communicate with the terminal  300 - 1  under control of the AP  200 - 1  (S 17 ). Therefore, in the present wireless communication system  10 , as compared with the case where communication with the subordinate terminal  300  is not performed since simultaneous communication may not be performed between the AP  200 - 1  and the adjacent AP  200 - 2 , the opportunity for transmission of the AP  200 - 1  increases and the throughput of the entire system can be improved. 
     Second Embodiment 
     Next, a second embodiment will be described. In the first embodiment, an example is described in which the central control station  100  sets the carrier sense threshold for the AP  200 - 1  for each pair of the subordinate terminal of the AP  200 - 1  and the adjacent AP  200 - 2 . The second embodiment represents an example in which the central control station  100  sets a carrier sense threshold for each pair of the subordinate terminals  300 - 1  and  300 - 3  of the AP  200 - 1  and the subordinate terminal  300 - 2  of the adjacent AP  200 - 2 . 
       FIG. 10  is a flowchart illustrating an operation example in the second embodiment. The flowchart illustrated in  FIG. 10  is performed, for example, in the carrier sense threshold calculation unit  130  of the central control station  100 . The example in  FIG. 6  will be described as in the first embodiment. 
     Upon starting the process (S 40 ), the central control station  100  sets i=1 and performs the following process for the AP( 1 ) (=AP  200 - 1 ) (S 41 ). 
     Next, the central control station  100  sets j=1 and performs the following process for the adjacent AP( 1 ) (=AP  200 - 2 ) of the AP  200 - 1  (S 42 ). 
     Next, the central control station  100  determines whether or not the received power value RSSI 1, 2  between adjacent APs is larger than the minimum NG power value (S 43 ). Since the minimum NG power value is not set at this stage, the process proceeds to S 44 . 
     In S 44 , the central control station  100  sets t=1 and performs the following process for the terminal STA 1  (=terminal  300 - 2 ) under control of the adjacent AP  200 - 2  (S 44 ). 
     Next, the central control station  100  determines whether or not the SINR of the terminal  300 - 2  at the time of performing simultaneous communication with the AP  200 - 1  is larger than the desired value (S 45 ). For example, the carrier sense threshold calculation unit  130  calculates the SINR in the terminal  300 - 2 , with the received power value for the adjacent AP  200 - 2  as S and the received power value for the AP  200 - 1  as I. In this case, it is assumed that the SINR of the terminal  300 - 2  is larger than the desired value. That is, even if the terminal  300 - 2  under control of the adjacent AP  200 - 2  performs simultaneous communication with the AP  200 - 1 , it is assumed that the terminal  300 - 2  can communicate with the adjacent AP  200 - 2  without an interference of the signal from the adjacent AP  200 - 1 . 
     When the SINR of the terminal  300 - 2  when performing simultaneous communication with the AP  200 - 1  is larger than the desired value (yes in S 45 ), the central control station  100  sets k=1, and performs the following process for the subordinate terminal STA  1  (=terminal  300 - 1 ) of the AP  200 - 1  (S 46 ). 
     Next, the central control station  100  determines whether or not the SINR when the subordinate terminal  300 - 1  of the AP  200 - 1  performs simultaneous communication with the adjacent AP  200 - 2  is larger than the desired value (S 47 ). For example, the carrier sense threshold calculation unit  130  calculates the SINR in the terminal  300 - 1 , with the received power value for the AP  200 - 1  as S and the received power value for the adjacent AP  200 - 2  as I. Here, it is assumed that the SINR of the terminal  300 - 1  is larger than the desired value. 
     When the SINR of the terminal  300 - 1  is larger than the desired value (yes in S 47 ), the central control station  100  sets the carrier sense threshold for the terminal  300 - 2  under control of the adjacent AP  200 - 2  of the terminal  300 - 1  to a value larger than the received power value SSI 1, 2  between adjacent APs (S 48 ). As described above, in this second embodiment, the central control station  100  sets a carrier sense threshold for each pair of the subordinate terminal  300 - 1  of the AP  200 - 1  and the subordinate terminal  300 - 2  of the adjacent AP  200 - 2 . 
     Next, the central control station  100  increments “k” and repeats the process of S 46  to S 48  for the other terminals  300  under control of the AP  200 - 1 . In the example of  FIG. 6 , the other terminals  300  under control of the AP  200 - 1  are only the terminal  300 - 3 , and the process of S 46  to S 48  is repeated for the terminal  300 - 3 . In this case, it is assumed that the SINR of the terminal  300 - 3  is less than the desired value. 
     When the SINR of the terminal  300 - 3  is less than the desired value (no in S 47 ), the central control station  100  sets the carrier sense threshold for the subordinate terminal  300 - 2  of the adjacent AP  200 - 2  of the terminal  300 - 3  to a value less than the received power value SSI 1, 2  between adjacent APs (S 50 ). In this case, simultaneous communication is not performed between the AP  200 - 1  and the subordinate terminal  300 - 3 , and the adjacent AP  200 - 2  and its subordinate terminal  300 - 2 . This is because the SINR of the terminal  300 - 3  is less than the desired value, and the terminal  300 - 3  may not normally receive the signal from the AP  200 - 1  due to an interference of the signal from the adjacent AP  200 - 2 , even if the simultaneous communication is performed. 
     Upon completion of the process for all the subordinate terminals  300 - 1  and  300 - 3  of the AP  200 - 1  (S 49 ), the central control station  100  increments “t”, and repeats the process from S 44  to S 50  for other subordinate terminals other than the subordinate terminal  300 - 2  of the adjacent AP  200 - 2 . In the example of  FIG. 6 , since the subordinate terminal of the adjacent AP  200 - 2  is only the terminal  300 - 2 , the loop process of the adjacent AP subordinate terminal (S 44  to S 51 ) is completed. 
     Upon completion of the loop process of the adjacent AP subordinate terminal, the central control station  100  repeats the process of S 42  to S 52  and S 54  to S 56  for other adjacent APs other than the adjacent AP  200 - 2  (S 52 ). In the example of  FIG. 6 , since the adjacent AP of the AP  200 - 1  is only the adjacent AP  200 - 2  and there is no other adjacent AP, the adjacent AP loop is completed. 
     Upon completion of the process for all adjacent APs of the AP  200 - 1  (S 52 ), the central control station  100  increments “i”, and repeats the process from S 42  to S 56  for the AP( 2 ) (=AP  200 - 2 ). 
     The process for the AP  200 - 2  is as follows. That is, since the minimum NG power value is not set, the central control station  100  determines whether or not the SINR during simultaneous communication is larger than the desired value, with respective to the subordinate terminal  300 - 1  of the adjacent AP  200 - 1  of the AP  200 - 2  (S 45 ). In this case, the central control station  100  assumes that the SINR of the terminal  300 - 1  is larger than the desired value (yes in S 45 ) and the SINR of the subordinate terminal  300 - 2  of the AP  200 - 2  is also larger than the desired value (yes in S 47 ). In this case, the central control station  100  sets the carrier sense threshold for the terminal  300 - 1  of the terminal  300 - 2  to a value larger than the received power value SSI 2, 1  between adjacent APs (S 48 ). That is, even if simultaneous communication is performed between AP  200 - 2  and its subordinate terminal  300 - 3 , and the adjacent AP  200 - 1  and its subordinate terminal  300 - 1 , communication with the AP becomes possible without an interference of signals from other APs. 
     On the other hand, with respect to the subordinate terminal  300 - 3  of the adjacent AP  200 - 1 , when the SINR is less than the desired value (no in S 45 ), the minimum NG power value is set to the received power value SSI 2, 1  between the adjacent APs (S 54 ). The central control station  100  sets the carrier sense threshold for the subordinate terminal  300 - 3  of the adjacent AP  200 - 1  of all the terminals  300 - 2  under control of the AP  200 - 2  to a value less than the received power value SSI 2, 1  between the adjacent APs (no in S 47 , and S 50 ). That is, simultaneous communication is not performed between the AP  200 - 2  and all the terminals  300 - 2  under its control, and the adjacent AP  200 - 1  and its subordinate terminal  300 - 3 . When such simultaneous communication is performed, this is because the terminal  300 - 3  may not be able to communicate with the AP  200 - 1  due to an interference of the signal from the AP  200 - 2 . 
     The central control station  100  completes the process for the subordinate terminals  300 - 1  and  300 - 3  of the adjacent AP  200 - 1  of the AP  200 - 2  (S 51 ), and performs the process for the subordinate terminal  300 - 4  of the adjacent AP  200 - 3 . 
     In this case, the central control station  100  assumes that the minimum NG power value is set and the received power value RSSI 2, 3  between the adjacent APs is larger than the minimum NG power value (=RSSI 2, 1 ). In this case, the central control station  100  determines yes in S 43 , sets the carrier sense threshold for all the terminals  300 - 4  under control of the AP  200 - 3  of all the terminals  300 - 2  under control of the AP  200 - 2  to a value less than RSSI 2, 3  (S 56 ). That is, the central control station  100  sets the carrier sense threshold such that simultaneous communication is impossible for the terminal  300 - 2  and the terminal  300 - 4 . Also in this case, the central control station  100  protects the subordinate terminal  300 - 3  of the AP  200 - 1  for which the AP  200 - 1  and the AP  200 - 2  are not able to perform simultaneous communication. 
     The central control station  100  completes the process for all the subordinate terminals  300 - 3  of the adjacent AP  200 - 3  (S 51 ), completes the process for the adjacent APs  200 - 1  and  200 - 3  (S 52 ), and increments “i”, and performs the process for the AP  200 - 3 . After that, since the process described above is repeated, the description thereof will be omitted. 
       FIG. 11A  and  FIG. 11B  respectively illustrate the setting examples of the carrier sense thresholds for the AP  200 - 1  and the AP  200 - 2 . As illustrated in  FIG. 11A  and  FIG. 11B , the central control station  100  sets a carrier sense threshold for each pair of the subordinate terminal  300 - 1  of the AP  200 - 1  and the subordinate terminal  300 - 2  of the adjacent AP  200 - 2 . The central control station  100  transmits the set carrier sense threshold to the AP  200 .  FIG. 11A  and  FIG. 11B  represent examples of the carrier sense thresholds stored in the carrier sense threshold storage units  220  of the respective AP  200 - 1  and  200 - 2 . 
       FIG. 12  is a flowchart illustrating an operation example of the AP  200  that receives the carrier sense threshold. In  FIG. 12 , the same process as in the first embodiment is denoted by the same reference numeral. 
     In this case, the carrier sense threshold is set for a pair of the subordinate terminals  300 - 1  and  300 - 3  of the AP  200 - 1  and the subordinate terminal  300 - 2  of the adjacent AP  200 - 2 . Therefore, the dynamic carrier sense operation unit  240  reads, for example, the carrier sense threshold from the carrier sense threshold storage unit  220  in the following manner. 
     That is, if the AP  200 - 2  receives the signal transmitted from the AP  200 - 1 , the dynamic carrier sense operation unit  240  receives the signal which is transmitted from the other AP  200 - 1  and is not addressed to the AP  200 - 2  (no in S 33 ). In this case, the dynamic carrier sense operation unit  240  receives the packet information extracted from the received signal from the wireless reception unit  230 , and extracts the destination (for example, the terminal  300 - 1 ) of the in-communication packet. Further, when the AP  200 - 2  transmits a packet to its own subordinate terminal  300 - 2 , the dynamic carrier sense operation unit  240  reads the destination (for example, the terminal  300 - 2 ) from the packet stored in the buffer. The dynamic carrier sense operation unit  240  specifies “subordinate terminal” corresponding to the destination (for example, the terminal  300 - 2 ) read from the packet stored in the buffer, and “adjacent AP subordinate terminal” corresponding to the destination (for example, the terminal  300 - 1 ) specified from the in-communication packet. The dynamic carrier sense operation unit  240  reads the carrier sense thresholds corresponding to the “subordinate terminal” (for example, STA  2 =the terminal  300 - 2 ) and “adjacent AP subordinate terminal” (for example, STA  1 =the terminal  300 - 1 ), from the carrier sense threshold storage unit  220 . 
     Thereafter, similarly to the first embodiment, the dynamic carrier sense operation unit  240  may determine whether transmission is possible or not (S 36  and S 37 ) using the read carrier sense threshold, and output the result to the wireless transmission unit  250 . 
     In this second embodiment, the central control station  100  sets a carrier sense threshold for each pair of the subordinate terminal of the AP  200 - 1  and the subordinate terminal of the adjacent AP  200 - 2  as a carrier sense threshold for the AP  200 - 1 . In this way, since the carrier sense threshold is set for each of the subordinate terminal and the subordinate terminal of the adjacent AP, the subordinate terminal  300 - 3  of the AP  200 - 1  is not allowed to perform simultaneous communication, while the subordinate terminal  300 - 1  of the AP  200 - 1  is allowed to perform simultaneous communication. For example, in the example of  FIG. 6 , the communication between the AP  200 - 1  and the terminal  300 - 3  and the communication between the AP  200 - 2  and the terminal  300 - 2  are not performed at the same time, while the communication between the AP  200 - 1  and the terminal  300 - 1  and the communication between the AP  200 - 2  and the terminal  300 - 2  can be performed at the same time. Therefore, in the present wireless communication system  10 , simultaneous communication may become possible in some cases as compared with the case where simultaneous communication is not performed, so that the opportunity for transmission of the AP  200 - 1  increases, and the throughput of the entire system can be improved. 
     Third Embodiment 
     Next, a third embodiment will be described. In the first and second embodiments, the central control station  100  compares the SINR with the desired value to evaluate the performance of simultaneous communication (for example, S 14  in  FIG. 7 ). In the third embodiment, the performance of simultaneous communication may be evaluated based on the communication capacity based on SINR. 
       FIG. 13  illustrates a configuration example of the wireless communication system  10  when the AP  200 - 2  evaluates simultaneous communication performance for the adjacent AP  200 - 1 . This case will be described below as an example. 
     For example, the carrier sense threshold calculation unit  130  evaluates the performance of simultaneous communication for the subordinate terminal  300 - 2  (S 15  to S 19 ) when the following is satisfied instead of S 14  in  FIG. 7 . 
     
       
         
           
             
               
                 
                   
                     C 
                      
                     
                       ( 
                       
                         
                           S 
                            
                           
                               
                           
                            
                           1 
                         
                         N 
                       
                       ) 
                     
                   
                   &lt; 
                   
                     
                       C 
                        
                       
                         ( 
                         
                           
                             S 
                              
                             
                                 
                             
                              
                             1 
                           
                           
                             N 
                             + 
                             
                               I 
                                
                               
                                   
                               
                                
                               2 
                             
                           
                         
                         ) 
                       
                     
                     + 
                     
                       C 
                        
                       
                         ( 
                         
                           
                             S 
                              
                             
                                 
                             
                              
                             2 
                           
                           
                             N 
                             + 
                             
                               I 
                                
                               
                                   
                               
                                
                               1 
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     In Expression (1), C ( ) represents the communication capacity based on the SINR. The whole Expression (1) represents that simultaneous communication is possible, if the communication capacity when the AP  200 - 1  and the AP  200 - 2  perform simultaneous communication (the right side of Expression (1)) is larger than the communication capacity when the AP  200 - 1  alone performs communication (the left side of Expression (1)). In  FIG. 7 , the carrier sense threshold calculation unit  130  proceeds to S 15  when Expression (1) is satisfied, and it proceeds to S 22  and proceeds with the process when Expression (1) is not satisfied. Even in S 16  of  FIG. 7 , the carrier sense threshold calculation unit  130  may make a determination by using Expression (1). In this case, in Expression (1), S1 is I1, S2 is I2, I1 is S1, and I2 is S2. 
     Fourth Embodiment 
       FIG. 14  is a diagram illustrating a configuration example of a wireless communication system  10  in a fourth embodiment. The wireless communication system  10  includes a control apparatus  100 , first and second base stations  200 - 1  and  200 - 2 , and a first terminal  300 - 1 . 
     The first and second base stations  200 - 1  and  200 - 2  are adjacent to each other. Further, the first base station  200 - 1  has the first terminal  300 - 1  as a subordinate terminal. 
     The control apparatus  100  corresponds to, for example, the central control station  100  in the first embodiment. Further, for example, the first base station  200 - 1  corresponds to the AP  200 - 1  in the first embodiment, and the second base station  200 - 2  corresponds to the AP  200 - 2  in the first embodiment. Further, the first terminal  300 - 1  corresponds to, for example, the terminal  300 - 1  in the first embodiment. 
     The control apparatus  100  includes a reception unit  110 , a carrier sense threshold calculation unit  130 , and a transmission unit  140 . 
     In the first terminal  300 - 1 , the reception unit  110  receives the first received power value for the first signal received from the first base station  200 - 1  and the second received power value for the second signal received from the second base station  200 - 2  adjacent to the first base station  200 - 1 . The reception unit  110  receives the first and second received power values from the first terminal  300 - 1 , through the first base station  200 - 1  or the second base station  200 - 2 . 
     The carrier sense threshold calculation unit  130  calculates the carrier sense threshold for each pair of the first terminal  300 - 1  under control of the first base station and the second base station, based on the received first and second received power values. 
     The example of  FIG. 14  represents an example in which there is one terminal under control of the first base station  200 - 1 . For example, if there are a plurality of terminals such as the first and second terminals, the carrier sense threshold calculation unit  130  calculates the carrier sense threshold also for the pair of the second terminal and the second base station  200 - 2 . Further, with respect to base stations adjacent to the first base station  200 - 1 , if there are a plurality of base stations such as the second and third base stations, the carrier sense threshold calculation unit  130  calculates the carrier sense threshold for each pair of the first terminal  300 - 1  and the third base station. 
     The transmission unit  140  transmits the carrier sense threshold to the first base station  200 - 1 . 
     As described above, in the fourth embodiment, with respect to the first base station  200 - 1 , the carrier sense threshold is set for each pair of the subordinate terminal  300 - 1  and the second base station  200 - 2  adjacent to the first base station  200 - 1 . Therefore, the first base station  200 - 1  may be able to transmit a wireless signal to the first terminal  300 - 1 , by simultaneous communication with the second base station  200 - 2 , depending on the set carrier sense threshold. Alternatively, the first base station  200 - 1  may not transmit a wireless signal to the other terminal under control of the first base station  200 - 1  by simultaneous communication with the second base station  200 - 2 , depending on the carrier sense threshold. In a case where simultaneous communication is not possible, as compared to a case where the first base station  200 - 1  may not transmit a wireless signal to all the terminals under its control, it is possible to transmit a wireless signal like the first terminal  300 - 1  in some cases, such that the transmission opportunity of the first base station  200 - 1  increases. This makes it possible to increase the throughput of the entire wireless communication system  10 . 
     Other Embodiments 
     In the first to fourth embodiments, a description is made in which the central control station  100  transmits the carrier sense threshold to the AP  200 . For example, the AP  200  transmits the carrier sense threshold received from the central control station  100  to the subordinate terminal  300 , and the subordinate terminal  300  may communicate with the AP  200  by using the carrier sense threshold. In this case, for example, the subordinate terminal  300  may perform wireless communication in the DL direction using the carrier sense threshold received from the AP  200  or may perform wireless communication in the UL direction. In the latter case, the AP  200  may perform wireless communication with the terminal  300  in the UL direction using the carrier sense threshold received from the central control station. 
     Further, in the first to fourth embodiments, an example is described in which the central control station  100  calculates the carrier sense threshold and transmits it to the AP  200 . For example, the AP  200 - 1  may calculate the carrier sense threshold, and transmit it to other APs  200 - 2  and  200 - 3 . In this case, the AP  200 - 1  may include a reception unit  110 , an adjacent AP identification information generation unit  120 , a carrier sense threshold calculation unit  130 , and a transmission unit  140 . The carrier sense threshold calculation unit  130  receives the received power values of the subordinate terminals  300 - 1  and  300 - 3  through the reception unit  110 , calculates a carrier sense threshold for each pair of the respective subordinate terminals  300 - 1  and  300 - 3  and the adjacent AP  200 - 2 , based on the received power value, and transmits it to the adjacent AP  200 - 2 . 
       FIG. 14A  to  FIG. 15  each illustrates a configuration example of a hardware block of each of the devices  100 ,  200 , and  300  in the wireless communication system  10 . Even with such a configuration example, it is possible to execute the operations described in the first and second embodiments. 
       FIG. 14A  is a diagram illustrating a hardware configuration example of the central control station  100 . The central control station  100  includes a memory  180 , a central processing unit (CPU)  181 , and a wired interface (IF)  182 . 
     The CPU  181  can realize the functions of the adjacent AP identification information generation unit  120  and the carrier sense threshold calculation unit  130 , described in the first and second embodiments, by reading the program stored in the memory  180  and executing the program. The CPU  181  corresponds to, for example, the adjacent AP identification information generation unit  120  and the carrier sense threshold calculation unit  130 . 
     The wired IF  182  is connected to the AP  200  and exchanges the received power value and the carrier sense threshold with the AP  200 . The wired IF  182  corresponds to, for example, the reception unit  110  and the transmission unit  140  in the first and second embodiments. 
       FIG. 14B  is a diagram illustrating a hardware configuration example of the AP  200 . The AP  200  includes a memory  280 , a CPU  281 , a wireless unit  282 , an antenna  283 , and a wired IF  284 . 
     The CPU  281  realizes the function of the dynamic carrier sense operation unit  240  in the first and second embodiments, by reading the program stored in the memory  280  and executing the program. The CPU  281  corresponds to, for example, the dynamic carrier sense operation unit  240 . 
     Further, the memory  280  corresponds to, for example, the carrier sense threshold storage unit  220  in the first embodiment. Further, the wireless unit  282  and the antenna  283  are a portion that performs a process for the wireless signal, and correspond to the wireless reception unit  230  and the wireless transmission unit  250  in the first and second embodiments. Further, the wired IF  284  exchanges the received power value, the carrier sense threshold, and the like with the central control station  100 , and corresponds to the control station-side reception unit  210  and the control station-side transmission unit  260  in the first and second embodiments. 
       FIG. 15  is a diagram illustrating a hardware configuration example of the terminal  300 . The terminal  300  includes a memory  380 , a CPU  381 , a wireless unit  382 , and an antenna  383 . 
     The CPU  381  executes the function of the information acquisition unit  320  in the first and second embodiments, by reading the program stored in the memory  380  and executing the program. The CPU  381  corresponds to, for example, the information acquisition unit  320 . Further, the wireless unit  382  and the antenna  383  correspond to, for example, the wireless reception unit  310  and the wireless transmission unit  330  in the first and second embodiments. 
     In addition, in the examples described above, the respective CPUs  181 ,  281 , and  381  may be, for example, a control unit, a controller, or a processor, such as a micro processing unit (MPU), a field-programmable gate array (FPGA), or a digital signal processor (DSP). Further, the wireless units  282  and  382  may be, for example, a control unit, a controller, or a processor, such as a DSP or an FPGA. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.