Patent Publication Number: US-2023142888-A1

Title: Wireless communication system, base station control device, communication control method, and communication control program

Description:
TECHNICAL FIELD 
     The present invention relates to a wireless communication system, a base station control device, a communication control method, and a communication control program. 
     BACKGROUND ART 
     In recent years, a wide spread of high-performance portable wireless terminals such as notebook computers and smartphones has led a wireless local area network (LAN) which complies with IEEE802.11 standards to be widely used also at home, let alone at companies and in publicly accessible spaces. 
     The wireless LAN which complies with IEEE802.11 standards includes a wireless LAN complying with IEEE802.11b/g/n standards in which a 2.4 GHz band is used and a wireless LAN complying with IEEE802.11a/n/ac standards in which a 5 GHz band is used. 
     In the wireless LAN complying with IEEE802.11b standards and IEEE802.11g standards, 13 channels are prepared at intervals of 5 MHz from 2400 MHz to 2483.5 MHz. However, in a case where a plurality of channels are used at the same location, it is possible to use up to three channels or four channels at the same time by using channels so that spectra do not overlap with each other to avoid interference. 
     In the wireless LAN complying with IEEE802.11a standards, a total of 19 channels including 8 channels and 11 channels which do not overlap with each other are respectively defined between 5170 MHz and 5330 MHz and between 5490 MHz and 5710 MHz in Japan. Note that in IEEE802.11a standards, a band width per channel is fixed at 20 MHz. 
     Maximum transmission speed of the wireless LAN is 11 Mbps in IEEE802.11b standards and is 54 Mbps in IEEE802.11a standards and IEEE802.11g standards. However, the transmission speed here is transmission speed on a physical layer. 
     Actually, transmission efficiency in a medium access control (MAC) layer is approximately from 50 to 70%, and thus, an upper limit value of throughput is approximately 5 Mbps in IEEE802.11b standards and is approximately 30 Mbps in IEEE802.11a standards and IEEE802.11g standards. Further, transmission speed further decreases if the number of wireless stations which try to transmit information increases. 
     Meanwhile, in a wired LAN, as well as a 100 Base-T interface of Ethernet (registered trademark), fiber to the home (FTTH) which uses an optical fiber has been spread also at every home, and high-speed lines on the order of 100 Mbps to 1 Gbps are provided. Thus, further higher transmission speed is desired also in the wireless LAN. 
     In IEEE802.11n standards for which standardization has been completed in 2009, a channel band width which has been fixed at 20 MHz so far is expanded to up to 40 MHz, and introduction of multiple input multiple output (MIMO) technique is determined. If transmission and reception are performed by applying all functions defined in IEEE802.11n standards, communication speed of up to 600 Mbps can be achieved in a physical layer. 
     Further, in IEEE802.11ac standards for which standardization has been completed in 2013, it is determined to expand a channel band width to 80 MHz or up to 160 MHz (or 80 + 80 MHz) and introduce a transmission method of multiuser MIMO (MU-MIMO) in which space division multiple access (SDMA) is applied. If transmission and reception are performed by applying all functions defined in IEEE802.11ac standards, communication speed of up to approximately 6.9 Gbps can be achieved in a physical layer. 
     Further, in IEEE802.11ax standards which are currently under formulation, orthogonal frequency division multiple access (OFDMA) which enables transmission and reception of frames by dividing channels of 20 MHz, 40 MHz, 80 MHz, 160 MHz and 80 + 80 MHz described above into finer sub-channels, is expected to be defined. Use of OFDMA enables simultaneous transmission by a plurality of wireless stations in unit of a resource unit by dividing the above-described channels into finer sub-channels. Further, a function of increasing communication opportunities while reducing interference from other peripheral cells by control of a carrier sense threshold (CCA threshold) is expected to be defined in IEEE802.11ax standards. 
     The wireless LAN complying with IEEE802.11 standards is operated in a frequency band of a 2.4 GHz band or a 5 GHz band for which license is not required. In this event, in a case where a base station complying with IEEE802.11 standards forms a wireless LAN cell (basic service set (BSS)), the base station selects one frequency channel from frequency channels which can be supported at the own station and operates the frequency channel. 
     A channel to be used at the own cell, setting values of a band width and other parameters, and other parameters which can be supported at the own station are described in a Beacon frame which is periodically transmitted, a Probe response frame for a Probe Request frame received from a wireless terminal, or the like. Further, the base station operates the cell by transmitting a frame on the frequency channel which the base station determines to operate and notifying subordinate wireless terminals and other peripheral wireless stations. 
     Examples of a method for selecting and setting a frequency channel, a band width and other parameters at the base station can include the following four methods:
     (1) a method in which default parameter values set in advance at the base station are used as is,   (2) a method in which values manually set by a user who operates the base station are used,   (3) a method in which parameter values are autonomously selected and set based on wireless environment information detected by each base station upon start-up, and   (4) a method in which parameter values determined by a central control station such as a wireless LAN controller are set.   

     Further, the number of channels which can be used at the same time at the same location is 3 in a wireless LAN of a 2.4 GHz band, and 2, 4, 9 or 19 in a wireless LAN of a 5 GHz band in accordance with a channel band width to be used for communication. Thus, in a case where a wireless LAN is actually introduced, it is necessary to select a channel to be used by the base station within the own BSS (see, for example, Non-Patent Literature 1 and Non-Patent Literature 2). 
     While the number of channels which can be used at the same time at the same location in a 5 GHz band is 19 channels in a case where the channel band width is 20 MHz, in a case where the channel band width is expanded to 40 MHz, 80 MHz, 160 MHz or 80 + 80 MHz, the number of channels which can be used at the same time at the same location in a 5 GHz band decreases to 9 channels, 4 channels, and 2 channels. In other words, the number of channels which can be used decreases as the channel band width increases. 
     In a wireless LAN dense environment where the number of channels which can be used is larger than the number of BSSs, a plurality of BSSs use the same channel (overlapping BSS (OBSS)). Thus, in the wireless LAN, autonomous distributed access control in which data is transmitted only in a case where a channel is available by carrier sense is used using carrier sense multiple access with collision avoidance (CSMA/CA). 
     Specifically, a wireless station at which a transmission request occurs first performs carrier sense in a predetermined sensing period (distributed interframe space (DIFS)) to monitor a state of a wireless medium, and if there is no transmission signal by other wireless stations in this period, performs random back-off. The wireless station continuously performs carrier sense also during a random back-off period, and in a case where there is no transmission signal by other wireless stations also in this period, obtains right to utilize a channel. 
     Note that whether there is transmission/reception by other wireless stations is determined by whether or not a signal greater than a carrier sense threshold set in advance is received. The wireless station which obtains the right to utilize the channel can transmit data to other wireless stations within the same BSS and can receive data from these other wireless stations. 
     In a case where such CSMA/CA control is performed, in a wireless LAN dense environment where the same channel is used, a channel becomes busy more frequently by carrier sense, which lowers throughput. It is therefore important to monitor a surrounding environment, select an appropriate channel and select a transmission power value and a carrier sense threshold which enable simultaneous transmission and reception. 
     Further, a method for selecting the above-described parameters such as a type of 2.4 GHz or 5 GHz which is an operating frequency band of the base station and a channel to be utilized in the operating frequency band is not defined in IEEE802.11 standards, and thus, respective vendors which supply base stations employ individual methods. 
     CITATION LIST 
     Non-Patent Literature 
     Non-Patent Literature 1: supervised by Masahiro Morikura, Shuji Kubota, “802.11 high-speed wireless LAN textbook”, revised third edition, Impress R&amp;D, March, 2008 
     SUMMARY OF THE INVENTION 
     Technical Problem 
     However, respective wireless stations select the above-described parameters in an autonomous distributed manner, which makes it impossible to achieve optimization as the whole system, and particularly, in an environment where there are a number of wireless stations, there is a case where user quality largely degrades. 
     Further, in recent years, the number of wireless stations at which a plurality of wireless modules are mounted has increased. This is because a band width to be used can be expanded and user throughput within a service area can be increased by mounting a plurality of wireless modules in the same chassis and using different frequency bands and different channels. 
     Further, in an environment where a plurality of wireless stations or wireless modules are operated, while a terminal station needs to select an appropriate wireless station to be connected and connect to the wireless station, factors such as environmental change may force the terminal station to switch the wireless station to which it is being connected. Due to this switching of the wireless station to be connected, there is a case where unnecessary control signals fly around and quality degrades in association with communication disconnection due to switching of the wireless station. 
     An object of the present invention is to provide a wireless communication system, a base station control device, a communication control method, and a communication control program, with which degradation of communication quality due to a terminal station switching a connection destination can be prevented and efficient wireless communication can be achieved as the whole system. 
     Means for Solving the Problem 
     A wireless communication system according to one aspect of the present invention is a wireless communication system which includes a plurality of base stations to which one or more terminal stations are connectable, and a base station control device which controls each of the base stations, the base station control device including an information collection unit configured to collect connection information regarding the terminal stations connected to the base stations from each of the base stations, a list generation unit configured to generate a white list indicating the terminal stations which are allowed to be connected to the base stations for each of the base stations based on the connection information, and a transmission unit configured to transmit the white list generated by the list generation unit to each of the base stations, and each of the base stations including a reception unit configured to receive the white list transmitted by the transmission unit, and a setting unit configured to perform setting so as to allow or inhibit connection of each of the terminal stations to the base station based on the white list received by the reception unit. 
     Further, a base station control device according to one aspect of the present invention is a base station control device which controls each of a plurality of base stations to which terminal stations are connectable, the base station control device including an information collection unit configured to collect connection information regarding the terminal stations connected to the base stations from each of the base stations, a list generation unit configured to generate a white list indicating the terminal stations which are allowed to be connected to the base stations for each of the base stations based on the connection information, and a transmission unit configured to transmit the white list generated by the list generation unit to each of the base stations. 
     Further, a communication control method according to one aspect of the present invention is a communication control method for controlling each of a plurality of base stations to which terminal stations are connectable, the communication control method including an information collection step of collecting connection information regarding the terminal stations connected to the base stations from each of the base stations, a list generation step of generating a white list indicating the terminal stations which are allowed to be connected to the base stations for each of the base stations based on the connection information, and a transmission step of transmitting the generated white list to each of the base stations. 
     Effects of the Invention 
     According to the present invention, it is possible to prevent degradation of communication quality due to a terminal station switching a connection destination and achieve efficient wireless communication as the whole system. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    illustrates a configuration example of a wireless communication system according to one embodiment. 
         FIG.  2    is a functional block diagram illustrating an example of functions of a terminal station. 
         FIG.  3    is a functional block diagram illustrating an example of functions of a base station. 
         FIG.  4    is a functional block diagram illustrating an example of functions of a base station control device according to one embodiment. 
         FIG.  5    is a flowchart illustrating an operation example of the base station control device according to one embodiment. 
         FIG.  6    illustrates an example of periodic operation to be performed by the wireless communication system according to one embodiment. 
         FIG.  7    is a flowchart illustrating a more detailed operation example of the wireless communication system according to one embodiment. 
         FIG.  8    illustrates results of the base station control device calculating one RSSI value for each of terminal stations for each of base stations. 
         FIG.  9    illustrates a list of connection destination candidate APs generated by the base station control device. 
         FIG.  10    illustrates a state where the base station control device performs processing using the list of connection destination candidate APs. 
         FIG.  11    illustrates a hardware configuration example of the base station control device according to one embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     One embodiment of a wireless communication system will be described below using the drawings.  FIG.  1    illustrates a configuration example of a wireless communication system  1  according to one embodiment. As illustrated in  FIG.  1   , the wireless communication system  1  is constituted by, for example, n base stations (APs)  2 - 1  to  2 - n  being respectively connected to a base station control device  4  via a network  10 . 
     For example, m terminal stations  6 - 1  to  6 - m  are located around the base stations  2 - 1  to  2 - n . Here, the terminal stations  6 - 1  to  6 - m  are connectable to at least one of the base stations  2 - 1  to  2 - n . 
     Note that while an example of a case will be described where the wireless communication system  1  operates while complying with, for example, IEEE802.11ax standards, the system is not limited to this, and the system may operate while complying with other communication standards. Hereinafter, a component which is not specified among a plurality of components as the base stations  2 - 1  to  2 - n  will be simply abbreviated as a base station  2 , or the like. 
     The terminal station  6  will be described first.  FIG.  2    is a functional block diagram illustrating an example of functions of the terminal station  6 . As illustrated in  FIG.  2   , the terminal station  6  includes, for example, a plurality of wireless communication units  60 , a collection unit  62 , a storage unit  64 , and a control unit  66 . 
     The wireless communication unit  60  includes a reception unit (acquisition unit)  600  and a transmission unit (notification unit)  602  and performs wireless communication with the base station  2  and other terminal stations  6 . 
     The reception unit  600  acquires information by receiving signals transmitted by, for example, the base station  2  and other terminal stations  6  and outputs the information to the collection unit  62 . The transmission unit  602  transmits (notifies) information, for example, stored in the storage unit  64  to the base station  2  and other terminal stations  6 . Note that the wireless communication units  60  may use different frequency bands or may employ different communication schemes or may perform communication using the same communication scheme. 
     The collection unit  62  collects wireless environment information, or the like, indicating wireless environments around, for example, the base station  2  and other terminal stations  6  via the wireless communication unit  60  and outputs the wireless environment information, or the like, to the storage unit  64 . It is assumed that the wireless environment information includes, for example, connection information regarding connection between the base station  2  and the terminal station  6 , such as strength of a received signal strength indicator (RSSI). The storage unit  64  stores the wireless environment information, or the like, collected by the collection unit  62 . 
     The control unit  66  includes a setting unit  660  and controls respective units which constitute the terminal station  6 . For example, the setting unit  660  performs setting for operation of the terminal station  6  based on information acquired by the wireless communication unit  60  from the base station  2 . 
     The base station  2  will be described next.  FIG.  3    is a functional block diagram illustrating an example of functions of the base station  2 . As illustrated in  FIG.  3   , the base station  2  includes, for example, a plurality of wireless communication units  20 , a collection unit  21 , a storage unit  22 , an own station information holding unit  23 , a network communication unit  24 , and a control unit  25 . 
     The wireless communication unit  20  includes a reception unit (acquisition unit)  200  and a transmission unit (notification unit)  202  and performs wireless communication with other base stations  2  and the terminal station  6 . 
     The reception unit  200  acquires information by receiving signals transmitted by, for example, other base stations  2  and the terminal station  6  and outputs the information to the collection unit  21 . The transmission unit  202  transmits (notifies) signals indicating, for example, information stored in the storage unit  64 , own station information (which will be described later) held by the own station information holding unit  23 , information acquired by the network communication unit  24  from the base station control device  4 , or the like, to other base stations  2  and the terminal station  6 . Note that the wireless communication units  20  may use different frequency bands or may employ different communication schemes or may perform communication using the same communication scheme. 
     The collection unit  21  collects wireless environment information, or the like, including a plurality of information items indicating, for example, wireless environments, or the like around other base stations  2  and the terminal station  6  from other base stations  2  and the terminal station  6  via the wireless communication units  20  and outputs the wireless environment information, or the like, to the storage unit  22 . Note that the wireless environment information includes, for example, information regarding communication and connection between the base station  2  and the terminal station  6  and information regarding an operation state of the base station  2 . The storage unit  22  stores the wireless environment information, or the like (including connection information), collected by the collection unit  21 . 
     The own station information holding unit  23  holds information regarding the base station  2 . For example, the own station information holding unit  23  holds own station information including specifications, functions, and the like, of the own station, such as a frequency band and a communication scheme to be used by the base station  2 , the number of connectable terminal stations and the number of wireless communication units  20 . 
     The network communication unit  24  includes a transmission unit (notification unit)  240  and a reception unit (acquisition unit)  242  and performs wired communication or wireless communication with the base station control device  4  via the network  10 . 
     The transmission unit  240  transmits (notifies) signals indicating, for example, the information stored in the storage unit  22  and the own station information held by the own station information holding unit  23  to the base station control device  4 . The reception unit  242  acquires information (for example, a white list, a parameter and the like, which will be described later) by receiving signals transmitted by the base station control device  4 . Further, the reception unit  242  outputs information which is received from the base station control device  4  and which should be transmitted to the terminal station  6 , to the wireless communication unit  20 . 
     The control unit  25  includes a setting unit  250  and controls respective units which constitute the base station  2 . For example, the setting unit  250  performs setting for operation of the base station  2  based on the information acquired by the network communication unit  24  from the base station control device  4 , the information acquired by the wireless communication unit  20  from the terminal station  6 , and the like. 
     For example, the setting unit  250  performs setting so as to allow or inhibit connection of each of the terminal stations  6  to the base station  2  based on a white list (WL) received by the reception unit  242 . Further, the setting unit  250  may perform setting for operation of the terminal station  6 . 
     The base station control device  4  will be described next.  FIG.  4    is a functional block diagram illustrating an example of functions of the base station control device  4  according to one embodiment. As illustrated in  FIG.  4   , the base station control device  4  includes, for example, an input unit  40 , an output unit  41 , a network communication unit  42 , an information collection unit  43 , a storage unit  44 , a list generation unit  45 , and a control unit  46 . 
     The input unit  40  accepts input (such as instructions and settings) by a worker with respect to the base station control device  4 . The output unit  41  outputs results, or the like, of processing by the base station control device  4  so as to show the results, or the like, to the worker. 
     The network communication unit  42 , which includes a reception unit (acquisition unit)  420  and a transmission unit (notification unit)  422 , performs wired communication or wireless communication with the base stations  2 - 1  to  2 - n  via the network  10 . 
     The reception unit  420  receives information respectively transmitted by the base stations  2 - 1  to  2 - n  and outputs the received information to the information collection unit  43 . For example, the reception unit  420  receives wireless environment information, or the like, (including the connection information) respectively collected by the base stations  2 - 1  to  2 - n  and outputs the wireless environment information, or the like, to the information collection unit  43 . The transmission unit  422  transmits information, or the like, processed by the base station control device  4  to the base stations  2 - 1  to  2 - n . For example, the transmission unit  422  transmits the white list generated by the list generation unit  45  to each of the base stations  2 - 1  to  2 - n . 
     The information collection unit  43  collects the information received by the reception unit  420  and outputs the information to the storage unit  44 . For example, the information collection unit  43  collects wireless environment information (including connection information) such as an operation log including a plurality of information items indicating wireless environments around each base station  2  and each terminal station  6  and a connection state from each of the base stations  2 - 1  to  2 - n  and causes the collected results to be stored in the storage unit  44 . 
     The information items included in the wireless environment information include, for example, strength of an RSSI, traffic, the number of terminal stations  6  connected to the base station  2  (the number of connected terminals), channel utilization, a data rate, a channel transition log, or the like. 
     The list generation unit  45  generates a white list (WL) indicating the terminal stations  6  which are allowed to be connected to the base station  2  for each of the base stations  2  based on, for example, the connection information stored in the storage unit  44 . Further, the list generation unit  45  may generate a black list (BL) indicating the terminal stations  6  which are inhibited from being connected to the base station  2  for each of the base stations  2 . 
     For example, the list generation unit  45  determines the base station  2  as a connection destination for the terminal station  6  which has been connected to only one of the base stations  2  and describes the terminal station  6  in the white list. 
     Further, the list generation unit  45  generates a white list so as to allow connection to one of the base stations  2  for each of the base stations  2  based on a threshold of signal strength determined in advance, the number of terminal stations which are allowed to be connected, a maximum number of connectable terminal stations, or the like, for the terminal station  6  which has been connected to two or more base stations  2 . 
     Further, in a case where there are a plurality of terminal stations  6  which have no reason to be inhibited from being connected to one of the base stations  2 , the list generation unit  45  generates a white list so as to set one of the base stations  2  to which the terminal station  6  has been connected last as a connection destination of the terminal station  6 . 
     In this event, the list generation unit  45  may generate a white list so as to specify one or more base stations  2  which become connection destinations for all the terminal stations  6  which are connected to the base stations  2 - 1  to  2 - n . 
     The control unit  46 , which includes the setting unit  460 , controls respective units which constitute the base station control device  4 . Further, the control unit  46  causes results obtained by respective units which constitutes the base station control device  4  processing information, to be stored in the storage unit  44 . 
     The setting unit  460  performs setting for the respective units which constitute the base station control device  4 . For example, the setting unit  460   performs setting for the information collection unit  43  and the list generation unit  45  based on settings input by the worker via the input unit  40 . 
     An operation example of the wireless communication system  1  will be described next.  FIG.  5    is a flowchart illustrating an operation example of the base station control device  4 . As illustrated in  FIG.  5   , the base station control device  4  collects wireless environment information including, for example, MAC addresses and RSSI values of the respective terminal stations  6  accommodated in the respective base stations (APs)  2  and own station information of the respective base stations  2  in an operation period during which connection is not restricted based on the white list (WL) or the black list (BL) (S 100 ). 
     Further, the base station control device  4  generates a white list of each base station  2  (S 102 ) and notifies each base station  2  of the white list (S 104 ). 
       FIG.  6    illustrates an example of periodic operation to be performed by the wireless communication system  1 . As illustrated in  FIG.  6   , the wireless communication system  1  periodically repeats an operation period A (information collection period) during which connection is not restricted based on the white list (or the black list) and an operation period B (connection restricted period: switching prevention period) during which connection is restricted based on the white list (or the black list). 
     In the operation period A, the base station control device  4  executes operation illustrated in  FIG.  5    to cause each of the base stations  2  to prepare for the operation period B. Each of the base stations  2  does not restrict switching of a connection destination of the terminal station  6 . In other words, the terminal station  6  itself determines the base station  2  which becomes a connection destination and connects to the base station  2 . 
     In this event, there is a possibility that the base station  2  which becomes a connection destination of the terminal station  6  may change in accordance with change, or the like, of a wireless environment. Further, there is also a possibility that a plurality of base stations  2  can accommodate one terminal station  6 . 
     In the operation period B, each of the base stations  2  restricts connection for each of the terminal stations  6  on the basis of the white list. Specifically, each base station  2  accommodates only the terminal stations  6  which are allowed in the white list notified from the base station control device  4  and inhibits connection from terminal stations other than the terminal stations  6  which are allowed in the white list. 
     A more detailed operation example of the wireless communication system  1  will be described next.  FIG.  7    is a flowchart illustrating a more detailed operation example of the wireless communication system  1 . 
     As illustrated in  FIG.  7   , first, each base station  2  to be controlled by the base station control device  4  creates a list of terminal stations  6  connected within an information collection period (S 200 ). 
     Each terminal station  6  collects wireless environment information including basic service set identifiers (BSSIDs) of the base stations  2  connected within the information collection period and signal strength information upon connection (S 202 ). The BSSID of the base station  2  is set as connection AP information. 
     Each base station  2  collects the wireless environment information from each connected terminal station  6  (S 204 ) and notifies the base station control device  4  of the collected wireless environment information along with own station information (S 206 ). 
     The base station control device  4  determines the base station  2  as a connection destination AP for the terminal station  6  including one piece of connection AP information (S 208 ). 
     The base station control device  4  sets the base station  2  which satisfies a condition of a “connection allowable terminal RSSI” set in advance as a connection destination candidate AP for other terminal stations  6  (S 210 ). The condition of the connection allowable terminal RSSI is, for example, a condition that a value of an RSSI is equal to or greater than a predetermined threshold. 
     The base station control device  4  sets the base station  2  to which the terminal station  6  has belonged last among the connection destination candidate APs as a temporal connection destination AP of the terminal station  6  (S 212 ). 
     The base station control device  4  calculates the number of terminal stations  6  which can be additionally connected for each of the base stations  2  and updates the list of the connection destination candidate APs (S 214 ). 
     The base station control device  4  determines the final connection destination AP for the terminal stations  6  in ascending order of the number of connection destination candidate APs of the terminal stations  6  and generates the WL (S 216 ). 
     The base station control device  4  notifies each base station  2  of the white list (S 218 ). 
     Then, each base station  2  performs setting (connection restriction: switching prevention) based on the white list (S 220 ). 
     Details of operation of the base station control device  4  creating a white list will be described next using a specific example. 
     The base station control device  4  generates a white list for each of the base stations  2  by performing the following processing from &lt;1&gt; to &lt;5&gt;. Here, description will be provided using an example of a case where four terminal stations  6 - 1  to  6 - 4  are located in an area where the base stations  2 - 1  to  2 - 5 , or the like, are provided. 
     &lt;1&gt; The base station control device  4  creates a list of the connection destination candidate APs for each of the terminal stations  6  by performing the following processing indicated from (A) to (C). 
     (A) Each of the terminal stations  6  collects BSSIDs (connection AP information) of all the base stations  2  connected within the information collection period and RSSI values upon connection. The base station control device  4  collects the connection AP information of the terminal stations  6  which belong to each of the base stations  2 - 1  to  2 - 5  and RSSI values within a past predetermined period (T WL _ RSSI _ AVG ) during which the terminal stations  6  are connected, from each of the base stations  2 - 1  to  2 - 5 . T WL _ RSSI _ AVG  is set at a period such as five minutes and ten minutes. 
     (B) The base station control device  4  calculates one RSSI value for each of the terminal stations  6 , for each of the base stations  2 . For example, the base station control device  4  selects the latest x (x = 0, 1, 2, ...) RSSI values from RSSI values acquired within T WL _ RSSI _ AVG  and calculates an average value of x RSSI values. In a case where an average value of all the RSSI values within T WL_RSSI_AVG  is calculated, x is set as x = 0. 
       FIG.  8    illustrates results of the base station control device  4  calculating one RSSI value for each of the terminal stations  6 , for each of the base stations  2 . 
     (C) In a case where the above-described condition (threshold) of the connection allowable terminal RSSI is set at -70 dBm, the base station control device  4  generates a list ( FIG.  9   ) of connection destination candidate APs for each of the terminal stations  6  using the results illustrated in  FIG.  8    and the condition of the connection allowable terminal RSSI. 
       FIG.  9    illustrates the list of the connection destination candidate APs generated by the base station control device  4 . The base station control device  4  generates a white list for each of the base stations  2  by performing the following processing from &lt;2&gt; to &lt;5&gt; using the list of the connection destination candidate APs illustrated in  FIG.  9   . 
       FIG.  10    illustrates a state (part of process) where the base station control device  4  performs processing from &lt;2&gt; to &lt;5&gt; using the list of the connection destination candidate APs illustrated in  FIG.  9   . 
     &lt;2&gt; For the terminal station  6  having only one connection destination candidate AP, the base station control device  4  determines the AP as the connection destination AP. In the example illustrated in  FIG.  10   , the connection destination candidate AP for the terminal station  6 - 2  is only the base station  2 - 2 , and thus, the base station control device  4  determines the base station  2 - 2  as the connection destination AP of the terminal station  6 - 1 . 
     &lt;3&gt; The base station control device  4  determines whether or not the base station  2  to which the terminal station  6  for which a connection destination has not been determined yet has belonged last within T WL _ RSSI _ AVG  is included in the connection destination candidate APs. In a case where the base station  2  to which the terminal station  6  has belonged last within T WL _ RSSI _ AVG  is included in the connection destination candidate APs, the base station control device  4  sets the base station  2  as the connection destination AP. This is because it eliminates the need of switching of the connection destination and stabilize wireless communication. 
     For example, in a case where information collected for the terminal station  6 - 3  is as follows, as illustrated in  FIG.  10   , the base station  2 - 1  is set as the connection destination AP of the terminal station  6 - 3 . 
     In a case where the terminal station  6 - 3  belongs to the base station  2 - 3  at collection time =  11 :30 
   (the base station  2 - 3  is included in the connection destination candidate APs)   
   In a case where the terminal station  6 - 3  belongs to the base station  2 - 2  at collection time =  11 :32 
   (the base station  2 - 2  is not included in the connection destination candidate APs)   
   In a case where the terminal station  6 - 3  belongs to the base station  2 - 1  at collection time =  11 :35 
   (the base station  2 - 1  is included in the connection destination candidate APs)   
   

     &lt;4&gt; The base station control device  4  calculates the remaining number of terminal stations which can be accommodated in each of the base stations  2  (the number of terminal stations which can be additionally connected) based on the number of connectable terminal stations included in the own station information of each of the base stations  2 . The base station control device  4  then updates the list of the connection destination candidate APs for the terminal station  6  for which a connection destination has not been determined yet based on the number of terminal stations which can be additionally connected to each of the base stations  2 . 
     Here, the number of terminal stations which can be connected to each of the base stations  2  is set at “one”, and the number of terminal stations which can be additionally connected to the base station  2  to which one terminal station  6  is connected is set at “0”. 
     &lt;5&gt; In this event, the terminal stations  6  to be connected to the base stations  2 - 3  to  2 - 5  are not determined. The base station control device  4  determines the connection destination AP for the terminal stations  6   for which connection destinations have not been determined yet in ascending order of the number of connection destination candidate APs of the terminal stations  6 . Here, in a case where there are a plurality of connection destination candidate APs, the base station control device  4  selects a connection destination AP which allows connection of the smallest number of terminal stations  6  in the white list. 
     For example, in the example illustrated in  FIG.  10   , only the base station  2 - 3  is left as the connection destination candidate AP of the terminal station  6 - 1 . Meanwhile, the base stations  2 - 4  and  2 - 5  are left as the connection destination candidate APs of the terminal station  6 - 4 . Thus, the base station control device  4  first determines the base station  2 - 3  as the connection destination AP of the terminal station  6 - 1 , and then, determines one of the base stations  2 - 4  and  2 - 5  as the connection destination AP of the base station  6 - 4 . 
     In this manner, the wireless communication system  1  generates a white list indicating terminal stations  6  which are allowed to be connected to the base station  2  for each of the base stations  2  based on the connection information, so that it is possible to prevent degradation of communication quality due to the terminal station  6  switching a connection destination and achieve efficient wireless communication as the whole system. 
     Note that part or all of respective functions of the base station  2 , the base station control device  4  and the terminal station  6  may be implemented with hardware such as a programmable logic device (PLD) and a field programmable gate array (FPGA) or may be implemented as a program to be executed by a processor such as a CPU. 
     For example, the base station control device  4  according to the present invention can be implemented using a computer and a program, and the program can be recorded in a storage medium or can be provided through a network. 
       FIG.  11    illustrates a hardware configuration example of the base station control device  4  (the base station  2 , the terminal station  6 ) according to one embodiment. As illustrated in  FIG.  11   , for example, the base station control device  4 , to which an input unit  500 , an output unit  510 , a communication unit  520 , a CPU  530 , a memory  540  and an HDD  550  are connected via a bus  560 , has functions as a computer. Further, the base station control device  4  can input/output data to/from a computer-readable storage medium  570 . 
     The input unit  500  is, for example, a keyboard, a mouse, or the like. The output unit  510  is, for example, a display device such as a display. The communication unit  520 , which is, for example, a wired or wireless network interface, can perform a plurality of wireless communications. 
     The CPU  530  controls respective units which constitute the base station control device  4  and performs calculation, or the like, described above. The memory  540  and the HDD  550  constitute the storage unit  44  described above which stores data. Particularly, the memory  540  stores respective pieces of data to be used for calculation described above. The storage medium  570  can store a wireless communication program, or the like, for executing functions of the base station control device  4 . Note that architecture constituting the base station control device  4  (the base station  2 , the terminal station  6 ) is not limited to the example illustrated in  FIG.  11   . 
     In other words, it is assumed that the “computer” described here includes hardware such as an OS and peripheral equipment. Further, the “computer-readable storage medium” indicates a storage device such as a portable medium such as a flexible disk, a magnetooptical disk, a ROM and a CD-ROM. 
     Further, the “computer-readable storage medium” may include a medium which dynamically holds a program in a short period, such as a communication line in a case where a program is transmitted via a network such as the Internet or a communication line such as a telephone line, and a medium which holds a program in a certain period, such as a volatile memory inside a computer which becomes a server or a client in that case. 
     While the embodiment of the present invention has been described above with reference to the drawings, the above-described embodiment is merely an example of the present invention, and it is obvious that the present invention is not limited to the above-described embodiment. Thus, components may be added, omitted, replaced, or changed within a range not deviating from the technical idea and the scope of the present invention. 
     REFERENCE SIGNS LIST 
     
         
           1  Wireless communication system 
           2 - 1  to  2 - n  Base station 
           4  Base station control device 
           6 - 1  to  6 - m  Terminal station 
           20  Wireless communication unit 
           21  Collection unit 
           22  Storage unit 
           23  Own station information holding unit 
           24  Network communication unit 
           25  Control unit 
           40  Input unit 
           41  Output unit 
           42  Network communication unit 
           43  Information collection unit 
           44  Storage unit 
           45  List generation unit 
           46  Control unit 
           60  Wireless communication unit 
           62  Collection unit 
           64  Storage unit 
           66  Control unit 
           200 ,  242 ,  420 ,  600  Reception unit (acquisition unit) 
           202 ,  240 ,  422 ,  602  Transmission unit (notification unit) 
           250 ,  460 ,  660  Setting unit 
           500  Input unit 
           510  Output unit 
           520  Communication unit 
           530  CPU 
           540  Memory 
           550  HDD 
           560  Bus 
           570  Storage medium