Patent Publication Number: US-2022225336-A1

Title: Communication control apparatus, communication control method and program

Description:
TECHNICAL FIELD 
     The present invention relates to a communication control device, a communication control method, and a program. 
     BACKGROUND ART 
     An example of a communication scheme LPWA (Low Power, Wide Area), which is a wireless communication technology enabling a wide range to be a target at a low power consumption, is LoRa (R) (see Non-Patent Literature 1). According to LoRa (R), for example, a control device (server) communicates with LPWA-compatible terminals (hereinafter, referred to as “LPWA terminals”) via a plurality of gateways. The LPWA terminals each perform a receiving operation on the basis of a reception timing notified from a gateway (a base station) by a beacon. In a case where a normal reception fails, data is retransmitted with further collision avoided by randomizing a transmission timing. 
     CITATION LIST 
     Non-Patent Literature 
     Non-Patent Literature 1: “LoRaWAN (R) 1.1 Specification”, [online] the LoRa Alliance Technical Committee, Oct. 11, 2017, [Searched on March 25, Heisei 31], the Internet &lt;URL:https://lora-alliance.org/sites/default/files/2018-04/lorawantm specification-v1.1.pdf&gt; 
     SUMMARY OF THE INVENTION 
     Technical Problem 
     In a case where a plurality of gateways are operated at the same frequency and geographically close ones of the gateways have a cell-overlapping area (hereinafter, referred to as “overlapping area”) therebetween, radio wave interference (hereinafter, also referred to simply as “interference”) occurs in the overlapping area. Thus, simultaneous transmission of data from a server to a large number of LPWA terminals would result in frequent occurrence of collision of packets in the overlapping area. In this case, the data is frequently retransmitted from the server to the LPWA terminals, which increases an amount of time required before all the LPWA terminals complete the reception of data. 
     The present invention has been made in view of such circumstances and an object thereof is to provide a technology enabling reducing an amount of time required for communication between a server and a terminal. 
     Means for Solving the Problem 
     An aspect of the present invention is a communication control device that transmits a packet to be transmitted to a terminal to a base station that performs wireless communication with the terminal, the communication control device including: a terminal classification unit that classifies, on the basis of a magnitude of radio wave interference from a base station different from a neighboring base station, a plurality of the terminals into a first terminal set and a second terminal set that is relatively larger in the magnitude of radio wave interference than the first terminal set; and a transmission unit that transmits, in a case where packets destinations of which are terminals belonging to the first terminal set are to be transmitted to respective neighboring base stations of the terminals, the packets at the same timing and transmits, in a case where packets destinations of which are terminals belonging to the second terminal set are to be transmitted to respective neighboring base stations of the terminals, the packets at timings different from each other. 
     In addition, an aspect of the present invention is the above-described communication control device further including a base station classification unit that groups base stations that cause no radio wave interference with each other together, in which in the case where the packets the destinations of which are the terminals belonging to the second terminal set are to be transmitted to the respective neighboring base stations of the terminals, the transmission unit transmits the packets at different timings for grouped base station sets. 
     In addition, an aspect of the present invention is the above-described communication control device further including a base station classification unit that groups base stations that cause no radio wave interference with each other together, in which in the case where the packets the destinations of which are the terminals belonging to the second terminal set are to be transmitted to the respective neighboring base stations of the terminals, when transmitting no packet to a base station belonging to a first base station set, the transmission unit transmits a packet a destination of which is a terminal that suffers radio wave interference from the base station belonging to the first base station set. 
     In addition, an aspect of the present invention is a communication control device that transmits a packet to be transmitted to a terminal to a base station that performs wireless communication with the terminal, the communication control device including: a first terminal classification unit that classifies, on the basis of a magnitude of radio wave interference from a base station different from a neighboring base station, a plurality of the terminals into a first terminal set and a second terminal set that is relatively larger in the magnitude of radio wave interference than the first terminal set; a second terminal classification unit that classifies, on the basis of a received signal intensity of a radio wave from the neighboring base station, a plurality of terminals included in the first terminal set into a third terminal set and a fourth terminal set that is relatively higher in the received signal intensity than the third terminal set; a transmission power control unit that reduces, in a case where a packet a destination of which is a terminal belonging to the fourth terminal set is to be transmitted, a transmission power for a radio wave from the neighboring base station of the terminal; and a transmission unit that transmits, in a case where packets destinations of which are terminals belonging to the third terminal set are to be transmitted to respective neighboring base stations of the terminals, the packets at the same timing and transmits, in a case where packets destinations of which are terminals belonging to the second terminal set are to be transmitted to respective neighboring base stations of the terminals, the packets at timings different from each other while transmitting packets destinations of which are terminals belonging to the fourth terminal set to respective neighboring base stations of the terminals. 
     In addition, an aspect of the present invention is the above-described communication control device, in which the transmission unit changes intervals of transmission of the packets in accordance with at least one of a communication quality during the wireless communication or the magnitude of radio wave interference between the base stations. 
     In addition, an aspect of the present invention is a communication control method of transmitting a packet to be transmitted to a terminal to a base station that performs wireless communication with the terminal, the communication control method including: a terminal classification step of classifying, on the basis of a magnitude of radio wave interference from a base station different from a neighboring base station, a plurality of the terminals into a first terminal set and a second terminal set that is relatively larger in the magnitude of radio wave interference than the first terminal set; and a transmission step of transmitting, in a case where packets destinations of which are terminals belonging to the first terminal set are to be transmitted to respective neighboring base stations of the terminals, the packets at the same timing and transmits, in a case where packets destinations of which are terminals belonging to the second terminal set are to be transmitted to respective neighboring base stations of the terminals, the packets at timings different from each other. 
     In addition, an aspect of the present invention is a communication control method of transmitting a packet to be transmitted to a terminal to a base station that performs wireless communication with the terminal, the communication control method including: a first terminal classification step of classifying, on the basis of a magnitude of radio wave interference from a base station different from a neighboring base station, a plurality of the terminals into a first terminal set and a second terminal set that is relatively larger in the magnitude of radio wave interference than the first terminal set; a second terminal classification step of classifying, on the basis of a received signal intensity of a radio wave from the neighboring base station, a plurality of terminals included in the first terminal set into a third terminal set and a fourth terminal set that is relatively higher in the received signal intensity than the third terminal set; a transmission power control step of reducing, in a case where a packet a destination of which is a terminal belonging to the fourth terminal set is to be transmitted, a transmission power for a radio wave from the neighboring base station of the terminal; and a transmission step of transmitting, in a case where packets destinations of which are terminals belonging to the third terminal set are to be transmitted to respective neighboring base stations of the terminals, the packets at the same timing and transmits, in a case where packets destinations of which are terminals belonging to the second terminal set are to be transmitted to respective neighboring base stations of the terminals, the packets at timings different from each other while transmitting packets destinations of which are terminals belonging to the fourth terminal set to respective neighboring base stations of the terminals. 
     In addition, an aspect of the present invention is a program for causing a computer to function as the communication control device 
     Effects of the Invention 
     According to the present invention, it is possible to reduce an amount of time required for communication between a server and a terminal. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  shows an example of an overall configuration of a communication control system  1  according to a first embodiment of the present invention. 
         FIG. 2  is a diagram of assistance in explaining an example of a case where radio wave interference and packet retransmission occur. 
         FIG. 3  is a diagram of assistance in explaining the example of the case where radio wave interference and packet retransmission occur. 
         FIG. 4  is a diagram of assistance in explaining a typical communication method. 
         FIG. 5  is a diagram of assistance in explaining a communication method according to the first embodiment of the present invention. 
         FIG. 6  is a diagram of assistance in explaining a configuration where packets are simultaneously transmitted on a gateway-group basis. 
         FIG. 7  is a block diagram showing a functional configuration of a server  10  according to the first embodiment of the present invention. 
         FIG. 8  is a sequence diagram showing update of terminal classification performed by the communication control system  1  according to the first embodiment of the present invention. 
         FIG. 9  is a flowchart showing an operation for terminal group update processing performed by the server  10  according to the first embodiment of the present invention. 
         FIG. 10  shows a configuration of a received signal intensity recording database Dl used by the server  10  according to the first embodiment of the present invention. 
         FIG. 11  shows a configuration of a terminal group database D 2  used by the server  10  according to the first embodiment of the present invention. 
         FIG. 12  is a flowchart showing an operation for simultaneous communication control processing performed by the server  10  according to the first embodiment of the present invention. 
         FIG. 13  shows a configuration of a database for transmission interval calculation D 3  used by the server  10  according to the first embodiment of the present invention. 
         FIG. 14  shows a configuration of a communication quality recording database D 4  used by the server  10  according to the first embodiment of the present invention. 
         FIG. 15  shows a configuration of another system interference recording database D 5  used by the server  10  according to the first embodiment of the present invention. 
         FIG. 16  is a flowchart showing an operation for the terminal group update processing performed by the server  10  according to the first embodiment of the present invention. 
         FIG. 17  shows a configuration of the topology information D 6  used by the server  10  according to the first embodiment of the present invention. 
         FIG. 18  shows a configuration of gateway group information D 7  used by the server  10  according to the first embodiment of the present invention. 
         FIG. 19  shows a configuration of a terminal group database D 8  used by the server  10  according to the first embodiment of the present invention. 
         FIG. 20  is a flowchart showing an operation for transmission processing performed by the server  10  according to the first embodiment of the present invention. 
         FIG. 21  shows a configuration of a terminal group extraction table D 9  used by the server  10  according to the first embodiment of the present invention. 
         FIG. 22  is a diagram of assistance in explaining a communication method according to a second embodiment of the present invention. 
         FIG. 23  is a diagram of assistance in explaining a configuration where packets are simultaneously transmitted on a gateway-group basis. 
         FIG. 24  is a flowchart showing an operation for transmission processing performed by the server  10  according to the second embodiment of the present invention. 
         FIG. 25  shows a configuration of a terminal group extraction table D 10  used by the server  10  according to the second embodiment of the present invention. 
         FIG. 26  is a flowchart showing an operation for terminal group update processing performed by the server  10  according to the second embodiment of the present invention. 
         FIG. 27  shows a configuration of a terminal group extraction table Dll used by the server  10  according to the second embodiment of the present invention. 
         FIG. 28  shows a configuration of a terminal group extraction table D 12  used by the server  10  according to the second embodiment of the present invention. 
         FIG. 29  is a diagram of assistance in explaining a communication method according to a third embodiment of the present invention. 
         FIG. 30  is a diagram of assistance in explaining a configuration where packets are simultaneously transmitted on a gateway-group basis. 
         FIG. 31  is a block diagram showing a functional configuration of the server  10  according to the third embodiment of the present invention. 
         FIG. 32  shows a configuration of a terminal group extraction table D 13  used by the server  10  according to the third embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
     Description will be made below on a first embodiment of the present invention with reference to the drawings. 
     [Configuration of Communication Control System] 
     Description will be made below on a configuration of a communication control system  1 . 
       FIG. 1  shows an example of an overall configuration of the communication control system  1  according to the first embodiment of the present invention. 
     As shown in  FIG. 1 , the communication control system  1  includes a server  10 , a gateway  20   b,  a gateway  20   g,  a gateway  20   r,  and a plurality of terminals (bb, bg, br, gg, gb, gr, rr, rb, and rg). The gateway  20   b,  the gateway  20   g,  and the gateway  20   r  are hereinafter referred to simply as “gateways  20 ” unless it is necessary to distinguish among them. Further, the plurality of terminals shown in  FIG. 1  are classified as follows. 
     The terminal bb, a neighboring base station of which is the gateway  20   b,  is a terminal located within a range of only a cell defined by the gateway  20   b.  The terminal bb is thus a terminal not affected by interference (or a terminal less affected by interference) from any gateway other than the gateway  20   b  (the neighboring base station). 
     The terminal bg, a neighboring base station of which is the gateway  20   b,  is a terminal located within an overlapping range between the range of the cell defined by the gateway  20   b  and a range of a cell defined by the gateway  20   g.  The terminal bg is thus a terminal that is likely to be affected by interference from the gateway  20   g.    
     The terminal br, a neighboring base station of which is the gateway  20   b,  is a terminal located within an overlapping range between the range of the cell defined by the gateway  20   b  and a range of a cell defined by the gateway  20   r.  The terminal br is thus a terminal that is likely to be affected by interference from the gateway  20   r.    
     The terminal gg, a neighboring base station of which is the gateway  20   g,  is a terminal located within a range of only a cell defined by the gateway  20   g.  The terminal gg is thus a terminal not affected by interference (or a terminal less affected by interference) from any gateway other than the gateway  20   g  (the neighboring base station). 
     The terminal gb, a neighboring base station of which is the gateway  20   g,  is a terminal located within an overlapping range between the range of the cell defined by the gateway  20   g  and the range of the cell defined by the gateway  20   b.  The terminal gb is thus a terminal that is likely to be affected by interference from the gateway  20   b.    
     The terminal gr, a neighboring base station of which is the gateway  20   g,  is a terminal located within an overlapping range between the range of the cell defined by the gateway  20   g  and the range of the cell defined by the gateway  20   r.  The terminal gr is thus a terminal that is likely to be affected by interference from the gateway  20   r.    
     The terminal rr, a neighboring base station of which is the gateway  20   r,  is a terminal located within a range of only a cell defined by the gateway  20   r.  The terminal rr is thus a terminal not affected by interference (or a terminal less affected by interference) from any gateway other than the gateway  20   r  (the neighboring base station). 
     The terminal rb, a neighboring base station of which is the gateway  20   r,  is a terminal located within an overlapping range between the range of the cell defined by the gateway  20   r  and the range of the cell defined by the gateway  20   b.  The terminal rb is thus a terminal that is likely to be affected by interference from the gateway  20   b.    
     The terminal rg, a neighboring base station of which is the gateway  20   r,  is a terminal located within an overlapping range between the range of the cell defined by the gateway  20   r  and the range of the cell defined by the gateway  20   g.  The terminal rg is thus a terminal that is likely to be affected by interference from the gateway  20   g.    
     The terminal bb, the terminal gg, and the terminal rr, which are terminals not affected by interference (or less affected by interference) from any base station other than the neighboring base stations, are hereinafter referred to collectively as “non-interference terminals”. Further, the terminal bg, the terminal br, the terminal gb, the terminal gr, the terminal rb, and the terminal rb, which are terminals likely to be affected by interference from any base station other than the neighboring base stations, are hereinafter referred to collectively as “interference terminals”. Further, the terminal bb, the terminal gg, the terminal rr, the terminal bg, the terminal br, the terminal gb, the terminal gr, the terminal rb, and the terminal rb are hereinafter referred to simply as “terminals” unless it is necessary to distinguish among them (in other words, it is not necessary to distinguish between the “non-interference terminals” and the “interference terminals”). 
     The server  10  is, for example, an information processing device such as a general-purpose computer. The server  10  is communicatively connected to the terminals via the gateways  20  to transmit and receive data. The server  10  has a communication control function to control a transmission timing at which data is to be transmitted to the terminals via the gateways  20 . It should be noted that a transmission/reception function and the communication control function for data may be implemented in the form of devices independent of each other. 
     The gateways  20 , which are relays that relay communication between the server  10  and each of the terminals, each function as a base station that communicates with the terminals. The gateways  20  and the terminals are communicatively connected by wireless communication (for example, an LPWA communication scheme such as LoRa (R)). For downstream communication from the gateways  20  to the terminals, the same frequency is used for all of the gateways  20 . It should be noted that the server  10  and each of the gateways  20  may be communicatively connected by wireless communication or may be communicatively connected by wire communication. 
     It should be noted that compared with a communication speed of communication between the server  10  and the gateways  20 , a communication speed between the gateways  20  and the terminals is slow. This frequently causes a situation where the plurality of gateways  20  overlap each other in terms of radio wave sending time. Therefore, in a case where a radio wave for downstream communication is sent to any of the interference terminals located in the overlapping areas between the cells of adjacent ones of the plurality of gateways  20 , interference frequently occurs. In particular, during simultaneous communication in terminal control requiring responses from the terminals to the server, packets for downstream communication are frequently retransmitted from the server  10  to the interference terminals. 
     Description will be made below on an example of a case where radio wave interference and packet retransmission occur with reference to  FIG. 2  and  FIG. 3 . As shown in  FIG. 2 , packets to be transmitted from the server  10  via the gateway  20   b  to the terminal bg, which is one of the interference terminals, include a packet “ 1 ”. Further, packets to be transmitted from the server  10  via the gateway  20   g  to the terminal gg, which is one of the non-interference terminals, include a packet “ 2 ” and a packet “ 3 ”. It should be noted that the numbers assigned to these packets represent the order of transmission from the server. 
     In this case, for example, radio wave interference and packet retransmission as shown in  FIG. 3  are likely to occur. In  FIG. 3 , an abscissa axis represents time. First, the server  10  transmits the packet “ 1 ” to the gateway  20   b.  After completing the transmission of the packet “ 1 ”, the server  10  subsequently transmits the packet “ 2 ” to the gateway  20   g.    
     When receiving the packet “ 1 ”, the gateway  20   b  transmits this packet “ 1 ” to the terminal bg. Likewise, when receiving the packet “ 2 ”, the gateway  20   g  transmits this packet “ 2 ” to the terminal gg. It should be noted that compared with the communication speed communication between the server  10  and the gateways  20 , the communication speed between the gateways  20  and the terminals is slow as described above. Thus, in  FIG. 3 , an amount of time required to transmit a packet from the gateways  20  to the terminals is longer than an amount of time required to transmit a packet from the server  10  to the gateways  20  even though the packets are the same. 
     The terminal bg starts receiving the packet “ 1 ”. Likewise, the terminal gg starts receiving the packet “ 2 ”. Here, the terminal bg is the interference terminal that suffers interference from the gateway  20   g.  Thus, as shown in  FIG. 3 , when the gateway  20   g  starts transmitting the packet “ 2 ”, a reception error (in  FIG. 3 , an “Err (1 +2)” spot) occurs in the terminal bg due to interference. Consequently, the terminal bg fails to receive the packet “ 1 ” and cannot return an ACK (Acknowledge; positive response) signal to the server  10 . 
     Meanwhile, the terminal gg, which is the non-interference terminal, does not suffer interference from the gateway  20   b.  Thus, as shown in  FIG. 3 , the terminal gg can normally receive the packet “ 2 ”, returning an ACK signal to the server  10  via the gateway  20   g  (in  FIG. 3 , a “ 2 ′” spot). The server  10  then receives the ACK signal. 
     Next, since having received no ACK signal in response to the packet “ 1 ” transmitted to the terminal bg, the server  10  retransmits another packet “ 1 ” to the gateway  20   b.  When completing the retransmission of the packet “ 1 ”, the server  10  subsequently transmits the packet “ 3 ” to the gateway  20   g,  since the ACK signal has been received in response to the packet “ 2 ”. 
     When receiving the packet “ 1 ”, the gateway  20   b  transmits this packet “ 1 ” to the terminal bg again. Likewise, when receiving the packet “ 3 ”, the gateway  20   g  transmits this packet “ 3 ” to the terminal gg. 
     The terminal bg starts receiving the packet “ 1 ”. Likewise, the terminal gg starts receiving the packet “ 3 ”. Here, as shown in  FIG. 3 , when the gateway  20   g  starts transmitting the packet “ 3 ”, a reception error (in  FIG. 3 , an “Err (1+3)” spot) occurs again in the terminal bg due to interference. Consequently, the terminal bg again fails to receive the packet “ 1 ” and cannot return an ACK signal to the server  10 . 
     As described above, in a case where a radio wave for downstream communication is sent to any of the interference terminals located in the overlapping areas between the respective cells defined by the plurality of gateways  20  adjacent to each other, a packet for downstream communication is likely to be frequently retransmitted to the interference terminal due to the frequent occurrence of interference. 
     A communication method according to the first embodiment will be described below in comparison with a typical communication method. 
       FIG. 4  is a diagram of assistance in explaining a typical communication method. In  FIG. 4 , an abscissa axis represents time. Further, in  FIG. 4 , “bb”, “bg”, “br”, “gg”, “gb”, “gr”, “rr”, “rb”, and “rg” denote packets to be transmitted from the gateways  20  to terminal sets or packets to be received by the terminal sets. For example, “bb” shown on a time axis of the gateway  20   b  denotes a packet to be transmitted to the terminal bb and “bg” denotes a packet to be transmitted to the terminal bg. Further, “bb” shown on a time axis of a gateway  20   b  terminal set denotes a packet to be received by the terminal bb and “bg” denotes a packet to be received by the terminal bg. 
     Further, in  FIG. 4 , “RBO” denotes a duration when communication between the gateways  20  and the terminals is suspended due to the randomization of the transmission timing and “Err” denotes the occurrence of a reception error. 
     As shown in  FIG. 4 , in the typical communication method, at a time tl, the gateway  20   b,  the gateway  20   g,  and the gateway  20   r  first simultaneously transmit packets to the terminal bb, the terminal gr, and the terminal rb, respectively. In this case, the packet “bb”, which is transmitted to one of the non-interference terminals, the terminal bb, suffers no interference, thus being normally received by the terminal bb. Meanwhile, the packet “gr”, which is transmitted to one of the interference terminals, the terminal gr, suffers interference from the gateway  20   r,  thus causing the terminal gr to have a reception error (“Err”). Likewise, the packet “rb”, which is transmitted to one of the interference terminals, the terminal rb, suffers interference from the gateway  20   b,  thus causing the terminal rb to have a reception error (“Err”). 
     Subsequently, as shown in  FIG. 4 , at a time t 2 , the gateway  20   b,  the gateway  20   g,  and the gateway  20   r  simultaneously transmit packets to the terminal bg, the terminal gg, and the terminal rg, respectively. In this case, the packet “gg”, which is transmitted to one of the non-interference terminals, the terminal gg, suffers no interference, thus being normally received by the terminal gg. Meanwhile, the packet “bg”, which is transmitted to one of the interference terminals, the terminal bg, suffers interference from the gateway  20 g, thus causing the terminal bg to have a reception error (“Err”). Likewise, the packet “rg”, which is transmitted to one of the interference terminals, the terminal rg, suffers interference from the gateway  20   g,  thus causing the terminal rg to have a reception error (“Err”). 
     Further, the above applies to a time t 3  and the subsequent times as shown in  FIG. 4 . As is understood from the above, the typical communication method is likely to frequently cause reception errors. 
     In contrast,  FIG. 5  is a diagram of assistance in explaining the communication method according to the first embodiment of the present invention. The server  10  according to the first embodiment first classifies all the target terminals into the non-interference terminals and the interference terminals. Then, for example, at the time tl, the server  10  simultaneously transmits packets to be transmitted to the non-interference terminals to the gateways  20 . For example, as shown in  FIG. 5 , the server  10  simultaneously transmits the packet “bb”, which is to be transmitted to one of the non-interference terminals, the terminal bb, to the gateway  20   b,  the packet “gg”, which is to be transmitted to another one of the non-interference terminals, the terminal gg, to the gateway  20   g,  and the packet “rr”, which is to be transmitted to still another one of the non-interference terminals, the terminal rr, to the gateway  20   r.  It should be noted that since all the packets transmitted at the time tl are packets to be transmitted to the non-interference terminals, no interference occurs at each of the terminals. 
     Next, the server  10  transmits packets to be transmitted to the interference terminals to the gateways  20 . In this regard, in transmitting the packets to the interference terminals, the server  10  transmits the packets according to a schedule instead of simultaneously transmitting the packets to the gateways  20 . 
     For example, as shown in  FIG. 5 , at the time t 2 , the server  10  transmits the packet “bg”, which is to be transmitted to one of the interference terminals that communicate with the gateway  20   b,  namely, the terminal bg, to the gateway  20   b.  At the time t 2 , the server  10  transmits no packet to any other gateway  20  (i.e., the gateway  20   g  and the gateway  20   r ). Thus, no radio wave is sent from the gateway  20   g  at the same timing, so that the terminal bg can receive the packet “bg” without being affected by interference from the gateway  20   g.    
     Subsequently, at the time t 3  to a time t 7 , the server  10  transmits the packets to be transmitted to the interference terminals to the respective gateways  20  in sequence, while at the same timing as the timing at which each of these packets is transmitted, transmitting no packet to any other gateway  20  that is likely to cause interference as described above. This allows each of the interference terminals to receive a desired packet without being affected by interference. 
     As described above, the server  10  according to the first embodiment can make radio wave interference unlikely to occur, allowing for reducing the frequency of the retransmission of packets. This makes communication between the server  10  and each of the terminals more efficient. 
     It should be noted that the number of the gateways  20  is usually not three but larger. Accordingly, for example, as shown in  FIG. 6 , the server  10  classifies the large number of gateways  20  into a gateway group of gateways  20   b,  a gateway group of gateways  20   g,  and a gateway group of gateways  20   r  on the basis of a position relationship between the ranges of the cells. Then, for example, the server  10  first simultaneously transmits packets to the gateway group of gateways  20   b,  simultaneously transmits packets to the gateway group of gateways  20   g  at the next transmission timing, and simultaneously transmits the gateway group of gateways  20   r  at the timing after the next. 
     [Functional Configuration of Server] 
     Description will be made below on a functional configuration of the server  10 . 
       FIG. 7  is a block diagram showing the functional configuration of the server  10  according to the first embodiment of the present invention. 
     The server  10  is a communication control device that transmits a packet to be transmitted to a terminal to the gateway  20  (the neighboring base station) that performs wireless communication with the terminal. The server  10  is, for example, an information processing device such as a general-purpose computer as described above. As shown in  FIG. 7 , the server  10  includes a control unit  100 , a data acquisition unit  101 , a terminal classification unit  102 , a base station classification unit  103 , a storage unit  104 , and a transmission unit  105 . 
     The control unit  100  controls operations of functional units of the server  10 . For example, the control unit  100  includes a processor such as a CPU (Central Processing Unit). It should be noted that, for example, the processor of the control unit  100  reads and executes a software program stored in the storage unit  104 , thereby implementing the functional units of the server  10 . 
     The data acquisition unit  101  acquires data for performing transmission to terminals from, for example, an external device or the like. The data acquisition unit  101  causes the storage unit  104  to store the acquired data. 
     The terminal classification unit  102  classifies, on the basis of the magnitude of radio wave interference from each of the gateways  20  (the base stations) different from the opposing gateway  20  (the neighboring base station), transmission targets, i.e., a plurality of terminals, into a first terminal group (a first terminal set: a terminal group of non-interference terminals) that is relatively smaller in magnitude of radio wave interference and a second terminal group (a second terminal set: a terminal group of non-interference terminals) that is relatively larger in magnitude of radio wave interference than the first terminal group. The terminal classification unit  102  causes the storage unit  104  to store information regarding the classified terminal groups. 
     The base station classification unit  103  groups the gateways  20  (the base stations) that cause no radio wave interference with each other together. The base station classification unit  103  causes the storage unit  104  to store information regarding the grouped gateways  20 . 
     The storage unit  104  temporarily stores data to be transmitted to the terminals. The storage unit  104  also stores the information regarding the classified terminal groups and the information regarding the grouped gateways  20  (the base station groups). The storage unit  104  includes, for example, a storage medium such as a RAM (Random Access Memory; a readable and writable memory), a flush memory, an EEPROM (Electrically Erasable Programmable Read Only Memory), or an HDD (Hard Disk Drive) or any combination of these storage media. 
     The transmission unit  105  acquires, from the storage unit  104 , data to be transmitted to the terminals. The transmission unit  105  then transmits packets by a different transmission method for each of the destinations of the packets (i.e., each of the terminal groups classified by the terminal classification unit  102 ). In a case where packets the destinations of which are the terminals belonging to the above-described first terminal group are to be transmitted to the respective gateways  20  (neighboring base stations) neighboring these terminals, the transmission unit  105  simultaneously transmits the packets at the same timing. 
     Further, in a case where packets the destinations of which are the terminals belonging to the above-described second terminal group are to be transmitted to the respective gateways  20  (neighboring base stations) neighboring these terminals, the transmission unit  105  transmits the packets at timings different from each other. Further, in the case where packets the destinations of which are the terminals belonging to the above-described second terminal group are to be transmitted to the respective gateways  20  (neighboring base stations) neighboring these terminals, the transmission unit  105  transmits the packets at different timings for the base station groups grouped by the base station classification unit  103 . 
     It should be noted that the transmission unit  105  includes, in addition to the functions that control the transmission of the packets as described above, a coding device that encodes data to be transmitted to the terminals and a communication interface (or an antenna) for communicative connection to the gateways  20 . 
     [Flow of Terminal Group Update Processing] 
     Description will be made below on an example of a flow of processing for updating the terminal groups. 
       FIG. 8  is a sequence diagram showing update of the terminal groups performed by the communication control system  1  according to the first embodiment of the present invention. It should be noted that  FIG. 8  shows a flow of terminal group update processing to be performed in a case where a terminal that will be affected by radio waves transmitted from the gateway  20   b  and the gateway  20   g  is classified. 
     As shown in  FIG. 8 , the terminal is first started (step S 001 ). When the terminal is started, the terminal outputs communication data for upstream communication (or a signal indicating a participation request) to a wireless system, trying to be communicatively connected to the wireless system (step S 002 ). In an example shown in  FIG. 8 , the outputted communication data is received by two of the gateways  20 , the gateway  20   b  and the gateway  20   g  (step S 003 ). 
     When receiving the communication data outputted from the terminal, the gateway  20   b  and the gateway  20   g  each transmit information indicating a received signal intensity upon reception to the server  10  along with the communication data (step S 004 ). The server  10  receives the communication data and the information indicating the received signal intensity upon reception transmitted from each of the gateway  20   b  and the gateway  20   g.  The server  10  then updates the terminal groups on the basis of the received information indicating the received signal intensity and updates topology information regarding network topology (step S 005 ). 
     [Detail of Terminal Group Update Processing] 
     A further detailed description will be made below on an example of the terminal group update processing performed by the server  10  in step S 005  described above. 
       FIG. 9  is a flowchart showing an operation for the terminal group update processing performed by the server  10  according to the first embodiment of the present invention. 
     The terminal classification unit  102  of the server  10  acquires information indicating all the gateways  20  (the gateway  20   b  and the gateway  20   g  in  FIG. 8 ) having received radio waves for upstream communication from the terminals and information indicating the received signal intensity upon reception (step S 101 ). The terminal classification unit  102  identifies one of the gateways  20  having received the radio wave from each of the terminals at the highest received signal intensity on the basis of the acquired information (step S 102 ). The terminal classification unit  102  then records the identified gateway  20  as a gateway for downstream communication to the terminal in a database (hereinafter, referred to as “received signal intensity recording database”) stored in the storage unit  104  (step S 103 ). It should be noted that a table configuration of the received signal intensity recording database will be described later. 
     The terminal classification unit  102  identifies any other gateway  20  having a received signal intensity that is different from the received signal intensity of the above-described identified gateway  20  for downstream communication by a predetermined value or less (for example, 10 [dB] or less) on the basis of the information indicating all the gateways  20  having received the radio waves for upstream communication from the terminals and the information indicating the received signal intensity upon reception (step S 104 ). When identifying any other the gateway  20  that satisfies the above-described condition, the terminal classification unit  102  records the identified gateway  20  as a gateway that will be an interference source in a received signal intensity recording database Dl (step S 105 ). 
     The terminal classification unit  102  classifies the terminals into a terminal group (the first terminal group) of non-interference terminals, that is, terminals for which no gateway  20  that will be an interference source during downstream communication exists, and a terminal group (the second terminal group) of interference terminals, that is, terminals for which the gateways  20  that will be interference sources during downstream communication exist on the basis of the received signal intensity recording database D 1 . The terminal classification unit  102  then records information indicating the terminals belonging to each of the terminal groups in a database (hereinafter, referred to as “terminal group database”) stored in the storage unit  104 . It should be noted that a table configuration of the terminal group database will be described later. 
     The operation of the terminal classification unit  102  of the server  10  shown in the flowchart of  FIG. 9  thus terminates. 
     [Configuration of Received Signal Intensity Recording Database] 
     Description will be made below on an example of the table configuration of the received signal intensity recording database D 1 .  FIG. 10  shows the configuration of the received signal intensity recording database D 1  used by the server  10  according to the first embodiment of the present invention. 
     As shown in  FIG. 10 , the received signal intensity recording database D 1  is data in the form of a table in which a terminal ID (Identifier), information indicating the gateway  20  for downstream communication and the received signal intensity thereof, and information indicating the gateway  20  that will be an interference source and the received signal intensity thereof are associated with each other. It should be noted that the information indicating the gateway  20  for downstream communication and the received signal intensity thereof is the information recorded in step S 103  described above. Meanwhile, the information indicating the gateway  20  that will be an interference source and the received signal intensity thereof is the information recorded in step S 105  described above. 
     As shown in  FIG. 10 , for example, the gateway  20   b  is associated as the gateway for downstream communication with a terminal with a terminal-identifying terminal ID of “ED- 1 ” (hereinafter, referred to as “terminal ED- 1 ”). Further, “−80 [dB]” is associated as a value indicating the received signal intensity of a radio wave from the gateway  20   b  at the terminal ED- 1 . Further, no gateway that will be an interference source is associated with the terminal ED- 1 . Further, as shown in  FIG. 10 , for example, the gateway  20   g  is associated as the gateway for downstream communication with a terminal with a terminal-identifying terminal ID of “ED- 3 ” (hereinafter, referred to as “terminal ED- 3 ”). Further, “−95 [dB]” is associated as a value indicating the received signal intensity of a radio wave from the gateway  20   g  at the terminal ED- 3 . Further, the gateway  20   b  is associated as the gateway that will be an interference source with the terminal ED- 1 . Further, “−102 [dB]” is associated as a value indicating the received signal intensity of a radio wave from the gateway  20   b  at the terminal ED- 3 . 
     [Configuration of Terminal Group Database] 
     Description will be made below on an example of the table configuration of a terminal group database D 2 . 
       FIG. 11  shows the configuration of the terminal group database D 2  used by the server  10  according to the first embodiment of the present invention. 
     As shown in  FIG. 11 , the terminal group database D 2  is data in the form of a table in which information indicating a terminal group and the terminal ID are associated with each other. It should be noted that the information indicating the terminal group and the terminal ID are the information recorded in step S 106  described above. 
     As shown in  FIG. 11 , the terminal group includes a terminal group of “non-interference terminals” and a terminal group of “interference terminals”. Further, as shown in  FIG. 11 , terminal IDs such as “ED- 1 ” are associated with the terminal group of “non-interference terminals” and terminal IDs such as “ED- 2 ” and “ED- 3 ” are associated with the terminal group of “interference terminals”. 
     [Simultaneous Communication Control by Server] 
     Description will be made below on an example of simultaneous communication control processing performed by server  10 . 
       FIG. 12  is a flowchart showing an operation for the simultaneous communication control processing performed by the server  10  according to the first embodiment of the present invention. 
     The transmission unit  105  of the server  10  calculates, on the basis of the above-descried received signal intensity recording database Dl (for example, see  FIG. 10 ), the number of terminals that perform downstream communication from each of the gateways  20  (i.e., the number of terminals per gateway  20 ) for each of the presence and the absence of the gateway  20  that will be an interference source (step S 201 ). Then, the transmission unit  105  determines, on the basis of a database for calculating an interval between times of transmission of packets from the transmission unit  105  during downstream communication (hereinafter, referred to as “database for transmission interval calculation”) and the calculated number of terminals per gateway  20 , an interval of transmission to each of the gateways  20  for each of the presence and the absence of the gateway  20  that will be an interference source (step S 202 ). It should be noted that a table configuration of the database for transmission interval calculation will be described later. 
     The transmission unit  105  calculates, on the basis of the calculated number of terminals per gateway  20  and an estimated output interval recorded in a database for transmission interval calculation D 3 , an estimated time of completion of output to all the terminals by each of the gateways  20  for each of the presence and the absence of the gateway  20  that will be an interference source (step S 203 ). 
     The transmission unit  105  corrects, on the basis of an average loss rate recorded in a database that records communication qualities of the terminals and the gateways  20  (hereinafter, referred to as “communication quality recording database”) and a value of another system interference rate recorded in a database that records other system interference of each of the gateways  20  (hereinafter, referred to as “other system interference recording database”), the interval of transmission to each of the gateways  20  (step S 204 ). For example, an input interval may be corrected by multiplying the input interval by 1/(1—average loss rate) or 1/(1—other system interference rate). It should be noted that table configurations of the communication quality recording database and the other system interference recording database will be described later. 
     The transmission unit  105  adds, in units of the gateways  20 , packets for downstream communication to the non-interference terminals to a queue  1  at the interval of transmission calculated above (step S 205 ). The transmission unit  105  extracts the packets for downstream communication to the gateways  20  from the queue  1  one by one. Then, the transmission unit  105  simultaneously transmits the extracted packets to the gateways  20  in parallel. Further, at the same time, the transmission unit  105  inserts the transmitted packet into an ACK-signal-waiting queue (step S 206 ). 
     The transmission unit  105  sets a time-out time for ACK signals on the basis of the estimated time of completion of output and waits for ACK signals to all the packets (step S 207 ). Here, in a case where the time-out time for ACK signals elapses and time-out occurs (step S 208 .Yes), the transmission unit  105  extracts the packet having been timed out from the ACK-signal-waiting queue and adds the packet again at an end of the above-described queue  1  (step S 209 ). Then, the process returns again to step S 205  described above. 
     In a case where ACK signals to all the packets for downstream communication to the non-interference terminals are received (step S 208 .No), the transmission unit  105  adds, in units of the gateways  20 , packets for downstream communication to the interference terminals to a queue  2  at the interval of transmission calculated above (step S 210 ). The transmission unit  105  extracts the packets for downstream communication to each of the gateways  20  from the queue  2  one by one. Then, the transmission unit  105  simultaneously transmits the extracted packets to the gateways  20  in parallel. Further, at the same time, the transmission unit  105  inserts the transmitted packet into the ACK-signal-waiting queue (step S 211 ). 
     The transmission unit  105  sets a time-out time for ACK signals on the basis of the estimated time of completion of output and waits for ACK signals to all the packets (step S 212 ). Here, in a case where the time-out time for ACK signals elapses and time-out occurs (step S 213 .Yes), the transmission unit  105  extracts the packet having been timed out from the ACK-signal-waiting queue and again adds the packet at an end of the above-described queue  2  (step S 214 ). Then, the process returns again to step S 210  described above. 
     When ACK signals to all the packets for downstream communication to the interference terminals are received (step S 213 .No), the operation of transmission unit  105  of the server  10  shown in the flowchart of  FIG. 12  terminates. 
     [Configuration of Database for Transmission Interval Calculation] 
     Description will be made below on an example of the table configuration of the database for transmission interval calculation D 3 . 
       FIG. 13  shows the configuration of the database for transmission interval calculation D 3  used by the server  10  according to the first embodiment of the present invention. As shown in  FIG. 13 , the database for transmission interval calculation D 3  is data in the form of a table in which information indicating the presence/absence of an interference source, the number of terminals per gateway, the interval of transmission, the estimated output interval, and a queue type are associated with each other. The transmission unit  105  can calculate, on the basis of the estimated output interval and the calculated number of the terminals per gateway  20  recorded in the database for transmission interval calculation D 3 , the estimated time of completion of output to all the terminals by each of the gateways  20 . 
     [Configuration of Communication Quality Recording Database] 
     Description will be made below on an example of the table configuration of a communication quality recording database D 4 . 
       FIG. 14  shows the configuration of the communication quality recording database D 4  used by the server  10  according to the first embodiment of the present invention. As shown in  FIG. 14 , the communication quality recording database D 4  is data in the form of a table in which information identifying the gateways  20 , the average loss rate at each of the gateways  20 , the terminal ID, and the average loss rate at each of the terminals are associated with each other. 
     [Configuration of Other System Interference Recording Database] 
     Description will be made below on an example of the table configuration of another system interference recording database D 5 .  FIG. 15  shows the configuration of the other system interference recording database D 5  used by the server  10  according to the first embodiment of the present invention. As shown in  FIG. 15 , the other system interference recording database D 5  is data in the form of a table in which information identifying the gateways  20  and the other system interference rate are associated with each other. The transmission unit  105  can correct the interval of transmission to each of the gateways  20  on the basis of the average loss rate recorded in the communication quality recording database D 4  and the value of the other system interference rate recorded in the other system interference recording database D 5 . 
     [Another Example of Terminal Group Update Processing] 
     A further detailed description will be made below on another example of the terminal group update processing performed by the server  10  in step S 005  described above. 
       FIG. 16  is a flowchart showing an operation for the terminal group update processing performed by the server  10  according to the first embodiment of the present invention. 
     The terminal classification unit  102  of the server  10  acquires information indicating all the gateways  20  (the gateway  20   b  and the gateway  20   g  in  FIG. 8 ) having received radio waves for upstream communication from the terminals and the information indicating the received signal intensity upon reception (step S 301 ). The terminal classification unit  102  identifies one of the gateways  20  having received the radio wave from each of the terminals at the highest received signal intensity on the basis of the acquired information (step S 302 ). The terminal classification unit  102  then records the identified gateway  20  as a gateway for downstream communication to the terminal in the received signal intensity recording database stored in the storage unit  104  (step S 303 ). It should be noted that the table configuration of the received signal intensity recording database is as described with reference to  FIG. 10 . 
     The terminal classification unit  102  identifies any other gateway  20  having a received signal intensity that is different from the received signal intensity of the above-described identified gateway  20  for downstream communication by a predetermined value or less (for example, 10 [dB] or less) on the basis of the information indicating all the gateways  20  having received the radio waves for upstream communication from the terminals and the information indicating the received signal intensity upon reception (step S 304 ). When identifying any other the gateway  20  that satisfies the above-described condition, the terminal classification unit  102  records the identified gateway  20  as a gateway that will be an interference source in the received signal intensity recording database D 1  (step S 305 ). 
     The terminal classification unit  102  records a combination of the gateway  20  for downstream communication and the gateway  20  that will be an interference source, which is recorded in the received signal intensity recording database D 1 , as an adjacent relationship in topology information D 6  (step S 306 ). The server  10  groups the gateways  20  such that the gateways  20  in the adjacent relationship do not belong to the same group on the basis of the topology information D 6  (step S 307 ). It should be noted that, for example, a typical technique such as a solution of a coloring problem using an ising model can be used for the grouping of the gateways  20 . It should be noted that configurations of the topology information D 6  and gateway group information D 7  will be described later. 
     The terminal classification unit  102  records a combination of the gateway group of the gateways  20  for downstream communication and the gateway group of the gateways  20  that will be an interference source with the terminal group assigned to the combination in a terminal group database D 8 . It should be noted that a table configuration of the terminal group database D 8  will be described later. The terminal classification unit  102  then assigns the terminal group to each of the terminals on the basis of the received signal intensity recording database D 1  and the gateway group information D 7  (step S 308 ). 
     The operation of the terminal classification unit  102  of the server  10  shown in the flowchart of  FIG. 16  thus terminates. 
     [Configuration of Topology Information] 
     Description will be made below on an example of the configuration of the topology information D 6 . 
       FIG. 17  shows the configuration of the topology information D 6  used by the server  10  according to the first embodiment of the present invention. As shown in  FIG. 17 , the topology information D 6  is data in the form of a table in which information identifying the gateways  20  and information identifying another one of the gateways  20  adjacent to the gateway  20  are associated with each other. The server  10  can group the gateways  20  such that the gateways  20  in the adjacent relationship do not belong to the same group on the basis of the topology information D 6 . 
     [Configuration of Gateway Group Information] 
     Description will be made below on an example of the configuration of the gateway group information D 7 . 
       FIG. 18  shows the configuration of the gateway group information D 7  used by the server  10  according to the first embodiment of the present invention. As shown in  FIG. 18 , the gateway group information D 7  is data in the form of a table in which the information identifying the gateways  20  and information identifying a gateway group to which the gateway  20  belongs are associated with each other. 
     [Configuration of Terminal Group Database] 
     Description will be made below on an example of the configuration of the terminal group database D 8 . 
       FIG. 19  shows the configuration of the terminal group database D 8  used by the server  10  according to the first embodiment of the present invention. As shown in  FIG. 19 , the terminal group database D 8  is data in the form of a table in which information identifying the terminal group, information identifying the gateway group for downstream communication, information identifying the gateway group that will be an interference source, and the terminal ID are associated with each other. The terminal classification unit  102  can assign a terminal group to each of the terminals on the basis of the terminal group database D 8 , the gateway group information D 7 , and the received signal intensity recording database D 1 . 
     [Operation of Server  10  for Transmission Processing] 
     Description will be made below on an example of transmission processing of packets performed by the server  10 . 
       FIG. 20  is a flowchart showing an operation for the transmission processing performed by the server  10  according to the first embodiment of the present invention. 
     First, assuming that E is a variable representing an entry number, the control unit  100  of the server  10  assigns  1  to the variable E (step S 401 ). It should be noted that entry herein refers to information corresponding to each row in a later-described terminal group extraction table, that is, one pattern of information in which information regarding the gateway group (set), information regarding the terminal group (set), etc. are associated with each other. The transmission unit  105  identifies the terminal group associated with the entry number E with reference to a gateway group extraction table D 9 . It should be noted that a configuration of the gateway group extraction table D 9  will be described later. The transmission unit  105  simultaneously transmits packets to the terminals belonging to the identified terminal group (step S 402 ). 
     The control unit  100  adds one to the variable E representing the entry number (step S 403 ). When the variable E is equal to or less than a maximum value of the entry number, the processing is continued back to step S 402 . Meanwhile, when the variable E is more than the maximum value of the entry number, the operation of the server  10  shown in the flowchart of  FIG. 20  terminates. 
     [Configuration of Terminal Group Extraction Table] 
     Description will be made below on an example of the configuration of the terminal group extraction table D 9 . 
       FIG. 21  shows the configuration of the terminal group extraction table D 9  used by the server  10  according to the first embodiment of the present invention. As shown in  FIG. 21 , the terminal group extraction table D 9  is data in the form of a table in which the entry number, information indicating the presence/absence of an interference source, information identifying the gateway group, and information identifying the terminal group (queue type) are associated with each other. 
     As described above, the server  10  according to the first embodiment of the present invention is a communication control device that transmits a packet to be transmitted to a terminal to the gateway  20  (the base station) that performs wireless communication with the terminal. The server  10  (the communication control device) includes the terminal classification unit  102  and the transmission unit  105 . The terminal classification unit  102  classifies, on the basis of the magnitude of radio wave interference from each of the gateways  20  (the base stations) different from the opposing gateway  20  (the neighboring base station), the plurality of terminals into the first terminal group (terminal set) and the second terminal group (the second terminal set) that is relatively larger in the magnitude of radio wave interference than the first terminal group (the first terminal set). In a case where packets the destinations of which are the terminals belonging to the above-described first terminal group (first terminal set) are to be transmitted to the respective opposing gateways  20  (neighboring base stations) of these terminals, the transmission unit  105  transmits the packets at the same timing, and in a case where packets the destinations of which are the terminals belonging to the above-described second terminal group (second terminal set) are to be transmitted to the respective opposing gateways  20  (neighboring base stations) of these terminals, the transmission unit  105  transmits the packets at timings different from each other. 
     Additionally, the server  10  according to the first embodiment further includes the base station classification unit  103 . The base station classification unit  103  groups the gateways  20  (the base stations) that cause no radio wave interference with each other together. In a case where packets the destinations of which are the terminals belonging to the above-described second terminal group (second terminal set) are to be transmitted to the respective opposing gateways  20  (neighboring base stations) of these terminals, the above-described transmission unit  105  transmits the packets at different timings for each of the grouped gateway groups (the base station sets). 
     The above-described transmission unit  105  also changes the intervals of transmission of the packets in accordance with at least one of the communication quality during the above-described wireless communication or the magnitude of radio wave interference between the above-described gateways  20  (base stations). 
     By virtue of the configuration as described above, the server  10  according to the first embodiment enables reducing the amount of time required for communication between the server  10  and each of the terminals. 
     Second Embodiment 
     Description will be made below on a second embodiment of the present invention with reference to the drawings. 
     A communication method according to the second embodiment will be described below in comparison with a typical communication method using retransmission at a randomized transmission timing as in the above-described first embodiment. The typical communication method using retransmission at a randomized transmission timing is as described above with reference to  FIG. 4 . 
     Unlike  FIG. 4 ,  FIG. 22 , is a diagram of assistance in explaining the communication method according to the second embodiment of the present invention. The server  10  according to the second embodiment first classifies all the target terminals into the non-interference terminals and the interference terminals as in the first embodiment. Then, for example, at the time t 1 , the server  10  simultaneously transmits packets to be transmitted to the non-interference terminals to the gateways  20 . For example, as shown in  FIG. 22 , the server  10  simultaneously transmits the packet “bb”, which is to be transmitted to one of the non-interference terminals, the terminal bb, to the gateway  20   b,  the packet “gg”, which is to be transmitted to another one of the non-interference terminals, the terminal gg, to the gateway  20   g,  and the packet “rr”, which is to be transmitted to still another one of the non-interference terminals, the terminal rr, to the gateway  20   r.  It should be noted that since all the packets transmitted at the time tl are packets to be transmitted to the non-interference terminals, no interference occurs at each of the terminals. 
     Next, the server  10  transmits packets to be transmitted to the interference terminals to the gateways  20 . In this regard, in transmitting the packets to the interference terminals, the server  10  transmits the packets according to a schedule instead of simultaneously transmitting the packets to the gateways  20 . 
     For example, as shown in  FIG. 22 , at the time t 2 , the server  10  transmits the packet “bg”, which is to be transmitted to the terminal gb, that is, the interference terminal that communicates with the gateway  20   g  and likely to suffer interference from the gateway  20   b,  to the gateway  20   g.  Further, in addition to this, at the time t 2 , the server  10  transmits the packet “rb”, which is to be transmitted to the terminal rb, that is, the interference terminal that communicates with the gateway  20   r  and likely to suffer interference from the gateway  20   b,  to the gateway  20   r.    
     At the time t 2 , the server  10  transmits no packet to the gateway  20   b  (in other words, stops the gateway  20 b). Thus, no radio wave is sent from the gateway  20   b  at the same timing (at the time t), so that the terminal gb can receive the packet “gb” without being affected by interference from the gateway  20   b  and the terminal rb can receive the packet “rb” without being affected by interference from the gateway  20   b.    
     Subsequently, at the time t 2  to a time t 4 , the server  10  transmits packets to the interference terminals in sequence, while transmitting, at the same timing as the timing at which each of these packets is transmitted, no packet to another gateway  20  that is likely to cause interference to occur (in other words, stopping another gateway  20  that is likely to cause interference to occur) as described above. This allows each of the interference terminals to receive a desired packet without being affected by interference. 
     As described above, the server  10  according to the second embodiment can make radio wave interference unlikely to occur, allowing for reducing the frequency of the retransmission of packets. This makes communication between the server  10  and each of the terminals more efficient. 
     It should be noted that the number of the gateways  20  is usually not three but larger. Accordingly, for example, as shown in  FIG. 23 , the server  10  classifies the large number of gateways  20  into a gateway group of gateways  20   b,  a gateway group of gateways  20   g,  and a gateway group of gateways  20   r  on the basis of a position relationship between the ranges of the cells. Then, for example, the server  10  first stops communication with the gateway group of gateways  20   b,  stops communication with the gateway group of gateways  20   g  at the next transmission timing, and stops communication with the gateway group of gateways  20   r  at the timing after the next. 
     [Operation of Server  10  for Transmission Processing] 
     Description will be made below on an example of transmission processing of packets performed by the server  10 . 
       FIG. 24  is a flowchart showing an operation for transmission processing performed by the server  10  according to the second embodiment of the present invention. 
     First, assuming that the number into which the gateways are grouped is m and the number of selected ones of the groups that do not perform transmission is a variable n, the control unit  100  of the server  10  assigns m+1 to the variable n (step S 501 ). The transmission unit  105  selects the variable n entries one by one with reference to a gateway group extraction table D 10  and identifies, upon selecting each entry, the terminal group associated with this entry. It should be noted that a configuration of the gateway group extraction table D 10  will be described later. The transmission unit  105  simultaneously transmits packets to the terminals belonging to the identified terminal group (step S 502 ). 
     The control unit  100  subtracts one from the variable n (step S 503 ). When the variable n is zero or larger, the processing is continued back to step S 502 . Meanwhile, when the variable n is smaller than zero, the operation of the server  10  shown in the flowchart of  FIG. 24  terminates. 
     [Configuration of Terminal Group Extraction Table] 
     Description will be made below on an example of the configuration of the terminal group extraction table D 10 . 
       FIG. 25  shows the configuration of the terminal group extraction table D 10  used by the server  10  according to the second embodiment of the present invention. As shown in  FIG. 25 , the terminal group extraction table D 10  is data in the form of a table in which the entry number, the variable n, information identifying the gateway group of the gateways  20  that are to be stopped, information identifying the gateway group of the gateways  20  to which packets are to be transmitted, and information identifying the terminal group (queue type) are associated with each other. The transmission unit  105  can select entries of the variable n one by one with reference to the gateway group extraction table D 10  and identify, upon selecting each entry, the terminal group associated with this entry. 
     As described above, the server  10  according to the second embodiment of the present invention is a communication control device that transmits a packet to be transmitted to a terminal to the gateway  20  (the base station) that performs wireless communication with the terminal. The server  10  (the communication control device) includes the terminal classification unit  102  and the transmission unit  105 . The terminal classification unit  102  classifies, on the basis of the magnitude of radio wave interference from each of the gateways  20  (the base stations) different from the opposing gateway  20  (the neighboring base station), the plurality of terminals into the first terminal group (the first terminal set) and the second terminal group (the second terminal set) that is relatively larger in the magnitude of radio wave interference than the first terminal group (the first terminal set). In a case where packets the destinations of which are the terminals belonging to the above-described first terminal group (first terminal set) are to be transmitted to the respective opposing gateways  20  (neighboring base stations) of these terminals, the transmission unit  105  transmits the packets at the same timing, and in a case where packets the destinations of which are the terminals belonging to the above-described second terminal group (second terminal set) are to be transmitted to the respective opposing gateways  20  (neighboring base stations) of these terminals, the transmission unit  105  transmits the packets at timings different from each other. 
     Additionally, the server  10  according to the second embodiment further includes the base station classification unit  103 . The base station classification unit  103  groups the gateways  20  (the base stations) that cause no radio wave interference with each other together. In a case where packets the destinations of which are the terminals belonging to the above-described second terminal group (second terminal set) are to be transmitted to the respective opposing gateways  20  (neighboring base stations) of these terminals, when no packet is transmitted to the gateways  20  (the second base stations) belonging to the first gateway group (first base station set), the above-described transmission unit  105  transmits packets the destinations of which are the terminals that suffer from radio wave interference from the gateways  20  (the base stations) belonging to the above-described first gateway group (first base station set). 
     By virtue of the configuration as described above, the server  10  according to the second embodiment enables further reducing the amount of time required for communication between the server  10  and each of the terminals. 
     [Another Example of Terminal Group Update Processing] 
     The terminal classification unit  102  of the server  10  according to the second embodiment may further have a function to combine two entries into one entry. Specifically, even in a case where any of the gateway groups registered as gateway groups that are to be stopped is selected in either of the two entries, unless it matches any of the gateway groups registered as gateway groups that are to be caused to transmit packets in the other one of the two entries, the terminal classification unit  102  combines the two entries. 
     A further detailed description will be made below on another example of the terminal group update processing performed by the server  10  in step S 005  described above. 
       FIG. 26  is a flowchart showing an operation for the terminal group update processing performed by the server  10  according to the second embodiment of the present invention. 
     The terminal classification unit  102  selects two entry A and entry B that are different from each other in steps S 601  to S 606 . Then, in step S 607 , the terminal classification unit  102  determines whether or not none of transmission gateway groups of the entry A is included in stopped gateway groups of the entry B and none of transmission gateway groups of the entry B is included in stopped gateway groups of the entry A. When a determination result is YES in step S 607 , the terminal classification unit  102  combines the respective stopped gateway groups, respective transmission gateway groups, and respective terminal groups of the entry A and the entry B with each other, forming one entry in step S 609 . The terminal classification unit  102  then reassigns new entry numbers to all the entries, thereby updating the terminal group extraction table. Afterward, the terminal classification unit  102  returns to step S 601 . 
       FIG. 27  shows a terminal group extraction table D 11  used by the server  10  according to the second embodiment of the present invention. Meanwhile,  FIG. 28  shows a terminal group extraction table D 12  after the entries are combined and the entry numbers are reassigned by performing the flowchart of  FIG. 26  on D 11 . 
     As shown in  FIG. 27 , in a case where a variable A and a variable B in  FIG. 26  are A=2 and B=6, respectively, a determination result is YES in step S 607 . This is because a transmission gateway group G with an entry number  2  is not included in stopped gateway groups R and C with an entry number  6  and a transmission gateway group B with the entry number  6  is not included in the stopped gateway group R with the entry number  2 . 
     Thus, the terminal classification unit  102  combines the entries with the entry number  2  and the entry number  6  with each other, thereby creating a new entry with an entry number  2 ′. It should be noted that the uncombined entries with the entry number  2  and the entry number  6  are deleted. The terminal group extraction table D 11  shown in  FIG. 27  is thus updated to the terminal group extraction table D 12  shown in  FIG. 28  by performing the processing shown in the flowchart of  FIG. 26 . 
     By virtue of the configuration described above, the server  10  according to the second embodiment increases the number of gateways allowed to simultaneously perform communication, thus making it possible to further reduce the amount of time required for communication between the server  10  and each of the terminals. 
     Third Embodiment 
     Description will be made below on a third embodiment of the present invention with reference to the drawings. 
     In the second embodiment, the description is made on the configuration where the gateways  20  that are likely to cause interference to occur are stopped in sequence, thereby reducing the occurrence of interference. However, each of the gateways  20  that are likely to cause interference to occur is not necessarily stopped but may be caused to perform communication with a transmission power reduced to a level at which no interference occurs. Instead of stopping each of the gateways  20  that are likely to cause interference to occur, a server  10   b  according to the third embodiment selects, from among terminals near the gateway  20  that is likely to cause interference to occur, a terminal with an especially high received signal intensity as a destination and causes the gateway  20  to perform communication with a transmission power for a radio wave transmitted from the gateway  20  reduced. 
       FIG. 29  is a diagram of assistance in explaining a communication method according to the third embodiment of the present invention. The server  10   b  according to the third embodiment first classifies all the target terminals into the non-interference terminals and the interference terminals. Further, the server  10 b classifies the non-interference terminals into a terminal with an especially high received signal power and the other terminals. For example, the server  10   b  may perform the above-described classification on the basis of whether or not a value of the received signal power is equal to or more than a predetermined threshold. 
     In  FIG. 29 , “bbw” denotes a packet to be transmitted to a terminal “bbw”, which is a terminal with an especially high received signal power among the non-interference terminals that communicate with the gateway  20   b.  Likewise, “ggw” and “rrw” denote packets to be transmitted to a terminal “bbw” and a terminal “rrw”, which are terminals with especially high received signal powers among the non-interference terminals that communicate with the gateway  20   g  and the gateway  20 r, respectively. 
     Further, in  FIG. 29 , “bbs” denotes packets to be transmitted to, among the non-interference terminals that communicate with the gateway  20   b,  the terminals other than the above-described terminal “bbw”. Likewise, “ggw” and “rrw” denote packets to be transmitted to, among the non-interference terminals that communicate with the gateway  20   g  and the gateway  20   r,  the terminals other than the above-described terminal “bbw” and terminal “rrw”, respectively. 
     Further, in  FIG. 29 , “bg”, “br”, “gb”, “gr”, “rb”, and “rg” are as described in the first embodiment. In other words, for example, “bg” is a packet to be transmitted to the terminal bg. Further, the terminal bg is a terminal the neighboring base station of which is the gateway  20   b  and that is located in the overlapping area between the range of the cell defined by the gateway  20   b  and the range of the cell defined by the gateway  20   g.    
     The terminal bg is thus one of the interference terminals that are likely to be affected by interference from the gateway  20   g.    
     The server  10   b  first simultaneously transmits packets to be transmitted to a terminal group of the non-interference terminals excluding a terminal with an especially high received signal power to each of the gateways  20 . For example, as shown in  FIG. 29 , at the time tl to the time t 7 , the server  10   b  transmits the packets “bbs”, which are to be transmitted to terminals bbs, that is, a terminal group excluding the terminal bbw with an especially high received signal power among the non-interference terminals that communicate with the gateway  20   b,  to the gateway  20   b.    
     Likewise, for example, as shown in  FIG. 29 , at the time tl to the time t 7 , the server  10   b  simultaneously transmits a packet “ggs” and a packet “rrs”, which are to be transmitted to terminals ggs and terminals rrs, that is, terminal groups excluding the terminal ggw and the terminal rrw with especially high received signal powers among the non-interference terminals that communicate with the gateway  20   g  and the gateway  20   r,  to the gateway  20   g  and the gateway  20   r,  respectively. It should be noted that since all the packets transmitted at the time t 1  to the time t 7  are packets to be transmitted to the non-interference terminals, no interference occurs at each of the terminals. 
     Next, at a time t 8 , the server  10   b  transmits, to the gateway  20   b,  instructions for causing the gateway  20   b  to perform communication with a transmission power reduced to a level at which no interference occurs at the interference terminals and transmits the packet “bbw” to be transmitted to the terminal bbw with an especially high received signal power. In addition to this, at the time t 8 , the server  10   b  transmits the packet “gb” to be transmitted to the terminal gb, which is the interference terminal, to the gateway  20   g.  Further, in addition to this, at the time t 8 , the server  10   b  transmits the packet “rb” to be transmitted to the terminal rb, which is the interference terminal, to the gateway  20   r.    
     Next to the above, at a time t 9 , the server  10   b  transmits, to the gateway  20   g,  instructions for causing the gateway  20   g  to perform communication with a transmission power reduced to a level no interference occurs at the interference terminals and transmits the packet “ggw” to be transmitted to the terminal ggw with an especially high received signal power in a similar manner to the above. In addition to this, at the time t 9 , the server  10   b  transmits the packet “bg” to be transmitted to the terminal bg, which is the interference terminal, to the gateway  20   b.  Further, in addition to this, at the time t 9 , the server  10   b  transmits the packet “rg” to be transmitted to the terminal rg, which is the interference terminal, to the gateway  20   r.    
     Further, next to the above, at a time t 10 , the server  10   b  transmits, to the gateway  20   r,  instructions for causing the gateway  20   r  to perform communication with a transmission power reduced to a level at which no interference occurs at the interference terminals and transmits the packet “rrw” to be transmitted to the terminal rrw with an especially high received signal power in a similar manner to the above. In addition to this, at the time t 10 , the server  10   b  transmits the packet “br” to be transmitted to the terminal br, which is the interference terminal, to the gateway  20   b.  Further, in addition to this, at the time t 10 , the server  10   b  transmits the packet “gr” to be transmitted to the terminal gr, which is the interference terminal, to the gateway  20   g.    
     As described above, the server  10   b  according to the third embodiment can make radio wave interference unlikely to occur, allowing for reducing the frequency of the retransmission of packets. This makes communication between the server  10   b  and each of the terminals more efficient. 
     It should be noted that the number of the gateways  20  is usually not three but larger. Accordingly, for example, as shown in  FIG. 30 , the server  10   b  classifies the large number of gateways  20  into a gateway group of gateways  20   b,  a gateway group of gateways  20   g,  and a gateway group of gateways  20   r  on the basis of a position relationship between the ranges of the cells. Then, for example, the server  10   b  first causes the gateway group of the gateways  20   b  to perform communication with a transmission power reduced, causes the gateway group of gateways  20   g  to perform communication with a transmission power reduced at the next transmission timing, and causes the gateway group of the gateways  20   r  to perform communication with a transmission power reduced at the timing after the next. 
     [Functional Configuration of Server] 
     Description will be made below on a functional configuration of the server  10   b.    
       FIG. 31  is a block diagram showing the functional configuration of the server  10   b  according to the third embodiment of the present invention. 
     The server  10   b  is a communication control device that transmits a packet to be transmitted to a terminal to the gateway  20  (the base station) that performs wireless communication with the terminal. The server  10   b  is, for example, an information processing device such as a general-purpose computer. 
     As shown in  FIG. 31 , the server  10   b  includes the control unit  100 , the data acquisition unit  101 , a terminal classification unit  102   b,  the base station classification unit  103 , the storage unit  104 , a transmission unit  105   b,  and a transmission power control unit  106 . Further, the terminal classification unit  102   b  includes a first terminal classification unit  102   b - 1  and a second terminal classification unit  102   b - 2 . It should be noted that the same reference signs are used to refer to functional units with functions equivalent to those of the functional units of the server  10  according to the first embodiment described with reference to  FIG. 7  and the description thereof is omitted below. 
     The first terminal classification unit  102   b - 1  classifies, on the basis of the magnitude of radio wave interference from each of the gateways  20  (the base stations) different from the opposing gateway  20  (the neighboring base station), a plurality of terminals into a first terminal group (a first terminal set: a terminal group of non-interference terminals) and a second terminal group (a second terminal set: a terminal group of non-interference terminals) that is relatively larger in magnitude of radio wave interference than the first terminal group. 
     The second terminal classification unit  102   b - 2  classifies, on the basis of a received signal intensity of a radio wave from the opposing gateway  20  (the neighboring base station), the plurality of terminals of the first terminal group into a third terminal group (a third terminal set: a terminal group with the terminals of a fourth terminal group excluded from the terminals of the terminal group of the non-interference terminals) and the fourth terminal group (a fourth terminal set: a terminal group of terminals with an especially high received signal intensity among the terminals of the terminal group of the non-interference terminals) that is relatively higher in the received signal intensity than the third terminal group (the third terminal set). 
     In a case where packets the destinations of which are terminals belonging to the fourth terminal group (the fourth terminal set) are to be transmitted, the transmission power control unit  106  reduces a transmission power for a radio wave from each of the opposing gateways  20  (the neighboring base stations) of the terminals. 
     In a case where packets the destinations of which are terminals belonging to the third terminal group (the third terminal set) are to be transmitted to the respective opposing gateways  20  (neighboring base stations) of the terminals, the transmission unit  105   b  transmits the packets at the same timing. In a case where packets the destinations of which are terminals belonging to the second terminal group (the second terminal set) are to be transmitted to the respective opposing gateways  20  (neighboring base stations) of the terminals, the transmission unit  105   b  transmits the packets at timings different from each other while transmitting the packets the destinations of which are terminals belonging to the fourth terminal group (the fourth terminal set) to the respective opposing gateways  20  (neighboring base stations) of the terminals. 
     [Operation of Server  10   b  for Transmission Processing] 
     For example, processing similar to the transmission processing described with reference to  FIG. 24  in the second embodiment can be used as transmission processing of packets performed by the server  10   b  according to the third embodiment. 
     [Configuration of Terminal Group Extraction Table] 
     Description will be made below on an example of the configuration of the terminal group extraction table D 13 . 
       FIG. 32  shows the configuration of the terminal group extraction table D 13  used by the server  10   b  according to the third embodiment of the present invention. 
     As shown in  FIG. 32 , the terminal group extraction table D 13  is data in the form of a table in which an entry number, a variable n, information identifying a gateway group of the gateways  20  to be reduced in transmission power (low-powered), information identifying a gateway group of the gateways  20  not to be reduced in transmission power (high-powered), information identifying a terminal group (queue type) corresponding to the gateway group of the gateways  20  to be low-powered, and information identifying a terminal group (queue type) corresponding to the gateway group of the gateways  20  to be high-powered are associated with each other. 
     As described above, the server  10   b  according to the third embodiment of the present invention is a communication control device that transmits a packet to be transmitted to a terminal to the gateway  20  (the base station) that performs wireless communication with the terminal. The server  10   b  (the communication control device) includes the first terminal classification unit  102   b - 1 , the second terminal classification unit  102   b - 2 , the transmission power control unit  106 , and the transmission unit  105   b.  The first terminal classification unit  102   b - 1  classifies, on the basis of the magnitude of radio wave interference from each of the gateways  20  (the base stations) different from the opposing gateway  20  (the neighboring base station), the plurality of terminals into the first terminal group (the first terminal set) and the second terminal group (the second terminal set) that is relatively larger in the magnitude of radio wave interference than the first terminal group (the first terminal set). The second terminal classification unit  102   b - 2  classifies, on the basis of a received signal intensity of a radio wave from the opposing gateway  20  (the neighboring base station), the plurality of terminals of the first terminal group (the first terminal set) into the third terminal group (the third terminal set) and the fourth terminal group (the fourth terminal set) that is relatively higher in the received signal intensity than the third terminal group (the third terminal set). In a case where packets the destinations of which are terminals belonging to the fourth terminal group (the fourth terminal set) are to be transmitted, the transmission power control unit  106  reduces a transmission power for a radio wave from each of the opposing gateways  20  (the neighboring base stations) of the terminals. In a case where packets the destinations of which are terminals belonging to the third terminal group (the third terminal set) are to be transmitted to the respective opposing gateways  20  (neighboring base stations) of the terminals, the transmission unit  105   b  transmits the packets at the same timing, and in a case where packets the destinations of which are terminals belonging to the second terminal group (the second terminal set) are to be transmitted to the respective opposing gateways  20  (neighboring base stations) of the terminals, the transmission unit  105   b  transmits the packets at timings different from each other while transmitting the packets the destinations of which are terminals belonging to the fourth terminal group (the fourth terminal set) to the respective opposing gateways  20  (neighboring base stations) of the terminals. 
     By virtue of the configuration as described above, the server  10   b  according to the third embodiment enables further reducing the amount of time required for communication between the server  10  and each of the terminals. 
     The server  10  and the server  10   b  in above-described embodiments may be partially or wholly implemented by a computer. In this case, the server  10  and the server  10   b  may each be implemented by recording a program for implementing this function in a computer-readable recording medium and causing a computer system to read and execute the program recorded in the recording medium. It should be noted that the “computer system” herein includes hardware such as OS and peripheral equipment. Further, the “computer-readable recording medium” refers to any one of portable media such as a flexible disk, a magneto-optical disk, a ROM, and a CD-ROM and storage devices such as a hard disk built in the computer system. In addition, the “computer-readable recording medium” may also include a medium that dynamically holds the program for a short period, such as a communication wire for transmitting the program through a network such as the Internet or a communication line such as a phone line and, in this case, a medium that holds the program for a certain period of time, such as a volatile memory within a server or a client, i.e., the computer system. Further, the above-described program may be intended to implement a part of the above-described function. Further, the above-described program may be configured to implement the above-described function in combination with a program having been recorded in the computer system or may be implemented with use of hardware such as a PLD (Programmable Logic Device) or an FPGA (Field Programmable Gate Array). 
     The embodiments of the present invention are described above with reference to the drawings, but the above-described embodiments are merely examples of the present invention and it is clear that the present invention is by no means limited to the above-described embodiments. Accordingly, addition, omission, replacement, and any other alteration of the components may be performed without departing from the technical idea and the scope of the present invention. 
     REFERENCE SIGNS LIST 
       1  . . . communication control system,  10 ,  10   b  . . . server,  20  . . . gateway,  100  . . . control unit,  101  . . . data acquisition unit,  102 ,  102   b  . . . terminal classification unit,  102   b - 1  . . . first terminal classification unit,  102   b - 2  . . . second terminal classification unit,  103  . . . base station classification unit,  104  . . . storage unit,  105  . . . transmission unit,  106  . . . transmission power control unit