Patent Publication Number: US-2023147391-A1

Title: Base station, base station system, and communication method

Description:
TECHNICAL FIELD 
     Embodiments relate to a base station, a base station system, and a communication method. 
     BACKGROUND ART 
     Wireless LAN terminals connect to networks such as the Internet via a base station. When a terminal moves outside the service area of a given base station, the terminal switches the base station to which a connection is to be made. Such switching is called “handover” or the like. 
     CITATION LIST 
     Non Patent Literature 
     
         
         NPL 1:IEEE Std 802.11-2016, “10.22.2 HCF contention based channel access (EDCA)”, 7 Dec. 2016 
       
    
     SUMMARY OF THE INVENTION 
     Technical Problem 
     To perform a handover, a terminal disconnects from the connected base station and re-connects to the next base station. Overhead arises during such base station switches. 
     Embodiments provide a base station, a base station system, and a communication method capable of suppressing overhead for switching base stations. 
     Means for Solving the Problem 
     According to an embodiment, in a base station system including a first base station and at least two second base stations, a base station functions as the first base station. The base station includes a MAC processing unit and a communication processing unit. The MAC processing unit performs MAC layer processing and generates a first MAC frame used in common between the first base station and the second base stations. The communication processing unit transmits the first MAC frame to the second base stations. 
     Advantageous Effects of the Invention 
     According to embodiments, a base station, a base station system, and a communication method capable of suppressing overhead for switching base stations can be provided. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a diagram illustrating an example of the configuration of a communication system according to an embodiment. 
         FIG.  2    is a diagram illustrating an example of the hardware configuration of a first base station. 
         FIG.  3    is a diagram illustrating an example of the hardware configuration of a second base station. 
         FIG.  4    is a diagram illustrating an example of the hardware configuration of a terminal. 
         FIG.  5    is a diagram illustrating MAC layer processing during communication between the first base station and second base station and the terminal. 
         FIG.  6    is a function block diagram of the first base station. 
         FIG.  7    is a function block diagram of the second base station. 
         FIG.  8    is a flowchart illustrating an example of transmission processing by the first base station. 
         FIG.  9    is a flowchart illustrating an example of transmission processing by the second base station. 
         FIG.  10    is a timing chart illustrating changes over time in a state of PHY processing units of second base stations during the processing illustrated in  FIG.  9   . 
         FIG.  11    is a flowchart illustrating an example of reception processing by the second base station. 
         FIG.  12    is a flowchart illustrating an example of reception processing by the first base station. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments will be described hereinafter on the basis of the drawings.  FIG.  1    is a diagram illustrating an example of the configuration of a communication system according to an embodiment. A communication system  1  includes a first base station  10 , second base stations  20 A,  20 B, and  20 C, and a terminal  30 . 
     In the embodiment, the first base station  10  and the second base stations  20 A,  20 B, and  20 C function as one access point (AP) for the terminal  30 . When communicating with the terminal  30 , the second base stations  20 A,  20 B, and  20 C use a MAC address set in a wireless module of the first base station  10  as their own MAC addresses. The first base station  10  is a base station serving as a higher-order AP that centrally manages MAC layer processing of the second base stations  20 A,  20 B, and  20 C. The first base station  10  is configured to be capable of connecting to a network  40 . The second base stations  20 A,  20 B, and  20 C are base stations serving as lower-order APs that transmit and receive wireless signals directly to and from the terminal  30 . Each of the second base stations  20 A,  20 B, and  20 C is configured to be capable of transmitting and receiving wireless signals to and from the terminal  30  within a predetermined service area. When communicating with the first base station  10 , the second base stations  20 A,  20 B, and  20 C may use MAC addresses set individually in the respective wireless modules of the second base stations  20 A,  20 B, and  20 C. Here, the second base stations  20 A,  20 B, and  20 C may be arranged such that overlapping areas are created in the service area, or may be arranged such that no overlapping areas are created. The following descriptions assume that the second base stations  20 A,  20 B, and  20 C are arranged such that overlapping areas are created in the service area. Additionally, three second base stations are illustrated in  FIG.  1   . It is sufficient for there to be at least one second base station. 
       FIG.  2    is a diagram illustrating an example of the hardware configuration of the first base station  10 . The first base station  10  includes a processor  11 , Read Only Memory (ROM)  12 , Random Access Memory (RAM)  13 , a Wide Area Network (WAN) module  14 , a routing module  15 , a wireless module  16 , and a wired module  17 . 
     The processor  11  is a processing device that controls the first base station  10  as a whole. The processor  11  is, for example, a Central Processing Unit (CPU). The processor  11  is not limited to a CPU. An Application Specific IC (ASIC) or the like may be used instead of a CPU. There may be two or more processors  11  instead of one. 
     The ROM  12  is a read-only storage device. The ROM  12  stores firmware, various types of programs, and the like necessary for the first base station  10  to operate. 
     The RAM  13  is a storage device that can be written to as desired. The RAM  13  is used as a work area for the processor  11 , and temporarily stores the firmware and the like held in the ROM  12 . 
     The WAN module  14  is a module including an interface for the first base station  10  to communicate with, for example, a server (not shown) over the network  40 . The WAN module  14  is configured to be capable of connecting to the network  40  via an optical line, for example. 
     The routing module  15  is connected to the WAN module  14 , and is configured to route IP packets from the WAN module according to address information thereof. Note that the first base station  10  does not absolutely have to include the routing module  15 . The first base station  10  may be configured to access a router provided outside the first base station  10  through wireless communication or wired communication and connect to the network  40  via this router. 
     The wireless module  16  is a module configured to perform processing necessary for wireless LAN communication. The wireless module  16  generates MAC frames by performing Logical Link Control (LLC) layer and Media Access Control (MAC) layer processing on data transferred from the processor  11 , for example. Each MAC frame includes a MAC header, which contains a MAC address used in common by the first base station  10  and the second base stations  20 A,  20 B, and  20 C, a MAC payload containing the data transferred from the processor  11  or the like, and error detection code (FCS). The MAC address used in common is, for example, the MAC address of the first base station  10 , which is the higher-order AP. The wireless module  16  extracts data from the MAC frames transmitted from the terminal  30  via any of the second base stations  20 A,  20 B, and  20 C and transfers that data to the processor  11 , for example. 
     The wired module  17  is a module configured to perform processing necessary for wired communication with the second base stations  20 A,  20 B, and  20 C. For example, the wired module  17  is connected to each of the second base stations  20 A,  20 B, and  20 C by a cable. A coaxial cable, Radio on Fiber (ROF), or the like is conceivable as the connection type. As long as the configuration is such that signals transmitted from the wired module  17  are received by the desired base station among the second base stations  20 A,  20 B, and  20 C, and signals transmitted from the second base stations  20 A,  20 B, and  20 C are received by the wired module  17 , the wired module  17  may be configured to be capable of communication using any desired wired communication method. 
       FIG.  3    is a diagram illustrating an example of the hardware configuration of the second base station  20 A. The second base stations  20 B and  20 C may have the same hardware configuration as the second base station  20 A. The following assumes that the hardware configurations of the second base stations  20 B and  20 C are the same as that of the second base station  20 A, and thus the hardware configurations of the second base stations  20 B and  20 C will not be described. 
     The second base station  20 A includes a processor  21 , Read Only Memory (ROM)  22 , Random Access Memory (RAM)  23 , a wireless module  24 , and a wired module  25 . 
     The processor  21  is a processing device that controls the second base station  20 A as a whole. The processor  21  is, for example, a Central Processing Unit (CPU). The processor  11  is not limited to a CPU. An Application Specific IC (ASIC) or the like may be used instead of a CPU. There may be two or more processors  21  instead of one. 
     The ROM  22  is a read-only storage device. The ROM  22  stores firmware, various types of programs, and the like necessary for the second base station  20 A. 
     The RAM  23  is a storage device that can be written to as desired. The RAM  23  is used as a work area for the processor  21 , and temporarily stores the firmware and the like held in the ROM  22 . 
     The wireless module  24  is a module configured to perform processing necessary for wireless LAN communication. The wireless module  24  performs physical layer processing on MAC frames received from the first base station  10  via the wired module  25  to convert the MAC frames into wireless signals. At this time, the wireless module  24  adds a physical header to the MAC frame. For example, in the IEEE 802.11 frame format, the physical header includes a Physical Layer Convergence Protocol (PLCP) preamble and a PLCP header. The wireless module  24  converts the MAC frame to which the physical header is added into a wireless signal. The wireless module  24  then transmits the wireless signal to the terminal  30 . The wireless module  24  also extracts MAC frames from wireless signals received from the terminal  30 . 
     The wired module  25  is a module configured to perform processing necessary for wired communication with the first base station  10 . For example, the wired module  25  is connected to the first base station  10  by a cable. A coaxial cable, Radio on Fiber (ROF), or the like is conceivable as the connection type. As long as the configuration is such that signals transmitted from the wired module  25  are received by the first base station  10 , and signals transmitted from the first base station  10  are received by the wired module  25 , the wired module  25  may be configured to be capable of communication using any desired wired communication method. 
       FIG.  4    is a diagram illustrating an example of the hardware configuration of the terminal  30 . The terminal  30  is a terminal device (station) such as a smartphone or the like. The terminal  30  may be a mobile terminal, a terminal installed in a moving body, or a fixed terminal. 
     The terminal  30  includes a processor  31 , ROM  32 , RAM  33 , a wireless module  34 , a display  35 , and storage  36 . 
     The processor  31  is a processing device that controls the terminal  30  as a whole. The processor  31  is, for example, a CPU. The processor  31  is not limited to a CPU. An ASIC or the like may be used instead of a CPU. There may be two or more processors  31  instead of one. 
     The ROM  32  is a read-only storage device. The ROM  32  stores firmware, various types of programs, and the like necessary for the terminal  30  to operate. 
     The RAM  33  is a storage device that can be written to as desired. The RAM  33  is used as a work area for the processor  31 , and temporarily stores the firmware and the like held in the ROM  32 . 
     The wireless module  34  is a module configured to perform processing necessary for wireless LAN communication. The wireless module  34  constructs MAC frames for wireless communication by performing MAC layer processing on data transferred from the processor  31 , for example, converts the MAC frames into a wireless signal by performing physical layer processing on the constructed MAC frames, and transmits the wireless signal to the first base station  10 . As will be described later, the wireless signal destined for the first base station  10  can be received by the second base stations  20 A,  20 B, and  20 C. The wireless module  34  also receives wireless signals from the second base stations  20 A,  20 B, and  20 C, extracts data from the received wireless signals, and transfers the data to the processor  21 , for example. 
     The display  35  is a display device that displays various types of screens. The display  35  may be a liquid crystal display, an organic EL display, or the like. The display  35  may also include a touch panel. 
     The storage  36  is a storage device such as a hard disk or the like. The storage  36  stores various types of applications executed by the processor  31 , for example. 
       FIG.  5    is a diagram illustrating MAC layer processing during communication between the first base station  10  and second base stations  20 A,  20 B, and  20 C, and the terminal  30 . The MAC layer processing in  FIG.  5    follows the IEEE 802.11 standard.  FIG.  5    illustrates both transmission-side processing and reception-side processing. When the wireless module of one of the first base station  10  and second base stations  20 A,  20 B, and  20 C and the terminal  30  performs the transmission-side processing, the wireless module of the other performs the reception-side processing. In the following example, the transmission-side and reception-side wireless modules will be described without making distinctions between the two. 
     The transmission-side processing will be described first. In step S 10 , the wireless module performs A-MSDU aggregation. Specifically, the wireless module concatenates multiple LLC packets input from the LLC layer to generate an Aggregate-MAC service data unit (A-MSDU). 
     In step S 11 , the wireless module assigns a sequence number (SN) to the A-MSDU. The sequence number is a unique number for identifying the A-MSDU. 
     In step S 12 , the wireless module fragments the A-MSDU into multiple MAC protocol data units (MPDU). 
     In step S 13 , the wireless module encrypts each MPDU to generate encrypted MPDUs. 
     In step S 14 , the wireless module adds a MAC header and error detection code (FCS) to each encrypted MPDU. The error detection code is, for example, Cyclic Redundancy Check (CRC) code. 
     In step S 15 , the wireless module performs A-MPDU aggregation. Specifically, the wireless module concatenates multiple MPDUs to generate an Aggregate-MAC protocol data unit (A-MPDU) as a MAC frame. 
     After step S 15 , the wireless module performs physical layer processing on the MAC frame. 
     In the above-described transmission-side processing, when the transmission side is the first base station  10  and the second base stations  20 A,  20 B, and  20 C, the wireless module  16  of the first base station  10  performs the MAC layer processing from step S 11  to step S 15 . The wireless modules  24  of the second base stations  20 A,  20 B, and  20 C perform the physical layer processing. On the other hand, when the transmission side is the terminal  30 , the wireless module  34  of the terminal  30  performs the MAC layer processing from step S 10  to step S 15  and the physical layer processing. 
     The reception-side processing will be described next. When a wireless signal is received, the wireless module performs physical layer processing and extracts a MAC frame from the wireless signal. The wireless module then performs the MAC layer processing illustrated in  FIG.  5   . 
     In step S 20 , the wireless module performs A-MPDU deaggregation. Specifically, the wireless module fragments the A-MPDU into units of MPDUs. 
     In step  521 , the wireless module performs error detection. For example, the wireless module performs a CRC to determine whether or not the reception of the wireless signal has succeeded. If the reception of the wireless signal has failed, the wireless module may make a retransmission request. At this time, the wireless module may request the retransmission in units of MPDUs. On the other hand, if the reception of the wireless signal has succeeded, the wireless module performs the next processing. 
     In step S 22 , the wireless module performs address detection. At this time, the wireless module determines whether or not the MPDU which has been sent thereto is addressed to that wireless module itself on the basis of the address recorded in the MAC header of each MPDU. If not addressed to that wireless module itself, the wireless module does not perform the next processing. If addressed to that wireless module itself, the wireless module performs the next processing. 
     In step S 23 , the wireless module decrypts the encrypted MPDU. 
     In step S 24 , the wireless module defragments the MPDUs. In other words, the wireless module reconstructs the A-MSDU from multiple MPDUs. 
     In step S 25 , the wireless module performs A-MSDU deaggregation. Specifically, the wireless module reconstructs the LLC packets in units of MPDUs from the A-MSDU. 
     After step S 25 , the wireless module outputs the LLC packets to the layer above the MAC layer. The higher layer is the LLC layer, for example. 
     In the above-described reception-side processing, when the reception side is the first base station  10  and the second base stations  20 A,  20 B, and  20 C, the wireless modules  24  of the second base stations  20 A,  20 B, and  20 C perform the physical layer processing and the MAC layer processing from step S 20  to step S 22 . On the other hand, the wireless module  16  of the first base station  10  performs the MAC layer processing from step S 23  to step S 25 . On the other hand, when the reception side is the terminal  30 , the wireless module  34  of the terminal  30  performs the physical layer processing and the MAC layer processing from step S 20  to step S 25 . 
       FIG.  6    is a function block diagram of the first base station  10 . The first base station  10  includes an LLC processing unit  101 , an LLC interface (LLC-IF)  102 , a MAC processing unit  103 , network interfaces (NW-IF)  104 A,  104 B, and  104 C, a physical (PHY) selection unit  105 , and a carrier sense control unit  106 . The LLC processing unit  101 , the LLC interface (LLC-IF)  102 , the MAC processing unit  103 , the network interfaces (NW-IF)  104 A,  104 B, and  104 C, the physical (PHY) selection unit  105 , and the carrier sense control unit  106  are implemented by, for example, the processor  11 , the wireless module  16 , and the wired module  17 . 
     The LLC processing unit  101  performs LLC layer processing on data transferred from a server on the network  40 , for example, and generates LLC packets. The LLC layer processing includes, for example, processing for adding a Destination Service Access Point (DSAP) header, a Source Service Access Point (SSAP) header, and the like to the data. The LLC processing unit  101  also extracts data from the LLC packets and outputs the extracted data to a higher layer. The higher layer is, for example, the application layer. 
     The LLC-IF  102  is an interface for mediating signals between the LLC processing unit  101  and the MAC processing unit  103 . The LLC-IF  102  has a queue, for example. This queue temporarily stores the LLC packets generated by the LLC processing unit  101 . This queue also temporarily stores LLC packets extracted from the MAC frames by the MAC processing unit  103 . 
     When transmitting data from the first base station  10 , the MAC processing unit  103  generates MAC frames from LLC packets input via the LLC-IF  102 . The generation of the MAC frames is performed according to the processing of step S 10  to step S 15  illustrated in  FIG.  5   . The MAC processing unit  103  also extracts the LLC packets by performing processing based on the MAC headers of the MAC frames input from the NW-IFs  104 A,  104 B, and  104 C, and outputs the extracted LLC packets to the LLC-IF  102 . The extraction of the LLC packets from the MAC frames is performed according to the processing of step S 22  to step S 25  illustrated in  FIG.  5   . Note that the MAC processing unit  103  may have a queue for each of access categories (AC). Access categories may include, for example, VO (Voice), VI (Video), BE (Best effort), and BK (Background). 
     The NW-IF  104 A performs processing necessary for communicating signals between the first base station  10  and the second base station  20 A. The NW-IF  104 B performs processing necessary for communicating signals between the first base station  10  and the second base station  20 B. The NW-IF  104 C performs processing necessary for communicating signals between the first base station  10  and the second base station  20 C. Each of the NW-IFs  104 A,  104 B, and  104 C has a queue, for example. These queues temporarily store the signals (MAC frames) generated by the MAC processing unit  103 . These queues also temporarily store signals transmitted from the second base stations  20 A,  20 B, and  20 C, respectively. The NW-IFs  104 A,  104 B, and  104 C perform processing according to the respective communication methods thereof. For example, if the communication method is ROF, the NW-IFs  104 A,  104 B, and  104 C convert electrical signals into optical signals through EO conversion (electrical-to-optical conversion) and transmit the optical signals to the desired base stations among the second base stations  20 A,  20 B, and  20 C. Additionally, the NW-IFs  104 A,  104 B, and  104 C convert optical signals from the second base stations  20 A,  20 B, and  20 C into electrical signals through OE (optical-to-electrical conversion) and output the electrical signals to the MAC processing unit  103 . Additionally, the NW-IFs  104 A,  104 B, and  104 C output information for performing carrier sensing, from the corresponding second base stations, to the carrier sense control unit  106 . 
     The PHY selection unit  105  selects one of the second base stations  20 A,  20 B, and  20 C as the base station to use for communication with the terminal  30  in accordance with information, obtained by the carrier sense control unit  106 , for performing carrier sensing from the second base stations  20 A,  20 B, and  20 C. The selection by the PHY selection unit  105  will be described in detail later. 
     The carrier sense control unit  106  performs various types of control for Enhanced Distribution Channel Access (EDCA). For example, the carrier sense control unit  106  determines the state of channels between the second base stations  20 A,  20 B, and  20 C and the terminal using Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). The state of the channels is determined on the basis of information for carrying out carrier sensing, obtained via the NW-IFs  104 A,  104 B, and  104 C. The carrier sense control unit  106  then outputs the result of determining the state of the channels to the PHY selection unit  105 . Here, the carrier sense control unit  106  may have access parameters for each AC. 
       FIG.  7    is a function block diagram of the second base station  20 A. The second base stations  20 B and  20 C may have the same functions as the second base station  20 A. The following assumes that the functions of the second base stations  20 B and  20 C are the same as those of the second base station  20 A, and thus the functions of the second base stations  20 B and  20 C will not be described. 
     The second base station  20 A includes a network interface (NW-IF)  201 , a physical interface (PHY-IF)  202 , a physical (PHY) processing unit  203 , an error detection unit  204 , and an ACK generation unit  205 . 
     The NW-IF  201  performs processing necessary for communication between the second base station  20 A and the first base station  10 . The NW-IF  201  has a queue. This queue temporarily stores signals (MAC frames) transmitted by the first base station  10 . The queue also temporarily stores signals output from the error detection unit  204 . The NW-IF  201  performs processing according to the communication method. For example, if the communication method is ROF, the NW-IF  201  converts electrical signals from the PHY-IF  202  into optical signals through. EO conversion and transmits the optical signals to the first base station  10 . Additionally, the NW-IF  201  converts optical signals from the first base station  10  into electrical signals through OE conversion and outputs the electrical signals to the PHY-IF  202 . Additionally, the NW-IF  201  sequentially transmits information for performing carrier sensing, sent from the PHY processing unit  203  via the PHY-IF  202 , to the first base station  10 . 
     The PHY-IF  202  is an interface for mediating communication between the NW-IF  201  and the PHY processing unit  203 , mediating signals between the PHY processing unit  203  and the error detection unit  204 , and mediating signals between the PHY processing unit  203  and the ACK generation unit  205 . The PHY-IF  202  has a queue, for example. This queue temporarily stores the signals (MAC frames) temporarily stored in the NW-IF  201 . Signals extracted by the PHY processing unit  203  are also temporarily stored. 
     The PHY processing unit  203  primarily performs physical layer processing for wireless LAN communication with the terminal  30 . For example, the PHY processing unit  203  performs physical layer processing on the MAC frames stored in the PHY-IF  202  to convert the MAC frames into a wireless signal, and transmits the wireless signal to the terminal  30 . The PHY processing unit  203  also measures information necessary for performing carrier sensing, and communicates that information to the first base station  10 . The carrier sensing may be performed using Clear Channel Assessment (CCA), for example. CCA is a method for determining whether or not a channel is clear based on the reception power (RSSI) of the PHY processing unit  203 . In this case, the PHY processing unit  203  measures the RSSI. As described earlier, the information necessary for performing carrier sensing is sent to the carrier sense control unit  106  of the first base station  10 . Additionally, the PHY processing unit  203  extracts the MAC frames from the wireless signal received from the terminal  30  and outputs the MAC frames to the PHY-IF  202 . The PHY processing unit  203  also broadcasts beacon frames. The MAC address included in the beacon frames is a MAC address used in common by the first base station  10  and the second base stations  20 A,  20 B, and  20 C. 
     The error detection unit  204  detects errors in the MAC frames in order to determine whether or not the data transmitted from the terminal  30  has been received correctly. The error detection is performed using FCS. The error detection may be carried out in units of MPDUs. When there is no error in a MAC frame, the error detection unit  204  outputs the MAC frame to the NW-IF  201 , and requests the ACK generation unit  205  to generate an acknowledge (ACK) indicating that the reception has been performed normally. On the other hand, when there is an error in a MAC frame, the error detection unit  204  discards the MAC frame. 
     The ACK generation unit  205  generates the ACK in response to the request from the error detection unit  204 , and outputs the generated ACK to the PHY processing unit  203  via the PHY-IF  202 . In this case, the PHY processing unit  203  transmits the ACK to the terminal  30 . Here, the ACK may be a block ACK. A block ACK is, for example, an ACK for a reception confirmation response in units of MPDUs. In this case, the block ACK is generated on the basis of a bitmap created by the error detection unit  204  in accordance with the result of error detection on each of the data in units of MPDUs included in the MAC frame. 
     Operations of the communication system  1  will be described next. First, operations performed when a base station (the first base station  10 , the second base stations  20 A,  20 B, and  20 C) transmit a wireless signal and the terminal  30  receives the wireless signal will be described. 
       FIG.  8    is a flowchart illustrating an example of transmission processing by the first base station  10 . In step S 31 , the MAC processing unit  103  determines whether or not data to be transmitted via the LLC processing unit  101  and the LLC-IF  102  has been input from a higher layer such as the application layer. When it is determined in step S 31  that no data has been input, the processing of  FIG.  8    ends. When it is determined in step S 31  that data has been input, the processing moves to step S 32 . 
     In step S 32 , the MAC processing unit  103  performs the MAC layer processing illustrated in  FIG.  5    on the input data (LLC packets) to generate a MAC frame. 
     In step S 33 , the carrier sense control unit  106  determines a clear channel on the basis of the RSSI input from the second base stations  20 A,  20 B, and  20 C via the NW-IFs  104 A,  104 B, and  104 C. 
     In step S 34 , the carrier sense control unit  106  determines whether or not the channels of a plurality of base stations among the second base stations  20 A,  20 B, and  20 C are clear on the basis of a result of the carrier sensing. For example, the carrier sense control unit  106  determines that the channel of a corresponding base station is clear when the reception power at the PHY processing unit has dropped below a threshold over a carrier sense period that adds a random backoff period to an Arbitration Inter Frame Space (AIFS). Here, AIFS is an access parameter indicating a transmission interval of the wireless signals. In other words, in the embodiment, the access parameters are common across the second base stations  20 A,  20 B, and  20 C. When it is determined in step S 34  that the channels of a plurality of base stations are clear, the processing moves to step S 35 . When it is determined in step S 34  that only the channel of a single base station is clear, the carrier sense control unit  106  notifies the PHY selection unit  105  of, for example, an ID of the base station for which the channel is clear. The PHY selection unit  105  notifies the MAC processing unit  103  of this ID as a selection result. The processing then moves to step S 40 . Note that in step S 34 , it is possible that the channels of all the base stations are determined not to be clear. In this case, the carrier sense control unit  106  may suspend the processing. 
     In step S 35 , the carrier sense control unit  106  transmits a sounding frame via the NW-IF corresponding to any base station, among the second base stations  20 A,  20 B, and  20 C, for which the channel is clear. Note that the sounding frame need not be transmitted when the second base station to be used is associated with the terminal serving as the transmission destination. For example, if the carrier sense control unit  106  or the PHY selection unit  105  holds the reception power from the terminal  30  for each of the second base stations, when it is determined that channels are clear for a plurality of the base stations, the base station having the highest reception power can be selected. 
     In step S 36 , the carrier sense control unit  106  determines whether or not sounding response signal reception results have been obtained from the NW-IFs, among the NW-IFs  104 A,  104 B, and  104 C, that correspond to base stations having a channel that is clear. These reception results are the reception powers of the sounding response signal. When it is determined in step S 36  that sounding response signal reception results have not been obtained, the carrier sense control unit  106  suspends the processing. When it is determined in step S 36  that sounding response signal reception results have been obtained, the carrier sense control unit  106  outputs those reception results to the PHY selection unit  105 . The processing then moves to step S 37 . 
     In step S 37 , the PHY selection unit  105  selects a base station to be used to transmit data to the terminal  30  in accordance with the sounding response signal reception results. For example, the PHY selection unit  105  selects the base station having the highest reception power for the sounding response signal. After the selection, the PHY selection unit  105  notifies the MAC processing unit  103  of, for example, the ID of the selected base station as a selection result. 
     In step S 38 , the MAC processing unit  103  transmits the MAC frame using the NW-IF corresponding to the selected base station. 
     In step S 39 , the MAC processing unit  103  determines whether or not an ACK has been received. If it is determined in step S 39  that an ACK has been received, the processing of  FIG.  8    ends. If it is determined in step S 39  that an ACK has not been received, the MAC processing unit  103  suspends the processing. A time over determination may be made if it is determined that an ACK has not been received for a predetermined amount of time. In this case, the MAC processing unit  103  may retransmit the MAC frame. Additionally, the MAC processing unit  103  may retransmit the MAC frame by receiving a notification of transmission failure from the second base stations  20 A,  20 B, and  20 C. 
     In step S 40 , the MAC processing unit  103  transmits the MAC frame using the NW-IF corresponding to the selected base station. The processing then moves to step S 39 . 
       FIG.  9    is a flowchart illustrating an example of transmission processing by the second base station  20 A. The second base stations  20 B and  20 C may perform the same processing as the second base station  20 A. The following assumes that the operations of the second base stations  20 B and  20 C are the same as those of the second base station  20 A, and thus the operations of the second base stations  20 B and  20 C will not be described. 
     In step S 51 , the PHY processing unit  203  obtains information for performing carrier sensing. For example, the PHY processing unit  203  measures the RSSI for CCA. 
     In step S 52 , the PHY processing unit  203  transmits a CCA result to the first base station  10  using the NW-IF  201 . 
     In step S 53 , the PHY processing unit  203  determines whether or not a signal such as a MAC frame has been input. If it is determined in step S 53  that a signal has not been input, the PHY processing unit  203  suspends the processing. If it is determined in step S 53  that a signal has been input, the processing moves to step S 54 . 
     In step S 54 , the PHY processing unit  203  transmits a wireless signal. For example, when a MAC frame has been input, the PHY processing unit  203  performs physical layer processing on the MAC frame to convert the MAC frame into a wireless signal. The PHY processing unit  203  then transmits the wireless signal to the terminal  30 . Additionally, when a sounding frame has been input, the PHY processing unit  203  performs physical layer processing to convert the sounding frame into a sounding signal, which is a wireless signal. The PHY processing unit  203  then transmits the sounding signal to the terminal  30 . In this case, the PHY processing unit  203  waits to receive the sounding response signal, which includes a sounding response frame, from the terminal  30 . Then, when the sounding response signal is received, the PHY processing unit  203  transmits the sounding response signal reception result to the first base station  10 . The reception result is, for example, the reception power of the sounding response signal. 
     In step S 55 , the PHY processing unit  203  determines whether or not an ACK has been received from the terminal  30 . If it is determined in step S 55  that an ACK has been received from the terminal  30 , the processing moves to step S 56 . If it is determined in step S 55  that an ACK has not been received, the MAC processing unit  103  suspends the processing. A time over determination may be made if it is determined that an ACK has not been received for a predetermined amount of time. In this case, the MAC processing unit  103  may notify the first base station  10  that the transmission has failed. 
     In step S 56 , the PHY processing unit  203  transmits an ACK to the first base station  10  using the NW-IF  201 . The processing of  FIG.  9    then ends. 
     In this manner, when transmitting a wireless signal, the second base station  20 A converts the MAC frame received from the first base station  10  into a wireless signal and transmits the wireless signal as-is to the terminal  30 . The same applies to the second base stations  20 B and  20 C. 
     The selection of a base station using the sounding signal will be described further hereinafter.  FIG.  10    is a timing chart illustrating changes over time in a state of the PHY processing units of the second base stations  20 A,  20 B, and  20 C during the processing illustrated in  FIG.  9   . In the example of  FIG.  10   , since the reception power of the PHY processing unit  203  in both the second base station  20 A and the second base station  20 B is below the threshold during the carrier sense period, the base stations are in a clear state. On the other hand, the second base station  20 C is in a busy state during the carrier sense period. 
     In response to such a result, the carrier sense control unit  106  of the first base station  10  transmits a sounding frame. The base station that has received the sounding frame transmits a sounding signal to the terminal  30 . In the example of  FIG.  10   , the second base station  20 A transmits the sounding signal (SD) to the terminal  30 . 
     Having received the sounding signal, the terminal  30  returns a sounding response signal (RE). The sounding response signal is received by both the second base station  20 A and the second base station  20 B. The PHY processing unit  203  of each of the second base station  20 A and the second base station  20 B measures the reception power as a sounding response signal reception result. Then, each PHY processing unit  203  transmits the reception power as a sounding response signal reception result to the first base station  10 . The first base station  10  selects a base station to be used to transmit data on the basis of the respective sounding signal reception results of the second base station  20 A and the second base station  20 B. In the example in  FIG.  10   , the second base station  20 B. Accordingly, the second base station  20 B transmits a wireless signal including data to the terminal  30 . 
     In this manner, in this embodiment, if the channels of a plurality of second base stations are clear when transmitting data from a base station, the second base station having the best communication state is selected as the base station for transmitting the data. 
     Operations performed when the terminal  30  transmits data and the base stations (the first base station  10 , the second base stations  20 A,  20 B, and  20 C) receive the data will be described next. Here, the terminal  30  specifies the MAC address used in common by the first base station  10  and the second base stations  20 A,  20 B, and  20 C as the destination MAC address when transmitting the wireless signal. The transmission of the data by the terminal  30  is performed using the same EDCA as the base station, for example. The operations through which the terminal  30  transmits the data will not be described in detail. 
       FIG.  11    is a flowchart illustrating an example of reception processing by the second base station  20 A. The second base stations  20 B and  20 C may perform the same processing as the second base station  20 A. The following assumes that the operations of the second base stations  20 B and  20 C are the same as those of the second base station  20 A, and thus the operations of the second base stations  20 B and  20 C will not be described. 
     In step S 71 , the PHY processing unit  203  determines whether or not a wireless signal has been received. If it is determined in step S 71  that a wireless signal has not been received, the processing of  FIG.  11    ends. If it is determined in step S 71  that a wireless signal has been received, the processing moves to step S 72 . 
     In step S 72 , the PHY processing unit  203  performs physical layer processing to extract a MAC frame from the wireless signal. The error detection unit  204  then detects error on the basis of the FCS of the MAC frame. 
     In step S 73 , the error detection unit  204  determines whether or not there is an error in the data. If it is determined in step S 73  that there are no errors in the data, the processing moves to step S 74 . If it is determined in step S 73  that there is an error in the data, the error detection unit  204  discards the data. In this case, the processing of  FIG.  11    ends. Note that when there is an error in the data, the error detection unit  204  may determine that the data is addressed to its own base station, and notify the first base station  10  that there is an error. This makes it possible for the first base station  10  to determine whether a retransmission request is necessary. 
     In step S 74 , the error detection unit  204  determines whether or not the received data is addressed to its own base station. In other words, the error detection unit  204  determines whether or not the MAC address is an address used in common for the first base station  10  and the second base stations  20 A,  20 B, and  20 C. If it is determined in step S 74  that the received data is addressed to its own base station, the processing moves to step S 75 . If it is determined in step S 74  that the received data is not addressed to its own base station, the error detection unit  204  discards the data. In this case, the processing of  FIG.  11    ends. 
     In step S 75 , the error detection unit  204  instructs the ACK generation unit  205  to generate an ACK. In response, the ACK generation unit  205  generates the ACK, and transmits the ACK to the terminal  30  using the PHY processing unit  203 . 
     In step S 76 , the error detection unit  204  transmits the MAC frame to the first base station  10  using the NW-IF  201 . The processing of  FIG.  11    then ends. 
     In this manner, if, when a wireless signal is received, the data can be received without errors and the data is addressed to the base station itself, the second base station  20 A transmits the MAC frame as-is to the first base station  10 . The same applies to the second base stations  20 B and  20 C. 
       FIG.  12    is a flowchart illustrating an example of reception processing by the first base station  10 . In step S 81 , the MAC processing unit  103  determines whether or not a MAC frame has been input via at least one of the NW-IFs  104 A,  104 B, and  104 C. When it is determined in step S 81  that no MAC frame has been input, the processing of  FIG.  12    ends. When it is determined in step S 81  that a MAC frame has been input, the processing moves to step S 82 . 
     In step S 82 , the MAC processing unit  103  determines whether or not a plurality of MAC frames have been input. When there are overlapping areas in the service areas of the second base stations  20 A,  20 B, and  20 C, a wireless signal may be received by the plurality of base stations at the same time or with a time difference. In this case, the same MAC frame is sent to the MAC processing unit  103  from the plurality of base stations. Step S 82  is a determination of this state. When it is determined in step S 82  that a plurality of MAC frames have been input, the processing moves to step S 83 . When it is determined in step S 82  that a plurality of MAC frames have not been input, the processing moves to step S 84 . Note that when a notification that the data received from the second base stations  20 A,  20 B, and  20 C has an error has been made, a retransmission request may be made as necessary. 
     In step S 83 , the MAC processing unit  103  selects one of the MAC frames. For example, the MAC processing unit  103  selects the MAC frame from the base station received at the highest reception power. Because the same MAC frame is transmitted from each base station, the MAC processing unit  103  may select the MAC frames in a predetermined priority order, such as the order of the second base stations  20 A,  20 B, and  20 C. Note that in order to avoid a situation in which a plurality of second base stations transmit the ACK simultaneously, a second base station that is primary may be set for each terminal, and the second base stations may transmit the ACK when reception has succeeded, the destination is itself, and the transmission source is the terminal for which the second base station itself is primary. Which second base station is to be primary may be the second base station having the highest reception power from the terminal. 
     In step S 84 , the MAC processing unit  103  performs the MAC layer processing illustrated in  FIG.  5    to extract the LLC packet. The MAC processing unit  103  then outputs the extracted LLC packet to the LLC processing unit  101 . 
     In step S 85 , the LLC processing unit  101  extracts the data from the LLC packet. 
     In step S 86 , the LLC processing unit  101  outputs the extracted data to the higher layer. The higher layer is, for example, the application layer. The processing of  FIG.  12    then ends. 
     According to the embodiment described thus far, the MAC layer processing of one or more second base station is performed collectively by a single first base station. This makes it possible to consider the first base station and the one or more second base stations as a single base station. Accordingly, it is not necessary to switch the connection at the time of a handover when the terminal  30  moves from the service area of the second base station  20 A to the service area of the second base station  20 B, for example. This makes it possible to reduce overhead arising from the switching of connections at the time of a handover. In the embodiment, the service area can be broadened substantially by increasing the number of second base stations that are installed. 
     Each second base station also performs error detection for the received MAC frames. This makes it possible to support protocols having stricter time limits than when the error detection is performed by the first base station. 
     Furthermore, when the channels of the plurality of second base stations are clear, a single second base station for transmitting the wireless signal is selected in accordance with a result of sounding, for example. As a result, communication with a terminal can be performed using the base station that has the channel in the best state. 
     Additionally, communication between the first base station and the second base stations is performed through wired communication. This makes it easier to stabilize the communication between the first base station and the second base stations. 
     [Variations] 
     Variations on the embodiment will be described next. In the embodiment, the first base station is not provided with a PHY processing unit. In other words, in the embodiment, the first base station does not perform wireless communication with the terminal directly. As opposed to this, the first base station may also include a PHY processing unit. 
     The embodiment also describes an example in which a single second base station is provided with a single PHY processing unit. However, a plurality of different PHY processing units, e.g., a PHY processing unit for the 2.4 GHz band and a PHY processing unit for the 5 GHz band, may be provided in a single second base station. In this case, carrier sensing may be performed for each of the PHY processing units. The PHY selection unit  105  may then select a single PHY processing unit on the basis of the CCA result obtained for each PHY processing unit. 
     Additionally, in the embodiment, the error detection is performed by the second base stations. However, the error detection may be performed by the first base station. In this case, the error detection unit of the first base station detects errors in the data received by each of the three second base stations. The error detection unit then transmits a block ACK based on the error in the data from a single second base station. 
     Each of the processes in the above-described embodiment can be stored as a program which can be executed by a processor serving as a computer. The programs can be stored in a storage medium of an external storage device such as a magnetic disk, an optical disk, semiconductor memory, or the like. The processor can then load the programs stored in the storage medium of the external storage device and execute the above-described processing by controlling the operations by the loaded programs. 
     Note that the present invention is not limited to the above embodiments, and can be modified in various ways without departing from the scope thereof at the implementation stage. In addition, embodiments may be combined as appropriate, in which case combined effects can be achieved. Furthermore, the foregoing embodiments include various inventions, and various inventions can be extracted by selecting combinations of the multiple constituent elements disclosed herein. For example, even if several of the constituent elements described in the embodiments are removed, a configuration in which those constituent elements have been removed can be extracted as an invention as long as the problem can be solved and the effect can be achieved. 
     REFERENCE SIGNS LIST 
     
         
           1  Communication system 
           10  First base station 
           11  Processor 
           12  ROM 
           13  RAM 
           14  WAN module 
           15  Routing module 
           16  Wireless module 
           17  Wired module 
           20 A,  20 B,  20 C Second base station 
           21  Processor 
           22  ROM 
           23  RAM 
           24  Wireless module 
           25  Wired module 
           30  Terminal 
           31  Processor 
           32  ROM 
           33  RAM 
           34  Wireless module 
           35  Display 
           36  Storage 
           40  Network 
           101  LLC processing unit 
           102  LLC-IF 
           103  MAC processing unit 
           104 A,  104 B,  104 C NW-IF 
           105  PHY selection unit 
           106  Carrier sense control unit 
           201  NW-IF 
           202  PHY-IF 
           203  PHY processing unit 
           204  Error detection unit 
           205  ACK generation unit