Patent Publication Number: US-2023141726-A1

Title: Communication apparatus and communication method

Description:
TECHNICAL FIELD 
     The present disclosure relates to a communication apparatus and a communication method. 
     BACKGROUND ART 
     Patent Literature (hereinafter, referred to as “PLT”) 1 discloses a method in which a radio communication apparatus mounted on a vehicle switches between a plurality of modes such as a Station (STA) mode and an Access Point (AP) mode and communicates with another radio communication apparatus. 
     In Non-Patent Literature (hereinafter, referred to as “NPL”) 1, a Personal Basic Service Set (PB SS) is specified as a method for communicating between terminals without a base station. In the PBSS, a PBSS Control Point (PCP) that performs a role of determining the scheduling is determined among one or more neighboring communicable terminals. 
     Incidentally, a traffic accident is likely to occur at a place such as an intersection. Thus, allowing communication for information, such as a video image of a camera, between vehicles entering an intersection may enhance safety at the intersection. 
     CITATION LIST 
     Patent Literature 
     PTL 1 
     
         
         WO2016/009481 
       
    
     Non-Patent Literature 
     NPL 1 
     
         
         IEEE802.11-2016 
       
    
     SUMMARY OF INVENTION 
     However, for communication between vehicles (hereinafter, may be referred to as “inter-vehicle communication”) based on a PBSS, establishing a radio link between vehicles entering the intersection may be difficult. 
     Non-limiting and exemplary embodiments of the present disclosure facilitate providing a communication apparatus and a communication method each capable of easily establishing a radio link between vehicles entering an intersection. 
     A communication apparatus according to an embodiment of the present disclosure is a communication apparatus mounted on a vehicle, the communication apparatus including: a first communication circuit that operates as an access point of an infrastructure mode and transmits and receives a radio wave toward and from a front side of the vehicle; and a second communication circuit that operates as a station of an infrastructure mode and transmits and receives a radio wave toward and from the front side of the vehicle. 
     A communication method according to an embodiment of the present disclosure is a communication method for a communication apparatus mounted on a vehicle, the communication method including: performing, by a first communication circuit, an operation as an access point of an infrastructure mode; transmitting and receiving, by the first communication circuit, a radio wave toward and from a front side of the vehicle; performing, by a second communication circuit, an operation as a station of an infrastructure mode; and transmitting and receiving, by the second communication circuit, a radio wave toward and from the front side of the vehicle. 
     It should be noted that a general or specific embodiment may be implemented as a system, an apparatus, a method, an integrated circuit, a computer program, a storage medium, or any selective combination thereof. 
     According to an embodiment of the present disclosure, it is possible to easily establish a radio link between vehicles entering an intersection. 
     Additional benefits and advantages of embodiments of the present disclosure will become apparent from the specification and drawings. The benefits and/or advantages may be individually obtained by some embodiments and features described in the specification and drawings, which need not all be provided in order to obtain one or more of such features. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    illustrates exemplary inter-vehicle communication using a PBSS mode; 
         FIG.  2    illustrates another exemplary inter-vehicle communication using a PB SS mode; 
         FIG.  3    is a block diagram illustrating an exemplary configuration of a communication apparatus according to Embodiment 1; 
         FIG.  4    illustrates exemplary directivity of antennae; 
         FIG.  5    illustrates exemplary inter-vehicle communication according to Embodiment 1; 
         FIG.  6    illustrates another exemplary inter-vehicle communication according to Embodiment 1; 
         FIG.  7    illustrates still another exemplary inter-vehicle communication according to Embodiment 1; 
         FIG.  8    is a flow chart illustrating an exemplary operation of inter-vehicle communication according to Embodiment 1; 
         FIG.  9    is a block diagram illustrating another exemplary configuration of a communication apparatus; 
         FIG.  10 A  is a flow chart illustrating an exemplary operation of inter-vehicle communication according to Embodiment 2; 
         FIG.  10 B  is another flow chart illustrating an exemplary operation of inter-vehicle communication according to Embodiment 2; 
         FIG.  11    illustrates an exemplary probe response frame; 
         FIG.  12    illustrates a block diagram illustrating an exemplary configuration of a communication apparatus according to Embodiment 3; 
         FIG.  13    illustrates exemplary directivity of antennae; and 
         FIG.  14    illustrates exemplary inter-vehicle communication according to Embodiment 3. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings as appropriate. However, a detailed description more than necessary may be omitted, such as a detailed description of a well-known matter and a duplicate description for a substantially identical configuration, to avoid unnecessary redundancy of the following description and to facilitate understanding by a person skilled in the art. 
     Note that, the accompanying drawings and the following description are provided for a person skilled in the art to sufficiently understand the present disclosure, and are not intended to limit the subject matter described in the claims. 
     Embodiment 1 
       FIG.  1    illustrates exemplary inter-vehicle communication using a PBSS mode.  FIG.  1    illustrates vehicles  10 A,  10 B,  10 C, and  10 D. Vehicles  10 A,  10 B,  10 C, and  10 D may be a vehicle that travels on a road or near a road, such as an automobile, a motorcycle, a bicycle, or a tram. Note that the following drawings are described in the case of a left-hand traffic, but the present disclosure can be applied to a right-hand traffic as well. 
     Vehicle  10 A is equipped with communication apparatus  1000   a . Vehicle  10 B is equipped with communication apparatus  1000   b . Vehicle  10 C is equipped with communication apparatus  1000   c . Vehicle  10 D is equipped with communication apparatus  1000 D. 
     Communication apparatuses  1000   a  to  1000   d  perform communication based on a PBSS mode. In the following description, communication performed between vehicles traveling in a back-and-forth relationship in the same traveling direction may be referred to as in-line inter-vehicle communication. 
     For example, vehicle  10 A and vehicle  10 B travel in the same traveling direction on the road. Communication apparatus  1000   a  and communication apparatus  1000   b  perform in-line inter-vehicle communication. 
     For example, communication apparatus  1000   a  finds (detects) communication apparatus  1000   b , and establishes (connects) a radio link with communication apparatus  1000   b  based on a PBSS. Communication apparatus  1000   a  and communication apparatus  1000   b  determine either of the two apparatuses to be a PCP based on a method described in NPL 1. For example, communication apparatus  1000   a  and communication apparatus  1000   b  determine that communication apparatus  1000   b  is a PCP and communication apparatus  1000   a  is a non-PCP. In this case, non-PCP communication apparatus  1000   a  participates in a PBSS of PCP communication apparatus  1000   b.    
     For example, vehicle  10 C and vehicle  10 D travel in the same traveling direction on the road. Communication apparatus  1000   c  and communication apparatus  1000   d  perform in-line inter-vehicle communication. 
     For example, communication apparatus  1000   c  finds (detects) communication apparatus  1000   d , and establishes (connects) a radio link with communication apparatus  1000   d  based on a PBSS. Communication apparatus  1000   c  and communication apparatus  1000   d  determine either of the two apparatuses to be a PCP based on a method described in NPL 1. For example, communication apparatus  1000   c  and communication apparatus  1000   d  determine that communication apparatus  1000   c  is a PCP and communication apparatus  1000   d  is a non-PCP. In this case, non-PCP communication apparatus  1000   d  participates in a PBSS of PCP communication apparatus  1000   c.    
     In this situation, at a place where vehicles cross each other, such as an intersection, performing inter-vehicle communication meeting each other may enhance safety. For example, transmitting a video image of a camera (not illustrated) mounted on vehicle  10 C from communication apparatus  1000   c  to communication apparatus  1000   a  allows vehicle  10 A to find a pedestrian (not illustrated) in a right-turn destination when turning right, which may result in enhancing safety at right-turn. 
     As an example of establishing communication at an intersection, communication apparatus  1000   a  of vehicle  10 A entering the intersection finds communication apparatus  1000   c  of vehicle  10 C entering the intersection. To initiate communication with the found communication apparatus  1000   c , communication apparatus  1000   a  leaves (disconnects) from the PBSS of communication apparatus  1000   b  and participates in the PBSS of communication apparatus  1000   c.    
     This allows communication apparatus  1000   a  of vehicle  10 A to communicate with communication apparatus  1000   c  of vehicle  10 C that communication apparatus  1000   a  meets at the intersection, and to receive the video image of the camera mounted on vehicle  10 C. Further, non-PCP communication apparatus  1000   d  performing in-line inter-vehicle communication with PCP communication apparatus  1000   c  can communicate with communication apparatus  1000   a  by transmitting a relay request to communication apparatus  1000   c.    
     However, communication apparatus  1000   a  does not perform in-line inter-vehicle communication with communication apparatus  1000   b  because communication apparatus  1000   a  has left from the PBSS of communication apparatus  1000   b . Thus, it is difficult for vehicle  10 A to find a vehicle approaching from the rear side, which may result in lowering the safety. 
     For example, when a camera (not illustrated) mounted on vehicle  10 B finds two-wheeled vehicle  10 H approaching from the rear side, communication apparatus  1000   b  does not need to transmit the video camera image of two-wheeled vehicle  10 H to communication apparatus  1000   a , and thus the safety of vehicle  10 A when turning right is lowered. 
     Further, communication apparatus  1000   a  cancels communication with communication apparatus  1000   b  before establishing a radio link with communication apparatus  1000   c , and after leaving from the PBSS of communication apparatus  1000   b , communication apparatus  1000   a  executes a procedure for participating in the PBSS of communication apparatus  1000   c . Thus, it takes a time to complete the establishment of the radio link between communication apparatus  1000   a  and communication apparatus  1000   c , the initiation of the transmission of the video camera image of vehicle  10 C to communication apparatus  1000   a  is delayed, and thus the safety of vehicle  10 A when turning right is lowered. 
       FIG.  2    illustrates another exemplary inter-vehicle communication using a PBSS mode. In  FIG.  2   , the same components as those in  FIG.  1    are denoted by the same reference numerals. 
     For example, vehicle  10 A and vehicle  10 B travel in the same traveling direction on the road. Communication apparatus  1000   a  and communication apparatus  1000   b  establish a radio link with each other based on a PBSS, and perform in-line inter-vehicle communication with each other. For example, communication apparatus  1000   a  determines a non-PCP and communication apparatus  1000   b  determines a PCP. 
     For example, vehicle  10 C and vehicle  10 D travel in the same traveling direction on the road. Communication apparatus  1000   c  and communication apparatus  1000   d  establish a radio link with each other based on a PBSS and perform in-line inter-vehicle communication with each other. For example, communication apparatus  1000   c  determines a PCP and communication apparatus  1000   d  determines a non-PCP 
     In this case, when vehicle  10 D enters an intersection and turns right after vehicle  10 C passes the intersection, two-wheeled vehicle  10 G on the oncoming lane is hidden behind vehicle  10 E and thus is located at the blind area with respect to a driver of vehicle  10 D. 
     Meanwhile, a sensor (not illustrated) such as radar or a camera mounted on vehicle  10 B can detect two-wheeled vehicle  10 G. When communication apparatus  1000   b  can notify information on two-wheeled vehicle  10 G detected by the sensor of vehicle  10 B to communication apparatus  1000   d  of vehicle  10 D, the safety when vehicle  10 D turns right can be enhanced. 
     However, because communication apparatus  1000   d  participates in the PBSS of communication apparatus  1000   c , it takes a time to establish a radio link with communication apparatus  1000   b . For example, because communication apparatus  1000   d  cancels communication with communication apparatus  1000   c  and executes a procedure for participating in the PBSS of communication apparatus  1000   b  after leaving from the PBSS of communication apparatus  1000   c , it takes a time to establish a radio link with communication apparatus  1000   b . When it takes a time to establish a radio link, the finding of a vehicle such as two-wheeled vehicle  10 G approaching from the front side by vehicle  10 D may be delayed, and thus the safety may be lowered. 
     A communication apparatus according to the present disclosure facilitates establishing a radio link between vehicles in radio communication between vehicles, for example, at a location such as an intersection. 
       FIG.  3    is a block diagram illustrating an exemplary configuration of communication apparatus  100  according to Embodiment 1. As illustrated in  FIG.  3   , communication apparatus  100  includes: AP radio device  101 ; non-AP radio device  102 ; non-AP radio device  103 ; control devices  104  and  105 ; connection circuit  106 ; and antennae  111 ,  112 , and  113 . 
     Antennae  111  and  112  change directivity in a certain range on the front side of the vehicle. Antenna  113  changes directivity in a certain range on the rear side of the vehicle. Antennae  111 ,  112 , and  113  may be array antennae or phased array antennae including a plurality of antenna elements. 
     AP radio device  101 , non-AP radio device  102 , and AP radio device  103  operate in an infrastructure mode compliant with the standards of IEEE802.11 series. The infrastructure mode may be referred to as an infrastructure BSS mode. 
     AP radio device  101  operates based on an AP mode. Antenna  111  is connected to AP radio device  101 , and AP radio device  101  performs communication in a certain range on the front side of the vehicle. AP radio device  101  performs communication using, for example, a millimeter wave. 
     Non-AP radio device  102  operates based on a non-AP mode. Antenna  112  is connected to non-AP radio device  102 , and non-AP radio device  102  performs communication in a certain range on the front side of the vehicle. Non-AP radio device  102  performs communication using, for example, a millimeter wave. 
     Non-AP radio device  103  operates based on a non-AP mode. Antenna  113  is connected to non-AP radio device  103 , and non-AP radio device  103  performs communication in a certain range on the front side of the vehicle. Non-AP radio device  103  performs communication using, for example, a millimeter wave. 
     Note that the AP may be referred to as a base station or a parent unit. The non-AP may be referred to as a STA, a terminal, a client, or a child unit. 
     AP radio device  101  and non-AP radio device  102  may be physically separated from each other. Further, AP radio device  101  and non-AP radio device  102  may be physically one radio device, and one radio device may have functions of AP radio device  101  and non-AP radio device  102 . 
     Non-AP radio devices  102  and  103  may be physically separated from each other. Further, non-AP radio devices  102  and  103  may be physically one radio device, and one radio device may have functions of non-AP radio devices  102  and  103 . 
     Control devices  104  and  105  perform routing control in a subnet (BSS) in which AP radio device  101 , non-AP radio device  102 , and non-AP radio device  103  participate. Further, control device  104  determines a network address of a BSS serviced by AP radio device  101 . Control devices  104  and  105  may be configured by a processor such as a Central Processing Unit (CPU) or a Digital Signal Processor (DSP). 
     Control device  104  is connected to AP radio device  101  and non-AP radio device  102  via an interface such as Universal Serial Bus (USB), Peripheral Component Interconnect Express (PCIe), or Ethernet. Control device  105  is connected to non-AP radio device  103  via an interface such as USB, PCIe or Ethernet. 
     Control device  104  and control device  105  are connected to connection circuit  106  via an interface such as Ethernet or Controller Area Network (CAN). Connection circuit  106  connects control device  104  and control device  105  with each other. Connection circuit  106  may be a device or a circuit such as a hub, a switch, a router, a wire harness, or a switch box. Further, connection circuit  106  may be an access point, a child unit, a USB dongle, or an extension-board based on a radio communication system such as wireless Local Area Network (LAN), Wireless Gigabit (WiGig), or Bluetooth. 
     Note that position detection device  200  that detects a position of a vehicle equipped with communication apparatus  100  is connected to communication apparatus  100 . Position detection device  200  may detect the position of the vehicle by a navigation system using a Global Navigation Satellite System (GNSS). Position detection device  200  may be incorporated in communication apparatus  100 . Further, communication apparatus  100  may be carried by a pedestrian (not illustrated). 
       FIG.  4    illustrates exemplary directivity of antennae  111 ,  112 , and  113 . Antennae  111  and  112  radiating radio waves on the front side of the vehicle may have directivity of, for example, a half-value angle of 5° as illustrated by sector  201 . As an example, antennae  111  and  112  may change the direction of the directivity (beam or sector) in a range of 120° in the front direction of the vehicle (60° on each of the left and right sides with respect to the front surface of the vehicle) as illustrated by sector  202 . 
     For example, antenna  113  radiating a radio wave to the rear side of the vehicle may have directivity of a half-value angle of 5° as illustrated by sector  203 . For example, antenna  113  may change the direction of the directivity in a range of 120° on the rear side of the vehicle (60° on each of the left and right sides with respect to the rear surface of the vehicle) as illustrated by sector  204 . 
     Note that the movable ranges of the directions of the directivity indicated by sectors  202  and  204  are not limited to 120°. The movable ranges of the directions of the directivity may be 90° or 180°. In millimeter wave communications, multiple antenna elements are placed on a board or a module to form a compact array antenna having high directivity. Thus, the communication range of one antenna module is often within 180°. 
       FIG.  5    illustrates exemplary inter-vehicle communication according to Embodiment 1. Vehicles  10 A,  10 B,  10 C, and  10 D illustrated in  FIG.  5    are each equipped with communication apparatus  100  illustrated in  FIG.  3   . 
     In the following description, in order to distinguish communication apparatus  100  mounted on vehicles  10 A,  10 B,  10 C, and  10 D, a communication apparatus mounted on vehicle  10 A may be referred to as communication apparatus  100   a , a communication apparatus mounted on vehicle  10 B may be referred to as communication apparatus  100   b , a communication apparatus mounted on vehicle  10 C may be referred to as communication apparatus  100   c , and a communication apparatus mounted on vehicle  10 D may be referred to as communication apparatus  100   d.    
     Further, in order to distinguish each unit included in each of communication apparatuses  100   a ,  100   b ,  100   c , and  100   d , the reference numerals of the units included in communication apparatuses  100   a ,  100   b ,  100   c , and  100   d  may be denoted with suffixes of a, b, c, and d, respectively. For example, AP radio device  101  of communication apparatus  100   a  may be referred to as AP radio device  101   a . AP radio device  101  of communication apparatus  100   b  may be referred to as AP radio device  101   b.    
     For example, vehicle  10 A and vehicle  10 B travel in the same traveling direction on the road. Non-AP radio device  103   a  of vehicle  10 A connects to (establishes a radio link) to AP radio device  101   b  of following vehicle  10 B, and performs in-line inter-vehicle communication. For example, non-AP radio device  103   a  of vehicle  10 A and AP radio device  101   b  of vehicle  10 B performs AP to non-AP communication with each other using a distribution service described in NPL 1. Thus, communication apparatus  100   a  of vehicle  10 A and communication apparatus  100   b  of vehicle  10 B perform data communication with each other based on an Internet Protocol (IP). 
     Note that communication apparatus  100  does not perform communication between non-AP radio devices. In other words, communication apparatus  100  does not perform communication between terminals (between non-AP radio device  102  and non-AP radio device  103 ). 
     Further, AP radio device  101  transmits a beacon frame, and non-AP radio device  103  transmit no beacon frame. As a result, communication apparatus  100  can reduce the total number of beacon frames, and thus enhances throughput of the data communication due to interference between the communication apparatuses and occupation of the radio band. The beacon frame may be referred to as a beacon signal. 
     Further, AP radio device  101  may provide one BSS service and configure one subnet (a unit of a local area network in IP communication). 
     In addition, control device  104  may have a server function of Dynamic Host Configuration Protocol (DHCP). For example, when non-AP radio device  103  connects to AP radio device  101 , control device  104  may assign one of IP addresses belonging to the subnet of AP radio device  101  to non-AP radio device  103 . 
     For example, vehicle  10 C and vehicle  10 D, for example, travel in the same traveling direction on the road. Non-AP radio device  103   c  of vehicle  10 C and AP radio device  101   d  of following vehicle  10 D also perform in-line inter-vehicle communication with each other as well as above-described non-AP radio device  103   a  of vehicle  10 A and AP radio device  101   b  of vehicle  10 B. 
     At this time, non-AP radio device  102   d  of vehicle  10 D enters the intersection while scanning a beacon frame. Non-AP radio device  102   d  of vehicle  10 D finds AP radio device  101   b  of vehicle  10 B, which is an AP, and connects to AP radio device  101   b . Note that AP radio device  101   b  of vehicle  10 B can establish a radio link with a plurality of non-APs (non-AP radio device  103   a  of vehicle  10 A and non-AP radio device  102   d  of vehicle  10 D). 
     This allows communication apparatus  100   d  of vehicle  10 D to communicate with communication apparatus  100   b  of vehicle  10 B via non-AP radio device  102   d  without disconnecting the radio link (disconnecting in-line inter-vehicle communication) with communication apparatus  100   c  of vehicle  10 C. Further, communication apparatus  100   d  of vehicle  10 D can communicate with communication apparatus  100   a  of vehicle  10 A via communication apparatus  100   b  (AP radio device  101   b ) of vehicle  10 B. 
     Control device  104   d  of vehicle  10 D can operate as a router. For example, control device  104   d  of vehicle  10 D may perform routing between a subnet corresponding to a BSS of AP radio device  101   d  and a subnet corresponding to a BSS of radio device  101   b  of vehicle  10 B, which is the connection destination of non-AP radio device  102   d.    
     This allows communication apparatus  100   c  of vehicle  10 C to communicate with communication apparatus  100   b  of vehicle  10 B via communication apparatus  100   d  (router) of vehicle  10 D. Further, communication apparatus  100   c  of vehicle  10 C can communicate with communication apparatus  100   a  of vehicle  10 A via communication apparatus  100   b  of vehicle  10 B. In other words, communication apparatus  100   c  of vehicle  10 C can be connected to a network of in-line inter-vehicle communication different from the in-line inter-vehicle communication with communication apparatus  100   d  of following vehicle  10 D. 
     Note that control device  104  may determine a communication path (routing path) using a routing protocol such as Routing Information Protocol (RIP), Open Shortest Path First (OSPF), or Border Gateway Protocol (BGP). 
     Control device  104  may also perform routing between AP radio device  101  and non-AP radio device  103 . Control device  104  may perform routing between non-AP radio device  102  and non-AP radio device  103 . 
     In addition, communication apparatus  100  may configure a mesh network using an existing routing protocol in the infrastructure mode. 
     Moreover, control device  104  may randomly set the network address of the subnet that AP radio device  101  uses. For example, control device  104  may randomly select one network address among network addresses of 10.0.0.0/28, 10.0.0.16/28, 10.0.0.32/28, . . . , 10.0.0.240/28, . . . , 10.0.1.0/28, . . . , 10.0.0.240/28, . . . , 10.0.255.0/28, . . . , 10.1.0.0/28, . . . , 10.255.0.0/28, . . . , 10.255.255.0/28, . . . , 10.255.255.240/28 (256×256×16 ways). 
     Thus, it is possible to reduce the probability that the network address of the subnet of AP radio device  101  overlaps with the network address of the subnet to which non-AP radio devices  102  and  103  are connected. Further, it is possible to reduce the probability that the network address of the subnet of AP radio device  101  overlaps with the network address of the subnet connected by the routing between non-AP radio devices  102  and  103  and control device  104  of another vehicle. Communication between a large number of vehicles can be achieved via the routing between AP radio device  101 , non-AP radio device  102 , and non-AP radio device  103  mounted on vehicle  10 . 
     Further, when the network addresses of AP radio device  101  and non-AP radio devices  102  and  103  overlap with each other, control device  104  may perform bridging instead of routing. When the network addresses of AP radio device  101  and non-AP radio devices  102  and  103  overlap with each other and the subnet mask is different from each other, control device  104  may connect the subnet based on Proxy ARP (ARP: Address Resolution Protocol). Control device  104  may select one of routing, bridging, or Proxy ARP depending on whether the network addresses of AP radio device  101  (AP) and non-AP radio devices  102  and  103  overlap with each other, and/or whether the subnet masks match with each other. 
     In addition, control device  104  may use the site-local address of IPv6 (IP version 6) to configure a subnet for each BSS. Further, control device  104  may use a unique IPv6 network address predetermined for each AP radio device  101 . 
     Furthermore, when non-AP radio device  102  of a first vehicle is connected to AP radio device  101  of a second vehicle, non-AP radio device  102  of the first vehicle may notify information on AP radio device  101  of the first vehicle, such as BSS Identifier (BSSID), MAC address, and Service Set Identifier (SSID), to AP radio device  101  of the second vehicle. Non-AP radio device  102  of the second vehicle may receive the information on AP radio device  101  of the first vehicle from AP radio device  101  of the second vehicle, and may exclude AP radio device  101  of the first vehicle from the connection target. 
     For example, in  FIG.  5   , non-AP radio device  102   d  of vehicle  10 D connects to AP radio device  101   b  of vehicle  10 B. Non-AP radio device  102   d  of vehicle  10 D notifies the SSID of AP radio device  101   d  of vehicle  10 D to AP radio device  101   b  of vehicle  10 B. Non-AP radio device  102   b  of vehicle  10 B excludes the SSID of AP radio device  101   d  of vehicle  10 D received by AP radio device  101   b  from the scanning target and the association target. Further, non-AP radio device  102   b  of vehicle  10 B may exclude the BSSID of AP radio device  101   d  of vehicle  10 D from the scanning target and the association target. 
       FIG.  6    illustrates another exemplary inter-vehicle communication according to Embodiment 1. In  FIG.  6   , the same components as those in  FIG.  5    are denoted by the same reference numerals. 
     In  FIG.  6   , roadside devices  400   a ,  400   b ,  400   c , and  400   d  are installed at the intersection. Roadside devices  400   a ,  400   b ,  400   c , and  400   d  are APs that operate based on an infrastructure mode. Roadside devices  400   a ,  400   b ,  400   c , and  400   d  are connected to each other by a backhaul line (not illustrated) and relay communication between non-AP radio devices by relaying, bridging, or routing. The backhaul line may be configured by wire or by radio. In the following description, when not distinguished from each other, roadside devices  400   a ,  400   b ,  400   c , and  400   d  may be described as roadside device  400 . 
     When non-AP radio devices  102   a ,  102   b ,  102   c , and  102   d  connect to roadside devices  400   a ,  400   b ,  400   c , and  400   d , intercommunication between non-AP radio devices  102   a ,  102   b ,  102   c , and  102   d  is possible. 
     As an example, non-AP radio device  102   b  of vehicle  10 B entering the intersection and non-AP radio device  102   d  of vehicle  10 D entering the intersection connect to roadside device  400 . Non-AP radio device  102   b  of vehicle  10 B and non-AP radio device  102   d  of vehicle  10 D communicate with each other via roadside device  400 . 
     In a case where no connection is made to roadside device  400  when the vehicle enters the intersection, non-AP radio device  102  of communication apparatus  100  may start scanning AP radio device  101 . That is, in a case where a radio link with roadside device  400  has been established when the vehicle enters the intersection, communication device  100  may not execute the establishment process of a radio link described in  FIG.  5   . In other words, non-AP radio device  102  may give higher priority to establishment of a radio link with the roadside device than establishment of a radio link with the AP radio device of the vehicle. 
     Note that, when non-AP radio device  102   x  (not illustrated) of vehicle X (not illustrated) following vehicle  10 B does not connect to roadside device  400  (e.g., when non-AP radio device  102   x  is outside the communication area of roadside device  400 ), non-AP radio device  103   b  of vehicle  10 B may connect to AP radio device  101  of the following vehicle X. Then, non-AP radio devices  102   b  and  103   b  of vehicle  10 B may perform routing. 
     This allows non-AP radio device  102   x  of vehicle X following vehicle  10 B to communicate with roadside device  400  via communication apparatus  100   b  of vehicle  10 B even when non-AP radio device  102   x  of vehicle X is outside the radio communication range of roadside device  400 . That is, communication apparatus  100   b  of vehicle  10 B expands the communication area of roadside device  400 . 
       FIG.  7    illustrates still another exemplary inter-vehicle communication according to Embodiment 1. In  FIG.  7   , the same components as those in  FIG.  5    are denoted by the same reference numerals. In  FIG.  7   , vehicle  10 C travels on the oncoming lane of the lane on which vehicles  10 A and  10 B travels. 
     It is assumed that non-AP radio device  103   a  of vehicle  10 A perform in-line inter-vehicle communication with AP radio device  101   b  of following vehicle  10 B. It is assumed that non-AP radio device  102   b  of vehicle  10 B communicates with AP radio device  101   c  of vehicle  10 C traveling on the oncoming lane. It is assumed that non-AP radio device  102   c  of vehicle  10 C does not establish a radio link with AP radio device  101  of another vehicle  10 . It is assumed that AP radio device  101   d  and non-AP radio device  102   d  of vehicle  10 D do not establish a radio link with AP radio device  101  and non-AP radio device  103  of another vehicle  10 . 
     In a case where non-AP radio device  102  has established a radio link with AP radio device  101  of another vehicle  10  before vehicle  10  enters the intersection, non-AP radio device  102  may disconnect the radio link with AP radio device  101  of another vehicle  10 . 
     For example, non-AP radio device  102   b  of vehicle  10 B illustrated in  FIG.  7    has established a radio link with AP radio  101   c  of vehicle  10 C traveling on the oncoming lane. Non-AP radio device  102   b  of vehicle  10 B may disconnect the radio link with AP radio device  101   c  of vehicle  10 C before entering the intersection. 
     Non-AP radio device  102   b  of vehicle  10 B that has disconnected the radio link with AP radio device  101   c  of vehicle  10 C starts scanning beacon frames. Non-AP radio device  102   b  of vehicle  10 B finds AP radio device  101   c  of vehicle  10 C and AP radio device  101   d  of vehicle  10 D based on the scanning of beacon frames. 
     Non-AP radio device  102   b  of vehicle  10 B establishes a radio link with AP radio device  101  having the better radio quality of a beacon frame of the found AP radio devices  101   c  and  101   d . Note that the radio quality may be Received Signal Strength Indicator (RSSI). 
     In this case, vehicle  10 C is an oncoming vehicle for vehicle  10 B and travels in the opposite direction with respect to vehicle  10 B. Vehicle  10 D travels in a lateral direction with respect to the traveling direction of vehicle  10 B. Thus, the relative speed between vehicle  10 B and vehicle  10 C is faster than the relative speed between vehicle  10 B and vehicle  10 D, and the radio quality of AP radio device  101   d  of vehicle  10 D is better than that of AP radio device  101   c  of vehicle  10 C. Therefore, of the found AP radio devices  101   c  and  101   d , non-AP radio device  102   b  of vehicle  10 B establishes a radio link with AP radio device  101   d  of vehicle  10 D. 
     That is, non-AP radio device  102   b  of vehicle  10 B establishes a radio link with AP radio device  101   d  of vehicle  10 D which non-AP radio device  102   b  of vehicle  10 B meets at the intersection (meets for the first time at the intersection). In other words, non-AP radio device  102   b  of vehicle  10 B disconnects the radio link with AP radio device  101   c  of vehicle  10 C traveling the oncoming lane, and gives priority to establishment of a radio link with AP radio device  101   d  of vehicle  10 D entering the intersection from a road different from the road on which vehicle  10 B travels. 
     Note that non-AP radio device  102   c  of vehicle  10 C that has not established any radio link finds AP radio device  101   b  of vehicle  10 B and AP radio device  101   d  of vehicle  10 D. Further, non-AP radio device  102   d  of vehicle  10 D that has not established any radio link finds AP radio device  101   b  of vehicle  10 B and AP radio device  101   c  of vehicle  10 C. 
     Non-AP radio device  102   d  of vehicle  10 D may connect to AP radio device  101   c  of vehicle  10 C of the found AP radio device  101   b  of vehicle  10 B and AP radio device  101   c  of vehicle  10 C. This is because non-AP radio device  102   b  of vehicle  10 B and AP radio device  101   d  of vehicle  10 D has established a radio link with each other. Non-AP radio device  102   d  of vehicle  10 D may notify the BSSID of AP radio device  101   d  to AP radio device  101   c  of vehicle  10 C that has established the radio link with non-AP radio device  102   d.    
     When obtaining BS SID of AP radio device  101   d  of vehicle  10 D, non-AP radio device  102   c  of vehicle  10 C may not connect to AP radio device  101   d  of vehicle  10 D, and may connect to AP radio device  101   b  of vehicle  10 B. 
     Further, when non-AP radio device  102   c  of vehicle  10 C is communicable with communication apparatus  100   b  of vehicle  10 B by the routing of communication apparatus  100   d  of vehicle  10 D, non-AP radio device  102   c  of vehicle  10 C may not connect to AP radio device  101   b  of vehicle  10 B. Non-AP radio device  102   c  of vehicle  10 C may connect to an AP radio device (not illustrated) of a preceding vehicle traveling in front of vehicle  10 C. 
       FIG.  8    is a flow chart illustrating an exemplary operation of inter-vehicle communication according to Embodiment 1.  FIG.  8    illustrates an exemplary operation of communication apparatus  100   b  mounted on vehicle  10 B. Communication apparatus  100   b  periodically executes the processing of a flow chart illustrated in  FIG.  8   . 
     Control device  104   b  detects the (expected) entry of vehicle  10 B into the intersection based on the position detection of position detection device  200   b  (S 1001 ). For example, control device  104   b  detects that vehicle  10 B enters a predetermined area including the intersection (e.g., in a circle having a radius of  50   m  with the intersection being centered) based on the current position of vehicle  10 B detected by position detection device  200   b  and the map information. 
     Note that control device  104   b  may detect the entry of vehicle  10 B into the intersection using ON of the direction indicator as a trigger. 
     Control device  104   b  determines whether the connection destination of non-AP radio device  102   b  is roadside device  400  (S 1002 ). When determining that the connection destination of non-AP radio device  102   b  is roadside device  400  (“Yes” in S 1002 ), control device  104   b  proceeds the process to S 1009 . 
     When determining that the connection destination of non-AP radio device  102   b  is not roadside device  400  (“No” in S 1002 ), control device  104   b  controls non-AP radio device  102   b  and starts scanning beacon frames (S 1003 ). 
     Note that, in a case where non-AP radio device  102   b  connects to AP radio device  101  of another vehicle  10  before starting scanning beacon frames in S 1003 , control device  104   b  can disconnect the communication with AP radio device  101  of another vehicle  10 . In other words, control device  104   b  may set non-AP radio device  102   b  that is to be connected to AP radio device  101  of another vehicle  10  to be temporality free (a state in which non-AP radio device  102   b  does not connect to any AP radio device  101 ) on the front side of vehicle  10 B. Thus, non-AP radio device  102   b  of vehicle  10 B, for example, disconnects the communication with AP radio device  101   c  of vehicle  10 C traveling on the oncoming lane, and can connect to AP radio device  101   d  of vehicle  10 D (vehicle  10 D that vehicle B meets at the intersection) entering from another road different from the road on which vehicle  10 B travels. 
     Control device  104   b  determines whether AP radio device  101  of another vehicle  10  has been detected based on the starting of the scanning of beacon frames in S 1003  (S 1004 ). 
     When control device  104   b  does not detect AP radio device  101  of another vehicle  10  (“No” in S 1004 ), the process shifts to S 1003 . 
     When detecting AP radio device  101  of another vehicle  10  (“Yes” in S 1004 ), control device  104   b  selects AP radio device  101  of the connection destination (S 1005 ). For example, control device  104   b  selects AP radio device  101  with the best radio quality of the beacon frame. 
     Control device  104   b  performs association with AP radio device  101  selected in S 1005  and determines an IP address (S 1006 ). 
     Control device  104   b  transmits a BSSID of AP radio device  101   b  to AP radio device  101  on which association is performed in S 1006  via non-AP radio device  102   b  (S 1007 ). Note that control device  104   b  may determine an IP address based on DHCP when performing communication based on IPv4. Control device  104   b  may determine an IP address based on StateLess Address Auto Configuration (SLAAC) or DHCPv6 when performing communication based on IPv6. 
     Control device  104   b  configures routing between AP radio device  101   b  and non-AP radio device  102   b , and transmits the configured routing information via AP radio device  101   b  (S 1008 ). 
     Control device  104   b  notifies the IP address of the network in which the radio link has been established to the connected network via AP radio device  101   b  (S 1009 ). 
     Note that control device  104   b  may find AP radio device  101  of another vehicle  10  by using, for example, multicast Domain Name System (mDNS) or Bonjour in the processes of S 1008  and S 1009 . 
     Further, control device  104   b  may inform an IP address and service information using mDNS or Bonjour. The service information may include, for example, application information such as collision avoidance information, route information, a dynamic map, and an Internet connection gateway. In addition, the service information may include protocol information such as Message Queuing Telemetry Transport (MQTT), Web Socket, or a Robot OS (ROS) message. 
     Control device  104   b  receives IP address information of communication apparatus  100  of another vehicle  10  via AP radio device  101   b  and non-AP radio device  102   b  (S 1010 ). 
     Control device  104   b  determines a transmission destination IP address and performs data communication via AP radio device  101   b  and non-AP radio devices  102   b  and  103   b  (S 1011 ). Note that control device  104   b  may broadcast or multicast the data to the routed subnet. 
     As described above, communication apparatus  100  includes AP radio device  101  and antenna  111  that operate as an AP of an infrastructure mode and transmit and receive a radio wave toward and from the front side of vehicle  10 , and includes non-AP radio device  102  and antenna  112  that operate as a station of an infrastructure mode and transmit and receive a radio wave toward and from the front side of vehicle  10 . Thus, communication apparatus  100  can easily establish a radio link at the place such as an intersection where roads intersect with each other. 
     For example, while AP radio device  101  maintains in-line inter-vehicle communication with the communication apparatus of vehicle  10  traveling ahead, communication apparatus  100  can further establish communication with the communication apparatus of the vehicle entering the intersection by non-AP radio device  102 . That is, communication apparatus  100  can easily establish communication with the communication device of the vehicle entering the intersection by non-AP radio device  102  without performing the disconnection of the in-line inter-vehicle communication of AP radio device  101 . 
     In addition, control device  104  controls non-AP radio device  102  to select AP radio device  101  with good radio quality (AP radio device  101  of the vehicle having a small relative speed). A radio link with vehicle  10  having a small relative speed has a low risk to be early disconnected, and thus communication apparatus  100  can perform a large number of data communications. 
     In addition, because communication apparatus  100  includes two radio devices of AP radio device  101  and non-AP radio device  102  communicating on the front side of vehicle  10 , in-line inter-vehicle communication with low delay can be achieved. 
     In addition, even in a mobility environment in which the position and direction of vehicle  10  change from moment to moment, communication apparatus  100  can form a mesh network using millimeter-wave communication having directivity, and can communicate with a large number of vehicles. 
     Note that position detection device  200  may detect the entry of a vehicle into the intersection using ADAS. The ADAS may detect the entry of vehicle  10  into the intersection with at least one sensor such as a camera, Light Detection and Ranging, Laser Imaging Detection and Ranging (LiDAR), or radar, for example. 
     Further, control device  104  executes the process of S 1002  when detecting the entry of the vehicle into the intersection in S 1001  in  FIG.  8   , but the present disclosure is not limited thereto. Control device  104  may execute the process of S 1002  when a blind area exists in a predetermined angle (e.g., 120°) on the front side of vehicle  10  based on Advanced driver-assistance systems (ADAS). The blind area on the front side of the vehicle may be detected by a sensor such as a camera, LiDAR, or radar. 
     In addition, communication apparatus  100  may transmit a list of network addresses or routing information using a radio system different from a radio system that AP radio device  101  and non-AP radio devices  102  and  103  use. For example, communication apparatus  100  may transmit a list of network addresses or routing information by a radio system such as Dedicated Short Range Communications (DSRC), Long Term Evolution (LTE)—Vehicle-to-Everything (V2X), or Wi-Fi. 
     Furthermore, communication apparatus  100  illustrated in  FIG.  3    includes two control devices  104  and  105  and connection circuit  106 , but is not limited thereto. Communication apparatus  100  may include one control device. 
       FIG.  9    is another block diagram illustrating an exemplary configuration of communication apparatus  100 . In  FIG.  9   , the same components as those in  FIG.  3    are denoted by the same reference numerals. As illustrated in  FIG.  9   , communication apparatus  100  includes one control device  114 . 
     Control device  114  performs routing control between subnets (BSS) in which AP radio device  101 , non-AP radio device  102 , and non-AP radio device  103  participate. Further, control device  114  determines a network address of BSS serviced by AP radio device  101 . Control device  114  may be configured by a processor such as a CPU or a DSP, for example. 
     Embodiment 2 
     In Embodiment 1, control device  104  selects AP radio device  101  with the best radio quality when detecting AP radio device  101  of another vehicle  10 . In Embodiment 2, control device  104  selects AP radio device  101  having a large increase in the number of communicable subnets. In other words, control device  104  excludes AP radio device  101  that has been already reachable by routing. In the following description, portions different from those of Embodiment 1 will be described. 
       FIGS.  10 A and  10 B  are flow charts illustrating an exemplary operation of inter-vehicle communication according to Embodiment 2. Circled A and B illustrated in  FIG.  10 A  lead to circled A and B illustrated in  FIG.  10 B . In  FIGS.  10 A and  10 B , the same processes as those described in  FIG.  8    are denoted by the same reference numerals, and the description thereof is omitted. For example, communication apparatus  100   b  periodically executes the processing of the flow chart illustrated in  FIGS.  10 A and  10 B . 
     Control device  104   b  controls non-AP radio device  102   b  to receive a Directional Multi Gigabit (DMG) beacon frame transmitted from AP radio device  101   c  of vehicle  10 C (S 1101 - 1 ). 
     After receiving the DMG beacon frame, control device  104   b  controls non-AP radio device  102   b  to transmit a probe request to AP radio device  101   c  of vehicle  10 C (S 1102 - 1 ). 
     Control device  104   b  controls non-AP radio device  102   b  to receive a probe response including a neighbor report from AP radio device  101   c  of vehicle  10 C (S 1103 - 1 ). The neighbor report, which will be described later, is information on AP radio device  101  of another communication apparatus  100  connectable directly or by routing (e.g., multi-hop) (e.g., a list of BSSIDs of connectable AP radio devices  101 ). 
     Control device  104   b  controls non-AP radio device  102   b  to receive a DMG beacon frame transmitted from AP radio device  101   d  of vehicle  10 D (S 1101 - 2 ). 
     After receiving the DMG beacon frame, control device  104   b  controls non-AP radio device  102   b  to transmit a probe request to AP radio device  101   d  of vehicle  10 D (S 1102 - 2 ). 
     Control device  104   b  controls non-AP radio device  102   b  to receive a probe response including a neighbor report from AP radio device  101   d  of vehicle  10 D (S 1103 - 2 ). 
     Control device  104   b  selects AP radio device  101  of the connection destination after the scanning time of the beacon frame is expired (S 1105 ). 
     For example, control device  104   b  selects either AP radio device  101   c  of vehicle  10 C from which the DMG beacon frame has been received in S 1101 - 1  or AP radio device  101   d  of vehicle  10 D from which the DMG beacon frame has been received in S 1101 - 2 . 
     At this time, when control device  104   b  performs association with AP radio device  101   c , control device  104   b  can communicate with AP radio device  101   c  and AP radio device  101  whose BSSID is on the list received in S 1103 - 1 . Thus, from the list received in S 1103 - 1 , control device  104   b  can obtain the number of connectable subnets in the case of performing association with AP radio device  101   c . Similarly, from the list received in S 1103 - 2 , control device  104   b  can obtain the number of connectable subnets in the case of performing association with AP radio device  101   d.    
     Control device  104   b  obtains the number of BSSIDs currently communicable (the number of connectable subnets) in S 1112 , which will be described later. Control device  104   b  compares the number of BSSIDs obtained in S 1112  (e.g., obtained in the previous flow chart processing) and the number of communicable subnets obtained in S 1103 - 1  in the case of connecting to AP radio device  101   c . Control device  104   b  compares the number of BSSIDs obtained in S 1112  and the number of communicable subnets obtained in S 1103 - 2  in the case of connecting to AP radio device  101   d . Control device  104   b  selects AP radio device  101   c  or AP radio device  101   d  whichever having a large increase (the increased number is the largest) in the number of subnets to be newly connectable (the number of AP radio devices to be newly connectable). 
     Control device  104   b  controls AP radio device  101   b  to obtain a list of AP radio devices  101  connectable directly or by routing (a list of BSSIDs) from communication apparatuses  100   a ,  100   c , and  100   d  of vehicle  10 A,  10 C, and  10 D (S 1112 ). 
     When obtaining a list of BSSIDs in S 1112 , control device  104   b  transmits a probe response including a neighbor report (S 1113 ). The neighbor report includes a list of BSSIDs obtained in S 1112 . The neighbor report may be transmitted, for example, using a probe response. 
       FIG.  11    illustrates an exemplary probe response frame.  FIG.  11    illustrates a probe response frame (probe response signal) that AP radio device  101   b  of vehicle  10 B transmits. Descriptions of blank field elements illustrated in  FIG.  11    is omitted. As illustrated in  FIG.  11   , a probe response includes a frame body field. 
     The frame body field includes an element of a neighbor report. In the example of  FIG.  11   , the frame body field includes elements of the neighbor reports of AP radio devices  101   a ,  101   c , and  101   d . The neighbor reports of AP radio devices  101   a ,  101   c , and  101   d  includes BSSIDs of AP radio devices  101   a ,  101   c , and  101   d , respectively. 
     As described above, control device  104  may select an AP radio device to which non-AP radio device  102  connects based on the increase number of subnets. Thus, communication apparatus  100  can establish a radio link with an AP radio device of another vehicle entering the intersection. 
     Further, when AP radio device  101  transmits routing information in advance, non-AP radio device  102   b  can perform a routing setting with low delay after starting the connection process. 
     Note that, when the routing passes to the Internet (the routing passes to the roadside device, resulting in the routing passing to the Internet), control device  104  may select non-AP radio device  102  having a small hop count to the roadside device (or Intelligent Transport Systems (ITS) server), which is a gateway of the Internet. AP radio device  101  may notify the hop count using the neighbor report of the probe response. 
     In addition, for example, when entering the intersection, control device  104  may obtain the relative speed of another vehicle, and may select AP radio device  101  of the vehicle whose obtained relative speed is smaller than a predetermined threshold. The relative speed of the vehicle may be obtained using a beacon frame or a probe response frame. Further, information on absolute speed and an azimuth angle of the vehicle may be notified by another communication system, and the relative speed of the vehicle may be obtained (calculated) from the notified information. 
     Furthermore, control device  104  may connect to AP radio device  101  having a large relative speed, disconnect the connection after performing a predetermined amount of communication, and connect to another AP radio device. 
     Embodiment 3 
     In Embodiments 1 and 2, AP radio device  101  perform communication on the front side of the vehicle. In Embodiment 3, AP radio device also communicates on the rear side of the vehicle. 
       FIG.  12    illustrates a block diagram illustrating an exemplary configuration of communication apparatus  300  according to Embodiment 3. As illustrated in  FIG.  12   , communication apparatus  300  includes AP radio device  301 , non-AP radio devices  102 ,  303 , and  323 , control device  314 , and antennae  111 ,  311 ,  321 ,  112 ,  313 , and  333 . Position detection device  200  is connected to communication apparatus  300 . 
     Antennae  111 ,  311 , and  321  are connected to AP radio device  301 . Antennae  112 ,  313 , and  333  are connected to non-AP radio device  102 ,  303 ,  323 , respectively. 
     Control device  314  perform routing control of a subnet (BSS) in which AP radio device  301  and non-AP radio devices  102 ,  303 , and  323  participate. Further, control device  314  determines the network address of the BSS serviced by AP radio device  301 . Control device  314  may be configured by a processor such as a CPU or a DSP, for example. 
     Note that communication apparatus  300  may include a plurality of control devices. For example, the plurality of control devices may be provided one each for AP radio device  301  and non-AP radio devices  102 ,  303 , and  323 . The plurality of control devices may be connected by connection circuit  106  described in  FIG.  3   . 
     Further, non-AP radio devices  102 ,  303 , and  323  may be physically separated from each other. In addition, non-AP radio devices  102 ,  303 , and  323  may be physically one radio device, and one radio device may have functions of non-AP radio devices  102 ,  303 , and  323 . 
     Moreover, AP radio device  301  and non-AP radio devices  102 ,  303 , and  323  may be physically separated from each other. AP radio device  301  and non-AP radio devices  102 ,  303 , and  323  may be physically one radio device, and one radio device may have functions of AP radio device  101  and non-AP radio devices  102 ,  303 , and  323 . 
       FIG.  13    illustrates exemplary directivity of antennae  111 ,  112 ,  311 ,  313 ,  321 , and  333 . Antennae  111  and  112  that radiate radio waves on the front side of the vehicle may have, for example, directivity of a half-value angle of 5° as illustrated by sector  201 . Antennae  111  and  112  may change the direction of the directivity, for example, in a range of 120° in the front direction of the vehicle (60° on each of the left and right sides with respect to the front surface of the vehicle) as illustrated by sector  202 . 
     Antenna  311  connected to AP radio device  301  and antenna  313  connected to non-AP radio device  303  may have directivity of a half-value angle of 5°, as illustrated by sector  205 . Antennae  311  and  313  may change the direction of the directivity, for example, in a range of 120° in the left rear direction of the vehicle as illustrated by sector  206 . 
     Antenna  321  connected to AP radio device  301  and antenna  333  connected to non-AP radio device  323  may have directivity of a half-value angle of 5°, as illustrated by sector  207 . Antennae  321  and  333  may change the direction of the directivity, for example, in a range of 120° in the right rear direction of the vehicle as illustrated by sector  208 . 
     AP radio device  301  communicates with a vehicle (non-AP radio device) in the surrounding 360° of vehicle  10  by switching or simultaneously using antennae  111 ,  311 , and  321 . Further, communication apparatus  300  includes non-AP radio devices  102 ,  303 , and  323  to which antennae  112 ,  313 ,  333  are connected, respectively, and thus communication apparatus  300  communicates with a vehicle (AP radio device) in the surrounding 360° of vehicle  10 . Communication apparatus  300  communicates with at least one communication apparatus (AP radio device) in each of sectors  202 ,  206 , and  208  illustrated in  FIG.  13   . 
     Note that antennae  111 ,  112 ,  311 ,  313 ,  321 , and  333  may cover an angular range of 120° or more (e.g., 140°) and may overlap with each other in the covering range. 
     Further, for example, four antennae may be connected to AP radio device  301 , and each antenna may cover an angular range of 90° at a corresponding one of four corners of vehicle  10 . Communication apparatus  300  may include four non-AP radio devices, and each non-AP radio device may cover an angular range of 90° at a corresponding one of four corners of vehicle  10 . 
       FIG.  14    illustrates exemplary inter-vehicle communication according to Embodiment 3. Vehicles  10 A,  10 B,  10 C, and  10 D illustrated in  FIG.  14    are each equipped with communication apparatus  300  illustrated in  FIG.  13   . 
     In the following description, in order to distinguish each communication apparatus  300  mounted on each of vehicles  10 A,  10 B,  10 C, and  10 D, a communication apparatus mounted on vehicle  10 A may be referred to as communication apparatus  300   a , a communication apparatus mounted on vehicle  10 B may be referred to as communication apparatus  300   b , a communication apparatus mounted on vehicle  10 C may be referred to as communication apparatus  300   c , and a communication apparatus mounted on vehicle  10 D may be referred to as communication apparatus  300   d.    
     Further, in order to distinguish each unit included in each of communication apparatuses  300   a ,  300   b ,  300   c , and  300   d , the reference numerals of the units included in each of communication apparatuses  300   a ,  300   b ,  300   c , and  300   d  may be denoted with suffixes of a, b, c, and d, respectively. 
     Communication apparatuses  300   a ,  300   b ,  300   c , and  300   d  mounted on vehicles  10 A,  10 B,  10 C, and  10 D, respectively, perform communication based on an infrastructure mode. 
     Vehicles  10 A and  10 B travel in the same direction on the road, for example. Non-AP radio device  303   a  of vehicle  10 A connects to AP radio device  301   b  of following vehicle  10 B and performs in-line inter-vehicle communication with AP radio device  301   b.    
     Non-AP radio devices  303   a  and  323   a  of vehicle  10 A cover communication on the left rear side and the right rear side of vehicle  10 A. For example, non-AP radio device  323   a  of vehicle  10 A communicates with AP radio device  301   c  of vehicle  10 C traveling on the oncoming lane. 
     AP radio device  301   c  of vehicle  10 C covers communication on the front side, the right rear side, and the left rear side of vehicle  10 C. For example, AP radio device  301   c  of vehicle  10 C communicates with non-AP radio device  323   a  of vehicle  10 A traveling on the oncoming lane. Further, for example, AP radio device  301   c  of vehicle  10 C communicates with non-AP radio device  102   b  of vehicle  10 B traveling on the oncoming lane. 
     The operation of communication apparatus  300  is the same as the operation described in the flow chart in  FIG.  8   , and the description thereof is omitted. 
     As described above, AP radio device  301  and antennae  111 ,  311 , and  321  may transmit and receive a radio wave toward and from the front side, the right rear side, and the left rear side of vehicle  10 . Thus, communication apparatus  100  can communicate with communication apparatuses of vehicles  10  traveling in the front, the rear, the right side and the left side. 
     In the above-described embodiments, the term “portion” or “device” used for the name of a component may be replaced with another term such as “circuitry”, “assembly”, “device”, “unit”, or “module”. 
     The description has been given of embodiments with reference to the drawings, but the present disclosure is not limited to the examples. It is apparent that variations or modifications in the category described in the claims may be conceived of by a person skilled in the art. It is to be understood that such variations or modifications fall within the technical scope of the present disclosure. In addition, component elements in the embodiments may be optionally combined without departure from the spirit of the present disclosure. 
     The present disclosure can be realized by software, hardware, or software in cooperation with hardware. Each functional block used in the description of each embodiment described above can be partly or entirely realized by an LSI such as an integrated circuit, and each process described in the embodiment may be controlled partly or entirely by the same LSI or a combination of LSIs. The LSI may be individually formed as chips, or one chip may be formed so as to include a part or all of the functional blocks. The LSI may include a data input and output coupled thereto. The LSI here may be referred to as an IC, a system LSI, a super LSI, or an ultra LSI depending on a difference in the degree of integration. 
     The technique of implementing an integrated circuit is not limited to the LSI and may be realized by using a dedicated circuit, a general-purpose processor, or a special-purpose processor. In addition, a Field Programmable Gate Array (FPGA) that can be programmed after the manufacture of the LSI or a reconfigurable processor in which the connections and the settings of circuit cells disposed inside the LSI can be reconfigured may be used. The present disclosure can be realized as digital processing or analogue processing. 
     If future integrated circuit technology replaces LSIs as a result of the advancement of semiconductor technology or other derivative technology, the functional blocks could be integrated using the future integrated circuit technology. Biotechnology can also be applied. 
     The present disclosure can be realized by any kind of apparatus, device or system having a function of communication, which is referred to as a communication apparatus. The communication apparatus may comprise a transceiver and processing/control circuitry. The transceiver may comprise and/or function as a receiver and a transmitter. The transceiver, as the transmitter and receiver, may include a radio frequency (RF) module and one or more antennae. The RF module may include an amplifier, an RF modulator/demodulator, or the like. Some non-limiting examples of such a communication apparatus include a phone (e.g., cellular (cell) phone, smart phone), a tablet, a personal computer (PC) (e.g., laptop, desktop, netbook), a camera (e.g., digital still/video camera), a digital player (digital audio/video player), a wearable device (e.g., wearable camera, smart watch, tracking device), a game console, a digital book reader, a telehealth/telemedicine (remote health and medicine) device, and a vehicle providing communication functionality (e.g., automotive, airplane, ship), and various combinations thereof. 
     The communication apparatus is not limited to be portable or movable, and may also include any kind of apparatus, device or system being non-portable or stationary, such as a smart home device (e.g. an appliance, lighting, smart meter, control panel), a vending machine, and any other “things” in a network of an “Internet of Things (IoT)”. 
     The communication may include exchanging data through, for example, a cellular system, a wireless LAN system, a satellite system, etc., and various combinations thereof. 
     The communication apparatus may comprise a device such as a controller or a sensor which is coupled to a communication apparatus performing a function of communication described in the present disclosure. For example, the communication apparatus may comprise a controller or a sensor that generates control signals or data signals which are used by a communication apparatus performing a communication function of the communication apparatus. 
     The communication apparatus also may include an infrastructure facility, such as a base station, an access point, and any other apparatuses, devices or systems that communicate with or control apparatuses such as those in the above non-limiting examples. 
     Summary of Embodiments 
     A communication apparatus according to the present disclosure is a communication apparatus mounted on a vehicle, the communication apparatus including: a first communication circuit that operates as an access point of an infrastructure mode and transmits and receives a radio wave toward and from a front side of the vehicle; and a second communication circuit that operates as a station of an infrastructure mode and transmits and receives a radio wave toward and from the front side of the vehicle. 
     In the communication apparatus according to the present disclosure, in a case where a radio link with a communication apparatus of a first vehicle has established when the vehicle enters a predetermined area including an intersection, the second communication circuit disconnects the radio link with the communication apparatus of the first vehicle. 
     In the communication apparatus according to the present disclosure, the second communication circuit establishes a radio link with a communication apparatus of a second vehicle different from the communication apparatus of the first vehicle. 
     In the communication apparatus according to the present disclosure, the second communication circuit selects, based on a quality of a beacon signal, a communication apparatus of a second vehicle with which the second communication circuit establishes a radio link. 
     In the communication apparatus according to the present disclosure, the second communication circuit selects, based on a number of increased subnets, a communication apparatus of a second vehicle with which the second communication circuit establishes a radio link. 
     In the communication apparatus according to the present disclosure, the second communication circuit gives higher priority to establishment of a radio link with a roadside device than establishment of a radio link with a communication apparatus of another vehicle. 
     In the communication apparatus according to the present disclosure, the first communication circuit transmits a probe response signal including information on an access point of a communication apparatus of another vehicle. 
     In the communication apparatus according to the present disclosure, the first communication circuit transmits and receives a radio wave toward and from a rear side of the vehicle. 
     In the communication apparatus according to the present disclosure, the second communication circuit transmits and receives a radio wave toward and from a rear side of the vehicle. 
     A communication method according to the present disclosure is a communication method for a communication apparatus mounted on a vehicle, the communication method including: performing, by a first communication circuit, an operation as an access point of an infrastructure mode; transmitting and receiving, by the first communication circuit, a radio wave toward and from a front side of the vehicle; performing, by a second communication circuit, an operation as a station of an infrastructure mode; and transmitting and receiving, by the second communication circuit, a radio wave toward and from the front side of the vehicle. 
     The disclosure of Japanese Patent Application No. 2020-121424, filed on Jul. 15, 2020, including the specification, drawings and abstract, is incorporated herein by reference in its entirety. 
     INDUSTRIAL APPLICABILITY 
     The present disclosure is suitable, for example, for radio communication between vehicles or between roadside devices and vehicles. 
     REFERENCE SIGNS LIST 
     
         
           10 ,  10 A,  10 B,  10 C,  10 D Vehicle 
           101 ,  101   a ,  101   b ,  101   c ,  101   d  AP radio device 
           301 ,  301   a ,  301   b ,  301   c ,  301   d  AP radio device 
           102 ,  102   a ,  102   b ,  102   c ,  103   d  Non-AP radio device 
           103 ,  103   a ,  103   b ,  103   c ,  103   d  Non-AP radio device 
           303 ,  303   a ,  303   b ,  303   c ,  303   d  Non-AP radio device 
           323 ,  323   a ,  323   b ,  323   c ,  323   d  Non-AP radio device 
           104 ,  105 ,  114 ,  314  Control device 
           106  Connection circuit 
           111 ,  112 ,  113 ,  311 ,  321 ,  313 ,  333  Antenna 
           200  Position detection device