Patent Publication Number: US-9426758-B2

Title: Radio communication device, radio communication system, congestion control method, and record medium

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a Continuation of U.S. application Ser. No. 14/004,975, which is a National Stage of International Application No. PCT/JP2012/056402 filed Mar. 13, 2012, claiming priority based on Japanese Patent Application No. 2011-055032 filed Mar. 14, 2011, the contents of all of which are incorporated herein by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a radio communication device, a radio communication system, a congestion control method, and a record medium. 
     BACKGROUND ART 
     Inter vehicle communication systems that communicate with each vehicle are known. In an inter vehicle communication system, a radio communication device mounted on a vehicle exchanges information (current position that is information about running) with radio communication devices respectively mounted on other vehicles (communication partners). 
     In a radio communication system where radio communication devices directly communicate with each other, not through a base station, such as an inter vehicle communication system, as the radio communication device moves, its communication state changes over time. For example, a communication state, in which there are no communication partners in the communication area its own radio communication device, might change to another communication state in which there are a few communication partners in the communication area its own radio communication device, and then to another communication state in which there are many communication partners in the communication area its own radio communication device. 
     If there are a large number of communication partners, namely a large number of radio communication devices, in the communication area its own radio communication device, communication of radio communication devices might congest and thereby the throughput might become lower. 
     Patent Literature 1 describes a radio communication device that can handle this problem. The radio communication device described in Patent Literature 1 controls the transmission power based on the transmission loss of a signal received from the communication partner so as to adjust the size of the communication area. The radio communication device described in Patent Literature 1 adjusts the size of the communication area so as to limit the number of radio communication devices that become communication partners. As a result, while the radio communication device described in Patent Literature 1 maintains a radio link with neighbor radio communication devices, this device can alleviate congestion and the decrease of throughput. 
     RELATED ART LITERATURE 
     Patent Literature 
     Patent Literature 1: JP2008-244960A, Publication 
     SUMMARY OF THE INVENTION 
     Problem to be Solved by the Invention 
     The radio communication device described in Patent Literature 1 controls transmission power and also congestion without taking into consideration the running states of individual vehicles. Thus, the radio communication device described in Patent Literature 1 cannot control congestion based on the running states of the individual vehicles. 
       FIG. 1  is a schematic diagram showing that vehicle  200  having on-board communication device  100  that is a radio communication device is running in low traffic lane  300 , vehicles  201  to  204  respectively having on-board communication devices  101  to  104  are running in jammed traffic lane  400 . 
     In the state shown in  FIG. 1 , it is preferable that on-board communication device  100  in traffic lane  300  does not suppress the congestion (for example, not to decease the transmission power) if possible so as to ensure communication with other on-board communication devices (not shown) on traffic lane  300 . In contrast, since on-board communication devices  101  to  104  are closer to each other on traffic lane  400 , it is preferable that they suppress the congestion. 
       FIG. 2  is a schematic diagram showing that vehicle  600  having on-board communication device  500  has broken down and stops in traffic lane  700  and that vehicles  601  to  604  respectively having on-board communication devices  501  to  504  are running in traffic lane  700 . 
     In the state shown in  FIG. 2 , it is preferable that on-board communication device  500  does not to suppress the congestion if possible so as to prevent a collision from occurring. In contrast, since on-board communication devices  501  to  504  are closer to each other on traffic lane  700 , it is preferable that they suppress the congestion. 
     As shown in  FIG. 1  and  FIG. 2 , it would be desirable to control the congestion in consideration of the running states of individual vehicles, namely the moving states of individual radio communication devices. 
     An object of the present invention is to provide radio communication devices, radio communication systems, congestion control methods, and record mediums that can solve the foregoing problem. 
     Means that Solve the Problem 
     A radio communication device according to the present invention is a radio communication device that communicates with other radio communication devices that become communication partners, including: first detection means that detects moving states of said other radio communication devices; second detection means that detects a moving state of its own device; third detection means that detects a congestion level of communication between said other radio communication devices or a congestion level of communication between said other radio communication devices and said own device; transmission means that transmits a radio signal from said own device to said other communication devices; and control means that controls said transmission means to transmit the radio signal based on the moving states of said other radio communication devices, the moving state of said own device, and said congestion level. 
     A radio communication system according to the present invention is a radio communication system, including: a plurality of the foregoing radio communication devices, wherein said plurality of radio communication devices communicate with each other. 
     A congestion control method according to the present invention is a congestion control method for a radio communication device that communicates with other radio communication devices that become communication partners, including: detecting moving states of said other radio communication devices; detecting a moving state of its own device; detecting a congestion level of communication between said other radio communication devices or a congestion level of communication between said other radio communication devices and said own device; transmitting a radio signal from said own device to said other communication devices; and controlling transmission of the radio signal based on the moving states of said other radio communication devices, the moving state of said own device, and said congestion level. 
     A record medium according to the present invention is a non-transitory computer readable record medium on which a program that causes a computer to execute procedures is recorded, the procedures including: a first detection procedure that detects moving states of said other radio communication devices that become communication partners; a second detection procedure that detects a moving state of said computer; a third detection procedure that detects a congestion level of communication between said other radio communication devices or a congestion level of communication between said other radio communication devices and said computer; a transmission procedure that transmits a radio signal from said computer to said other communication devices; and a control procedure that controls transmission of said radio signal based on the moving states of said other radio communication devices, the moving state of said computer, and said congestion level. 
     Effect of the Invention 
     According to the present invention, the congestion can be controlled by taking into consideration the moving states of individual radio communication devices. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram showing an example of the running states of individual vehicles. 
         FIG. 2  is a schematic diagram showing another example of the running states of individual vehicles. 
         FIG. 3  is a block diagram showing an inter vehicle communication system according to a first exemplary embodiment of the present invention. 
         FIG. 4  is a block diagram showing on-board communication device  1   
         FIG. 5  is a schematic diagram showing neighbor vehicle information. 
         FIG. 6  is a block diagram showing on-board communication section  1 X. 
         FIG. 7  is a block diagram showing on-board communication section  1 Y. 
     
    
    
     MODES THAT CARRY OUT THE INVENTION 
     Next, with reference to the accompanying drawings, exemplary embodiments of the present invention will be described. 
     (First Exemplary Embodiment) 
       FIG. 3  is a block diagram showing an inter vehicle communication system that is a radio communication system according to a first exemplary embodiment of the present invention. 
     In  FIG. 3 , the inter vehicle communication system includes a plurality of on-board communication devices  1  (on-board communication devices  1   a  and  1   b ). Although  FIG. 3  shows only two on-board communication devices  1 , the number of on-board communication devices  1  is not limited to two, but may be greater than two. 
     On-board communication devices  1   a  and  1   b  may be generally referred to as radio communication devices. On-board communication devices  1   a  and  1   b  mutually and directly communicate with each other. 
     On-board communication device  1   a  is connected to on-board application system  2   a . On-board communication device  1   b  is connected to on-board application system  2   b.    
     On-board application systems  2   a  and  2   b  are, for example, known car navigation systems or known safe driving support systems. On-board application systems  2   a  and  2   b  are provided with a GPS (Global Positioning System) and various types of on-board sensors. 
       FIG. 4  is a block diagram showing an on-board communication device  1  that is used as each of on-board communication devices  1   a  and  1   b . In  FIG. 4 , on-board communication device  1  includes radio section  11 , process section  12 , and transmission power control section  13 . 
     Radio section  11  transmits and receives radio signals. Radio section  11  includes transmission section  111  and reception section  112 . 
     Transmission section  111  can be generally referred to as transmission means. 
     Transmission section  111  modulates transmission data that are transmitted to on-board communication devices respectively mounted on neighbor vehicles (hereinafter, these communication devices are simply referred to as “other on-board communication devices”). Transmission section  111  transmits modulated transmission data as a radio signal to other on-board communication devices. 
     Transmission data include identification information that serves to identify the own device (on-board communication device  1 ) (hereinafter, this identification information is referred to as “ID”), position information that represents the position of its own device, moving direction information that represents the moving direction of its own device, moving speed information that represents the moving speed of its own device, and transmission power setup value that represents the transmission power of its own device. 
     Reception section  112  can be generally referred to as first detection means. 
     Reception section  112  receives radio signals transmitted from other on-board communication devices. When reception section  112  receives a radio signal, reception section  112  measures the reception power of the radio signal, acquires the reception time, demodulates the radio signal, and generates reception data. 
     The reception data include IDs of the other on-board communication devices, position information of the other on-board communication devices, moving direction information of the other on-board communication devices, moving speed information of the other on-board communication devices, and transmission power setup values of the other on-board communication devices. Thus, reception section  112  generates the reception data so as to detect the moving states of the other on-board communication devices (moving directions or moving speeds). 
     Reception section  112  correlates the reception data, the measured values of the reception powers, and the reception time information that represents the reception times so as to generate neighbor vehicle information. 
       FIG. 5  is a schematic diagram showing neighbor vehicle information. 
     In  FIG. 5 , neighbor vehicle information  301  includes ID  301   a , position information  301   b , moving direction information  301   c , moving speed information  301   d , transmission power setup value  301   e , reception power measurement value  301   f , and reception time information  301   g.    
     Reception section  112  outputs neighbor vehicle information  301  to process section  12 . 
     Process section  12  receives neighbor vehicle information  301  from radio section  11 . In addition, process section  12  acquires the position of the vehicle having its own device, namely the position information that represents the position of its own device, from on-board application system  2  at intervals of a predetermined time period (hereinafter referred to as “predetermined time interval”). 
     Process section  12  includes detection sections  121  and  122 . 
     Detection section  121  can be generally referred to as second detection means. Detection section  121  detects the moving state of its own device. According to this exemplary embodiment, detection section  121  detects the moving direction and moving speed of its own device as the moving state of its own device. 
     Detection section  121  detects the moving direction of its own device based on the positions represented by the position information received from on-board application system  2 . For example, whenever detection section  121  acquires position information, detection section  121  stores it and detects the direction from the position represented by the former position information to the position represented by the latest position information as the moving direction of its own device. If the former position information is not present, detection section  121  does not detect the moving direction of its own device. 
     In addition, detection section  121  detects the moving speed of its own device based on the position represented by the position information received from on-board application system  2  and time intervals at which the position information is acquired (predetermined time intervals). For example, whenever detection section  121  acquires the position information, detection section  121  obtains the distance from the position represented by the former position information to the position represented by the latest position information, divides the distance by the predetermined time intervals, and thereby detects the moving speed of its own device. If the former position information is not present, detection section  121  does not detect the moving speed of its own device. 
     Detection section  121  outputs the moving direction and moving speed of its own device to transmission power control section  13  together with the ID of its own device stored in detection section  121 . 
     In addition, detection section  121  outputs communication data that contain the position information received from on-board application system  2 , the moving direction information that represents the moving direction of its own device, the moving speed information that represents the moving speed of its own device, and the ID of its own device to transmission section  111 . 
     When transmission power control section  13  sets up the transmission power, if transmission power control section  13  does not use the moving direction, detection section  121  may not detect the moving direction of its own device. In this case, the communication data and neighbor vehicle information  301  do not include the moving direction information. 
     When transmission power control section  13  sets up the transmission power, if transmission power control section  13  does not use the moving speed, detection section  121  may not detect the moving speed of its own device. In this case, the communication data and neighbor vehicle information  301  do not contain the moving speed information. 
     Detection section  122  can be generally referred to as third detection means. 
     Detection section  122  receives neighbor vehicle information  301  from reception section  112 . Detection section  122  outputs neighbor vehicle information  301  to transmission power control section  13  and on-board application system  2 . 
     In addition, detection section  122  detects the congestion level that is the congestion level of communication between the other on-board communication devices and the congestion level of communication between the other on-board communication devices and its own device. In this case, detection section  122  detects the congestion level of radio channels on which its own device and the other on-board communication devices communicate with each other or the number or density of other on-board communication devices. 
     For example, detection section  122  detects the congestion level that is the “congestion level of radio channels” that represents the use of a plurality of radio channels in the radio communication system (inter vehicle communication system) and that is detected using a known carrier sense function or the like or the “congestion level of on-board communication devices” that represents the number or density of on-board communication devices with which its own device can communicate and that is obtained from each of neighbor vehicles. 
     Detection section  122  outputs the congestion level to transmission power control section  13 . 
     Transmission power control section  13  can be generally referred to as control means. 
     Transmission power control section  13  stores neighbor vehicle information  301 . If transmission power control section  13  newly receives neighbor vehicle information  301  having the same ID (referred to latest neighbor vehicle information  301 ) as neighbor vehicle information  301  that has been stored in transmission power control section  13  (referred to as former neighbor vehicle information  301 ), transmission power control section  13  updates former neighbor vehicle information  301  with latest neighbor vehicle information  301 . Transmission power control section  13  may delete neighbor vehicle information  301  that has not been updated for a predetermined time period. 
     Transmission power control section  13  controls transmission section  111  to transmit a radio signal based on the moving states of the other on-board communication devices represented by neighbor vehicle information  301 , the moving state of its own device detected by detection section  121 , and the congestion level of communication detected by detection section  122 . 
     Transmission power control section  13  controls the transmission power of the radio signal based on the moving states of the other on-board communication devices, the moving state of its own device, and the congestion level of communication. 
     According to this exemplary embodiment, transmission power control section  13  determines whether or not the ratio of the number of on-board communication devices that move in the same direction or in nearly the same direction as its own device to the number of other on-board communication devices is equal to or greater than a threshold (hereinafter, this ratio is referred to as “first ratio”). The threshold is, for example, 80%. It should be noted that the threshold may be adequately changed to other than 80%, for example, 50% or greater. 
     The moving direction that is the same or nearly the same as the moving direction of its own device is a moving direction in which the deviation angle to the moving direction of its own device is n (where n is any number that is equal to or greater than 0 and that is less than 45). n is a number used to detect the on-board communication devices that move on that same land as its own device of other on-board communication devices. n may be adequately selected from a value that is equal to or greater than 0 and that is less than 45. 
     If the first ratio is equal to or greater than the threshold, when the congestion level exceeds a reference value, transmission power control section  13  decreases the transmission power setup value of the radio signal by equal to or greater than a first predetermined value. In contrast, if the first ratio is less than the threshold, when the congestion level exceeds the reference value, transmission power control section  13  decreases the transmission power setup value of the radio signal by a value that is equal to or greater than 0 and that is less than the first predetermined value. The threshold, the reference value, and the first predetermined value can be adequately changed and are stored in transmission power control section  13 . 
     Transmission power control section  13  may control the transmission power of the radio signal based on the moving speeds of the other on-board communication devices, the moving speed of its own device, and the congestion level of communication. 
     In this case, transmission power control section  13  determines whether or not the ratio of the number of other on-board communication devices that move at a moving speed that differs from its own device in a predetermined range to the number of other on-board communication devices is equal to or greater than the threshold (hereinafter, this ratio is referred to as “second ratio”). The predetermined range is for example from 0 km/h to 50 km/h. It should be noted that the predetermined range may be adequately changed to other than 0 km/h to 50 km/h. 
     If the second ratio is equal to or greater than the threshold, when the congestion level exceeds a reference value, transmission power control section  13  decreases the transmission power setup value of the radio signal by equal to or greater than a first predetermined value. In contrast, if the second ratio is less than the threshold, when the congestion level exceeds the reference value, transmission power control section  13  decreases the transmission power setup value of the radio signal by a value that is equal to or greater than 0 and that is less than the first predetermined value. In this case, likewise, the threshold, the reference value, and the first predetermined value can be adequately changed and are stored in transmission power control section  13 . 
     Transmission power control section  13  outputs the transmission power setup value to transmission section  111 . 
     Transmission section  111  adds the transmission power setup value to the communication data received from detection section  121 , obtains transmission data, modulates the transmission data, and transmits the modulated transmission data as a radio signal having the transmission power represented by the transmission power setup value to the other on-board communication devices. 
     Next, the operation of this exemplary embodiment will be described. 
     When reception section  112  receives radio signals from the other on-board communication devices, reception section  112  measures the reception powers of the radio signals, acquires the reception times, demodulates the radio signals, and generates reception data. 
     Reception section  112  correlates the reception data, the measured values of the reception powers, and the reception time information that represents the reception times so as to generate neighbor vehicle information  301 . Thereafter, reception section  112  outputs neighbor vehicle information  301  to process section  12 . 
     In process section  12 , detection section  121  detects the moving direction of its own device based on the positions represented by the position information received from on-board application system  2 . In addition, detection section  121  detects the moving speed of its own device based on the positions represented by the position information received from on-board application system  2  and time intervals at which the position information is acquired (predetermined time intervals). 
     Detection section  121  outputs the moving direction information and moving speed information of its own device to transmission power control section  13  together with the ID of its own device stored in detection section  121 . In addition, detection section  121  outputs communication data that include the position information received from on-board application system  2 , the moving direction information, the moving speed information of its own device, and the ID of its own device to transmission section  111 . 
     When detection section  122  receives neighbor vehicle information  301  from reception section  112 , detection section  122  outputs neighbor vehicle information  301  to transmission power control section  13  and on-board application system  2 . 
     In addition, detection section  122  detects the congestion level of communication between other on-board communication devices and its own device and outputs the congestion level to transmission power control section  13 . 
     Transmission power control section  13  controls the transmission power setup value of the radio signal based on the moving states (moving directions or moving speeds) of the other on-board communication devices represented by neighbor vehicle information  301 , the moving state of its own device detected by detection section  121 , and the congestion level of communication detected by detection section  122 . 
     If the first ratio is equal to or greater than the threshold, when the congestion level exceeds a reference value, transmission power control section  13  decreases the transmission power setup value of the radio signal by a value that is equal to or greater than a first predetermined value. In contrast, if the first ratio is less than the threshold, when the congestion level exceeds the reference value, transmission power control section  13  decreases the transmission power setup value of the radio signal by a value that is equal to or greater than 0 and that is less than the first predetermined value. 
     When on-board communication device  1  is turned on, transmission power control section  13  sets a default value to the transmission power setup value. 
     Transmission section  111  adds the transmission power setup value to the communication data received from detection section  121 , obtains transmission data, modulates the transmission data, and transmits the modulated transmission data as a radio signal having the transmission power represented by the transmission power setup value to the other on-board communication devices. 
     Next, an example of transmission power control will be described. 
     In the inter vehicle communication system, as shown in  FIG. 1 , a plurality of on-board communication devices such as on-board communication devices  100  to  104  might be closer to each other and they might transmit and receive radio signals to and from each other. In the following description, each of on-board communication devices  100  to  104  has the same structure as on-board communication device  1 . 
     As the number of neighbor vehicles increases, the congestion level of the radio transmission path increases. Each of on-board communication devices  100  to  104  detects the congestion level of radio channels, for example, using the carrier sense technique or detects the congestion level of the radio transmission path based on information that represents the density of neighbor on-board communication devices obtained from neighbor vehicle information  301 . 
     While on-board communication devices  101  to  104  run in the same direction, only on-board communication device  100  runs in the opposite direction. 
     In this case, even if the congestion level of the radio transmission path is high, the traffic of traffic lane  300 , where on-board communication device  100  is moving, is low. Thus, when on-board communication device  100  communicates with other on-board communication devices (not shown) that are moving in the same direction, it is preferable that on-board communication device  100  not decrease the transmission power, but increase the communication distance as much as possible. 
     Thus, according to this exemplary embodiment, even if on-board communication devices  101  to  104  decrease the transmission power as the congestion level of the radio communication path increases, on-board communication device  100  does not decrease the transmission power. Alternatively, on-board communication device  100  decreases the transmission power in such a manner that the amount of decrease of the transmission power of on-board communication device  100  is less than that of each of on-board communication devices  101  to  104 . 
     In addition, as shown in  FIG. 2 , a plurality of on-board communication devices such as on-board communication devices  500  to  504  might be closer to each other and they might transmit and receive radio signals to and from each other. In the following description, each of on-board communication devices  500  to  504  has the same structure as on-board communication device  1 . 
     While on-board communication devices  501  to  504  are moving nearly at the same moving speed, only on-board communication device  500  stops. 
     In this case, the speed difference between on-board communication device  500  and each of the other on-board communication devices is large and their moving states differ. Thus, even if the congestion level of the radio transmission path is high, it is preferable that on-board communication device  500  not decrease the transmission power, but increase the communication distance as much as possible so as to prevent a collision or the like from occurring. 
     Thus, according to this exemplary embodiment, even if on-board communication devices  501  to  504  decrease the transmission power as the congestion level of the radio communication path increases, on-board communication device  500  does not decrease the transmission power. Alternatively, on-board communication device  500  decreases the transmission power in such a manner that the amount of decrease of the transmission power of on-board communication device  500  is lower than that of each of on-board communication devices  501  to  504 . 
     Next, the effects of this exemplary embodiment will be described. 
     According to this exemplary embodiment, reception section  112  detects the moving states of the other on-board communication devices. Detection section  121  detects the moving state of its own device. Detection section  122  detects the congestion level of communication between other on-board communication devices and the congestion level of communication between other on-board communication devices and the own device. Transmission section  111  transmits a radio signal from its own device to other on-board communication devices. Transmission power control section  13  controls transmission section  111  to transmit the radio signal based on the moving states of other on-board communication devices, the moving state of its own device, and the congestion level. 
     Thus, the congestion level of communication can be controlled by taking into consideration the moving states of the individual on-board communication devices. 
     According to this exemplary embodiment, transmission power control section  13  determines whether or not the first ratio (the ratio of the number of other on-board communication devices that move in the same direction or in nearly the same direction as the own device to the number of other on-board communication devices) is equal to or greater than the threshold. If the first ratio is equal to or greater than the threshold, when the congestion level exceeds a reference value, transmission power control section  13  decreases the transmission power setup value of the radio signal by the value that is equal to or greater than the first predetermined value. In contrast, if the first ratio is less than the threshold, when the congestion level exceeds the reference value, transmission power control section  13  decreases the transmission power setup value of the radio signal by a value that is equal to or greater than zero and that is less than the first predetermined value. 
     Thus, even if a state in which the radio transmission path congestion is detected, when a small number of vehicles are smoothly running in a low traffic lane, each of on-board communication devices mounted on the vehicles does not decrease the transmission power so as to ensure a communication distance at which they can smoothly run. Alternatively, the amount of decrease of the transmission power of each of these on-board communication devices that move on the low traffic lane can be lower than that of the on-board communication devices that move in the jammed traffic lane. 
     If the second ratio (the ratio of the number of other on-board communication devices that move at a moving speed that differs from the own device in a predetermined range to the number of other on-board communication devices) is equal to or greater than the threshold, when the congestion level exceeds a reference value, transmission power control section  13  decreases the transmission power setup value of the radio signal by the value that is equal to or greater than the first predetermined value. In contrast, if the second ratio is less than the threshold, when the congestion level exceeds the reference value, transmission power control section  13  decreases the transmission power setup value of the radio signal by a value that is equal to or greater than 0 and that is less than the first predetermined value. As a result, the following effects can be realized. 
     An on-board communication device mounted on a vehicle that is running in a different running state from other vehicles or stops increases the priority of information of its own device over information received from other on-board communication devices based on information of the moving speeds. Thus, even if the congestion level of the radio communication path increases, the on-board communication device does not decrease the transmission power. Alternatively, the amount of decrease of the transmission power of the on-board communication device can be lower than that of the other on-board communication devices. 
     As described above, the transmission power can be adequately controlled corresponding to the running states of vehicles and neighboring traffic states based on information of the moving speeds and moving directions of on-board communication devices so as to decrease the congestion level of the radio transmission path. 
     On-board communication device  1  may be realized by a computer. In this case, the computer reads a program from a non-transitory computer readable record medium such as a CD-ROM (Compact Disk Read Only Memory) and then executes the program such that the computer operates as radio section  11 , process section  12 , and transmission power control section  13 . The record medium is not limited to a CD-ROM, but can be changed to any appropriate non-transitory recording medium. 
     (Second Exemplary Embodiment) 
       FIG. 6  is a block diagram showing on-board communication device  1 X used for an inter vehicle communication system that is a radio communication system according to a second exemplary embodiment of the present invention. 
     On-board communication device  1 X shown in  FIG. 6  is different from on-board communication device  1  shown in  FIG. 4  in that the former has transmission period control section  14  instead of transmission power control section  13 . Next, differences between on-board communication device  1 X and on-board communication device  1  will be mainly described. 
     In  FIG. 6 , transmission period control section  14  can be generally referred to as control means. 
     Transmission period control section  14  receives neighbor vehicle information  301  from detection section  122  and stores neighbor vehicle information  301 . According to this exemplary embodiment, neighbor vehicle information  301  may not include transmission power setup value  301   e . If transmission period control section  14  newly receives neighbor vehicle information  301  having the same ID (referred to as latest neighbor vehicle information  301 ) as neighbor vehicle information  301  that has been stored in transmission period control section  14  (referred to as former neighbor vehicle information  301 ), transmission period control section  14  updates former neighbor vehicle information  301  with latest neighbor vehicle information  301 . Transmission period control section  14  may delete neighbor vehicle information  301  that has not been updated for a predetermined time period. 
     Transmission period control section  14  controls the transmission period of the radio signal transmitted by transmission section  111  based on the moving states of the other on-board communication devices represented by neighbor vehicle information  301 , the moving state of the own device detected by detection section  121 , and the congestion level of communication detected by detection section  122 . 
     According to this exemplary embodiment, like transmission power control section  13 , transmission period control section  14  determines whether or not a first ratio is equal to or greater than a threshold. 
     If the first ratio is equal to or greater than the threshold, when the congestion level exceeds a reference value, transmission period control section  14  increases the transmission period setup value of a radio signal by a value that is equal to or greater than a second predetermined value. In contrast, if the first ratio is less than the threshold, when the congestion level exceeds the reference value, transmission period control section  14  increases the transmission period setup value by a value that is equal to or greater than 0 and that is less than the second predetermined value. The threshold, the reference value, and the second predetermined value can be adequately changed and are stored in transmission period control section  14 . 
     Transmission period control section  14  may control the transmission period of the radio signal based on the moving speeds of the other on-board communication devices, the moving speed of the own device, and the congestion level of communication. 
     In this case, transmission period control section  14  determines whether or not a second ratio is equal to or greater than a threshold. 
     If the second ratio is equal to or greater than the threshold, when the congestion level exceeds the reference value, transmission period control section  14  increases the transmission power setup value of the radio signal by a value that is equal to or greater than the second predetermined value. In contrast, if the second ratio is less than the threshold, when the congestion level exceeds the reference value, transmission period control section  14  increases the transmission period setup value by a value that is equal to or greater than 0 and that is less than the second predetermined value. In this case, the threshold, the reference value, and the second predetermined value can be adequately changed and are stored in transmission period control section  14 . 
     Transmission period control section  14  outputs the transmission period setup value to transmission section  111 . 
     When on-board communication device  1 X is turned on, transmission period control section  14  sets a default value to the transmission period setup value. 
     Transmission section  111  modulates communication data received from detection section  121 , obtains the modulated transmission data, and transmits the modulated transmission data as a radio signal having the transmission period represented by the transmission period setup value to the other on-board communication devices. 
     According to this exemplary embodiment, transmission period control section  14  controls the transmission period of the radio signal based on the moving states (moving directions or moving speeds) of the other on-board communication devices, the moving state of its own device, and the congestion level of communication. Thus, the second exemplary embodiment can achieve the same congestion control as the first embodiment. 
     On-board communication device  1 X may be realized by a computer. In this case, the computer reads a program from a non-transitory computer readable record medium and then executes the program such that the computer operates as radio section  11 , process section  12 , and transmission period control section  14 . 
     (Third Exemplary Embodiment) 
       FIG. 7  is a block diagram showing on-board communication device  1 Y used for an inter vehicle communication system that is a radio communication system according to a third exemplary embodiment of the present invention. 
     On-board communication device  1 Y shown in  FIG. 7  is different from on-board communication device  1  shown in  FIG. 4  in that the former has transmission speed control section  15  instead of transmission power control section  13 . Next, differences between on-board communication device  1 Y and on-board communication device  1  will be mainly described. 
     In  FIG. 7 , transmission speed control section  15  can be generally referred to as control means. 
     Transmission speed control section  15  receives neighbor vehicle information  301  from detection section  122  and stores neighbor vehicle information  301 . According to this exemplary embodiment, neighbor vehicle information  301  may not include transmission power setup value  301   e . If transmission speed control section  15  newly receives neighbor vehicle information  301  having the same ID (referred to as latest neighbor vehicle information  301 ) as neighbor vehicle information  301  that has been stored in transmission speed control section  15  (referred to as former neighbor vehicle information  301 ), transmission speed control section  15  will update former neighbor vehicle information  301  with latest neighbor vehicle information  301 . Transmission speed control section  15  may delete neighbor vehicle information  301  that has not been updated for a predetermined time period. 
     Transmission speed control section  15  controls the transmission speed of the radio signal transmitted by transmission section  111  based on the moving states of the other on-board communication devices represented by neighbor vehicle information  301 , the moving state of the own device detected by detection section  121 , and the congestion level of communication detected by detection section  122 . 
     According to this exemplary embodiment, like transmission power control section  13 , transmission speed control section  15  determines whether or not a first ratio is equal to or greater than a threshold. 
     If the first ratio is equal to or greater than the threshold, when the congestion level exceeds a reference value, transmission speed control section  15  increases the transmission speed setup value of a radio signal by a value that is equal to or greater than a third predetermined value. In contrast, if the first ratio is less than the threshold, when the congestion level exceeds the reference value, transmission speed control section  15  increases the transmission speed setup value by a value that is equal to or greater than 0 and that is less than the third predetermined value. The threshold, the reference value, and the third predetermined value can be adequately changed and are stored in transmission speed control section  15 . 
     Transmission speed control section  15  may control the transmission speed of the radio signal based on the moving speeds of the other on-board communication devices, the moving speed of its own device, and the congestion level of communication. 
     In this case, transmission speed control section  15  determines whether or not a second ratio is equal to or greater than the threshold. 
     If the second ratio is equal to or greater than the threshold, when the congestion level exceeds the reference value, transmission speed control section  15  increases the transmission speed setup value of the radio signal by a value that is equal to or greater than the third predetermined value. In contrast, if the second ratio is less than the threshold, when the congestion level exceeds the reference value, transmission speed control section  15  increases the transmission speed setup value by a value that is equal to or greater than 0 and that is less than the third predetermined value. In this case, the threshold, the reference value, and the third predetermined value can be adequately changed and are stored in transmission speed control section  15 . 
     Transmission speed control section  15  outputs the transmission speed setup value to transmission section  111 . 
     When on-board communication device  1 Y is turned on, transmission speed control section  15  sets the default value to the transmission speed setup value. 
     Transmission section  111  modulates communication data received from detection section  121 , obtains the modulated transmission data, and transmits the modulated transmission data as a radio signal having the transmission speed represented by the transmission speed setup value to the other on-board communication devices. 
     According to this exemplary embodiment, transmission speed control section  15  controls the transmission speed of the radio signal based on the moving states (moving directions or moving speeds) of the other on-board communication devices, the moving state of its own device, and the congestion level of communication. Thus, the third exemplary embodiment can achieve the same congestion control as the first exemplary embodiment. 
     On-board communication device  1 Y may be realized by a computer. In this case, the computer reads a program from a computer readable record medium and then executes the program such that the computer operates as radio section  11 , process section  12 , and transmission speed control section  15 . 
     According to each of the foregoing exemplary embodiments, reception section  112  demodulates a radio signal so as to detect the moving directions of the other on-board communication devices. Alternatively, detection section  122  may detect the moving directions of the other on-board communication devices based on position information  301   b  of neighbor vehicle information  301  corresponding to the other on-board communication devices. 
     For example, when detection section  122  receives neighbor vehicle information  301 , detection section  122  reads position information  301   b  corresponding to ID 301   a  included therein from neighbor vehicle information  301  stored in transmission power control section  13 , transmission period control section  14 , or transmission speed control section  15 . 
     Thereafter, detection section  122  detects the direction from the position represented by position information  301   b  of neighbor vehicle information  301  that has been read to the position represented by position information  301   b  of neighbor vehicle information  301  that has been received as the moving direction of the on-board communication device identified by ID  301   a  of neighbor vehicle information  301  that has been received. 
     Thereafter, detection section  122  outputs moving direction information that represents the moving direction of the on-board communication device to transmission power control section  13 , transmission period control section  14 , or transmission speed control section  15 . In this case, detection section  122  also operates as first detection means. 
     In this case, transmission data may not include moving direction information. 
     According to each of the foregoing exemplary embodiments, reception section  112  demodulates a radio signal so as to detect the moving speeds of other on-board communication devices. Alternatively, detection section  122  may detect the moving speeds of the other on-board communication devices based on position information  301   b  of neighbor vehicle information  301  corresponding to the other on-board communication devices and received time interval of position information  301   b.    
     For example, when detection section  122  receives neighbor vehicle information  301 , detection section  122  reads position information  301   b  corresponding to ID 301   a  included in neighbor vehicle information  301  from neighbor vehicle information  301  stored in transmission power control section  13 , transmission period control section  14 , or transmission speed control section  15  (hereinafter, this position information is referred to as “former information”). 
     Thereafter, detection section  122  calculates the distance from the position represented by position information  301   b  of neighbor vehicle information  301  that has been read to the position represented by position information  301   b  of neighbor vehicle information  301  that has been received. 
     Thereafter, detection section  122  calculates an elapsed time period (received time interval) from the date and time represented by reception time information  301   g  of the former information to the date and time represented by reception time information  301   g  of neighbor vehicle information  301  that has been received. 
     Thereafter, detection section  122  divides the calculated distance by the calculated elapsed time period so as to detect the moving speed of the on-board communication device identified by ID  301   a.    
     Thereafter, detection section  122  outputs moving speed information that represents the moving speed of the on-board communication device to transmission power control section  13 , transmission period control section  14 , or transmission speed control section  15 . In this case, detection section  122  also operates as first detection means. 
     In this case, the transmission data may not include the moving speed information. 
     According to each of the foregoing exemplary embodiments, detection section  122  outputs neighbor vehicle information  301  to on-board application system  2 . Alternatively, detection section  122  may output only information required by on-board application system  2  of neighbor vehicle information  301  to on-board application system  2 . In this case, information that is not necessary to control the congestion and that is not required by on-board application system  2  may be deleted from the transmission data. 
     With reference to the exemplary embodiments, the present invention has been described. However, it should be understood by those skilled in the art that the structure and details of the present invention may be changed in various manners without departing from the scope of the present invention. 
     The present application claims a priority based on Japanese Patent Application JP 2011-055032 filed on Mar. 14, 2011, the entire contents of which are incorporated herein by reference in its entirety. 
     DESCRIPTION OF REFERENCE NUMERALS 
       1 ,  1 X,  1 Y,  1   a ,  1   b  On-board communication devices 
       11  Radio section 
       111  Transmission section 
       112  Reception section 
       12  Process section 
       121 ,  122  Detection sections 
       13  Transmission power control section 
       14  Transmission period control section 
       15  Transmission speed control section 
       2 ,  2   a ,  2   b  On-board application systems