Abstract:
An acquisition unit determines the location of an object. A setting unit sets a wait time if the determined location satisfies predetermined conditions. The range of wait time that the setting unit can set is narrower than the range of wait time that can be set by other kinds of terminal devices capable of broadcasting packet signals by means of carrier sensing. A carrier sense unit performs a carrier sense over the set wait time. A modulator-demodulator unit and RF unit broadcast a packet signal together with the results of the carrier sense.

Description:
RELATED APPLICATIONS 
     This application is the U.S. National Phase under 35 U.S.C. §371 of International Application No. PCT/JP2011/070704, filed on Sept. 12, 2011, which in turn claims the benefit of Japanese Application No. 2011-063863, filed on Mar. 23, 2011, the disclosures of which Applications are incorporated by reference herein. 
     TECHNICAL FIELD 
     The present invention relates to a communication technology, more particularly, to a terminal device that outputs a signal which contains predetermined information. 
     BACKGROUND ART 
     To prevent a collision accident at an intersection, between-road-and-vehicle communication is under study. In the between-road-and-vehicle communication, information related to an intersection situation is communicated between a road-side device and a vehicle device. In the between-road-and-vehicle communication, installation of the road-side device is necessary, which requires much working and cost. In contrast to this, in between-vehicles communication, that is, in a form of communicating information between vehicle devices, the installation of the road-side device becomes unnecessary. In this case, for example, the current location information is detected by means of a GPS (Global Positioning System) and the like, and the location information is exchanged between the vehicle devices, whereby it is determined on which roads one vehicle and the other vehicle are situated to enter an intersection (e.g., see a patent document 1). 
     CITATION LIST 
     Patent Literature 
     PLT1: JP-A-2005-202913 
     SUMMARY OF INVENTION 
     Technical Problem 
     In a wireless LAN (Local Area Network) in conformity with standards such as the IEEE802.11 and the like, an access control function called CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) is used. Because of this, in the wireless LAN, the same wireless channel is shared with a plurality of terminal devices. In the CSMA/CA, it is confirmed by carrier sense that another packet signal is not transmitted, thereafter, a packet signal is transmitted. In a case where a wireless LAN is applied to the between-vehicles communication of an ITS (Intelligent Transport Systems) and the like, it is necessary to transmit information to a terminal device that is mounted in each of many unspecified vehicles, accordingly, it is desirable to transmit a signal in a broadcast manner. As a result of this, the terminal device receives the signal transmitted in the broadcast manner, detects an approach of another vehicle, notifies the driver of the approach, thereby urging the driver to take caution to prevent a collision accident between the vehicles. 
     It is desired not only to prevent a collision accident between vehicles but also to prevent a collision accident between a vehicle and a pedestrian and the like. To deal with this, the terminal device is also carried by other pedestrians. To prevent a pedestrian from being struck from behind by a vehicle, the terminal device carried by the pedestrian notifies the terminal device of the vehicle of the location where the pedestrian is situated. On the other hand, the terminal device carried by the pedestrian is driven by a battery, accordingly, it is needed to reduce a processing amount compared with the terminal device of the vehicle. For example, an approach of another vehicle is not notified to the pedestrian. Even in a case where the terminal device carried by the pedestrian transmits a packet signal in the broadcast manner, it is desirable that influence, which is given to a packet signal transmitted in the broadcast manner from the vehicle terminal device, is small. Besides, in light of the purpose that the pedestrian notifies their location, it is desirable that the terminal device carried by the pedestrian is able to output a packet signal more preferentially than the vehicle terminal device. However, if a packet signal is preferentially output constantly, the influence given to the vehicle terminal device is likely to become large. 
     The present invention has been made in light of this situation, and it is an object of the present invention to provide a technology that identifies timing for preferentially outputting a packet signal while reducing influence given to a packet signal output from another terminal device. 
     Solution to Problem 
     To solve the above issue, a terminal device according to the present invention includes: a positioning portion that positions a location of an object; a setting portion that sets a waiting time period in a case where the location positioned by the positioning portion meets a predetermined condition; a carrier sense portion that executes carrier sense for the waiting time period set by the setting portion; and an output portion that outputs a packet signal based on a result of the carrier sense by the carrier sense portion. A range of the waiting time period which the setting portion is able to set is narrower than a range of a waiting time period that another kind of terminal device, which is able to output the packet signal based on the carrier sense, is able to set. 
     In the meantime, an arbitrary combination of the above constituent components and exchanges of expressions of the present invention among a method, a device, a system, a recording medium, a computer program and the like are also effective as forms of the present invention. 
     Advantageous Effects of Invention 
     According to the present invention, it is possible to identify the timing for preferentially outputting a packet signal while reducing the influence given to a packet signal output from the other terminal device. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a view showing a structure of a communication system according to an embodiment of the present invention. 
         FIG. 2  is a view showing a structure of a base station device in  FIG. 1 . 
         FIG. 3A  is a view showing a format of a frame defined in the communication system in FIG  1 ,  FIG. 3B  is a view showing a structure of a frame generated by a 1st base station device  10   a ,  FIG. 3C  is a view showing a structure of a frame generated by a 2nd base station device  10   b , and  FIG. 3D  is a view showing a structure of a frame generated by a 3rd base station device  10   c.    
         FIG. 4A  is a view showing a structure of a sub-frame in  FIGS. 3A-3D   
         FIG. 4B  is a view showing disposition of packet signals in a between-road-and-vehicle transmission period. 
         FIG. 5A  is a view showing a format of a MAC frame contained in a packet signal defined in the communication system in  FIG. 1 , and  FIG. 5B  is a view showing a structure of a message header of the MAC frame contained in the packet signal defined in the communication system in  FIG. 1 . 
         FIG. 6  is a view showing a structure of a vehicle terminal device mounted in a vehicle in  FIG. 1 . 
         FIG. 7  is a view showing a structure of a mobile terminal device carried by a pedestrian in  FIG. 1 . 
         FIG. 8  is a view showing operation of the mobile terminal device in  FIG. 7 . 
         FIG. 9  is a flow chart showing a transmission procedure in the mobile terminal device in  FIG. 7 . 
         FIG. 10  is a view showing a structure of a mobile terminal device according to a modification of the present invention. 
         FIG. 11  is a flow chart showing a transmission procedure in the mobile terminal device in  FIG. 10 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Before specifically describing the present invention, an overview is described. An embodiment of the present invention relates to a communication system that executes between-vehicles communication between terminal devices (hereinafter, called a “vehicle terminal device”) mounted in vehicles and also executes between-road-and-vehicle communication from a base station device disposed at an intersection and the like to a vehicle terminal device. As the between-vehicles communication, the vehicle terminal device transmits a packet signal in a broadcast manner that contains information (hereinafter, called “data”) of a speed, location and the like of the vehicle. Besides, another vehicle terminal device receives the packet signal and recognizes an approach and the like of a vehicle based on the data. The approach of a vehicle is notified to drivers, whereby the drivers are urged to take caution. To reduce interference of the between-vehicles communication and the between-road-and-vehicle communication, the base station device repeatedly defines a frame that contains a plurality of sub-frames. The base station device selects any one of the plurality of sub-frames for the between-road-and-vehicle communication, and transmits a packet signal, which contains control information and the like, in the broadcast manner during a period of a header portion of the selected sub-frame. The control information contains information related to a period (hereinafter, called a “road-vehicle transmission period”) during which the base station device transmits the packet signal in the broadcast manner. 
     The vehicle terminal device identifies a road-vehicle transmission period based on the control information and transmits a packet signal during a period other than the road-vehicle transmission period. In this way, the between-road-and-vehicle communication and the between-vehicles communication are multiplexed in time division, accordingly, a likelihood of a packet signal collision between both is reduced. In the meantime, the between-vehicles communication is performed during a period (hereinafter, called a “vehicle-vehicle transmission period”) for executing the between-vehicles communication other than the road-vehicle transmission period by means of a CSMA protocol. This terminal device is carried by a pedestrian as well (hereinafter, a terminal device carried by a pedestrian is called a “mobile terminal device.”). The mobile terminal device is driven by a battery, and low electric power consumption is needed. Because of this, the mobile terminal device transmits only a packet signal containing the data in the broadcast manner, but does not notify the pedestrian of an approach of a vehicle. 
     Even in the case where the mobile terminal device transmits a packet signal in the broadcast manner, it is needed to reduce influence given to the packet signal that is transmitted in the broadcast manner from the vehicle terminal device. Besides, the purpose of the mobile terminal device transmitting the packet signal in the broadcast manner is to notify drivers of a location of the pedestrian. Because of this, it is desired that the mobile terminal device is able to perform the transmission more preferentially than the vehicle terminal device when necessary. To deal with this, the communication system according to the present embodiment executes the following process. In the meantime, hereinafter, even the mobile terminal device refers to the between-vehicles communication and the between-road-and-vehicle communication. Besides, there is a case where the “terminal device” is used without distinguishing the vehicle terminal device and the mobile terminal device from each other, and there is also a case where the vehicle terminal device and the mobile terminal device are collectively called the “terminal device.” 
     Like the vehicle terminal device, the mobile terminal device also executes the CSMA protocol during the vehicle-vehicle transmission period. Here, the mobile terminal device is required to have electric power consumption lower than the vehicle terminal device, accordingly, an information amount transmitted in the broadcast manner from the mobile terminal device is made to be lesser than an information amount transmitted in the broadcast manner from the vehicle terminal device. As a result of this, a packet signal length of the former is set shorter than a packet signal length of the latter. In the CSMA protocol, the length of a contention window is variable, and carrier sense is executed during the period. Despite the short packet signal length, if the maximum value of the contention window is the same, a waiting time period for the packet signal length becomes long. To deal with this, the maximum value of the contention window in the mobile terminal device is defined to become shorter than the maximum value of the contention window in the vehicle terminal device. In the meantime, if such transmission is executed all the time, a transmission opportunity for the vehicle terminal device decreases. Because of this, a distance between the pedestrian and the vehicle is measured, and if the distance becomes smaller than a threshold value, the above transmission is executed. 
       FIG. 1  shows a structure of a communication system  100  according to the embodiment of the present invention. This shows one intersection viewed from top. The communication system  100  includes: a base station device  10 ;a 1st vehicle  12   a , a 2nd vehicle  12   b , a 3rd vehicle  12   c , a 4th vehicle  12   d , a 5th vehicle  12   e , a 6th vehicle  12   f , a 7th vehicle  12   g , and an 8th vehicle  12   h  that are collectively called a vehicle  12 ; a 1st pedestrian  16   a , and a 2nd pedestrian  16   b  who are collectively called a pedestrian  16 . In the meantime, each vehicle  12  is provided with a not-shown vehicle terminal device, while each pedestrian  16  carries a not-shown mobile terminal device. Besides, an area  212  is formed around the base station terminal  10 , and an area  214  is formed outside the area  212 . 
     As shown in the figure, a road running in a horizontal direction of the figure, that is, a left-right direction and a road running in a vertical direction of the figure, that is, a top-bottom direction cross each other at a central portion. Here, the upper side of the figure corresponds to a “North” direction, the left side corresponds to a “West” direction, the lower side corresponds to a “South” direction, and the right side corresponds to a “East” direction. Besides, the crossing portion of the two roads is an “intersection.” The 1st vehicle  12   a  and the 2nd vehicle  12   b  are traveling from left to right, while the 3rd vehicle  12   c  and the 4th vehicle  12   d  are traveling from right to left. Besides, the 5th vehicle  12   e  and the 6th vehicle  12   f  are traveling from top to bottom, while the 7th vehicle  12   g  and the 8th vehicle  12   h  are traveling from bottom to top. 
     The base station device  10  controls the communication between the terminal devices. Based on a signal received from a GPS satellite (not shown) and a frame formed in the other base station device  10 , the base station device  10  repeatedly generates a frame that contains a plurality of sub-frames. Here, definition is performed such that a road-vehicle transmission period is able to be set into the header portion of each sub-frame. From the plurality of sub-frames, the base station device  10  selects a sub-frame in which a road-vehicle transmission period is not set by another base station device  10 . The base station device  10  sets a road-vehicle transmission period into the header portion of the selected sub-frame. The base station device  10  outputs a packet signal during the set road-vehicle transmission period. 
     The vehicle  12  is driven by an engine and incorporates a vehicle terminal device. Based on the control information contained in a received packet signal, the vehicle terminal device generates a frame. As a result of this, the frame generated by each of the plurality of vehicle terminal devices synchronizes with the frame generated by the base station device  10 . Besides, the vehicle terminal device executes the CSMA/CA during the vehicle-vehicle transmission period to output the packet signal. The vehicle terminal device sets, for example, information related to a location into the packet signal. Besides, the vehicle terminal device sets the control information as well into the packet signal. In other words, the control information transmitted from the base station device  10  is transferred by the vehicle terminal device. 
     On the other hand, a vehicle terminal device that is not able to receive the packet signal from the base station device  10 , that is, a vehicle terminal device situated outside the area  214  executes the CSMA/CA irrespective of a frame structure to output a packet signal. Further, the vehicle terminal device receives a packet signal from another vehicle terminal device to notify the driver of an approach of the vehicle in which the other vehicle terminal device is mounted. 
     The pedestrian  16  carries a mobile terminal device. The mobile terminal device executes a process similar to the vehicle terminal device. However, to facilitate the process, the mobile terminal device does not notify approaches of a vehicle and the like. Here, the mobile terminal device acquires its own location, and upon receiving the packet signal from the vehicle terminal device, acquires the location of the vehicle as well. If the distance between its own location and the location of the vehicle is smaller than the threshold value, the mobile terminal device sets a contention window such that an average waiting time period when executing the CSMA/CA becomes shorter than an average waiting time period in the vehicle terminal device. On the other hand, if the distance is equal to the threshold value or more, the mobile terminal device sets a contention window like the vehicle terminal device. In the meantime, transmission electric power for the mobile terminal device is set to become lower than transmission electric power for the other device. 
       FIG. 2  shows a structure of the base station device  10 . The base station device  10  includes: an antenna  20 ; a RF portion  22 ; a modulation/demodulation portion  24 ; a process portion  26 ; a network communication portion  28 ; and a control portion  30 . The process portion  26  includes: a frame definition portion  32 ; a selection portion  34 ; and a generation portion  36 . 
     As a reception process, the RF portion  22  receives packet signals from a not-shown terminal device and another base station device  10  by means of the antenna  20 . The RF portion  22  applies frequency conversion to the received packet signal having a radio frequency to generate a base band packet signal. Further, the RF portion  22  outputs the base band packet signal to the modulation/demodulation portion  24 . Generally, the base band packet signal is formed of an in-phase component and an orthogonal component, accordingly, two signal lines should be shown; however, here, for the sake of making the figure clear, only one signal line is shown. The RF portion  22  further includes: a LNA (Low Noise Amplifier); a mixer; an AGC; and an A/D conversion portion. 
     As a transmission process, the RF portion  22  applies the frequency conversion to the base band packet signal input from the modulation/demodulation portion  24  to generate a radio frequency packet signal. Further, the RF portion  22  transmits the radio frequency packet signal from the antenna  20  during the road-vehicle transmission period. Besides, the RF portion  22  further also includes: a PA (Power Amplifier); a mixer; and an D/A conversion portion. 
     As a reception process, the modulation/demodulation  24  applies demodulation to the base band packet signal from the RF portion  22 . Further, the modulation/demodulation  24  outputs the demodulation result to the process portion  26 . Besides, as a transmission process, the modulation/demodulation  24  applies modulation to the data from the process portion  26 . Further, the modulation/demodulation  24  outputs the modulation result as a base band packet signal to the RF portion  22 . Here, the communication system  100  is compatible with the OFDM (Orthogonal Frequency Division Multiplexing) modulation method, accordingly, the modulation/demodulation  24  executes also the FFT (Fast Fourier Transform) as the reception process and executes also the IFFT (Inverse Fast Fourier Transform) as the transmission process. 
     The frame definition portion  32  receives a signal from the not-shown GPS satellite and acquires time point information based on the received signal. In the meantime, a conventional technology may be used for the acquisition of the time point information, accordingly, here, description is skipped. Based on the time point information, the frame definition portion  32  generates a plurality of frames. For example, based on timing shown by the time point information, the frame definition portion  32  divides a period of “1 sec.” into 10 segments to generate 10 frames of “100 msec.” By repeating this process, the frame is defined repeatedly. 
     In the meantime, the frame definition portion  32  may detect the control information from a demodulation result to generate a frame based on the detected control information. This process corresponds to generating a frame that synchronizes with the timing of the frame formed by the other base station device  10 .  FIGS. 3A-3D  show frame formats defined by the communication system  100 .  FIG. 3A  shows a frame structure. The frame is formed of N sub-frames shown by a 1st sub-frame to an Nth sub-frame. For example, in a case where the frame length is 100 msec. and N is  8 , sub-frames each having a length of 12.5 msec. are defined.  FIGS. 3B-3D  are described later. 
     In  FIG. 2 , the selection portion  34  selects, from the plurality of sub-frames contained in the frame, a sub-frame for setting the road-vehicle transmission period. Specifically, the selection portion  34  receives the frame defined by the frame definition portion  32 . The selection portion  34  receives demodulation results from the not-shown other base station device  10  or terminal device via the RF portion  22  and the modulation/demodulation  24 . The selection portion  34  extracts, from the received demodulation results, a demodulation result from the other base station device  10 . The selection portion  34  identifies a sub-frame that receives the demodulation result to identify a sub-frame that does not receive the demodulation result. This corresponds to identifying a sub-frame in which the road-vehicle transmission period is not set by the other base station device  10 , that is, an unused sub-frame. In a case where there are a plurality of unused sub-frames, the selection portion  34  selects one sub-frame at random. In a case where there are no unused sub-frames, that is, in a case where all of the plurality of sub-frames are used, the selection portion  34  acquires a reception electric power value that is the magnitude of electric power of a packet signal received by the RF portion  22  and demodulated by the modulation/demodulation portion  24  and preferentially selects a sub-frame that has a small reception electric power value. 
       FIG. 3B  shows a structure of a frame generated by a 1st base station device  10   a . The 1st base station device  10   a  sets a road-vehicle transmission period into a header portion of the 1st sub-frame. Besides, the 1st base station device  10   a  sets a vehicle-vehicle transmission period into the 1st sub-frame after the road-vehicle transmission period. The vehicle-vehicle transmission period is a period during which the vehicle terminal device is able to output the packet signal. In other words, definition is performed such that the 1st base station device  10   a  is able to output the packet signal during the road-vehicle transmission period that is a header period of the 1st sub-frame and the vehicle terminal device is able to output the packet signal during the vehicle-vehicle transmission period rather than the road-vehicle transmission period. Further, the 1st base station device  10   a  sets vehicle-vehicle transmission periods only into the 2nd sub-frame to the N-th sub-frame. 
       FIG. 3C  shows a structure of a frame generated by a 2nd base station device  10   b . The 2nd base station device  10   b  sets a road-vehicle transmission period into a header portion of the 2nd sub-frame. Besides, the 2nd base station device  10   b  sets vehicle-vehicle transmission periods into the 3rd sub-frame to the N-th sub-frame after the road-vehicle transmission periods in the 1st sub-frame and the 2nd sub-frame.  FIG. 3D  shows a structure of a frame generated by a 3rd base station device  10   c . The 3rd base station device  10   c  sets a road-vehicle transmission period into a header portion of the 3rd sub-frame. Besides, the 3rd base station device  10   c  sets vehicle-vehicle transmission periods into the 4th sub-frame to the N-th sub-frame after the road-vehicle transmission periods in the 1st sub-frame, the 2nd sub-frame and the 3rd sub-frame. As described above, the plurality of base station devices  10  select the sub-frames different from one another and set the road-vehicle transmission period into the header portion of the selected sub-frame. Back to 
       FIG. 2 . The selection portion  34  outputs a number of the selected sub-frame to the generation portion  36 . 
     The generation portion  36  sets a road-vehicle transmission period into the sub-frame that has the sub-frame number received from the selection portion  34  and generates a RSU packet signal to be output during the road-vehicle transmission period. In the meantime, in the following description, the RSU packet signal and the packet signal are used without being distinguished from each other. 
       FIGS. 4A and 4B  show a structure of a sub-frame.  FIG. 4A  shows a sub-frame in which a road-vehicle transmission period is set. As shown in the figure, one sub-frame is formed in an order from a road-vehicle transmission period to a vehicle-vehicle transmission period. 
       FIG. 4B  shows disposition of packet signals in the road-vehicle transmission period. As shown in the figure, a plurality of RSU packet signals are ranged in the road-vehicle transmission period. Here, the adjacent packet signals are spaced by a SIFS (Short Interframe Space). 
     Here, a structure of a RSU packet signal is described.  FIGS. 5A and 5B  show a format of a MAC frame contained in the packet signal that is defined by the communication system  100 .  FIG. 5A  shows the format of the MAC frame. The MAC frame successively disposes, from the header, a “MAC header,” a “LLC header,” a “message header,” “data payload,” and a “FCS.” Information contained in the “data payload” is described later. FIG. 5B  is a view showing a structure of a message header generated by the generation portion  36 . The message header contains a base portion. 
     The base portion includes: a “protocol version,” a “transmission node type,” a “reuse number,” a “TSF timer,” a “RSU transmission period length.” The protocol version shows a version of a corresponding protocol. The transmission node type shows a sender of the packet signal that contains the MAC frame. For example, “0” shows the terminal device, while “1” shows the base station device  10 . In the meantime, in a case where the vehicle terminal device and the mobile terminal device are distinguished from each other, the transmission node type is shown by two bits. In a case where the selection portion  34  extracts, from the input demodulation results, a demodulation result from the other base station device  10 , the selection portion  34  uses a value of the transmission node type. The reuse number shows an index of validity in a case where the message header is transferred by the terminal device, and the TSF timer shows a transmission time point. The RSU transmission period length is information related to the road-vehicle transmission period which shows a length of a road-vehicle transmission period. Back to  FIG. 2 . 
     The network communication portion  28  is connected to a not-shown network  202 . The network communication portion  28  receives jam information from the network  202 . The generation portion  36  acquires the jam information from the network communication portion  28  and stores the jam information into the data payload to generate the above RSU packet signal. The control portion  30  controls a process of the entire base station device  10 . 
     It is possible to achieve this structure as hardware by means of a CPU of an arbitrary computer; a memory; and another LSI, and as software by means of a program and the like loaded in the memory; here, function blocks achieved by a combination of them are shown. Accordingly, it is understandable by those skilled in the art that these function blocks are achievable in various forms by the hardware only, the software only, or a combination of them. 
       FIG. 6  shows a structure of a vehicle terminal device  14  mounted in the vehicle  12 . The vehicle terminal device  14  includes: an antenna  40 ; a RF portion  42 ; a modulation/demodulation portion  44 ; a process portion  46 ; and a control portion  48 . The process portion  46  includes: a timing identification portion  50 ; a transfer decision portion  56 ; an acquisition portion  58 ; a notification portion  60 ; and a generation portion  62 , and the timing identification portion  50  includes: an extraction portion  52 ; and a carrier sense portion  54 . The antenna  40 , the RF portion  42  and the modulation/demodulation portion  44  execute a process similar to the antenna  20 , the RF portion  22  and the modulation/demodulation portion  24  in  FIG. 2 . Because of this, here, difference is mainly described. 
     The modulation/demodulation portion  44  and the process portion  46  receive the packet signal from the other terminal device (not shown) and the other base station device  10  (not shown). In the meantime, as described above, the modulation/demodulation portion  44  and the process portion  46  receive the packet signal from the base station device  10  during the road-vehicle transmission period. As described above, the modulation/demodulation portion  44  and the process portion  46  receive the packet signal from the other vehicle terminal device  14  during the vehicle-vehicle transmission period. Further, although details are decribed later, the modulation/demodulation portion  44  and the process portion  46  receive the packet signal from the not-shown mobile terminal device irrespective of the road-vehicle transmission period and the vehicle-vehicle transmission period. 
     In a case where a demodulation result from the modulation/demodulation portion  44  is the packet signal from the not-shown base station device  10 , the extraction portion  52  identifies timing of a sub-frame in which the road-vehicle transmission period is disposed. At this time, the extraction portion  52  infers that the base station device  10  is situated in the area  212  of  FIG. 1 . The extraction portion  52  generates a frame based on the timing of the sub-frame and the content of the message header of the packet signal, specifically, the content of the RSU transmission period length. In the meantime, the generation of the frame may be performed like in the above frame definition portion  32 , accordingly, here, description is skipped. As a result of this, the extraction portion  52  generates a frame that synchronizes with the frame formed by the base station device  10 . 
     On the other hand, in a case where the RSU packet signal is not received, the extraction portion  52  infers that the base station device  10  is situated in the area  214  of  FIG. 1 . In the case where the extraction portion  52  infers that the base station device  10  is situated in the area  214 , the extraction portion  52  selects the vehicle-vehicle transmission period. When the extraction portion  52  infers that the base station device  10  is situated outside the area  214 , the extraction portion  52  selects timing irrelevant to the frame structure. In the case where the extraction portion  52  selects the vehicle-vehicle transmission period, the extraction portion  52  outputs the timing of the frame and sub-frame and the information related to the vehicle-vehicle transmission period to the carrier sense portion  54 . When the extraction portion  52  selects the timing irrelevant to the frame structure, the extraction portion  52  instructs the carrier sense portion  54  to execute carrier sense. 
     The carrier sense portion  54  receives the timing of the frame and sub-frame and the information related to the vehicle-vehicle transmission period from the extraction portion  52 . The carrier sense portion  54  executes carrier sense during the vehicle-vehicle transmission period to measure interference electric power. Besides, the carrier sense portion  54  decides transmission timing during the vehicle-vehicle transmission period based on the interference electric power. Specifically, the carrier sense portion  54  stores beforehand a predetermined threshold value, and compares the interference electric power and the threshold value with each other. The carrier sense portion  54  decides the transmission timing. In a case where the carrier sense portion  54  is instructed to execute the carrier sense by the extraction portion  52 , the carrier sense portion  54  executes the CSMA without considering the frame structure to decide the transmission timing. The carrier sense portion  54  notifies the generation portion  62  of the decided transmission timing. 
     The acquisition portion  58  includes: a GPS receiver, a gyroscope, a vehicle speed sensor and the like that are not shown, and based on data supplied from them, acquires a location, traveling direction, traveling speed and the like (hereinafter, collectively called “location information”) of the vehicle terminal device  14 . In the meantime, the location is shown by latitude and longitude. A conventional technology may be used to acquire the location information, accordingly, description is skipped. The acquisition portion  58  outputs the location information to the generation portion  62 . 
     The transfer decision portion  56  controls transfer of a message header. The transfer decision portion  56  extracts message headers from the packet signals. In a case where the packet signal is directly transmitted from the base station device  10 , the reuse number is set at “0,” however, in a case where the packet signal is transmitted from the other vehicle terminal device  14 , the reuse number is set at a value of “1 or more.” The transfer decision portion  56  selects a message header to be transferred from the extracted message headers. Here, for example, the message header whose reuse number is the smallest is selected. Besides, the transfer decision portion  56  may generate a new message header by synthesizing the contents contained in the plurality of message headers. The transfer decision portion  56  outputs the message header of the selected target to the generation portion  62 . At this time, the transfer decision portion  56  augments the reuse number by “1.” 
     The generation portion  62  receives the location information from the acquisition portion  58  and receives the message header from the transfer decision portion  56 . The generation portion  62  uses the MAC frame shown in  FIGS. 5A and 5B  to store the location information into the data payload. The generation portion  62  generates a packet signal which contains the MAC frame, and transmits the generated packet signal in the broadcast manner at the transmission timing decided by the carrier sense portion  54  via the modulation/demodulation portion  44 , the RF portion  42  and the antenna  40 . This corresponds to the between-vehicles communication. In the meantime, the transmission timing is contained in the vehicle-vehicle transmission period. 
     The notification portion  60  acquires the packet signal from the not-shown base station device  10  and the packet signal from the not-shown other vehicle terminal device  14  via the extraction portion  52 . As a process for the acquired packet signal, the notification portion  60  notifies the driver of approaches and the like of another not-shown vehicle  12  and pedestrian  16  via a monitor and a speaker in accordance with the data content contained in the packet signal. Further, the notification portion  60  notifies the driver of the jam information and the like via the monitor and the speaker. 
       FIG. 7  shows a structure of the mobile terminal device  18  carried by the pedestrian  16 . The mobile terminal device  18  includes: an antenna  70 ; a RF portion  72 ; a modulation/demodulation portion  74 ; a process portion  76 ; and a control portion  78 . Besides, the process portion  76  include: an acquisition portion  80 ; a generation portion  82 ; and a timing identification portion  84 , and the timing identification portion  84  includes: an extraction portion  86 ; a setting portion  88 ; and a carrier sense portion  90 . The acquisition portion  80  acquires location information like the acquisition portion  58  in  FIG. 6 . This is a location of the present mobile terminal device  18  and a location of the pedestrian  16 . The acquisition portion  80  outputs the location information to the setting portion  88  and the generation portion  82 . 
     The modulation/demodulation portion  74  and the process portion  76  receive the packet signals from the other terminal device (not shown) and the other base station device  10  (not shown) like the modulation/demodulation portion  44  and the process portion  46  in  FIG. 6 . Especially, the modulation/demodulation portion  74  and the process portion  76  receive, during the road-vehicle transmission period, the packet signal from the base station device  10 , that is, the packet signal containing the information related to the frame structure. Besides, the modulation/demodulation portion  74  and the process portion  76  receive, during the vehicle-vehicle transmission period, the packet signal from the vehicle terminal device  14 , that is, the packet signal containing information related to the location information of the vehicle terminal device  14 . 
     In a case where a demodulation result from the modulation/demodulation portion  74  is the packet signal from the not-shown base station device  10 , like the extraction portion  52 , the extraction portion  86  identifies timing of a sub-frame in which the road-vehicle transmission period is disposed. Besides, the extraction portion  86  identifies the vehicle-vehicle transmission period and outputs the timing of the frame and sub-frame and the information related to the vehicle-vehicle transmission period to the setting portion  88 . On the other hand, when the extraction portion  86  selects timing irrelevant to the frame structure, the extraction portion  86  notifies the setting portion  88  that a frame is not defined. Further, the extraction portion  86  extracts the location information from the packet signal that is received from the vehicle terminal device  14 . The extraction portion  86  outputs the location information to the setting portion  88 . 
     When the setting portion  88  receives the timing of the frame and sub-frame and the information related to the vehicle-vehicle transmission period from the extraction portion  86 , the setting portion  88  sets a waiting time period for carrier sense during the vehicle-vehicle transmission period. Here, as the setting of the waiting time period, preferential transmission and usual transmission are defined. To select either of them, the setting portion  88  calculates the distance between the location acquired by the acquisition portion  80  and the location contained in the packet signal that is received by the reception portion from the extraction portion  86 . In a case where the distance is smaller than a threshold value, the setting portion  88  selects the preferential transmission, while in a case where the distance is equal to the threshold value or more, the setting portion  88  selects the usual transmission. This corresponds to selecting the preferential transmission in a case where the location positioned by the acquisition portion  80  meets a predetermined condition. 
     Here, an overview of the CSMA operation by means of the carrier sense is described.  FIG. 8  shows operation of the mobile terminal device  18 . The horizontal axis shows the time. A busy state shows a state in which a signal from another device is received. After the busy state ends, waiting is performed for a Distributed Coordination Function Inter Frame Space, or DIFS (DCF IFS). Further, after the DIFS ends, the waiting is also performed for a contention window. In a case where a signal is not received during this time, a packet signal is transmitted. Here, the contention window is composed of a plurality of slots. The size of a slot is  13  μsec. Besides, the number of slots is defined by random numbers 0 to N in the case where the carrier sense in the vehicle terminal device  14  and the carrier sense in the usual transmission are executed. As described above, in the usual transmission, the contention window is set like in the vehicle terminal device  14 . 
     The length of the packet signal output from the mobile terminal device  18  is shorter than the length of the packet signal output from the vehicle terminal device  14 . Because of this, if the period of the contention window is the same in both, the former has a waiting time period for the packet signal length longer than the latter. To deal with this, a random number range, which the carrier sense portion  90  is able to set for the carrier sense, is set to be narrower than a random number range which the vehicle terminal device  14  is able to set for the carrier sense. For example, a random number range able to be set for the carrier sense in the preferential transmission is defined by 0 to N/2. This corresponds to that a waiting time period range, which the vehicle terminal device  14  is able to be set, is shorter than a waiting time period which the mobile terminal device  18  is able to set. Besides, the maximum value of the waiting time period, which the setting portion  88  is able to set, is smaller than the maximum value of the waiting time period which the vehicle terminal device  14  is able to set for the carrier sense. Here, by decreasing the maximum value of the contention window in the preferential transmission only, influence on the vehicle terminal device  14  is reduced. Back to  FIG. 7 . 
     In the meantime, in the case where the setting portion  88  notifies that a frame is not defined, likewise, the setting portion  88  sets likewise a contention window irrespective of the frame structure. The carrier sense portion  90  executes the carrier sense for the waiting time period set by the setting portion  88 . The modulation/demodulation portion  74  and the RF portion  72  output a packet signal based on the result of the carrier sense by the carrier sense portion  90 . 
     Operation of the communication system  100  having the above structure is described.  FIG. 9  is a flow chart showing a transmission procedure in the mobile terminal device  18 . The modulation/demodulation portion  74  and the process portion  76  receive a packet signal (S 10 ). The extraction portion  86  extracts location information of the vehicle  12  (S 12 ). The acquisition portion  80  acquires a location of the pedestrian  16  (S 14 ). The setting portion  88  calculates the distance between the vehicle  12  and the pedestrian  16  (S 16 ). If the distance is smaller than the threshold (Y in S 18 ), the setting portion  88  selects the preferential transmission (S 20 ). On the other hand, if the distance is not smaller than the threshold (N in S 18 ), the setting portion  88  selects the usual transmission (S 22 ). 
     Next, a modification of the present invention is described. The modification of the present invention, like the embodiments, relates to a mobile terminal device carried by a pedestrian. In the embodiments, based on the distance between the vehicle and the pedestrian, the execution of the preferential transmission is decided. On the other hand, in the modification, based on an area where the pedestrian is situated, the execution of the preferential transmission is decided. For example, in a case where the pedestrian is walking on a vehicle road or a road side zone, the execution of the preferential transmission is decided. The communication system  100 , base station device  10 , and vehicle terminal device  14  according to the modification are of the type similar to  FIG. 1 ,  FIG. 2  and  FIG. 6 . Here, difference is mainly described. 
       FIG. 10  shows a structure of the mobile terminal device  18  according to the modification of the present invention. A storage portion  92  is added to the mobile terminal device  18  shown in  FIG. 7 . The storage portion  92  stores location information of a predetermined area. As described above, the predetermined area is a vehicle road or a road side zone where a risk of a collision with the vehicle  12  is high. Such location information also is shown by latitude and longitude. In a case where the location positioned by the acquisition portion  80  is covered by the area stored in the storage portion  92 , the setting portion  88  selects the preferential transmission. On the other hand, in a case where the location positioned by the acquisition portion  80  is not covered by the area stored in the storage portion  92 , the setting portion  88  selects the usual transmission. 
     Operation of the communication system  100  having the above structure is described.  FIG. 11  is a flow chart showing a transmission procedure in the mobile terminal device  18 . The acquisition portion  80  acquires a location of the pedestrian  16  (S 40 ). In a case where the acquired location is in the predetermined area (Y in S 42 ), the setting portion  88  selects the preferential transmission (S 44 ). On the other hand, in a case where the acquired location is not in the predetermined area (N in S 42 ), the setting portion  88  selects the usual transmission (S 46 ). 
     According to the embodiments of the present invention, the carrier sense is executed during the vehicle-vehicle transmission period identified by the frame information that is output from the base terminal device, accordingly, even in the case where the packet signal is output from mobile terminal device, it is possible to reduce the influence given to the packet signal that is output from the vehicle terminal device. Besides, when a predetermined condition is met, the waiting time period range is set to be narrower than the waiting time period range of the vehicle terminal device, accordingly, it is possible to preferentially transmit the packet signal during a shorter waiting time period. Besides, it becomes easy to transmit the packet signal during the shorter waiting time period, accordingly, it is possible to quickly output the location. Besides, the preferential transmission is executed only when the predetermined condition is met, accordingly, it is possible to alleviate a reduction in the transmission opportunity of the vehicle terminal device. Besides, the preferential transmission is executed only when the predetermined condition is met, accordingly, it is possible to identify the timing for preferentially outputting the packet signal while reducing the influence given to the packet signal output from the vehicle terminal device. 
     Besides, transmission electric power for the packet signal output from the mobile terminal device is set to be smaller than transmission electric power for the packet signal output from the base station device and the vehicle terminal device, accordingly, it is possible to reduce the influence given to the latter. Besides, the transmission electric power is set to be small, accordingly, it is possible to reduce electric power consumption. Besides, the electric power consumption is reduced, accordingly, it is possible to prolong the drive time period. Besides, when a vehicle approaches, the preferential transmission is executed, accordingly, it is possible to perform a notification about the location when the risk of a collision is high. Besides, when entering an area where the risk is high, the preferential transmission is executed, accordingly, it is possible to perform a notification about the location when the risk of a collision is high. 
     Hereinbefore, the description is performed based on the embodiments. These embodiments are mere examples, and it is understandable to those skilled in the art that various modifications are possible in a combination of all the constituent components of them and various treatment processes; and such modifications also are in the scope of the present invention. 
     In the embodiments of the present invention, in the preferential transmission, the maximum value of the waiting time period, which the mobile terminal device  18  is able to set, is set to be smaller than the maximum value of the waiting time period which the vehicle terminal device  14  is able to set for the carrier sense. However, this is not limiting, and the waiting time period range which the setting portion  88  is able to set may be deviated from the waiting time period range which the vehicle terminal device  14  is able to set for the carrier sense. In an example, the contention window by the setting portion  88  is defined from “0” to “15,” while the contention window by the vehicle terminal device  14  is defined from “16” to “63.” According to the present modification, the waiting time period is deviated, accordingly, it is possible to reduce the collision likelihood of the packet signal. 
     In the embodiments of the present invention, in the preferential transmission, the maximum value of the waiting time period, which the mobile terminal device  18  is able to set, is set to be smaller than the maximum value of the waiting time period which the vehicle terminal device  14  is able to set for the carrier sense. However, this is not limiting, and the waiting time period, which the mobile terminal device  18  is able to set, may be a fixed value, while the waiting time period, which the vehicle terminal device  14  is able to set for the carrier sense, may be a variable value. In an example, the setting portion  88  sets the SIFS and the like. According to the present modification, it is possible to preferentially output the packet signal from the mobile terminal device  18 . 
     In the embodiments of the present invention, the setting portion  88  selects the execution of the preferential transmission or the usual transmission based on the location acquired by the acquisition portion  80 . However, this is not limiting, and the mobile terminal device  18  may be provided with a button, and if the setting portion  88  detects that the button is pushed down by a pedestrian, the setting portion  88  may select the preferential transmission. Further, if a predetermined time period passes after the button is pushed down, the setting portion  88  may select the usual transmission. According to the present modification, it is possible to execute the preferential transmission according to intention of the pedestrian. 
     REFERENCE SIGNS LIST 
       10  base station device 
       12  vehicle 
       14  vehicle terminal device 
       16  pedestrian 
       18  mobile terminal device 
       20  antenna 
       22  RF portion 
       24  modulation/demodulation portion 
       26  process portion 
       28  network communication portion 
       30  control portion 
       32  frame definition portion 
       34  selection portion 
       36  generation portion 
       40  antenna 
       42  RF portion 
       44  modulation/demodulation portion 
       46  process portion 
       48  control portion 
       50  timing identification portion 
       52  extraction portion 
       54  carrier sense portion 
       56  transfer decision portion 
       58  acquisition portion 
       60  notification portion 
       62  generation portion 
       70  antenna 
       72  RF portion 
       74  modulation/demodulation portion 
       76  process portion 
       78  control portion 
       80  acquisition portion 
       82  generation portion 
       84  timing identification portion 
       86  extraction portion 
       88  setting portion 
       90  carrier sense portion 
       100  communication system