Wireless communication apparatus and wireless communication system

A wireless communication apparatus is provided. The wireless communication apparatus is provided with: a control part performing repetitive transmission of a repetitive signal, that is, a signal that needs to be repeatedly transmitted, from a transmission part in the form of an electric wave; a reception part receiving an electric wave; and a signal collision determination part determining, on the basis of the electric wave received by the reception part while the repetitive signal is being transmitted by the control part, whether or not the repetitive signal transmitted from the transmission part is in collision with a signal transmitted by a different device. When the signal collision determination part has determined that the repetitive signal is in collision with another signal, the control part changes the transmission timing of the repetitive signal.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Phase Application under 35 U.S.C. 371 of International Application No. PCT/JP2016/075933 filed on Sep. 5, 2016 and published in Japanese as WO 2017/061203 A1 on Apr. 13, 2017. This application is based on and claims the benefit of priority from Japanese Patent Application No. 2015-200536 filed on Oct. 8, 2015. The entire disclosures of all of the above applications are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to a wireless communication apparatus and a wireless communication system provided with the multiple wireless communication apparatus, and particularly, to a technique for avoiding collision of signals transmitted from the wireless communication apparatus.

BACKGROUND ART

CSAMA/CA is widely known as a technique for avoiding collision of signals transmitted by wireless communication apparatus. CSAMA/CA is an abbreviation for “carrier sense multiple access/collision avoidance.”

Further, as widely known as a hidden terminal problem, even when the technique of CSAMA/CA is used, signals may collide with each other. In Patent Literature 1, while a subject terminal is transmitting a packet, collision of the packet with a packet transmitted by another terminal is monitored. When the packet collision has been detected, the subject terminal stops the packet transmission.

PRIOR ART LITERATURES

Patent Literature

Patent Literature 1: JP 2007-96902 A

SUMMARY OF INVENTION

Patent Literature 1 discloses a technique to stop the transmission of a signal when collision of the transmitted signal has been detected. According to the technique disclosed in Patent Literature 1, however, the collision of the signal cannot be avoided. When multiple signals collide with each other, all of the colliding signals cannot be decoded by a receiver. Thus, the transmitted signals become wasted. Therefore, as a matter of course, avoiding collision of signals may be more desired than detecting collision of signals and stopping transmission of the signals.

It is an object of the present disclosure to provide a wireless communication apparatus and a wireless communication system that enable to reduce collision of signals.

A wireless communication apparatus according to one aspect of the present disclosure includes: a transmission part that transmits an electric wave; a transmission control part that transmits, as an electric wave, a repetitive signal that needs to be repeatedly transmitted from the transmission part within a transmission time slot that is periodically repeated at a transmission timing determined, based on a start time of the transmission time slot and repeatedly transmits the repetitive signal by repetition of the transmission time slot; a reception part that receives an electric wave; and a signal collision determination part that determines whether the repetitive signal transmitted from the transmission part is in collision with a signal transmitted by a different device, based on an electric wave that is received by the reception part while the transmission control part is transmitting the repetitive signal. The transmission control part changes the transmission timing of the repetitive signal, based on a determination that there is a collision of the repetitive signal by the signal collision determination part.

The wireless communication apparatus includes the signal collision determination part. While the transmission control part is transmitting the repetitive signal, the signal collision determination part determines whether the repetitive signal is in collision with a signal transmitted by a different device. When the signal that is in collision with the repetitive signal is a repetitive signal transmitted by the different device, there is a high possibility that the different device transmits a signal at the same transmission timing also in the next transmission time slot. Thus, the transmission control part changes the transmission timing of the repetitive signal, based on the determination that there is a collision of the repetitive signal by the signal collision determination part. Accordingly, in the next transmission of the repetitive signal, even when the different device also transmits the repetitive signal, there is a high possibility that the repetitive signal has no collision with the repetitive signal transmitted by the different device. Thus, it may be possible to reduce collision of signals.

DESCRIPTION OF EMBODIMENTS

Hereinbelow, an embodiment will be described with reference to the drawings. As illustrated inFIG. 1, a wireless communication system1according to a first embodiment is provided with a roadside machine2and an onboard machine3. The roadside machine2corresponds to a wireless communication apparatus, and the onboard machine3corresponds to a service receiving terminal.

[Overview Configuration of Wireless Communication System1]

FIG. 1illustrates two roadside machines2A,2B. However, three or more roadside machines2may be provided. When the multiple roadside machines2A,2B are not distinguished from each other, the roadside machines2A,2B are referred to as the roadside machines2. Further,FIG. 1illustrates only one onboard machine3. However, multiple onboard machines3may be provided. The roadside machine2and the onboard machine3communicate with each other in compliance with the WAVE standards. WAVE is an abbreviation for “wireless access in vehicular environments.”

Because of compliance with the WAVE standards, the wireless communication system1sets one control channel and multiple service channels as communication channels. The control channel and the service channels are set to predetermined frequency channels different from each other. All of the control channel and the service channels belong to a 5.8 GHz band or a 5.9 GHz band.

The roadside machine2notifies a WSA that refers to “wave service advertisement” through the control channel. The WSA corresponds to service start information and includes various pieces of information that needs to be acquired by the onboard machine3to start a service. For example, the WSA includes channel information which designates a service channel. Further, the roadside machine2transmits service execution information through a service channel designated by the WSA. The roadside machine2may be fixed to a road side or may be a movable device.

The service execution information needs to be communicated between the roadside machine2and the onboard machine3to execute a service. The service execution information includes information transmitted by the roadside machine2and information transmitted by the onboard machine3. However, for example, in some services such as a road traffic information distribution service, the service execution information includes only information transmitted by the roadside machine2.

The onboard machine3is mounted on a vehicle4. Thus, the onboard machine3is a movable device. The onboard machine3is capable of receiving the WSA and the service execution information and also capable of performing vehicle-to-vehicle communication with another onboard machine3.

The roadside machine2performs road-to-vehicle communication with the onboard machine3that is present in a wireless communication area6formed by the roadside machine2and executes a predetermined service by transmitting various pieces of information to the onboard machine3and acquiring various pieces of information from the onboard machine3. The wireless communication area is an area within the reach of a signal transmitted by the roadside machine2with a predetermined strength detectable by the onboard machine3or higher, and corresponds to a transmission range.

The roadside machine2is disposed at a position suitable for a service provided by the roadside machine2. For example, the roadside machine2is disposed at an intersection, the middle of a road that connects intersections, or an entrance to a specific facility (for example, a parking lot, a store, or a toll road). The wireless communication area6of the roadside machine2can be set according to a service provided by the roadside machine2. In other words, an output of an electric wave transmitted by the roadside machine2can be set according to a service provided by the roadside machine2. When the set wireless communication area6is large, the set wireless communication area6may overlap the wireless communication area6of another roadside machine2.

FIG. 1illustrates a wireless communication area6A of the roadside machine2A and also illustrates a wireless communication area6B of the roadside machine2B. As illustrated inFIG. 1, these two wireless communication areas6A,6B partially overlap each other. In the example ofFIG. 1, the roadside machine2B is included in the wireless communication area6A of the roadside machine2A. On the other hand, the position of the roadside machine2A is outside the wireless communication area6B of the roadside machine2B. These two wireless communication areas6A,6B are directional areas. However, each of the wireless communication areas6may be a non-directional area, that is, an area having a perfect circle shape.

As illustrated inFIG. 2, the roadside machine2is provided with a roadside communication part21and a roadside control part22. The roadside communication part21and the roadside control part22are mutually communicably connected.

The roadside communication part21performs road-to-vehicle communication with the onboard machine3that is present in the wireless communication area6formed by the roadside machine2by short range communication. The short range communication is a communication system that performs direct communication through no relay device. The wireless communication area6formed by the roadside machine2is within a several hundred meter radius in many cases. However, the range of the wireless communication area6varies according to a service provided by the roadside machine2, and may be within an approximately 10 meter radius or an approximately 1 km radius.

The roadside communication part21includes two operation modes: a mode that performs communication using the control channel; and a mode that performs communication using the service channel. That is, the roadside communication part21performs both of the communications with the onboard machine3using the control channel and the service channel.

The roadside communication part21is provided with a roadside transmission part21A, a roadside reception part21B, a circulator21c, and an antenna21d. The roadside transmission part21A is capable of setting a transmission channel that is a frequency channel for transmitting an electric wave by selecting one channel from the control channel and the multiple service channels. The roadside transmission part21A modulates data input from the roadside control part22, further converts the modulated data to an electric wave having a frequency of the transmission channel, and transmits the electric wave. The roadside transmission part21A corresponds to a transmission part.

The roadside reception part21B sets a reception channel that is a frequency channel for receiving an electric wave to the same channel as the transmission channel, and receives an electric wave transmitted from the onboard machine3through the set reception channel. Further, the roadside reception part21B demodulates the received electric wave to extract a signal and outputs the signal to the roadside control part22. The roadside reception part21B corresponds to a reception part.

The circulator21cguides a signal from the roadside transmission part21A to the antenna21dand guides a signal representing an electric wave received by the antenna21dto the roadside reception part21B. Providing the circulator21cenables the roadside communication part21to receive a signal by the roadside reception part21B while transmitting a signal from the roadside transmission part21A.

The roadside control part22is configured as a common computer and provided with a CPU, a non-volatile memory such as a ROM or a flash memory, a volatile memory such as a RAM, an I/O, and a bus line which connects these components which are all known.

A roadside memory22M provided in the roadside control part22is a non-volatile storage medium and implemented by, for example, a flash memory. Program modules and data for executing various processes and a terminal ID assigned to the roadside machine2are stored in the roadside memory22M. Further, information for generating a WSA and information for generating service execution information are also stored in the roadside memory22M. Furthermore, a received signal strength of a signal received by the roadside reception part21B is also temporarily stored in the roadside memory22M.

As illustrated inFIG. 3, the roadside control part22is provided with a time synchronization part221, a roadside communication control part222, a signal collision determination part225, and a service processing part226as functional blocks implemented by executing the above program modules. Some or all of the functions executed by the roadside control part22may be configured as hardware by one or more ICs.

The time synchronization part221performs a process for synchronizing time information held by the roadside control part22with a reference time. The time information held by the roadside control part22is measured, based on a clock signal of the CPU. However, when time is measured, based on the clock signal, the time may be gradually deviated from the reference time. Thus, the time synchronization part221synchronizes the time information held by the roadside control part22with the reference time. The reference time is, for example, a time used in a global navigation satellite system (hereinbelow, referred to as GNSS). Hereinbelow, the time is referred to as a GNSS time. The time synchronization part221communicates with a server that is provided outside, and acquires the reference time. Alternatively, the roadside machine2may be provided with a GNSS receiver, the GNSS receiver may receive a signal including the GNSS time from a GNSS artificial satellite, and the time synchronization part221may acquire the GNSS time from the GNSS receiver.

The roadside communication control part222controls the operation of the roadside communication part21and performs a switch between communication through the control channel and communication through the service channel. The roadside communication control part222generates information that corresponds to the operation mode of the roadside communication part21and should be transmitted from the roadside communication part21and causes the roadside communication part21to transmit the information. Further, the roadside communication control part222acquires data received by the roadside communication part21and provides the acquired data to the service processing part226.

The roadside communication control part222is provided with a CCH communication control part223and an SCH communication control part224as smaller functional blocks. In each of the parts, CCH refers to the control channel and SCH refers to the service channel.

The CCH communication control part223takes charge of control of communication using the control channel. The CCH communication control part223generates a WSA, sets the transmission channel of the roadside communication part21to the control channel, and causes the roadside communication part21to transmit the generated WSA in a broadcast system. Further, the CCH communication control part223acquires data that is received by the roadside communication part21by communication using the control channel and provides the acquired data to the service processing part226.

FIG. 4illustrates the configuration of the WSA generated by the CCH communication control part223. As illustrated inFIG. 4, the WSA includes a header, a PSID, priority, and channel information. The header is information for recognizing that received data is a WSA in the onboard machine3, which is a reception side device. The header includes, for example, information indicating a version of the WAVE standards and information for distinguishing the WSA from another information such as the service execution information.

The PSID is information that determines the type of a service provided by a service provider through the roadside machine2. The priority is information that determines the priority of a service specified by the PSID among various services provided by various roadside machines2. The channel information is a channel number of multiple service channels that is used by the roadside machine2for providing a service. The service channel may be determined according to the provided service, and multiple services may be associated with the same service channel.

The time at which the CCH communication control part223causes the WSA to be transmitted corresponds to a control channel time slot. The roadside communication control part222divides time into the control channel time slot and a service channel time slot.

FIG. 5illustrates a change with time of the communication channel of the roadside communication part21. The communication channel of the roadside communication part21refers to the transmission channel of the roadside transmission part21A and the reception channel of the roadside reception part21B. The transmission channel and the reception channel are constantly set to the same frequency channel.

In the example ofFIG. 5, the control channel and the service channel are alternately set. A time slot in which the communication channel is set to the control channel is referred to as the control channel time slot, and a time slot in which the communication channel is set to the service channel is referred to as the service channel time slot.

The length of the control channel time slot and the length of the service channel time slot are equal to each other and, for example, each 50 milliseconds. Further, a start time of the control channel time slot and a start time of the service channel time slot are times determined by a reference time system. The control channel time slot and the service channel time slot are periodically repeated time slots. The control channel time slot corresponds to a transmission time slot.

The CCH communication control part223repeatedly transmits the WSA each control channel time slot. Thus, the WSA corresponds to a repetitive signal, and the CCH communication control part223which controls the transmission of the WSA corresponds to a transmission control part.

The SCH communication control part224takes charge of control of communication using a predetermined service channel. The SCH communication control part224generates service execution information, sets the transmission channel of the roadside communication part21to a service channel that is determined according to the type of a service, and causes the roadside communication part21to transmit the generated service execution information. The time at which the SCH communication control part224causes the roadside communication part21to transmit the generated service execution information corresponds to the service channel time slot. A transmission system may be any of broadcast, unicast, and multicast. Which transmission system is used is determined according to the type of a service. Further, the SCH communication control part224acquires data received by the roadside communication part21by communication using the service channel, and provides the acquired data to the service processing part226.

When the CCH communication control part223controls the roadside transmission part21A so that the roadside transmission part21A transmits a WSA, the signal collision determination part225determines whether the WSA is in collision with a signal transmitted by a different device. A method for the determination will be described with reference to the flowchart illustrated inFIG. 8.

The service processing part226provides a predetermined service to the onboard machine3, based on data provided from the roadside communication control part222. Examples of the service to be provided include an automatic fare collection service during a travel on a toll road, an automatic parking fee collection service during parking, a traffic information distribution service, a positional information notification service, and an advertisement distribution service.

Next, the configuration of the onboard machine3will be described. As illustrated inFIG. 6, the onboard machine3is provided with a control part31, a short range communication part32, and a GNSS receiver33. The control part31is mutually communicably connected to the short range communication part32and the GNSS receiver33.

The short range communication part32performs short range communication with the roadside communication part21of the roadside machine2and the short range communication part32provided in another onboard machine3. A communication range of the short range communication part32is, for example, approximately several hundred meters. Further, the short range communication of the present embodiment uses, for example, the control channel or the service channel described above. The short range communication part32is provided with a short range reception part32B that demodulates a signal received by an antenna and outputs the demodulated signal to the control part31and a short range transmission part32A that modulates data input from the control part31, further converts the modulated data to an electric wave, and transmits the electric wave to surroundings.

The short range reception part32B selects one channel from the control channel and the service channels, sets the selected channel as a reception channel, and receives an electric wave transmitted from the roadside machine2through the set reception channel. Then, the short range reception part32B demodulates the received electric wave to extract a signal and outputs the signal to the roadside control part22.

The short range transmission part32A sets a transmission channel to the same frequency channel as the reception channel, modulates data input from the roadside control part22, further converts the modulated data to an electric wave having the frequency of the transmission channel, and transmits the electric wave.

The GNSS receiver33calculates the current position of the GNSS receiver33by receiving an electric wave from a satellite used in GNSS. The current position calculated by the GNSS receiver33is, for example, represented by latitude and longitude. Information indicating the current position calculated by the GNSS receiver33is sequentially (for example, each 100 milliseconds) provided to the control part31.

Further, the GNSS receiver33outputs a PPS signal each one second while acquiring the GNSS satellite. PPS is an abbreviation for “pulse per second.” The PPS signal functions as a signal indicating the timing of an integer second, in other words, the timing when second is switched.

The control part31is configured as a common computer and provided with a CPU, a non-volatile memory such as a ROM or a flash memory, a volatile memory such as a RAM, an I/O, and a bus line which connects these components which are all known.

A memory31M provided in the control part31is a non-volatile storage medium and implemented by, for example, a flash memory or a ROM. Program modules and data for executing various processes and a terminal ID assigned to the onboard machine3are stored in the memory31M. Further, a WSA received by the short range reception part32B is temporarily stored in the memory31M.

As illustrated inFIG. 7, the control part31is provided with a time synchronization part311, a channel control part312, and a service execution part313as functional blocks implemented by executing the above program modules. Some or all of the functions executed by the control part31may be configured as hardware by one or more ICs.

The time synchronization part311performs a process for synchronizing time information held by the control part31with the reference time. Since the onboard machine3is provided with the GNSS receiver33, the time synchronization part311acquires a signal including the GNSS time received by the GNSS receiver33from the GNSS artificial satellite to perform the synchronization process.

The channel control part312sets a communication channel of the short range communication part32to either the control channel or the service channel. Specifically, when a WSA is received through the control channel as the communication channel, the communication channel is switched to a service channel determined by the WSA from a start time of the first service channel time slot after the reception of the WSA. Thereafter, during the execution of a service, the communication channel is set to the service channel in the service channel time slot and set to the control channel in the control channel time slot. In the service channel time slot during the execution of the service, the short range communication part32transmits and receives information for executing the service to and from the roadside machine2.

When the transmission and reception of the information for executing the service are finished, the channel control part312fixes the communication channel to the control channel. However, when a WSA relating to a service that differs from the service under execution is received during the execution of the service, the channel control part312changes the service channel to be opened to a service channel determined by the received WSA. Then, the channel control part312continues control for alternately opening the service channel and the control channel.

When the short range reception part32B receives a WSA while the channel control part312sets the communication channel to the control channel, the service execution part313determines a service to be executed, based on the received WSA. Further, the service execution part313acquires service execution information received by the short range reception part32B while the channel control part312sets the communication channel to the service channel and determines data requested by the roadside machine2, based on the acquired service execution information. Then, the service execution part313causes the short range transmission part32A to transmit the determined data to the roadside machine2.

[Processing of Roadside Control Part22]

FIG. 8andFIG. 9are flowcharts describing processing executed by the roadside control part22. In the roadside control part22, in addition to the processing illustrated inFIG. 8andFIG. 9, the time synchronization part221periodically performs the synchronization process.

The roadside control part22repeatedly executes the processing illustrated inFIG. 8andFIG. 9while power is ON. The processing ofFIG. 8is performed with the communication channel set to the control channel and thus referred to as CCH processing. The processing ofFIG. 9is performed with the communication channel set to the service channel and thus referred to as SCH processing.

First, the CCH processing illustrated inFIG. 8will be described. InFIG. 8, step S6and step S7are processes performed by the signal collision determination part225, and the other steps are processes performed by the CCH communication control part223.

In step S1, the control channel is opened. That is, the transmission channel and the reception channel are set to the control channel.

In step S2, it is determined whether a transmission timing of transmitting a WSA has come. The WSA transmission timing is a timing that is determined, based on the start time of the control channel time slot. When the determination of step S2is NO, the determination of step S2is repeated. When the determination of step S2is YES, a move to step S3is made.

In step S3, carrier sensing is executed. In step S4, it is determined whether the result of the carrier sensing shows that the communication channel is free. When it is determined that the communication channel is busy, the determination of step S4becomes NO. When the determination of step S4is NO, a return to step S3is made, and the carrier sensing is continued. When the control channel time slot is over during the repetition of the processes of step S3and step S4, a move to the processing ofFIG. 9is made.

When it is determined that the communication channel is free in step S3, the determination of step S4becomes YES. As with known CSMA/CA, the carrier sensing executed in step S3performs the determination whether the communication channel is free for a certain determination time DI FS and then further determines a random back-off time when the communication channel is determined to be free. Also during the back-off time, it is determined whether the communication channel is free. Then, when the communication channel is continuously free until the back-off time is over, the determination of step S4becomes YES. When the determination of step S4becomes YES, a move to step S5is made.

In step S5, a WSA is transmitted from the roadside transmission part21A. In the following step S6, a received signal strength of a signal that is received by the roadside reception part21B, the received signal strength being stored in the memory31M, during the transmission of the WSA in step S5is acquired. In step S7, it is determined whether the WSA transmitted in step S5is in collision with a signal transmitted by a different device. When there is a period during which the received signal strength acquired in step S6is equal to or larger than a signal strength threshold set for determining collision and also when at least a part of the period overlaps the period during which the WSA is transmitted, it is determined that there is a collision of the WSA.

The roadside machine2executes the carrier sensing in step S3. Similarly, another roadside machine2executes the carrier sensing. Thus, when each of multiple roadside machines2can receive a WSA transmitted by another roadside machine2, no collision occurs between WSAs transmitted by the roadside machines2. However, in the example illustrated inFIG. 1, the roadside machine2B is located inside the wireless communication area6A of the roadside machine2A, but the roadside machine2A is located outside the wireless communication area6B of the roadside machine2B. Thus, when the roadside machine2A transmits a WSA, the roadside machine2B can determine that the communication channel is busy by the carrier sensing in step S3. However, even when the roadside machine2B transmits a WSA, the roadside machine2A cannot determine that the communication channel is busy by the carrier sensing in step S3.

Thus, as illustrated inFIG. 10, even when the roadside control part22of the roadside machine2A executes the processing ofFIG. 8between time ta1and time ta2which are included in a time slot in which the roadside machine2B transmits a WSA, the determination of step S4becomes YES. As a result, as illustrated inFIG. 10, the roadside machine2A may transmit a WSA from time ta2. Accordingly, the roadside control part22of the roadside machine2B determines that the WSAs are in collision with each other in a period from time ta2to time ta3. InFIG. 10, each down arrow conceptually represents that the WSA transmitted by the roadside machine2B does not reach the roadside machine2A, and each up arrow conceptually represents that the WSA transmitted by the roadside machine2A reaches the roadside machine2B. When it is determined that the WSAs are in collision with each other, the determination of step S7becomes YES, and a move to step S8is made.

In step S8, it is determined whether a remaining time in the control channel time slot is equal to or longer than an enough time to transmit the WSA. The enough time to transmit the WSA is the total of a time during which the WSA is transmitted and a time required for carrier sensing. The carrier sensing includes the back-off time, which is randomly set. Thus, the determination in step S8is performed assuming that the back-off time is set to a maximum value.

When the determination of step S8is YES, a return to step S3is made for re-transmitting the WSA. When the return to step S3is made, carrier sensing is performed during the determination time DIFS and the back-off time. The determination time DIFS and the back-off time at this time correspond to a waiting time.

When the determination of step S8is NO, a move to step S9is made. Additionally, when the determination of step S7is NO, that is, when it is determined that there is no collision of the transmitted WSA, a move to step S9is made.

In step S9, it is determined whether the service channel time slot has come. When the determination of step S9is NO, the determination of step S9is repeated. When the determination of step S9is YES, a move to the SCH processing illustrated inFIG. 9is made.

Next, the SCH processing illustrated inFIG. 9will be described. InFIG. 9, step S21and step S23are processes performed by the SCH communication control part224, step S22is a process performed by the service processing part226, and step S24and step S25are processes performed by the CCH communication control part223.

In step S21, the service channel is opened. In the following step S22, a service execution information transmission and reception process is performed. The service execution information includes both of information that needs to be transmitted to the onboard machine3by the roadside machine2for executing a service and information that needs to be transmitted to the roadside machine2by the onboard machine3for executing a service.

In step S23, it is determined whether the control channel time slot has come. When the determination is NO, the determination of step S23is repeated. When the determination is YES, a move to step S24is made.

In step S24, a CCH free time slot is determined. The CCH free time slot is a time slot in which the control channel is not busy in the immediately preceding control channel time slot. In the process of step S24, specifically, a received signal strength in the immediately preceding control channel time slot is acquired from the roadside memory22M, and a time slot in which the acquired received signal strength is lower than the signal strength threshold is determined as the CCH free time slot.

In step S25, a WSA transmission timing in the next control channel time slot is determined. Specifically, the WSA transmission timing in the next control channel time slot is set to a timing within the earliest time slot in the CCH free time slot determined in step S24. The earliest time slot is a time slot that starts from the earliest time of the CCH free time slot, for example, a time slot of approximately SIFS used in CSA/CA or a time slot of approximately several times as long as SIFS.

FIG. 11illustrates the next WSA transmission timing determined in step S25. In the example ofFIG. 11, time tb is a time, based on the start time t0of the control channel time slot.

In the example ofFIG. 11, the current transmission timing is time tb4, and steps S3to S5which are processes for transmitting the WSA are executed from time tb4to time tb5this time. Further, the CCH free time slot determined in step S24comes at and after time tb1. Thus, in step S25, time tb2is set as the next WSA transmission timing. Accordingly, in the next CCH processing, time tb2is set as the WSA transmission timing, the process for transmitting the WSA is started from time tb2, and the WSA is transmitted until time tb3. Time tb3varies according to the back-off time, which is randomly set.

<Summary of First Embodiment>

The roadside machine2of the first embodiment is provided with the signal collision determination part225. While the CCH communication control part223is transmitting a WSA, the signal collision determination part225determines whether the WSA is in collision with a signal transmitted by a different device. The frequency of the control channel is common between all roadside machines2that are in compliance with the WAVE standards, and all the roadside machines2transmit WSAs each control channel time slot. Thus, there is a high possibility that a signal that is in collision with the WSA transmitted by the roadside machine2that executes the processing ofFIG. 8is a WSA transmitted by another roadside machine2. Thus, when it is determined that the WSAs are in collision with each other, the CCH communication control part223changes the next WSA transmission timing. Accordingly, when the WSA is transmitted next time, there is a high possibility that the transmitted WSA has no collision with the WSA transmitted by another roadside machine2. Thus, it may be possible to reduce collision of the WSAs.

Further, in the present embodiment, the CCH communication control part223re-transmits the WSA when it is determined that there is a collision of the WSA and the remaining time in the control channel time slot is equal to or longer than the enough time to transmit the WSA after standby for the determination time DIFS and the back-off time. Accordingly, it may be possible that the onboard machine3receives the retransmitted WSA and opens the service channel that is determined by the WSA from the next service channel time slot.

However, also in the next and subsequent control channel time slots, when the WSA is transmitted with the same transmission timing as that in the retransmission, the WSA transmission timing may be shifted to a late time slot in the control channel time slot each time it is determined that there is a collision of the WSA. As a result, the WSA transmission timing may become close to the finish time of the control channel time slot.

The WSA is a signal that designates the service channel for the onboard machine3. The onboard machine3performs a process for decoding the WSA after receiving the WSA and a process for opening the service channel designated by the decoded WSA. As a matter of course, these processes require a certain amount of time. Thus, when the WSA transmission timing is close to the finish time of the control channel time slot, the opening of the service channel in the onboard machine3may be late for the start time of the service channel time slot.

Thus, in the present embodiment, in step S24and step S25, the WSA transmission timing in the next control channel time slot is set to the timing within the earliest time slot in the CCH free time slot. Accordingly, the WSA transmission timing is set to an early time slot in the control channel time slot. Thus, it may be possible to prevent the opening of the service channel in the onboard machine3from being late for the start time of the service channel time slot.

Next, a second embodiment will be described. In the description for the second embodiment and the subsequent modifications, an element having the same reference sign as a previously-used reference sign is identical with an element having the previously-used reference sign in the previous embodiment unless otherwise specifically noted. When only a part of the configuration is described, the previously-described embodiment can be applied to the other part of the configuration.

As illustrated inFIG. 12, a wireless communication system100in the second embodiment is provided with roadside machines2L,2M,2S. These roadside machines2L,2M,2S are fixed roadside machines. In addition, the wireless communication system100in the second embodiment is also provided with a roadside machine2mothat is a movable roadside machine and mounted on a vehicle5.

As illustrated inFIG. 12, among wireless communication areas6L,6M,6S of the roadside machines2L,2M,2S, the wireless communication area6L has the largest size, the wireless communication area6M has the second largest size, and the wireless communication area6S has the smallest size. Further, the wireless communication area6L includes the wireless communication areas6M,6S of the roadside machines2M,2S. The wireless communication areas6M,6S partially overlap each other. The roadside machine2M is located inside the wireless communication area6S, but the roadside machine2S is located outside the wireless communication area6M. Although the size of a wireless communication area6moof the roadside machine2mois not particularly limited to any size, for example, the size of the wireless communication area6momay be approximately equal to the size of the wireless communication area6S of the roadside machine2S.

Each of the roadside machines2L,2M,2S,2mohas the same hardware configuration as the roadside machine2of the first embodiment. Further, processing executed by a roadside control part22of each of the roadside machines2L,2M,2S,2mois the same as that in the first embodiment except step S25ofFIG. 9.

The first embodiment does not particularly refer to an initial value of the WSA transmission timing, and the initial value is not particularly limited to any value. On the other hand, in the second embodiment, in the fixed roadside machines2L,2M,2S, as conceptually illustrated inFIG. 13, as the wireless communication area6of the roadside machine2is larger, the initial value of the WSA transmission timing is set to an earlier timing.

On the other hand, an initial value of a WSA transmission timing of the movable roadside machine2mois a fixed value that does not depend on the size of the wireless communication area6moin the present embodiment. The WSA transmission timing of the roadside machine2mois set to a point after half the control channel time slot.

Further, in the second embodiment, a minimum value of a time slot in which the WSA transmission timing is settable is determined within the control channel time slot. The minimum value refers to the earliest time. As conceptually illustrated inFIG. 14, the minimum value in the present embodiment is the same as the initial value of the WSA transmission timing in all of the fixed roadside machines2L,2M,2S and the movable roadside machine2mo.

The initial value and the minimum value of the WSA transmission timing are set as illustrated inFIG. 13andFIG. 14. Thus, in step S25in the second embodiment, the next WSA transmission timing is set to a timing within the earliest time slot after the transmission timing minimum value in the CCH free time slot.

Next, an effect obtained by setting the initial value and the minimum value illustrated inFIG. 13andFIG. 14will be described. The roadside machines2L,2M,2S are not necessarily simultaneously installed and may be sequentially added. Here, a case in which the roadside machines2M,2S are first installed, and the roadside machine2L is added thereafter will be considered.

As described above, in the second embodiment, the initial value of the WSA transmission timing of each of the fixed roadside machines2M,2S is determined by the size of the wireless communication area6. The initial values of the WSA transmission timings of the roadside machines2M,2S are defined as time t1and time t2as illustrated inFIG. 15. Each time t illustrated inFIG. 15is a time, based on a start time t0of each control channel time slot.

The roadside machine2M starts carrier sensing from time t1. At this time, the control channel is not busy. Thus, the roadside machine2M transmits a WSA. At time t2, the roadside machine2S starts carrier sensing. At this time, the roadside machine2M is transmitting the WSA. However, the roadside machine2S is located outside the wireless communication area6M of the roadside machine2M and thus cannot detect the WSA transmitted by the roadside machine2M. As a result, the roadside machine2S also transmits a WSA. Thus, the two WSAs are in collision with each other from the start of the transmission of the WSA by the roadside machine2S to time t5at which the roadside machine2M finishes the transmission of the WSA.

The roadside machine2M is located inside the wireless communication area6S of the roadside machine2S. Thus, the roadside machine2M can determine that the WSAs are in collision with each other and can also detect that the roadside machine2S finishes the transmission of the WSA at time t6. In the example ofFIG. 15, at the time when the roadside machine2S finishes the transmission of the WSA, the time for transmitting the WSA is still left within the control channel time slot. Thus, the roadside machine2M re-transmits the WSA at time t7as a new WSA transmission timing. The retransmission of the WSA ends at time t10.

In the next control channel time slot, the WSA transmission timing of the roadside machine2M is time t7. Thus, the WSA transmitted by the roadside machine2S has no collision with the WSA transmitted by the roadside machine2M.

When the roadside machine2L is installed, as illustrated in a time slot after the installation of the roadside machine2L inFIG. 15, the roadside machine2L starts carrier sensing from time t0and transmits a WSA. The roadside machine2L finishes the transmission of the WSA at time t3. Thus, when the roadside machine2S starts carrier sensing from time t2in the same manner as the carrier sensing before the installation of the roadside machine2L, the roadside machine2S can detect the WSA transmitted by the roadside machine2L and determine that the control channel is busy.

As the wireless communication area6of a roadside machine2is larger, a WSA transmitted by the roadside machine2is more easily detected by another roadside machine2. In other words, as the wireless communication area6of a roadside machine2is smaller, a WSA transmitted by the roadside machine2is less easily detected by another roadside machine2. As a result, the possibility of a WSA collision becomes higher.

Thus, in the second embodiment, in order to make the WSA transmission timing earlier as the wireless communication area6is larger, the initial value and the minimum value are set to earlier times as the wireless communication area6is larger.

Although the roadside machine2S starts carrier sensing from time t2, the roadside machine2S cannot determine that the control channel is free until time t3. Thus, the WSA transmission timing of the roadside machine2S is time t4, and the roadside machine2S transmits the WSA until time t8.

Although the roadside machine2M also starts carrier sensing from time t7in the same manner as the carrier sensing before the installation of the roadside machine2L, the roadside machine2M cannot determine that the control channel is free until time t8. Thus, the WSA transmission timing of the roadside machine2M is time t9, and the roadside machine2M transmits the WSA until time t11.

In the example ofFIG. 15, the roadside machine2M first changes the WSA transmission timing. Thereafter, when the roadside machine2L is added, the roadside machine2S and the roadside machine2M change the respective WSA transmission timings. Accordingly, the WSAs transmitted by the respective three roadside machines2L,2M,2S have no collision with each other.

Next, a case in which the roadside machine2mopasses through the wireless communication area6L of the roadside machine2L will be considered. Before the roadside machine2moenters the wireless communication area6L of the roadside machine2L, as illustrated inFIG. 16, the roadside machines2L,2M,2S transmit WSAs in a time slot of time t0to time t3, a time slot of time t9to time t11, and a time slot of time t4to time t8, respectively. Thus, the WSAs transmitted by the respective roadside machines2L,2M,2S have no collision with each other.

The WSA transmission timing of each of the fixed roadside machines2L,2M,2S is set within the earliest time slot after the transmission timing minimum value in the CCH free time slot. Thus, the WSA transmission timing of each of the fixed roadside machines2L,2M,2S is likely to become an early time slot in the control channel time slot. On the other hand, the initial value and the minimum value of the WSA transmission timing of the movable roadside machine2moare set to a point after half the control channel time slot. Thus, in a time slot during the passage of the roadside machine2moinFIG. 16, the movable roadside machine2motransmits the WSA in a time slot from time t12to time t13. Time t12is after time t11when the roadside machine2M finishes the transmission of the WSA. Thus, even when the roadside machine2mopasses through the wireless communication areas6L,6M,6S, it is unnecessary for the roadside machines2L,2M,2S to change their WSA transmission timings.

Although the embodiments have been described above as examples, the present disclosure is not limited to the above embodiments. Various embodiments including the following modifications can be employed.

For example, in the above embodiments, when it is determined that there is no WSA collision, the CCH free time slot is determined in step S24, and the next WSA transmission timing is determined, based on the determined CCH free time slot in step S25. However, when it is determined that there is no WSA collision, the WSA transmission timing may be maintained.

In the above embodiments, the roadside machine2is provided with the circulator21cso that the roadside reception part21B can acquire an electric wave received by the antenna21dwhile the roadside transmission part21A is transmitting a signal. However, the present disclosure is not limited thereto. A transmission antenna and a reception antenna may be separately provided so that the roadside reception part21B can receive an electric wave while the roadside transmission part21A is transmitting a signal.

In the second embodiment, the WSA transmission timing of the movable roadside machine2mois set to the point after half the control channel time slot. However, the WSA transmission timing of the movable roadside machine2momay be a point after two thirds of the control channel time slot or may be a point after three quarters of the control channel time slot.