Wireless communication system and wireless communication method

Abstract

This wireless communication system includes a roadside device and an in-vehicle device mounted on a vehicle. The roadside device includes: a distribution unit that periodically distributes a downlink message including a usage status of each of a plurality of slots, which are obtained by dividing the transmission period of an uplink message of the in-vehicle device for each predetermined time; and an update unit that updates the usage status of the slots on the basis of the uplink message received from the in-vehicle device. The in-vehicle device includes: a time setting unit that determines a reference time on the basis of a satellite signal; a reception unit that receives the downlink message from the roadside device; and a transmission unit that randomly selects one slot that is not occupied by another in-vehicle device on the basis of the usage status of the slots included in the downlink message, and transmits an uplink message to the roadside device in the selected slot.

Description

Wireless communication system and wireless communication method

[0001] The present disclosure relates to a wireless communication system and a wireless communication method.

[0002] In recent years, road-to-vehicle communication, which transmits and receives data between roadside devices and in-vehicle devices, and vehicle-to-vehicle communication, which transmits and receives data between in-vehicle devices, have been used to perform billing processing, information distribution to vehicles, and information collection from vehicles (see, for example, Patent Document 1).

[0003] Japanese Patent No. 6340891

[0004] A roadside device communicates with multiple in-vehicle devices within its communication range. To prevent collisions caused by multiple vehicles (in-vehicle devices) simultaneously transmitting messages to the roadside device, the roadside device performs vehicle-to-vehicle communication to stagger communication timing. However, the communication range of roadside-to-vehicle communication may be wider than that of vehicle-to-vehicle communication. For example, two vehicles may be within the communication range of a roadside device, but the distance between the vehicles may be too far to allow vehicle-to-vehicle communication. If vehicle-to-vehicle communication is not possible, the process of staggering communication timing cannot be performed, and there is a possibility that two in-vehicle devices may simultaneously transmit messages to the roadside device. This results in the so-called hidden terminal problem, in which the roadside device cannot receive messages from these vehicles.

[0005] An object of the present disclosure is to provide a wireless communication system and a wireless communication method that can prevent collisions from occurring when multiple on-board devices simultaneously send messages in road-to-vehicle communication between a roadside device and multiple on-board devices.

[0006] According to one aspect of the present disclosure, a wireless communication system is a wireless communication system comprising a roadside device and an on-board device mounted on a vehicle, wherein the roadside device has a distribution unit that periodically distributes downlink messages including the usage status of each of a plurality of slots obtained by dividing the transmission period of the on-board device's uplink messages into predetermined time intervals, and an update unit that updates the usage status of the slots based on the uplink messages received from the on-board device, and the on-board device has a time setting unit that determines a reference time based on a satellite signal, a receiving unit that receives the downlink message from the roadside device, and a transmitting unit that randomly selects a slot that is not occupied by another on-board device based on the usage status of the slots included in the downlink message, and transmits an uplink message to the roadside device in the selected slot.

[0007] According to one aspect of the present disclosure, a wireless communication method is a wireless communication method using a roadside device and an on-board device mounted on a vehicle, and includes the steps of: the roadside device periodically distributing a downlink message including the usage status of each of a plurality of slots obtained by dividing the transmission period of the on-board device's uplink message into predetermined time intervals; the roadside device updating the usage status of the slots based on the uplink message received from the on-board device; the on-board device determining a reference time based on a satellite signal; the on-board device receiving the downlink message from the roadside device; and the on-board device randomly selecting a slot that is not occupied by another on-board device based on the usage status of the slots included in the downlink message, and transmitting an uplink message to the roadside device in the selected slot.

[0008] According to the above aspect, in road-to-vehicle communication between a roadside device and a plurality of in-vehicle devices, it is possible to prevent collisions from occurring due to the plurality of in-vehicle devices simultaneously transmitting messages.

[0009] 1 is a schematic diagram showing an overall configuration of a wireless communication system according to a first embodiment. FIG. 2 is a block diagram showing a functional configuration of a roadside device according to the first embodiment. FIG. 3 is a block diagram showing a functional configuration of an in-vehicle device according to the first embodiment. FIG. 4 is a first sequence diagram showing a processing example of a wireless communication system according to the first embodiment. FIG. 5 is a first diagram showing an example of a downlink message according to the first embodiment. FIG. 6 is a first diagram showing an example of an uplink message according to the first embodiment. FIG. 7 is a second diagram showing an example of a downlink message according to the first embodiment. FIG. 8 is a third diagram showing an example of a downlink message according to the first embodiment. FIG. 9 is a second sequence diagram showing a processing example of a wireless communication system according to the first embodiment. FIG. 10 is a third diagram showing an example of an uplink message according to the first embodiment. FIG. 11 is a fifth diagram showing an example of an uplink message according to the first embodiment. FIG. 12 is a fourth diagram showing an example of a downlink message according to the first embodiment. FIG. 13 is a third sequence diagram showing a processing example of a wireless communication system according to the first embodiment.

[0010] First Embodiment Hereinafter, an embodiment will be described in detail with reference to the drawings.

[0011] (Overall Configuration) Fig. 1 is a schematic diagram showing the overall configuration of a wireless communication system according to Embodiment 1. As shown in Fig. 1, the wireless communication system 1 includes a roadside device 10 and an in-vehicle device 20.

[0012] The roadside device 10 is installed on the side of a road and performs wireless communication with the vehicle-mounted device 20 located within a communication area R1.

[0013] The on-board device 20 is mounted on a vehicle V, and when the on-board device 20 enters the communication range R1 of the roadside device 10, it performs wireless communication with the roadside device 10. Furthermore, when an on-board device 20 of another vehicle exists within its own communication range, the on-board device 20 may perform vehicle-to-vehicle communication with this on-board device 20.

[0014] (Functional Configuration of Roadside Device) Fig. 2 is a block diagram showing the functional configuration of the roadside device according to Embodiment 1. As shown in Fig. 2, the roadside device 10 includes a processor 11, a memory 12, a storage 13, and a communication antenna 14.

[0015] The processor 11 operates in accordance with a predetermined program to function as a distribution unit 111 and an update unit 112 .

[0016] The distribution unit 111 periodically distributes a downlink message WSA including the usage status of each of a plurality of slots obtained by dividing the transmission period of the uplink message BSM of the in-vehicle device 20 into predetermined time slots.

[0017] The downlink message WSA is broadcast to each in-vehicle device 20 present within the communication area R1. The downlink message WSA may further include requests to each in-vehicle device 20 (such as requests to send probe data or data for billing processing) and information to be provided to each in-vehicle device 20 (such as traffic information). The uplink message BSM is data transmitted from the in-vehicle device 20 to the roadside device 10 at regular intervals (for example, every one second) and includes various information requested by the roadside device 10. The uplink message BSM includes vehicle probe data (such as identification information, GNSS positioning information) requested by the roadside device 10, information for billing processing, and the like.

[0018] The update unit 112 updates the slot usage status based on the uplink message BSM received from the in-vehicle device 20 .

[0019] The memory 12 has a memory area necessary for the operation of the processor 11 .

[0020] The storage 13 is a so-called auxiliary storage device, such as a hard disk drive (HDD), a solid state drive (SSD), etc. The storage 13 stores data that each part of the processor 11 acquires, generates, and references during processing.

[0021] The communication antenna 14 transmits and receives various data to and from the in-vehicle device 20 via wireless communication.

[0022] (Functional Configuration of In-Vehicle Device) Fig. 3 is a block diagram showing the functional configuration of the in-vehicle device according to Embodiment 1. As shown in Fig. 3, the in-vehicle device 20 includes a processor 21, a memory 22, a storage 23, a communication antenna 24, and a GNSS receiving unit 25.

[0023] The processor 21 operates according to a predetermined program to function as a time setting unit 211, a receiving unit 212, and a transmitting unit 213.

[0024] The time setting unit 211 determines the reference time based on the satellite signal.

[0025] The receiving unit 212 receives the downlink message WSA from the roadside unit 10 .

[0026] The transmitting unit 213 randomly selects one slot that is not occupied by another in-vehicle device 20 based on the usage status of the slot included in the downlink message WSA, and transmits the uplink message BSM to the roadside device 10 in the selected slot.

[0027] The memory 22 has a memory area necessary for the operation of the processor 21 .

[0028] The storage 23 is a so-called auxiliary storage device, such as a hard disk drive (HDD), a solid state drive (SSD), etc. The storage 23 stores data that each part of the processor 21 acquires, generates, and references during processing.

[0029] The communication antenna 24 transmits and receives various data to and from the roadside device 10 via wireless communication.

[0030] The GNSS receiving unit 25 calculates the position, speed, moving direction, etc. of the vehicle V based on the satellite signals received from the GNSS satellites, and outputs the calculated information as GNSS positioning information to the processor 21. The GNSS receiving unit 25 also outputs a 1 PPS (Pulse Per Second) signal for time synchronization to the processor 21.

[0031] (Processing Example 1 of Communication System) Fig. 4 is a first sequence diagram showing a processing example of the wireless communication system according to the first embodiment. Here, the flow of processing of the wireless communication system 1 will be described in detail with reference to Fig. 4 .

[0032] First, the roadside device 10 periodically distributes a downlink message WSA (step S101). FIG. 5 is a first diagram illustrating an example of a downlink message according to the first embodiment. The downlink message WSA includes at least slot usage status information. The slot usage status includes information on the slot number, occupancy flag, and identification information for each of n divided slots. The occupancy flag is represented as ON or OFF. When the occupancy flag is ON, it indicates that the slot is occupied by the in-vehicle device 20. When the occupancy flag is OFF, it indicates that the slot is empty. The identification information is information that can identify the in-vehicle device 20 (or vehicle V) that occupies the slot. FIG. 5 illustrates an example of a downlink message WSA1 when no in-vehicle device 20 is present within the communication area R1 of the roadside device 10. In the downlink message WSA1, all slots are empty (occupancy flag = OFF).

[0033] Also, assume that the in-vehicle device 20B enters the communication area R1 of the roadside device 10. In this case, the receiving unit 212 of the in-vehicle device 20B receives the downlink message WSA1 distributed by the roadside device 10 (step S102).

[0034] Upon receiving the downlink message WSA1, the time setting unit 211 of the in-vehicle device 20B sets the reference time of the transmission slot of the uplink message BSM based on the 1 pps signal output from the GNSS receiving unit 25 (step S103). A known technique may be used to set the reference time from the 1 pps signal. This allows precise time synchronization between multiple in-vehicle devices 20, preventing simultaneous data transmission between the in-vehicle devices 20 even if the slots are divided into very short time intervals on the order of milliseconds.

[0035] Next, the transmitter 213 of the on-board unit 20B checks the slot usage status of the received downlink message WSA1 and randomly selects one of the available slots (occupancy flag = OFF). The transmitter 213 of the on-board unit 20B then starts periodically transmitting an uplink message BSM, to which the identification information of the on-board unit 20B is attached, in the selected slot (step S104). FIG. 6 is a first diagram illustrating an example of an uplink message according to the first embodiment. FIG. 6 illustrates an example of an uplink message BSM1 transmitted by the transmitter 213 of the on-board unit 20B in step S104. The uplink message BSM includes a header portion and a message portion. The header portion includes the slot number selected by the on-board unit 20B, the slot occupation flag, and the identification information of the on-board unit 20B. In the example of FIG. 6, the transmitter 213 of the on-board unit 20B selects slot 1 and transmits the uplink message BSM1 to the roadside unit 10. The message section also includes any message, such as probe data (GNSS positioning information) requested by the roadside device 10 or information used for billing processing.

[0036] When the update unit 112 of the roadside device 10 receives the uplink message BSM1 from the in-vehicle device 20B (step S105), if the usage status of the slot number included in the uplink message BSM1 is empty, the update unit 112 updates the slot usage status based on the received uplink message BSM1 (step S106). Figure 7 is a second diagram showing an example of a downlink message according to the first embodiment. In the example of Figure 6, the in-vehicle device 20B selects slot 1. Therefore, as shown in Figure 7, the update unit 112 of the roadside device 10 changes the occupation flag for slot 1 in the slot usage status to ON and inputs the identification information (in-vehicle device 20B) received from the in-vehicle device 20B into the identification number.

[0037] Furthermore, the distribution unit 111 of the roadside unit 10 repeatedly distributes the downlink message WSA at regular intervals. Therefore, after the slot usage status is updated, the distribution unit 111 of the roadside unit 10 distributes a downlink message WSA2 ( FIG. 7 ) including the updated slot usage status at the next interval (step S107). After this, while the in-vehicle unit 20B continues to receive the downlink message WSA from the roadside unit 10, the in-vehicle unit 20B repeatedly transmits an uplink message BSM1 to the roadside unit 10 at regular intervals.

[0038] Assume also that the on-board unit 20A enters the communication region R1. The receiver 212 of the on-board unit 20A receives a downlink message WSA2 from the roadside unit 10 (step S108). The time setting unit 211 of the on-board unit 20A then performs the same process as in step S103 to set the reference time (step S109). The transmitter 213 of the on-board unit 20A checks the slot usage status of the received downlink message WSA2, randomly selects one of the available slots (occupancy flag = OFF), and starts periodic transmission of an uplink message BSM2 containing the identification information of the on-board unit 20A (step S110). Figure 8 is a second diagram illustrating an example of an uplink message according to the first embodiment. Figure 8 illustrates an example of the uplink message BSM2 transmitted by the transmitter 213 of the on-board unit 20A in step S110. In the example of FIG. 8, the transmitter 213 of the in-vehicle device 20A selects slot 5 and transmits the uplink message BSM2 to the roadside device 10.

[0039] When the update unit 112 of the roadside device 10 receives the uplink message BSM2 from the in-vehicle device 20A (step S111), it updates the slot usage status based on the received uplink message BSM2 (step S112). Figure 9 is a third diagram showing an example of a downlink message according to the first embodiment. In the example of Figure 8, the in-vehicle device 20A selects slot 5. Therefore, as shown in Figure 9, the update unit 112 of the roadside device 10 changes the occupation flag of slot 5 in the slot usage status to ON and inputs the identification information (in-vehicle device 20A) received from the in-vehicle device 20A into the identification number. At this point, of the multiple slots, slot 1 and slot 5 are occupied.

[0040] After the slot usage status is updated, the distribution unit 111 of the roadside unit 10 distributes a downlink message WSA3 (FIG. 9) including the updated slot usage status in the next cycle (step S113). After that, like the on-vehicle unit 20B, the on-vehicle unit 20A repeatedly transmits an uplink message BSM2 to the roadside unit 10 at regular intervals while continuing to receive the downlink message WSA from the roadside unit 10.

[0041] (Processing Example 2 of Communication System) Fig. 10 is a second sequence diagram showing a processing example of the wireless communication system according to the first embodiment. Fig. 10 shows a processing example when uplink messages BSM of two in-vehicle devices 20A and 20B collide.

[0042] First, the roadside unit 10 periodically distributes the downlink message WSA1 (step S101). At this point, it is assumed that all slots are empty, as shown in FIG.

[0043] Assume also that on-board units 20A and 20B enter the communication area R1 of roadside unit 10. The receiver 212 of on-board unit 20A receives downlink message WSA1 transmitted by roadside unit 10 (step S202). Upon receiving downlink message WSA1, the time setting unit 211 of on-board unit 20A sets the reference time for the transmission slot of uplink message BSM based on the 1 pps signal output from the GNSS receiver 25 (step S203). Thereafter, the transmitter 213 of on-board unit 20A randomly selects one of the available slots (occupancy flag = OFF) and starts periodic transmission of uplink message BSM3 ( FIG. 11 ) containing identification information of on-board unit 20A in the selected slot (step S204). FIG. 11 is a third diagram illustrating an example of an uplink message according to the first embodiment. As shown in FIG. 11, it is assumed that the transmitting unit 213 of the in-vehicle device 20A selects slot 1.

[0044] Similarly, the receiver 212 of the on-board unit 20B receives the downlink message WSA1 distributed by the roadside unit 10 (step S205). Upon receiving the downlink message WSA1, the time setting unit 211 of the on-board unit 20B sets the reference time of the transmission slot of the uplink message BSM based on the 1 pps signal output from the GNSS receiver 25 (step S206). Thereafter, the transmitter 213 of the on-board unit 20B randomly selects one of the slots that is in an empty state (occupancy flag = OFF) and starts periodically transmitting the uplink message BSM1 ( FIG. 6 ) containing the identification information of the on-board unit 20A in the selected slot (step S207). As shown in FIG. 6, the transmitter 213 of the on-board unit 20B selects slot 1.

[0045] 10, a collision occurs because both the in-vehicle units 20A and 20B specify slot 1 to transmit uplink messages. As a result, the roadside unit 10 cannot receive the uplink messages from the in-vehicle units 20A and 20B. In this case, the roadside unit 10 does not update the slot usage status. Therefore, the distribution unit 111 of the roadside unit 10 transmits the same downlink message WSA1 as that transmitted in step S201 (step S208).

[0046] Next, the receiver 212 of the on-board unit 20A receives the downlink message WSA1 from the roadside unit 10 (step S209). The transmitter 213 of the on-board unit 20A checks the slot usage status of the downlink message WSA1. If the on-board unit's identification information is not registered in the selected slot 1, the transmitter 213 randomly selects an available slot other than slot 1 and starts periodically transmitting an uplink message BSM4 ( FIG. 12 ) containing the on-board unit's identification information in the newly selected slot (step S210). In this way, the on-board unit 20A determines that the roadside unit 10 was unable to receive the on-board unit's uplink message BSM3 because its information is not registered in the slot usage status of the downlink message WSA1 received from the roadside unit 10, and can select a different slot and retry. As shown in FIG. 12, the transmitter 213 of the on-board unit 20A newly selects slot 7.

[0047] Meanwhile, the receiver 212 of the on-board unit 20B receives the downlink message WSA1 from the roadside unit 10 (step S211). The transmitter 213 of the on-board unit 20B checks the slot usage status of the downlink message WSA1, and if the on-board unit's identification information is not registered in the selected slot 1, the transmitter 213 randomly selects an available slot other than slot 1 and starts periodic transmission of an uplink message BSM5 ( FIG. 13 ) to which the on-board unit 20B's identification information is added in the newly selected slot (step S212). As shown in FIG. 13 , the transmitter 213 of the on-board unit 20B newly selects slot 13.

[0048] When the update unit 112 of the roadside device 10 receives the uplink messages BSM4 and BSM5 from the on-vehicle devices 20A and 20B, respectively (step S213), if the slot number usage status included in the uplink messages BSM4 and BSM5 is empty, the update unit 112 updates the slot usage status based on the received uplink messages BSM4 and BSM5 (step S214). FIG. 14 is a fourth diagram showing an example of a downlink message according to the first embodiment. In the example of FIG. 14, the on-vehicle device 20A selects slot 7, and the on-vehicle device 20B selects slot 13. Therefore, as shown in FIG. 14, the update unit 112 of the roadside device 10 changes the occupation flag of slot 7 in the slot usage status to ON and inputs the identification information of the on-vehicle device 20A into the identification number. Furthermore, the update unit 112 changes the occupation flag of slot 13 to ON and inputs the identification information of the on-vehicle device 20B into the identification number.

[0049] After the slot usage status is updated, the distribution unit 111 of the roadside unit 10 distributes a downlink message WSA4 (FIG. 14) including the updated slot usage status in the next cycle (step S215). After that, while continuing to receive the downlink message WSA4 from the roadside unit 10, the in-vehicle units 20A and 20B each repeatedly transmits an uplink message BSM4, BSM5 to the roadside unit 10 at regular intervals.

[0050] (Processing Example 3 of Communication System) Fig. 15 is a third sequence diagram showing a processing example of the wireless communication system according to the first embodiment. Fig. 15 shows a processing example of deleting information of the in-vehicle device 20 from the slot usage status.

[0051] 15, it is assumed that the on-vehicle units 20A and 20B have left the communication area R1 of the roadside unit 10. In this case, when the receivers 212 of the on-vehicle units 20A and 20B no longer receive the downlink message WSA4, they stop periodically transmitting the uplink messages BSM4 and BSM5 (steps S301 and S302).

[0052] Furthermore, if the roadside unit 10 fails to receive uplink messages BSM4 and BSM5 from the on-board units 20A and 20B, whose identification information is registered in the slot usage status, for a certain period of time or longer, the update unit 112 of the roadside unit 10 deletes the registration of the on-board units 20A and 20B and updates the slot usage status (step S304). The update unit 112 of the roadside unit 10 changes the occupancy flag for slot 7 in the slot usage status (FIG. 14) to OFF and deletes the identification information for on-board unit 20A. Similarly, the update unit 112 changes the occupancy flag for slot 13 in the slot usage status (FIG. 14) to OFF and deletes the identification information for on-board unit 20B. In this example, all slots become vacant, as shown in FIG. 5. After the slot usage status is updated, the distribution unit 111 of the roadside unit 10 distributes a downlink message WSA1 (FIG. 5) including the updated slot usage status in the next cycle (step S305). After this, the distribution unit 111 of the roadside device 10 distributes the downlink message WSA1 at regular intervals until it receives an uplink message BSM from a new in-vehicle device 20 that has entered the communication area R1. When a new in-vehicle device 20 enters the communication area R1, the distribution unit 111 determines the slot to be used by the in-vehicle device 20 according to the sequence of FIG. 4 or FIG. 10.

[0053] (Operation and Effect) As described above, the wireless communication system 1 according to this embodiment includes a roadside device 10 and an in-vehicle device 20 mounted on a vehicle V. The roadside device 10 includes a distribution unit 111 that periodically distributes a downlink message WSA including the usage status of each of a plurality of slots obtained by dividing the transmission period of an uplink message BSM of the in-vehicle device 20 into predetermined time intervals, and an update unit 112 that updates the slot usage status based on the uplink message BSM received from the in-vehicle device 20. The in-vehicle device 20 includes a time setting unit 211 that determines a reference time based on a satellite signal, a reception unit 212 that receives the downlink message WSA from the roadside device 10, and a transmission unit 213 that randomly selects a slot that is not occupied by another in-vehicle device based on the slot usage status included in the downlink message WSA, and transmits the uplink message BSM to the roadside device 10 in the selected slot.

[0054] In this way, each in-vehicle device 20 selects one of multiple slots to send an uplink message BSM, thereby preventing collisions caused by multiple in-vehicle devices simultaneously transmitting messages in the wireless communication system 1. Furthermore, by each in-vehicle device 20 randomly selecting an available slot rather than in order of slot number, the possibility of the slot being used by another in-vehicle device 20 entering the communication area R1 of the roadside device 10 at the same time can be reduced.

[0055] Furthermore, if the transmitting unit 213 of the in-vehicle device 20 is not registered in the selected slot in the downlink message WSA received after selecting the slot, the transmitting unit 213 newly selects another slot that is not occupied by another in-vehicle device 20 and transmits the uplink message BSM to the roadside device 10 in the newly selected slot.

[0056] In this way, even if the same slot is selected between the in-vehicle devices 20 and a collision occurs between the transmitted uplink messages BSM, the wireless communication system 1 can know that the slot was not occupied on the in-vehicle device 20 side based on the next received downlink message WSA. In this case, the in-vehicle device 20 can select a new slot and retry transmitting the uplink message BSM, thereby preventing the uplink message BSM from being lost due to a message collision. As a result, the occurrence of the hidden terminal problem can be prevented.

[0057] Furthermore, if the update unit 112 of the roadside device 10 does not receive an uplink message BSM from the vehicle-mounted device 20 for a certain period of time or more, it updates the slot occupied by the vehicle-mounted device 20 to an empty state.

[0058] By doing this, if the wireless communication system 1 is unable to receive the uplink message BSM due to reasons such as the vehicle-mounted device 20 leaving the communication area R1 of the roadside device 10, the wireless communication system 1 can quickly release this slot for another vehicle-mounted device 20.

[0059] Other Embodiments Although one embodiment has been described in detail above with reference to the drawings, the specific configuration is not limited to the above, and various design modifications are possible. That is, in other embodiments, the order of the above-described processes may be changed as appropriate. Furthermore, some processes may be executed in parallel.

[0060] <Additional Notes> The wireless communication system and wireless communication method described in the above-described embodiment can be understood, for example, as follows.

[0061] (1) According to a first aspect, the wireless communication system 1 includes a roadside device 10 and an in-vehicle device 20 mounted on a vehicle V. The roadside device 10 includes a distribution unit 111 that periodically distributes a downlink message WSA including the usage status of each of a plurality of slots obtained by dividing the transmission period of the uplink message BSM of the in-vehicle device 20 into predetermined time intervals, and an update unit 112 that updates the slot usage status based on the uplink message BSM received from the in-vehicle device 20. The in-vehicle device 20 includes a time setting unit 211 that determines a reference time based on a satellite signal, a receiving unit 212 that receives the downlink message WSA from the roadside device 10, and a transmitting unit 213 that randomly selects a slot that is not occupied by another in-vehicle device 20 based on the usage status of the slot included in the downlink message WSA, and transmits the uplink message BSM to the roadside device 10 in the selected slot.

[0062] In this way, each in-vehicle device 20 selects one of multiple slots to send an uplink message BSM, thereby preventing collisions caused by multiple in-vehicle devices simultaneously transmitting messages in the wireless communication system 1. Furthermore, by each in-vehicle device 20 randomly selecting an available slot rather than in order of slot number, the possibility of the slot being used by another in-vehicle device 20 entering the communication area R1 of the roadside device 10 at the same time can be reduced.

[0063] (2) According to the second aspect, in the wireless communication system 1 relating to the first aspect, if the transmitting unit 213 of the vehicle-mounted device 20 is not registered in the slot selected in the downlink message WSA received after selecting the slot, the transmitting unit 213 newly selects another slot that is not occupied by another vehicle-mounted device 20 and transmits the uplink message BSM to the roadside device 10 in the newly selected slot.

[0064] In this way, even if the same slot is selected between the in-vehicle devices 20 and a collision occurs between the transmitted uplink messages BSM, the wireless communication system 1 can know that the slot was not occupied on the in-vehicle device 20 side based on the next received downlink message WSA. In this case, the in-vehicle device 20 can select a new slot and retry transmitting the uplink message BSM, thereby preventing the uplink message BSM from being lost due to a message collision. As a result, the occurrence of the hidden terminal problem can be prevented.

[0065] (3) According to the third aspect, in the wireless communication system 1 relating to the first or second aspect, if no uplink message BSM is received from the vehicle-mounted device 20 for a certain period of time or more, the update unit 112 of the roadside device 10 updates the slot occupied by the vehicle-mounted device 20 to an empty state.

[0066] By doing this, if the wireless communication system 1 is unable to receive the uplink message BSM due to reasons such as the vehicle-mounted device 20 leaving the communication area R1 of the roadside device 10, the wireless communication system 1 can quickly release this slot for another vehicle-mounted device 20.

[0067] (4) According to a fourth aspect, a wireless communication method is a wireless communication method using a roadside device 10 and an in-vehicle device 20 mounted on a vehicle V, and includes the steps of: the roadside device 10 periodically distributing a downlink message WSA including the usage status of each of a plurality of slots obtained by dividing the transmission period of the in-vehicle device 20's uplink message BSM into predetermined time intervals; the roadside device 10 updating the slot usage status based on the uplink message BSM received from the in-vehicle device 20; the in-vehicle device 20 determining a reference time based on a satellite signal; the in-vehicle device 20 receiving the downlink message WSA from the roadside device 10; and the in-vehicle device 20 randomly selecting a slot that is not occupied by another in-vehicle device 20 based on the slot usage status included in the downlink message WSA, and transmitting the uplink message BSM to the roadside device 10 in the selected slot.

[0068] According to the above aspect, in road-to-vehicle communication between a roadside device and a plurality of in-vehicle devices, it is possible to prevent collisions from occurring due to the plurality of in-vehicle devices simultaneously transmitting messages.

[0069] REFERENCE SIGNS LIST 1 Wireless communication system 10 Roadside device 11 Processor 111 Distribution unit 112 Update unit 12 Memory 13 Storage 13 Slot 14 Communication antenna 20 On-vehicle device 20, 20A, 20B On-vehicle device 21 Processor 211 Time setting unit 212 Receiving unit 213 Transmitting unit 22 Memory 23 Storage 24 Communication antenna 25 GNSS receiving unit V Vehicle

Claims

1. A wireless communication system comprising a roadside device and an on-board device mounted on a vehicle, wherein the roadside device comprises: a distribution unit that periodically distributes downlink messages including the usage status of each of a plurality of slots obtained by dividing the transmission period of the on-board device's uplink messages into predetermined time intervals; and an update unit that updates the usage status of the slots based on the uplink messages received from the on-board device, and the on-board device comprises: a time setting unit that determines a reference time based on a satellite signal; a receiving unit that receives the downlink message from the roadside device; and a transmitting unit that randomly selects a slot that is not occupied by another on-board device based on the usage status of the slots included in the downlink message, and transmits an uplink message to the roadside device in the selected slot.

2. The wireless communication system according to claim 1, wherein the transmitting unit of the in-vehicle device, if the in-vehicle device is not registered in the selected slot in the downlink message received after selecting the slot, newly selects another slot that is not occupied by another in-vehicle device and transmits an uplink message to the roadside device in the newly selected slot.

3. The wireless communication system according to claim 1 or 2, wherein the update unit of the roadside device updates the slot occupied by the vehicle-mounted device to an empty state when no uplink message is received from the vehicle-mounted device for a certain period of time or more.

4. A wireless communication method using a roadside device and an on-board device mounted on a vehicle, comprising the steps of: the roadside device periodically distributing a downlink message including the usage status of each of a plurality of slots obtained by dividing the transmission cycle of the on-board device's uplink message into predetermined time intervals; the roadside device updating the usage status of the slots based on the uplink message received from the on-board device; the on-board device determining a reference time based on a satellite signal; the on-board device receiving the downlink message from the roadside device; and the on-board device randomly selecting a slot that is not occupied by another on-board device based on the usage status of the slots included in the downlink message, and transmitting an uplink message to the roadside device in the selected slot.

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