JP6136984B2 - In-vehicle wireless communication device, wireless communication method, and program - Google Patents

Description

  The present invention relates to an in-vehicle wireless communication device and a wireless communication method performed by the in-vehicle wireless communication device.

In recent years, research and development of advanced driving support systems (automatic driving systems) in which vehicles (on-vehicle wireless communication devices) directly exchange control information by wireless communication without being operated by a driver has been performed. As a standard for inter-vehicle communication, the Association of Radio Industries and Businesses (ARIB) has issued ARIB STD-T109. In this standard, CSMA / CA (Carrier Sense Multiple Access / Collision Avoidance) is adopted as a media access method. In the CSMA / CA system, in a communication environment where vehicles are crowded, communication quality is degraded due to communication congestion and hidden terminal problems. Therefore, the target of the communication success rate must be set to about 80%, but more reliable communication is desired in order to realize automatic driving.

If a time division multiple access (TDMA) method is adopted as a communication method, it is expected to improve communication congestion and communication quality. However, in inter-vehicle communication, there is no device that performs centralized management of communication resources, such as a mobile phone base station, and thus a device is required to realize the TDMA scheme.

  In Patent Document 1, each terminal transmits a time slot determined as not being used by any terminal as an empty slot list when communicating in an empty time slot. As a result, information on empty slots is shared between terminals.

JP 2004-274321 A

  The method of Patent Document 1 requires sharing of empty slots, and is difficult to apply to a large-scale inter-vehicle communication system. In a large-scale vehicle-to-vehicle communication system, it is necessary to perform TDMA radio resource allocation by a method with as little overhead as possible.

  The present invention has been made in consideration of the above-described problems, and an object of the present invention is to provide a wireless communication system capable of realizing time division multiple access with little overhead in a vehicle-to-vehicle communication system.

  An in-vehicle wireless communication apparatus according to the present invention is an in-vehicle wireless communication apparatus that performs wireless communication by a time division multiple access method, and performs time sense to determine a time slot to be used for communication, Based on the information received by the receiving means, the transmitting means for transmitting the transmission data in the time slot determined by the time slot determining means, the receiving means for receiving a plurality of time slots, and the communication collision based on the information received by the receiving means Detecting means for detecting the time slot in which the error occurs, and notifying means for notifying the time slot detected by the detecting means.

  According to such a configuration, time division multiple access with a small overhead can be realized, and efficient communication can be realized. In this method, there is room for a communication collision, but when a communication collision occurs, a notification to that effect is made immediately, so that the in-vehicle wireless communication device that made the transmission knows the occurrence of the collision. it can.

  It is also preferable that the in-vehicle wireless communication device according to the present invention includes transmission information including collision information that identifies a time slot in which a collision occurs. The information specifying the time slot can be, for example, a time slot number. In this way, it is possible to realize a system that requires a small amount of data for the collision notification and has a low overhead.

  Further, in the present invention, when it is determined that a collision has occurred in the time slot used for transmission by the transmission unit based on the collision information included in the data received by the reception unit, It is preferable that the time slot determination means re-determines a time slot to be used for communication. In this way, the communication conflict can be resolved by stopping the transmission in the time slot in which the collision occurs and transmitting in the new time slot.

  At this time, when the received signal strength of the data including the collision information notifying that a collision has occurred in the transmission by the own apparatus is equal to or higher than the first threshold, the time slot used for communication is re-determined, It is also preferable not to change the time slot used for communication when it is less than the first threshold. The low received signal strength of data including collision information means that a collision notification is being made from a distant on-vehicle wireless communication device. This is because when a collision is notified only from an in-vehicle wireless communication device located far away, the collision is not detected near the device itself, and therefore it is not always necessary to change the transmission time slot.

  It is also preferable that the detection means in the present invention determines that a communication collision has occurred in the time slot when demodulation has failed in the time slot that has been received up to the previous frame.

  Further, the detection means in the present invention has a communication collision in the time slot when the signal-to-interference ratio (SIR) in the reception time slot is equal to or less than the second threshold. It is also preferable to judge. In this case, before the signal can be demodulated in this time slot, it is possible to detect the presence of the interference signal and notify the surroundings of the collision. At this time, it is also preferable to determine the second threshold according to the number of in-vehicle wireless communication devices existing around. In particular, it is preferable to set the second threshold value smaller as the number of in-vehicle wireless communication devices present in the vicinity increases. It is necessary to increase the communication capacity in the system as the number of in-vehicle wireless communication devices existing in the surrounding area increases. By reducing the second threshold, collision detection can be reduced and the communication capacity of the system can be increased.

  Moreover, it is also preferable that the detection means in the present invention detects a communication collision for a time slot whose received signal strength is equal to or greater than a third threshold value. By making only the time slot having a received signal strength equal to or higher than the threshold value as the target of collision detection, collision detection can be performed only for communication performed by the in-vehicle wireless communication apparatus near the own apparatus. At this time, it is also preferable to determine the third threshold according to the number of in-vehicle wireless communication devices existing around. In particular, it is preferable to set the third threshold value larger as the number of in-vehicle wireless communication devices existing in the vicinity increases. It is necessary to increase the communication capacity in the system as the number of in-vehicle wireless communication devices existing in the surrounding area increases. By increasing the third threshold, collision detection can be reduced and the communication capacity of the system can be increased.

  Moreover, it is also preferable that the time slot determination means in the present invention determines the carrier sense level according to the number of in-vehicle wireless communication devices existing in the vicinity. In particular, it is preferable to set the carrier sense level higher as the number of in-vehicle wireless communication devices present in the surrounding area increases. It is necessary to increase the communication capacity in the system as the number of in-vehicle wireless communication devices existing in the surrounding area increases. By increasing the carrier sense level, the number of empty slots can be substantially increased, and the communication capacity can be increased. If the carrier sense level is increased, the collision between the strong signal and the weak signal increases somewhat, but even in this case, communication by the strong signal can be secured and communication between nearby vehicles is established.

  Further, in the present invention, when the detection unit detects that a communication collision occurs in a plurality of time slots, the notification unit performs a predetermined operation from a plurality of time slots in which the collision occurs. Preferably, the collision is notified for a predetermined number of time slots selected according to the criteria. This is because notifying all time slots in which a collision is detected increases the amount of data for notification and increases overhead. The number of time slots for notifying the collision (the predetermined number) may be set as appropriate according to the request of the system, but may be 1 to 3, for example.

  It should be noted that various criteria can be adopted as a selection criterion (the predetermined criterion) as to which slot is notified from among a plurality of time slots in which a collision occurs. For example, it is conceivable to preferentially notify about time slots having a large number of consecutive cases when collisions occur continuously over a plurality of frames. It is also conceivable to preferentially notify time slots having a small signal-to-interference ratio. It is also conceivable to preferentially notify a time slot with a high received signal strength. In addition, it is conceivable to preferentially notify the time slot having a received signal strength of a predetermined value or higher with a high interference signal strength. Further, it is conceivable to preferentially notify the received signal strength and other time slots having a predetermined interference value or less that have a high interference signal strength. It is also conceivable to preferentially notify a time slot with a large or small time slot number. Moreover, you may determine at random. When determining randomly, it may be determined randomly from all the time slots in which a collision is detected, or may be determined randomly from the results of narrowing down according to any of the above criteria .

  Further, in the present invention, the number of in-vehicle wireless communication devices (in the vicinity) based on the number of receptions of data transmitted from the surrounding in-vehicle wireless communication devices per unit time (for example, one frame) by the receiving means. The number of vehicles) can be determined. Thereby, various threshold values can be changed according to the number of in-vehicle wireless communication devices.

  In the present invention, it is preferable that the transmission means periodically transmits transmission data including at least one of a position, a moving speed, and a moving direction of a vehicle on which the in-vehicle wireless communication device is mounted. In this way, in each in-vehicle wireless communication device, it is possible to grasp information such as the position, moving speed, and moving direction of the in-vehicle wireless communication device existing in the vicinity.

  The in-vehicle wireless communication apparatus according to the present invention further includes a map information storage unit that stores map information in which one or a plurality of reference positions are defined, and a position information acquisition unit that acquires position information. With regard to such a reference position, it is also preferable that the notification means notifies the time slot in which a collision has occurred when the reference position is present in a place closer to the vehicle-mounted wireless communication apparatus existing in the vicinity. In this way, since only some in-vehicle wireless communication devices perform collision notification, overhead for collision notification can be reduced as a whole system. Note that, for example, the center position of an intersection is preferably used as the reference position.

  Further, the time slot determining means in the present invention acquires the moving direction of the vehicle on which the in-vehicle wireless communication device is mounted, and the time slot used for communication preferentially from the time slots associated with the moving direction. It is also preferable to determine In this way, in-vehicle wireless communication devices traveling in the same direction will preferentially use the same group of time slots, and in-vehicle wireless communication devices traveling in the opposite direction or crossing direction will have the same group of time slots. It can be avoided and the communication quality of the entire system is improved.

  Further, the time slot determination means in the present invention is a time slot used for communication from a time slot other than the time slot that is notified that a collision has occurred by the collision information included in the data received by the receiving means. Preferably, the slot is determined. Even if it is determined that the time slot is empty as a result of the carrier sense, a collision may be notified. Since the time slot in which the collision is notified means that it is used by another in-vehicle wireless communication device, the communication collision can be avoided beforehand by avoiding the use of this time slot.

  Further, the time slot determination means in the present invention is a time slot randomly selected from time slots determined to be free as a result of carrier sense, or a time slot determined to be free as a result of carrier sense. Of these, it is preferable to determine a time slot randomly selected from time slots having a received signal strength equal to or lower than a predetermined strength as a time slot to be used for communication. In this way, by selecting at random, it is possible to prevent a plurality of in-vehicle wireless communication devices from selecting the same time slot at the same time and starting communication. In addition, by selecting at random from time slots with low received signal strength, it is possible to prevent duplicate selection with other in-vehicle wireless communication devices while selecting a time slot with good communication quality.

  Further, the in-vehicle wireless communication device according to the present invention may be a wireless communication device provided in a vehicle, or may be a portable wireless communication device that can be carried on a vehicle.

  The present invention can also be understood as an in-vehicle communication device provided with at least a part of the above means. Moreover, this invention can also be grasped | ascertained as a vehicle-mounted radio | wireless communications system containing two or more said vehicle-mounted communication apparatuses. The present invention can also be understood as a wireless communication method that executes at least a part of the above processing. The present invention can also be understood as a computer program for causing a computer to execute this method, or a computer-readable storage medium in which this computer program is stored non-temporarily. Each of the above means and processes can be combined with each other as much as possible to constitute the present invention.

  According to the present invention, time division multiple access can be realized with a small overhead in a vehicle-to-vehicle communication system.

The figure which shows the flame | frame structure used with the vehicle-to-vehicle communication system concerning this embodiment. The schematic diagram explaining generation | occurrence | production of the collision of communication in the vehicle-to-vehicle communication system concerning this embodiment. The figure explaining the generation | occurrence | production and detection of a communication collision, the notification of a communication collision, (C) change of a time slot, respectively in this embodiment. The functional block diagram which shows the structure of the vehicle-mounted radio | wireless communication apparatus concerning this embodiment. The flowchart which shows the flow of the whole process of the radio | wireless communication process concerning this embodiment. (A) The flowchart which shows the flow of the collision detection and notification process in this embodiment, and (B) (C) The figure which shows the method of detecting the collision of communication. The flowchart which shows the flow of the collision notification detection and time slot change process in this embodiment. The flowchart which shows the flow of the time synchronous process in this embodiment. The figure explaining the threshold value in collision detection. The figure explaining the relationship between a threshold value parameter, a collision detection area, and a collision notification area. The figure explaining the modification of a collision detection and notification process. The figure explaining the modification of collision notification detection and a time slot change process. The figure explaining the modification of time slot allocation.

<Overview>
The inter-vehicle communication system according to the present embodiment is an autonomous distributed communication system, and is a system that does not have a management device that centrally manages communication resources. The inter-vehicle communication system according to the present embodiment employs a time division multiple access (TDMA) system as a radio access system and provides a fixed-length time slot. Each vehicle performs carrier sense and selects a time slot to be used autonomously while synchronizing time with each other, and periodically performs broadcast communication. When a plurality of vehicles select the same time slot, a communication collision occurs. In this embodiment, a vehicle that detects such a communication collision has a function of notifying other vehicles of the collision. Furthermore, when the occurrence of a collision is notified from another vehicle, a function of changing the time slot used in the host vehicle is provided. By providing such a time-synchronous inter-vehicle communication system with a collision detection function and a dynamic time slot allocation function, efficient and reliable communication can be realized.

  In the inter-vehicle communication system according to the present embodiment, each vehicle uses a single channel of a predetermined band (for example, a UHF band such as 700 MHz band) for each 100 ms (milliseconds). It is assumed that information such as speed information / control information (hereinafter referred to as vehicle information) is broadcast. At this time, by transmitting the same data a plurality of times, it is possible to reduce the requirements for the packet error rate per transmission. By repeatedly transmitting the same data N times, one packet error rate requirement condition can be raised to the 1 / Nth power.

  In this embodiment, as shown in FIG. 1, a TDMA system with a superframe of 100 ms is adopted, and two subframes of 50 ms are provided in one superframe. In one subframe, 1000 time slots of 50 μs (microseconds) are set. Each vehicle selects an empty slot for each of the two subframes, and repeatedly transmits the same information within one superframe. A packet error rate is reduced by repeatedly transmitting the same data twice.

  In this embodiment, a plurality of vehicles may collide using the same time slot. For example, in the situation shown in FIG. 2, since the vehicle A is located outside the communication range RC of the vehicle C, it is determined that the time slot used by the vehicle C is empty, and the same time slot is used. (Hidden terminal problem). A communication collision between the vehicle A and the vehicle C can be detected by the vehicle B located within the communication ranges RA and RC of both vehicles, and the vehicle B immediately notifies the collision when a communication collision is detected. . When the vehicle A and the vehicle C are notified of the collision, the collision is avoided by changing the time slot to be used.

FIG. 3 is a diagram for explaining a communication flow from the occurrence of a collision to the elimination thereof. FIG. 3A is a diagram illustrating a state of a frame (first frame) in which a communication collision has occurred. In subframe 1, vehicle A and vehicle C use the same time slot # 2, and a communication collision has occurred. The vehicle B monitors whether a communication collision has occurred in each time slot, and detects that a collision has occurred in the time slot # 2 of the subframe 1. Then, as shown in FIG. 3B, the vehicle B has the collision information that “the collision has occurred in the time slot # 2 of the subframe 1” in the transmission data of the own vehicle of the next frame (second frame). Send including The vehicles A and C can know that the communication from the own vehicle is colliding by receiving the collision information regarding the time slot used by the own vehicle. Therefore, as shown in FIG. 3C, in the next frame (third frame) that has received the collision notification, the time slot used in subframe 1 is changed and transmitted. By doing so, the collision can be resolved immediately.

<Configuration>
With reference to FIG. 4, the structure of the vehicle-mounted radio | wireless communication apparatus 100 mounted in the vehicle which comprises the vehicle-to-vehicle communication system of this embodiment is demonstrated. As illustrated in FIG. 4, the in-vehicle wireless communication device 100 includes a reception unit 111, a transmission unit 112, a carrier sense unit 121, a time slot determination unit 122, a reception processing unit 123, a collision detection unit 124, a collision notification unit 125, transmission data. A generation unit 126 is included. These functional units may be realized by an electric circuit or an electronic circuit, or a CPU (Central Processing Unit), a DSP (Digital Signal Processor), or an FPGA (Field Programmable Gate Array) executes a program stored in a storage device. It may be realized by doing. That is, the in-vehicle wireless communication device 100 may be realized by a combination of a computer and software, may be realized by a hardware circuit, or may be realized by a combination thereof.

The reception unit 111 performs reception radio processing (down-conversion, analog / digital (A / D) conversion, etc.) on a radio signal received in the reception band via the antenna, and the obtained signal is used as the carrier sense unit 121. And output to the reception processing unit 123 and the collision detection unit 124.
The transmission unit 112 performs transmission radio processing (up-conversion, digital / analog (D / A) conversion, etc.) on the signal generated by the transmission data generation unit 126 and transmits the signal via an antenna. The transmission unit 112 performs transmission in the time slot determined by the time slot determination unit 122. Note that time synchronization between in-vehicle wireless communication devices is basically performed based on GPS time (absolute time).

  The carrier sense unit 121 detects the reception signal level of the reception signal for each time slot. Here, the carrier sense unit 121 includes an RSSI circuit (Received Signal Strength Indicator). The carrier sense unit 121 determines that the time slot is vacant (available) when the received signal strength is equal to or lower than a predetermined threshold (carrier sense level). The carrier sense level may be a fixed value (for example, −85 dBm) or may be changed according to the surrounding situation. For example, in a situation where the number of surrounding vehicles is large or the communication quality is poor, the carrier sense level may be set large (for example, −60 dBm). By doing so, the communicable range is narrowed, but the number of time slots determined to be free increases, and a communication opportunity can be obtained.

Based on the carrier sense result of the carrier sense unit 121, the time slot determination unit 122 determines a time slot that the device itself uses for communication. For example, the time slot determination unit 122 randomly determines one time slot from the time slots determined to be free by the carrier sense unit 121. At this time, time slots are independently selected for the two subframes (subframe 1 and subframe 2).
Note that the time slot determination unit 122 selects a predetermined number of time slots having low received signal strength from the available time slots, and randomly determines a time slot to be used for communication from among them. It may be. However, the time slot selection method is preferably not stochastic but probabilistic. This is to prevent the plurality of vehicles from selecting the same time slot when a plurality of vehicles select the time slot at the same timing.

  The reception processing unit 123 performs demodulation processing or the like on the signal input from the reception unit 111 and acquires information transmitted from another vehicle. Information (such as vehicle information included in the reception data) obtained by demodulation by the reception processing unit 123 is passed to an upper layer. As will be described later, the received data may include collision information that identifies a time slot in which a communication collision has occurred. The reception processing unit 123 determines whether any received data includes collision information related to the time slot used by the own device for transmission, and if such notification is included, the time processing unit 123 122, the time slot used for communication is determined again.

  The collision detection unit 124 is a functional unit that detects a communication collision in any time slot based on a signal received from the reception unit 111. A method for detecting a communication collision will be described later. The time slot in which a collision is detected by the collision detection unit 124 is input to the collision notification unit 125.

  The collision notification unit 125 determines whether or not the own device transmits a collision notification and for which time slot the collision notification is performed when the collision notification is transmitted. If a collision is detected, a collision notification may always be sent, but if all vehicles send a collision notification, it is considered that the efficiency of communication will be reduced, so only some vehicles will send a collision notification. It is also preferable to make it. In addition, when a collision is detected for a plurality of time slots, a collision notification for all the time slots may be transmitted. However, since this consumes communication capacity, only a part of the time slots according to a predetermined standard is used. It is also preferable to send a collision notification for. The details of determining whether to transmit a collision notification and determining which time slot to transmit a collision notification will be described in detail later. When the collision notification unit 125 determines to perform a collision notification for a certain time slot, the collision notification unit 125 notifies the transmission data generation unit to that effect.

  The transmission data generation unit 126 acquires at least one of vehicle information such as vehicle ID, position information, movement speed, movement direction, and vehicle control information from the vehicle system, and generates transmission data including the vehicle information. In this embodiment, since new vehicle information is transmitted every 100 ms, the transmission data generation unit 126 acquires vehicle information from the vehicle system every 100 ms and generates transmission data. Note that, when the collision notification unit 125 notifies that a collision notification is made for a certain time slot, the collision notification is included in the transmission data. The collision notification may be arbitrary information as long as it is information that can identify the time slot in which the collision occurs. In this embodiment, the time slot number itself is used as the collision information.

  Note that the in-vehicle wireless communication device 100 according to the present embodiment receives position information and time information from the GPS device 200, can grasp the current position, and is time with other vehicles based on the GPS time. Synchronized. Further, the in-vehicle wireless communication device 100 receives information such as map information and destination information from the navigation device 300, and can perform processing based on the map information.

<Processing>
(1. Overall processing)
First, the overall processing generally performed by the in-vehicle wireless communication device 100 will be described with reference to the flowchart of FIG. In the following description, the in-vehicle wireless communication device and the vehicle on which the device is mounted are regarded as the same, and the vehicle communicates.
When the in-vehicle wireless communication device 100 starts system operation (S10), first, time synchronization processing is performed based on time information obtained from the GPS device 200 (S11). Then, the carrier sense unit 121 performs carrier sense processing for each time slot, and investigates the usage status of the time slot by surrounding vehicles (S12). Specifically, the carrier sense unit 121 determines time slots whose received signal strength is continuously equal to or lower than the carrier sense level for a predetermined number of frames as empty slots, and determines other time slots as active slots. To do.

  As described above, since the same data is transmitted in each of subframe 1 and subframe 2 in this embodiment, it is determined whether there are empty slots in both subframe 1 and subframe 2 (S13). Here, when there is no empty slot in any of the subframes (S13-NO), it waits for a certain time (S14) and performs carrier sense again. When there are empty slots in both subframe 1 and subframe 2 (S13-YES), time slot determination unit 122 randomly selects a time slot to be used for communication from the empty slots (S15). Here, for each of subframe 1 and subframe 2, it is preferable to randomly determine the time slot to be used, but it is also possible to select subframe 1 and subframe 2 to use different time slots. preferable. Further, when there is no empty slot for one of subframe 1 and subframe 2, transmission may be started only in the subframe with the empty space.

  Once a time slot to be used for communication is determined, periodic vehicle information broadcast transmission processing using the selected time slot, collision detection and collision notification processing in communication with other vehicles, presence of collision notification from other vehicles And the time slot changing process when the collision notification is received are repeatedly executed (S16). Details of these processes will be described later.

(2. Collision detection / notification processing)
The collision detection and collision notification processing in step S16 will be described with reference to FIG. FIG. 6A is a flowchart showing the flow of collision detection / notification processing, and FIGS. 6B and 6C are diagrams for explaining a collision detection method.

The collision detection unit 124 determines whether or not a communication collision has occurred for all time slots. In other words, the collision detection unit 124 determines whether there is a time slot that a plurality of vehicles are simultaneously using for communication (S20).
The collision detection can be performed as follows, for example. The first method is a method of determining that a collision has occurred in a time slot when reception has failed in a time slot in which reception (demodulation) has been performed up to the immediately preceding frame. For example, as shown in FIG. 6B, reception (demodulation) was possible in time slot # 2 in the previous frame and previous frame, and reception (demodulation) was not possible in time slot # 2 in the current frame. In this case, it can be determined that a collision has occurred in time slot # 2. In this case, it is also preferable to impose a condition that the received signal strength in the time slot is a predetermined value or more. When this method is adopted, it is necessary to store in which time slot the latest frame can be received.
In the second method, a collision occurs when a signal-to-interference ratio (SIR) of a received signal in each reception time slot is equal to or less than a predetermined threshold (second threshold). This is a determination method (see FIG. 6C). As this threshold value, a predetermined value (for example, 10 dB or 20 dB) may be used, or may be variable according to the number of surrounding vehicles and communication conditions. In calculating the SIR, the signal strength S can be calculated from the received signal strength RSSI. Further, the signal intensity I of the interference wave can be calculated from an error vector amplitude EVM (Error Vector Magnitude), or can be calculated from a signal obtained by subtracting a desired signal demodulated from a received signal. At this time, it is also preferable to impose a condition that the signal strength S is a predetermined value or more.
Note that only one of the first method and the second method may be employed, or both may be employed. However, the present invention does not limit the collision detection method, and may detect a communication collision by an arbitrary technique.

  If no collision of communication has occurred in any time slot as a result of collision detection (S21-NO), the processing in the current frame is terminated. On the other hand, when a communication collision occurs in any time slot (S21-YES), the collision notification unit 125 selects one time slot from the time slots in which the collision occurs ( In step S22, the transmission data generation unit 126 is instructed to transmit collision information regarding this time slot in the next frame. The transmission data generation unit 126 generates transmission data including the time slot number selected in step S22 as collision information at the transmission timing of the host vehicle in the next frame, and broadcasts the transmission data (S23).

  Here, although it has been described that a collision notification is always performed when a collision is detected, whether to perform a collision notification may be determined according to a predetermined condition. Further, one time slot to be notified is selected at random, but a time slot to be notified may be selected according to a predetermined standard other than this. Further, the number of time slots to be notified is not limited to one and may be plural. Details of the modified example regarding the notification will be described later.

(3. Collision notification detection / slot change processing)
The detection of the presence / absence of a collision notification from another vehicle and the time slot change process when receiving a collision notification in step S16 of the flowchart of FIG. 5 will be described with reference to the flowchart of FIG.

  When the reception processing unit 123 receives data from another vehicle (S30), it is determined whether or not the received data includes collision information regarding the time slot used by the host vehicle for transmission (S31). If such collision information does not exist in any received data (S31-NO), the processing in the current frame is terminated. On the other hand, when there is collision information regarding the time slot used by the own vehicle for transmission in any of the received data (S31-YES), the time slot used for transmission is changed. Based on the result of the carrier sense unit 121, it is determined whether there is an empty slot in the same subframe as the time slot to which the collision is notified (S32). Wait until it is possible (S33). When there is an empty slot (S32-YES), the time slot determining unit 122 randomly selects a time slot to be used for communication from the empty slots (S34). In the subsequent frame, broadcast transmission of transmission data is performed using the time slot selected in step S34.

(4. Time synchronization processing)
In the flowchart of FIG. 5, the time synchronization process is shown to be performed only when the system operation is started, but actually the time synchronization process is periodically executed while the system is operating. Here, the details of the time synchronization processing will be described with reference to the flowchart of FIG.

When the time simultaneous processing starts (S40), it is first determined whether absolute time information (GPS time information) can be received from the GPS device 200 (S41). If absolute time information is available (S41-YES), time synchronization with other vehicles can be performed using absolute time (S42). On the other hand, when the absolute time information cannot be used (S41-NO), it is determined whether transmission information from another vehicle can be received (S43). When transmission information from another vehicle can be received (S43—YES), time synchronization is performed in accordance with the transmission timing of this vehicle (S44). When absolute time cannot be received and transmission information from other vehicles cannot be received (S41-NO and S43)
-NO) waits for a predetermined time until any of these can be received (S45).

<Effect of this embodiment>
According to the present embodiment, in a vehicle-to-vehicle communication system without a centralized control device, a time slot to be used autonomously and distributedly can be determined based on carrier sense, and TDMA wireless communication can be realized. Unlike the CSMA / CA method, in which the communication efficiency decreases as the number of communication vehicles increases, the TDMA method can realize efficient communication even when the number of communication vehicles increases.

  In the present embodiment, it is impossible to completely prevent a plurality of vehicles from using the same time slot (communication collision), but when a communication collision occurs, a notification is immediately sent from another vehicle. The collision can be avoided by changing the time slot to be used.

<Modification>
The above description exemplifies the embodiment of the present invention. The present invention should not be construed as being limited to the above embodiment, and various modifications can be made within the scope of the technical idea of the present invention. Deformation is possible.

(Modification of collision detection)
In the above description of the embodiment, the example in which the collision is detected based on the signal-to-interference wave ratio (SIR) has been described. However, it is also preferable to detect the collision based on not only the SIR but also the SIR and the received signal strength. Specifically, in the collision detection process S20, the collision detection unit 124 generates a communication collision when the SIR is equal to or lower than the threshold ThSI and the received signal strength is equal to or higher than the threshold ThS, as shown in FIG. You may judge that you did.

FIG. 10A shows an example of a positional relationship when the vehicle A detects a collision of communication between the vehicle B and the vehicle C. In this example, the distance between AB is closer than the distance between AC. Therefore, the signal transmitted from the vehicle C becomes an interference signal with respect to the signal transmitted from the vehicle B. That is, the received signal strength is related to the distance between AB, and the strength of the interference signal is related to the difference between the distance between AB and the distance between AC.
If the threshold ThS of the received signal strength is increased, a distant vehicle will not detect a collision for communication. That is, the condition of being equal to or greater than the threshold ThS means that the transmission source vehicle B is located within a distance R (a distance corresponding to the threshold ThS) from the own vehicle. Therefore, collision detection and notification are performed only when the transmission source vehicle is located in the collision notification area 1001 within the distance R from the own vehicle as shown in FIG.
Further, if the SIR threshold ThSIR is increased, a very strong signal and a very weak signal are also considered as collisions. Conversely, if the SIR threshold ThSIR is increased, the collision is not considered in such a case. The condition that the SIR is equal to or less than the threshold ThSI means that a difference in distance between the two vehicles using the same time slot and the own vehicle is a distance d (a distance according to the threshold ThSIR). Therefore, it is determined that the vehicle C has collided with the collision detection area 1002 shown in FIG. 10C, and if it is outside the collision detection area 1002, it is determined that there is no collision. Note that the distance d serving as a collision detection threshold is not determined solely by the SIR threshold ThSIR, but also varies depending on the distance between the vehicles AB (that is, the received signal strength).

The threshold values ThS and ThSIR are preferably variable according to the number of surrounding vehicles, communication conditions, and the like.
For example, it is preferable to increase the threshold ThS as the number of surrounding vehicles increases. As the threshold ThS is increased, the collision notification area 1001 is narrowed to notify a collision only for a nearby vehicle, and not notify a collision for communication of a distant vehicle. Since the demand for communication quality changes according to the communication distance, signals from distant vehicles (weak signals)
On the other hand, since it is considered that the need to notify the collision is low, it can be said that such a threshold setting is effective.
It is also preferable to decrease the threshold ThSIR as the number of surrounding vehicles increases. As the threshold ThSIR is decreased, the collision detection area 1002 is narrowed, and even if interference between a strong signal and a weak signal occurs, it is not detected as a collision. Therefore, the probability of collision detection is reduced, and the number of available time slots, that is, the communication capacity of the system can be increased. Conversely, when the threshold ThSIR is increased, even interference between strong signals and weak signals is detected as a collision, so that communication quality can be ensured. By making the threshold ThSIR variable according to the number of vehicles, priority can be given to communication quality when the number of vehicles is small, and priority can be given to communication capacity when the number of vehicles is large.
Further, by combining the above two methods, the threshold ThS may be increased and the threshold ThSIR may be decreased as the number of surrounding vehicles increases. Further, these threshold values do not have to be changed continuously according to the number of vehicles. For example, these threshold values may be changed stepwise according to the number of vehicles. In this specification, the change of the threshold value according to the number of vehicles includes not only a continuous change but also a step change.

  The number of surrounding vehicles can be grasped by counting the number of broadcast transmissions received within one subframe period (unit time). This is because each vehicle broadcasts vehicle information once within one subframe period.

  Thus, by making the conditions for detecting a collision variable according to the number of surrounding vehicles, appropriate communication can be realized according to the situation. Specifically, when the number of surrounding vehicles increases, the number of communicable vehicles can be increased instead of limiting the communicable range of each vehicle.

(Modification of collision notification 1)
In the description of the above-described embodiment, when it is detected that a collision occurs in a plurality of time slots, a collision notification is performed for a randomly selected time slot. However, it is also preferable to select a time slot other than the random selection criterion for the collision notification.

  Although various criteria are assumed, for example, it is conceivable to preferentially notify time slots having a large number of consecutive cases when collisions occur continuously over a plurality of frames. This is because when the collisions are continuous, the collisions are resolved as soon as possible. Moreover, priority may be given to those having a low signal-to-interference ratio (SIR), and priority may be given to those having a high interference wave signal strength. Both represent the strength of interference and give priority to those in which strong interference occurs. Furthermore, priority may be given to a signal having a high interference wave signal strength among time slots satisfying the condition that the received signal strength is equal to or higher than a predetermined strength or the distance from the host vehicle is within a predetermined distance. The purpose is to prioritize those in which strong interference has occurred in nearby vehicles. Further, based on the slot number, for example, a slot with a smaller slot number may be given priority, or a slot with a larger slot number may be given priority. Alternatively, when the time slots to be used are grouped according to the traveling direction of the vehicle, priority may be given to the time slots in the group used by the host vehicle. This is to give priority to a vehicle that travels in the same direction and is expected to have a long communication duration. It is also possible to combine a plurality of selection criteria described here.

  In the above, “priority” does not only mean selecting the one that best satisfies the condition (for example, the time slot having the highest interference wave intensity), but weights according to the degree of matching with the condition. Selection with a probability obtained, and determination at random from a predetermined number that satisfies the conditions most.

By making the selection criteria for the time slot for performing the collision notification as described above, it is possible to preferentially resolve the collision that matches the criteria.

(Modification 2 of collision notification)
In the description of the above embodiment, a vehicle that has detected a collision always makes a collision notification (see FIG. 6). However, it is not always necessary for all vehicles to make a collision notification, and only some of the vehicles may make a collision notification.

  For example, each vehicle may randomly select whether or not to make a collision notification. At this time, it is also preferable to increase the probability of performing the collision notification as the number of surrounding vehicles is smaller. However, if it is determined probabilistically, it is conceivable that notification is not performed even though a collision occurs. Therefore, it is also preferable to employ the following method.

  That is, it is preferable that a reference position with respect to the road is defined in advance, and the collision notification is performed when the host vehicle is closest to the reference position as compared with the vehicles existing around. The reference position may be an arbitrary position, and for example, the center position of the intersection can be adopted. The map information in which such a reference position is defined is stored in the map information storage unit of the car navigation device. Then, map information is obtained via the car navigation device, and it is determined whether or not the own vehicle is closest to the reference position from the position information transmitted from the surrounding vehicles and the position information obtained from the GPS device of the own vehicle. do it.

  The collision detection / notification process according to this method will be described with reference to FIG. FIG. 11A is a flowchart showing the flow of collision detection / notification processing according to this modification. A difference from FIG. 6A is that steps S1101 and S1102 are provided. When it is determined that there is a collision (S21-YES), a reference position (intersection center) in the vicinity is extracted with reference to the map information (S1101). Then, it is determined whether or not the own vehicle is closest to the reference position as compared with surrounding vehicles, and if it is closest (S1101-YES), a collision notification is performed. In the example of FIG. 11B, only the vehicle A closest to the intersection center position 1103 performs a collision notification, and the vehicles B to D do not perform a collision notification because the vehicle A is closer to the intersection center position 1103. Become. When there are a plurality of reference positions near the own vehicle, all the reference positions are extracted, and if the vehicle is the closest to any reference point, a collision notification may be performed.

  In this way, it is possible to reduce the number of vehicles that perform the collision notification, to suppress the communication overhead associated with the collision notification, and to ensure that any vehicle performs the collision notification reliably.

(Modification of time slot change processing)
In the description of the above embodiment, the time slot to be used is always changed when notified that a collision has occurred in the time slot used by the host vehicle (FIG. 7). However, if the received signal strength of the signal for notifying the collision information of the time slot used by the host vehicle is equal to or lower than the threshold value, the communication may be continued without changing the time slot. FIG. 12A is a flowchart showing the flow of collision notification detection / slot change processing according to this modification. Step S1201 for determining whether or not the received signal strength is greater than or equal to a threshold value is compared with FIG. The only difference is that it is provided in.

When the received signal strength of the collision information is small, it means that a collision notification is given from a distant vehicle. The fact that a collision notification is made only from a distant vehicle means that a nearby vehicle has not detected a collision. In such a case, it is not necessary to change the time slot from the viewpoint of ensuring communication near the host vehicle. Rather, it can be said that it is better not to change the time slot in consideration of the effect of changing the time slot. For example, in the situation shown in FIG. 12B, it is assumed that the vehicle B has detected that communication between the vehicle A and the vehicle C has collided. Vehicle A will receive a collision notification from vehicle B,
When the distance between the vehicle A and the vehicle B is large, it can be said that there is little need for the vehicle A to immediately change the time slot as described above. As shown in FIG. 12B, when the vehicle A and the vehicle C are traveling in different directions, since the communication collision is naturally resolved thereafter, no inconvenience occurs. On the other hand, when the vehicle A and the vehicle C are traveling in the direction in which the vehicle A approaches, the received signal strength of the collision notification increases thereafter, so the time slot used at this point may be changed.

  According to this modification, unnecessary time slot changes can be prevented, and adverse effects associated with time slot changes can be avoided.

  When the distance is not the received signal strength of the collision notification but the distance information (position information of the transmission source vehicle) transmitted together with the collision notification and the position information of the host vehicle, and this distance is equal to or greater than the threshold value Even if the time slot is not changed, the same effect can be obtained.

(Modification of time slot selection processing)
Next, a modified example of processing for selecting a time slot used for communication will be described. First, it is conceivable to change the criteria for the empty slot determination process (carrier sense), which is a precondition for time slot selection. That is, it is preferable that the threshold value (carrier sense level) of the received signal strength for determining that the time slot is vacant is variable according to the number of surrounding vehicles. For example, it is conceivable that the carrier sense threshold is set to −80 dBm when the number of vehicles is small, and −60 dBm when the number of surrounding vehicles increases. By doing this, it is possible to increase the communication capacity of the system by increasing the number of slots that are pseudo-free. As a result, collisions between strong and weak signals are expected to increase somewhat. It can be said that there is no problem when compared.

  In the description of the above embodiment, a time slot is randomly selected from time slots determined to be free as a result of carrier sense. However, it is also preferable to select a time slot used for communication according to a criterion other than random. For example, it is conceivable to select a predetermined number of time slots whose received signal strength is equal to or less than a predetermined threshold, and select at random from among them. By such two-step selection, a time slot with more stable communication quality can be used. However, it is not preferable to narrow down the number of candidates by the first stage selection. This is because if the selected time slot becomes too deterministic, the possibility that a plurality of vehicles simultaneously select the same time slot increases.

  In selecting a time slot, it is also preferable not to select a time slot that has been notified of the latest collision. This is because, even if it is determined that the time slot in which the collision notification is made is free as a result of the carrier sense of the host vehicle, it is understood that there is a vehicle used for communication. It is also preferable to use different time slots for subframe 1 and subframe 2 as much as possible. For example, time slots with the same slot number may not be used in subframe 1 and subframe 2, or the slot numbers used in subframe 1 and subframe 2 are selected so as to be equal to or greater than a predetermined value. May be. This is because if the same time slot is used in different subframes, transmission between subframes may have a correlation, and transmission in both subframes may fail.

It is also preferable to group time slots used for communication according to the traveling direction (azimuth angle) of each vehicle. FIGS. 13A and 13B show examples in which time slots are divided into four groups. As shown in the figure, north is 0 degree, 315 degrees to 360 degrees, 0 degrees to 45 degrees (-45 degrees to 45 degrees), 45 degrees to 135 degrees, 135 degrees to 225 degrees, 225 degrees to 315 degrees The four groups have time slot numbers 1-250 and 251-500, respectively.
, 501-750, 751-1000 slots. Here, the range of time slots used by each group in subframes 1 and 2 is the same, but it is also preferable that the range of time slots assigned to each group be different for each subframe. By grouping the time slots to be used according to the direction of travel in this way, it is possible to avoid using the same time slot as a vehicle traveling in the opposite direction or a vehicle located on a road that intersects 90 degrees. Communication quality is improved. The traveling direction may be acquired from a magnetic sensor (electronic compass) or may be acquired from a change in GPS position information.

(Other)
The above description of the embodiment and the modifications is an exemplification, and various modifications are possible. For example, the length of one superframe, the number of subframes per superframe, the number of time slots per subframe number, the length of one time slot, and the like can be changed. In particular, if the number of subframes per superframe is increased, the reliability of communication can be improved instead of reducing the substantial communication capacity.

  The description of the above-described embodiment and various modifications can be combined as much as possible within a range where there is no technical contradiction.

DESCRIPTION OF SYMBOLS 100 In-vehicle wireless communication apparatus 111: Reception part 112: Transmission part 121: Carrier sense part 122: Time slot determination part 123: Reception processing part 124: Collision detection part 125: Collision notification part 126: Transmission data generation part

Claims (18)

An in-vehicle wireless communication device that performs wireless communication by a time division multiple access method,
Time slot determination means for performing carrier sense and determining a time slot used for communication;
Transmitting means for transmitting transmission data in the time slot determined by the time slot determining means;
Receiving means for receiving for a plurality of time slots;
Detecting means for detecting a time slot in which a communication collision occurs based on information received by the receiving means;
Notification means for notifying the time slot detected by the detection means;
I have a,
The notification means includes collision information for identifying a time slot in which a collision occurs, and transmits the collision information included in the transmission data.
In-vehicle wireless communication device. When it is determined that a collision has occurred in the time slot used for transmission by the transmission unit based on the collision information included in the data received by the reception unit, the time slot determination unit includes: Re-determine the time slot used for communication,
The in-vehicle wireless communication device according to claim 1 . When the reception signal strength of the data for notifying that a collision has occurred in the time slot used for transmission by the transmission means is equal to or higher than the first threshold, the time slot used for communication is re-determined. ,
The in-vehicle wireless communication device according to claim 2 . The detection means determines that a communication collision has occurred in the time slot when demodulation has failed in the time slot that has been received until immediately before,
The in-vehicle wireless communication device according to any one of claims 1 to 3 . The detection means detects a communication collision for a time slot whose received signal strength is equal to or greater than a third threshold corresponding to the received signal strength of a signal from a predetermined distance .
The in-vehicle wireless communication device according to any one of claims 1 to 4 . The third threshold value is set to a larger value as the number of in-vehicle wireless communication devices existing around is larger.
The in-vehicle wireless communication device according to claim 5 . The time slot determining means sets the carrier sense level to a larger value as the number of in-vehicle wireless communication devices existing in the surrounding area increases, and performs carrier sense.
The in-vehicle wireless communication device according to any one of claims 1 to 6 . When the detection means detects that a communication collision occurs in a plurality of time slots, the notification means is selected from a plurality of time slots in which a collision occurs according to a predetermined criterion. For a predetermined number of timeslots ,
The predetermined criterion is:
Priority is given to time slots with a large number of consecutive cases when collisions occur continuously over multiple frames.
Give priority to time slots with a low signal-to-interference ratio,
Give priority to time slots with high received signal strength,
Of the time slots whose received signal strength is equal to or greater than a predetermined value, priority is given to the one having a high interference signal strength.
Among received signal strength and other time slots below a predetermined value, priority is given to the one having a high interference signal strength.
Priority is given to time slots with a large or small time slot number.
Randomly determined from the results of narrowing down according to any of the above criteria.
Randomly determined from all time slots in which a collision is detected,
Or at least one of
The in-vehicle wireless communication device according to any one of claims 1 to 7 . According to the number of receptions per unit time by the receiving means, determine the number of in-vehicle wireless communication devices present in the surroundings,
The in-vehicle wireless communication device according to any one of claims 1 to 8 . The transmission means periodically transmits transmission data including at least one of a position, a moving speed, and a moving direction of a vehicle on which the in-vehicle wireless communication device is mounted.
Vehicle wireless communication device according to any one of claims 1-9. Map information storage means for storing map information in which center positions of one or more intersections are defined;
Position information acquisition means for acquiring position information;
Further comprising
When there is a location closer to the vehicle-mounted wireless communication device existing around the center position of any of the intersections, the notification means notifies the time slot in which a collision occurs.
The in-vehicle wireless communication device according to claim 10 . The time slot determining means acquires a moving direction of a vehicle on which the in-vehicle wireless communication device is mounted, and determines a time slot to be used for communication preferentially from time slots associated with the moving direction.
Vehicle wireless communication device according to any one of claims 1 to 11. The time slot determination unit determines a time slot to be used for communication from a time slot other than the time slot in which it is reported that a collision has occurred according to the collision information included in the data received by the reception unit. ,
The in-vehicle wireless communication device according to any one of claims 1 to 12 . The time slot determination means is a time slot randomly selected from time slots determined to be free as a result of carrier sense, or a received signal among time slots determined to be free as a result of carrier sense. A time slot randomly selected from time slots whose strength is equal to or lower than a predetermined strength is determined as a time slot to be used for communication.
The in-vehicle wireless communication device according to any one of claims 1 to 13 . The transmission means repeatedly transmits the same transmission data a plurality of times,
The time slot determining means determines a time slot that is different for each of the plurality of transmissions as a time slot to be used for communication.
The in-vehicle wireless communication device according to any one of claims 1 to 14 . A wireless communication method executed by an in-vehicle wireless communication device that performs wireless communication by a time division multiple access method,
A time slot determination step of performing carrier sense and determining a time slot used for communication;
A transmission step of transmitting transmission data in the time slot determined in the time slot determination step;
A receiving step for receiving for a plurality of time slots;
A detection step of detecting a time slot in which a communication collision occurs based on the information received in the reception step;
A notification step of notifying the time slot detected in the detection step;
Only including,
In the notifying step, collision information specifying a time slot in which a collision occurs is included in the transmission data and transmitted.
Wireless communication method. Based on the collision information included in the data received in the reception step, when it is determined that a collision has occurred in the time slot used for transmission in the transmission step, the time slot used for communication is Redetermined,
The wireless communication method according to claim 16 . A program for causing a computer to execute the wireless communication method according to claim 16 or 17 .

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