JP5527375B2 - Wireless communication system and time correction method thereof - Google Patents

Description

  The present invention relates to a radio communication system suitable as, for example, an intelligent transport system (ITS). More specifically, the present invention relates to a time correction method between a transmitter and a receiver constituting the wireless communication system.

In recent years, for the purpose of promoting traffic safety and preventing traffic accidents, advanced road traffic systems that improve the safety of vehicles by receiving information from infrastructure devices installed on the road and utilizing this information have been studied. (For example, refer to Patent Document 1).
Such an intelligent road traffic system is mainly composed of a plurality of roadside communication devices which are wireless communication devices on the infrastructure side and a plurality of in-vehicle communication devices which are wireless communication devices mounted on each vehicle.

  In this case, a combination of communication performed between communication subjects includes road-to-road communication between road-side communication devices, road-to-vehicle (or vehicle-road) communication between road-side communication devices and vehicle-mounted communication devices, and vehicle-mounted communication. Vehicle-to-vehicle communication performed between aircraft.

Japanese Patent No. 2806801

In the above-mentioned intelligent road traffic system, including communication between vehicles, road-to-vehicle communication, road-to-road communication, and road-to-step communication, in order to coexist these communications, what kind of communication is used by effectively utilizing the bandwidth. The issue is whether to perform control.
In view of this, it has been studied that multiple access is used to perform communication between roads, road vehicles, and vehicles within a limited frequency band.

As this multi-access method, there are frequency division multiplexing (FDMA) and code division multiple access (CDMA), but communication with only a small number of in-vehicle communication devices is assumed in mountainous areas. As a multi-access method for inter-vehicle communication, it is preferable to adopt an autonomous random access method represented by CSMA (Carrier Sense Multiple Access), for example.
However, road-to-vehicle communication, road-to-road communication, and vehicle-to-vehicle communication coexist in an area where roadside communication devices exist.

In this case, since the priority of the information handled by the roadside communication device on the infrastructure side is generally high, a mechanism for performing road-to-vehicle communication and road-to-road communication preferentially over vehicle-to-vehicle communication is necessary. is there.
As described above, in order to preferentially transmit information of a roadside communication device, time division multiplexing (TDMA) in which a frequency band used for communication is time-divided at regular intervals and a time slot dedicated to transmission of the roadside communication device is provided. : Multiple access by Time Division Multiple Access) is enabled.

Therefore, for example, assuming a communication system composed of a plurality of roadside communication device groups installed at each intersection, a time slot dedicated to wireless transmission from the roadside that the roadside communication device wirelessly transmits is allocated by the TDMA method and remains. It is considered that it is a rational communication system to use time slots for inter-vehicle communication by the CSMA method.
In such an intelligent road traffic system, for example, slot information related to the time slot used by each roadside communication device is notified to the in-vehicle communication device by a downlink signal, and the in-vehicle communication device adopts the CSMA method in a time zone other than the time slot. Wireless transmission can be considered.

  In this case, if accurate time synchronization is not achieved between the roadside communication devices and between the roadside communication device and the in-vehicle communication device, the time slot start time and the end time grasped by each roadside communication device and the in-vehicle communication device are shifted. As a result, radio interference occurs between the downlink signal of the roadside communication device and the transmission signal of the in-vehicle communication device.

On the other hand, as a method of time synchronization between roadside communication devices, for example, GPS synchronization for adjusting its own local time to GPS (Global Positioning System) time, or time information notified from other roadside communication devices of its own local time Air synchronization that matches (time stamp) can be considered.
Among these, the synchronization method based on the GPS time is excellent for accuracy, but if the GPS receiver breaks down, or if there is a problem such as ice or snow adhering to the antenna of the GPS receiver, the GPS receiver An accurate reference time cannot be obtained.

For this reason, in consideration of such a device failure or malfunction, GPS synchronization is not always advantageous. If the failure or malfunction is left as it is, the slot information notified by the roadside communication device remains mad. Road-to-vehicle communication will continue.
Therefore, there is a demand for a wireless communication system that can detect the above-described failure and malfunction and can accurately perform time synchronization by air synchronization even if the communication device does not include a GPS receiver.

  In view of such a problem, the present invention determines whether time synchronization between the transmitter and the time synchronization between the two is accurately performed by wirelessly transmitting a time stamp to the receiver. An object of the present invention is to provide a wireless communication system capable of accurately performing the above.

(1) a first wireless communication system of the present invention is a radio communication system comprising a transmitter and a receiver, the transmitter timestamp while fixing the position from the beginning of the data portion A PDU generation unit that generates a stored PDU, a modulation transmission unit that modulates a carrier wave using the PDU and transmits a radio wave from a transmission antenna, and a transmission delay time of (a) set in advance in the transmitter itself A first time correction unit that obtains the time stamp by adding to the local time of the receiver, and the receiver includes a demodulation receiving unit that extracts the PDU from a radio wave signal that has reached a reception antenna, and the PDU The SDU playback unit for playing back the SDU and the time stamp from the SDU, and adding the total value of the reception delay time (b) and the playback delay time (c) to the time stamp to obtain the corrected time value. A second time correction unit.

(A) Transmission delay time from the time of setting the time stamp to the time of radio transmission of the transmitting antenna (b) Reception delay time from the time of receiving the radio wave to the arrival of the preamble to the SDU playback unit (c) Preamble to the SDU playback unit Playback delay time from the time of arrival to the time of time stamp acquisition

According to the wireless communication system of the present invention, the first time correction unit of the transmitter adds the transmission delay time to the local time of the transmission side to obtain a time stamp, so that the time stamp of the transmitter is the PDU It almost exactly coincides with the time when the radio wave is transmitted from the transmitting antenna.
On the other hand, the second time correction unit of the receiver obtains a correction time value by adding the total value of the reception delay time and the reproduction delay time to the time stamp, and the correction time value is calculated based on the local time of the transmitter. Matches almost exactly.

Therefore, on the receiver side, it is possible to determine whether or not the time synchronization between the transceivers using the corrected time value and the time synchronization between them are accurate.
(2) For example, when the transmitter performs GPS synchronization but the transmitter has higher local time accuracy, such as when the receiver does not have GPS, the second time The correction unit may adjust its own local time to the correction time value.

(3) However, when the second time correction unit performs a synchronization process that adjusts its own local time to the correction time value, the timings of the internal processes on the receiver side based on the local time become discontinuous all at once. Since shifting occurs, if the frequency of time synchronization is too high, the system on the receiver side may become unstable.
Therefore, only when a deviation that does not satisfy the predetermined communication condition occurs, the second time correction unit performs a synchronization process for adjusting the local time to the correction time value, and the local time of the receiver shifts discontinuously. It is preferable to reduce the frequency of destabilization as much as possible.

Therefore, the second time correction unit determines whether or not the difference between its local time and the correction time value is greater than or equal to a predetermined threshold, and on the condition that the determination result is affirmative, It is preferable to match its own local time with the corrected time value.
(4) Moreover, when the said determination result is affirmative, the said 2nd time correction | amendment part will perform the response | compatibility processing at the time of abnormality occurrence about the said transmitter or the said receiver, or both of these Also good.

As a response process when an abnormality occurs, assuming that both the transmitter and the receiver are installed in the roadside communication device, for example, the external (central device) is notified of the occurrence of an abnormality in the internal clock of the roadside communication device. Or a process of canceling the wireless transmission of the roadside communication device.
(5) In this case, the second time correction unit adjusts its local time to the correction time value, and then when the determination result is affirmative, the transmitter or the receiver, or these It is also possible to execute a response process when an abnormality occurs for both of the above.

(6) Moreover, when the modulation / demodulation method by OFDM is employ | adopted for the said transmitter and the said receiver, it is preferable that the said predetermined threshold value is set to the guard interval of an OFDM modulation wave.
In this case, since the time error between the transmitter and the receiver can be suppressed to a guard interval or less, for example, the time error of the transmission timing between roadside communication devices can be suppressed to a guard interval or less by air synchronization by roadside communication. And deterioration due to intersymbol interference can be effectively avoided.

Note that “setting the predetermined threshold value to the guard interval of the OFDM modulation wave” here is substantially the same value as the guard interval to ensure that the time error is equal to or less than the guard interval. It also includes setting to a value slightly smaller than the interval (for example, a value of about 80% or 90% of the guard interval value defined by the standard or the like).
Therefore, for example, if the guard interval value is 3.2 μsec, the predetermined threshold may be set to a value such as 3.1 μsec or 3.0 μsec.

(7) In the wireless communication system of the present invention, the second time correction unit is specifically based on a transmission rate of a communication frame that can be specified from information described in a portion before the time stamp in the PDU. Thus, the reproduction delay time can be obtained.
For example, when the PDU is a MAC frame, the modulation method is stored in the signal of the frame, and the transmission rate can be specified based on this modulation method. Therefore, if the receiver can detect the storage location of the time stamp, the reproduction delay time from the arrival of the preamble to the arrival of the time stamp can be obtained.

(8) Further, in the wireless communication system of the present invention, when the PDU includes location information of the transmitter, the second time correction unit further performs a transmission delay of the following (d) It is preferable to obtain the corrected time value by adding time to the time stamp.
(D) Transmission delay time according to the distance between the position of the transmitter determined from the position specifying information and the position of the receiver as its own device

In this case, since the second time correction unit adds the transmission delay time to the time stamp to obtain a correction time value, even if the positions of the transmitter and the receiver are changed, the second time correction unit accurately depends on the local time of the transmitter. The matched correction time value can be obtained.
For this reason, it is possible to more accurately perform time synchronization by air synchronization that matches the local time of the receiver to the transmitter side, and more accurately determine whether time synchronization between the two is correctly performed. Can do.

(9) In the wireless communication system of the present invention, when a modulation / demodulation method using OFDM is adopted in the transmitter and the receiver, the accuracy of the time stamp required by the first time correction unit (the time stamp concerned) Is preferably equal to or less than the guard interval of the OFDM-modulated wave.
In this case, for example, the time error of the transmission timing between roadside communication devices can be suppressed to a guard interval or less by air synchronization by roadside communication, and deterioration due to intersymbol interference can be effectively avoided.

  (10) A wireless communication system according to the present invention includes a plurality of roadside communication devices to which dedicated time slots are allocated in a time division manner and a plurality of mobile communication devices that are allowed to wirelessly transmit only in time zones other than the time slots. The transmitter is mounted on the roadside communication device, and the receiver is mounted on the roadside communication device, the mobile communication device, or both of them.

  According to this wireless communication system, the transmitter is mounted on a roadside communication device, and the receiver is mounted on a roadside communication device or a mobile communication device or both of them. For both, it is possible to determine whether the time synchronization between the two is accurately performed, and to accurately perform the time synchronization between the two.

  (11) The time correction method according to the present invention is a method for correcting by sharing the delay time generated when the transmitter and the receiver of the wireless communication system according to the present invention transmit and receive time stamps. The same operation effect as a communication system is produced.

  As described above, according to the present invention, the transmitter transmits a time stamp to the receiver, thereby determining whether time synchronization between the two is accurately performed and accurately performing time synchronization between the two. Can do.

It is a schematic perspective view which shows the whole structure of an intelligent road traffic system. It is a road top view which shows a part of jurisdiction area of an intelligent road traffic system. It is a block diagram which shows the internal structure of a roadside communication apparatus and a vehicle-mounted communication apparatus. It is a conceptual diagram which shows an example of the time slot of road-to-vehicle communication. It is a block diagram which shows the structure of the transmitter and receiver of a radio | wireless communications system in a protocol stack format. (A) is a time chart which shows the event which arises in transmission / reception of a time stamp, (b) is a table | surface which shows the content of the delay time between the events. (A) is a figure which shows an example of the format of a communication frame, (b) is a table | surface which shows the required time of frame reproduction | regeneration according to a modulation method and a transmission rate.

[Overall system configuration]
FIG. 1 is a schematic perspective view showing an overall configuration of an intelligent road traffic system (ITS) according to an embodiment of the present invention. In this embodiment, as an example of the road structure, a grid structure in which a plurality of roads in the north-south direction and the east-west direction intersect with each other is assumed.
As shown in FIG. 1, the intelligent transportation system of this embodiment is equipped with a traffic signal 1, a roadside communication device 2, an in-vehicle communication device 3 (see FIGS. 2 and 3), a central device 4, and an in-vehicle communication device 3. A vehicle 5 and a roadside sensor 6 including a vehicle detector and a monitoring camera are included.

The traffic signal 1 and the roadside communication device 2 are installed at each of a plurality of intersections Ji (i = 1 to 12 in the example), and are connected to the router 8 via a communication line 7 such as a telephone line. . This router 8 is connected to the central device 4 in the traffic control center.
The central device 4 constitutes a local area network (LAN) with the traffic signal device 1 and the roadside communication device 2 at each intersection Ji included in the area under its control. Therefore, the central device 4 can perform bidirectional communication with each traffic signal 1 and each roadside communication device 2. The central device 4 may be installed on the road instead of the traffic control center.

The roadside sensor 6 is installed in various places on the road in the jurisdiction area for the purpose of counting the number of vehicles flowing into each intersection Ji. The roadside sensor 6 includes a vehicle sensor that ultrasonically senses the vehicle 5 that passes underneath, or a monitoring camera that shoots traffic conditions on the road in time series. The sensing information S4 and the image data S5 are transmitted via the communication line 7. Is transmitted to the central device 4 via
In FIG. 1 and FIG. 2, only one signal lamp is depicted at each intersection Ji for the sake of simplicity of illustration, but each actual intersection Ji is used for ascending and descending roads that intersect each other. At least four signal lamps are installed.

[Central equipment]
The central device 4 has a control unit composed of a workstation (WS), a personal computer (PC), etc., and this control unit collects and processes various traffic information from the roadside communication device 2 and the roadside sensor 6. (Calculation)-Performs recording, signal control and information provision in an integrated manner.
Specifically, the control unit of the central device 4 performs system control for adjusting the traffic signal group 1 on the same road for the traffic signal 1 at the intersection Ji belonging to its own network, and this system control is applied to the road network. Extended wide area control (surface control) can be performed.

In addition, the central device 4 has a communication unit that is a communication interface connected to the LAN side via the communication line 7, and this communication unit includes a signal control command S1 relating to the lamp color switching timing of the signal lamp, Traffic information S2 including traffic jam information and the like is transmitted to the traffic signal device 1 and the roadside communication device 2 every predetermined time (see FIG. 1).
The signal control command S1 is transmitted every calculation period (for example, 1.0 to 2.5 minutes) of the signal control parameter when performing the system control and the wide area control, and the traffic information S2 is transmitted every 5 minutes, for example. Is done.

  Further, the communication unit of the central device 4 is generated when the vehicle passes through the vehicle information S3 including the current position of the vehicle 5 received by the communication device 2 from the in-vehicle communication device 3 from the roadside communication device 2 corresponding to each intersection Ji. Sensing information S4 of a vehicle sensor (not shown) consisting of a pulse signal and image data S5 consisting of digital information of a road photographed by a surveillance camera are received, and the control unit of the central device 4 Based on various information, the system control and the wide area control are executed.

[Wireless communication systems, etc.]
FIG. 2 is a road plan view showing a part of the jurisdiction area of the above intelligent road traffic system.
In FIG. 2, each of two roads intersecting each other is illustrated as one lane on one side in the up and down directions, but the road structure is not limited to this.
As shown also in FIG. 2, the intelligent transportation system of this embodiment includes a plurality of roadside communication devices 2 capable of wireless communication with the in-vehicle communication device 3, and other communication devices 2 and 3 using a carrier sense method. It also functions as a wireless communication system provided with the in-vehicle communication device 3 that is a kind of mobile wireless transceiver that performs wireless communication.

In the example of FIG. 1 and FIG. 2, the plurality of roadside communication devices 2 are installed at each roadside intersection Ji, and are attached to the pillars of the traffic signal device 1. On the other hand, the in-vehicle communication device 3 is mounted on a part or all of the vehicle 5 traveling on the road.
Each in-vehicle communication device 3 mounted on the vehicle 5 can receive the downlink signal in the downlink area A that is the reach range of the downlink signal from the roadside communication device 2.

In the present embodiment, it is assumed that the reach of the transmission signal of the in-vehicle communication device 3 is equal to or less than the reach of the downlink signal of the roadside communication device 2. Accordingly, each roadside communication device 2 can perform wireless communication with the in-vehicle communication device 3 that travels within the range of the downlink area A of the own device.
Thus, in the present embodiment ITS, communication between the vehicle-mounted communication devices 3 (vehicle-to-vehicle communication) and between the roadside communication device 2 and the vehicle-mounted communication device 3 (road-to-vehicle communication from “road” to “car”) (Including both inter-vehicle communication from “car” to “road”), wireless communication is used.

Further, when the installation positions of the adjacent roadside communication devices 2 are relatively close to each other and the downlink areas A overlap (may be partially overlapped or all overlapped), the wireless communication between the roadside communication devices 2 is performed. Communication (inter-road communication) is possible.
As described above, the central device 4 provided in the traffic control center is capable of two-way communication with each roadside communication device 2 by wire, but wireless communication may be performed between these devices.

The roadside communication device 2 allocates a dedicated time slot (first slot T1 in FIG. 4) for wireless transmission by the TDMA system, and a time zone other than this time slot (second slot T2 in FIG. 4). Does not perform wireless transmission.
That is, the time zone other than the time slot for the roadside communication device 2 is open as a transmission time by the CSMA method for the in-vehicle communication device 3.

The roadside communication device 2 has a time synchronization function with other roadside communication devices 2 in order to control its own transmission timing.
The time synchronization of the roadside communication device 2 is performed by, for example, GPS synchronization that adjusts its own clock to the time acquired from the GPS satellite, air synchronization that adjusts its own clock to a transmission signal from the other roadside communication device 2, and the like. Can do.

[Roadside communication device]
FIG. 3 is a block diagram showing the internal configuration of the roadside communication device 2 and the in-vehicle communication device 3.
The roadside communication device 2 includes a wireless communication unit (transmission / reception unit) 21 to which an antenna 20 for wireless communication is connected, a wired communication unit 22 that performs bidirectional communication with the central device 4, and a processor (CPU) that performs communication control thereof. A central processing unit) and a storage unit 24 connected to the control unit 23, such as a ROM or a RAM.

The storage unit 24 of the roadside communication device 2 stores a computer program for communication control executed by the control unit 23, communication device IDs of the communication devices 2 and 3, and the like.
The control unit 23 of the roadside communication device 2 includes a transmission control unit 23A that controls the transmission timing of the wireless communication unit 21 and received data of each of the communication units 21 and 22 as functional units that are achieved by executing the computer program. A data relay unit 23B that performs the relay process of FIG. 5 and a time correction unit 23C that generates a time stamp Ts obtained by correcting its own local time.

The data relay unit 23B of the roadside communication device 2 temporarily stores the traffic information S2 and the like from the central device 4 received by the wired communication unit 22 in the storage unit 24 and causes the wireless communication unit 21 to perform broadcast transmission.
In addition, the data relay unit 23B temporarily stores the vehicle information S3 received by the wireless communication unit 21 in the storage unit 24 and transfers the vehicle information S3 to the central device 4 via the wired communication unit 22.

The transmission control unit 23A of the roadside communication device 2 synchronizes the transmission timing with the other device, and the time slot T1 of the predetermined slot number i assigned to the own device (see FIG. 4; hereinafter, “slot i”). In this case, wireless transmission is performed for a predetermined transmission time.
That is, the storage unit 24 of the roadside communication device 2 stores, for example, slot information S6 including the following information a) and b). The slot information S6 is individually set for each roadside communication device 2.
a) Slot number i being used by the own device (see FIG. 4)
b) Start time and duration of the first slot T1 (see FIG. 4) with slot number i

Further, the storage unit 24 of the roadside communication device 2 stores a transmission time corresponding to the amount of information (transmission data amount) to be downlink transmitted from the own device and the transmission start time. The transmission start time and transmission time are individually set for each roadside communication device 2 so as to be within the time slot T1 assigned to the own device.
The transmission control unit 23A generates a downlink signal having the set transmission time length, and causes the radio communication unit 21 to transmit the downlink signal at the set transmission start time.

The transmission time of the roadside communication device 2 may be set to the maximum of the duration (slot length) of the time slot T1 assigned to the own device. In consideration of the information processing time, it is preferable that the slot length is set slightly shorter than the slot length with a predetermined margin (for example, a guard time of the order of 10 μs).
Further, the transmission time of the roadside communication device 2 can be set to an arbitrary time length within the range of the slot length assigned to the own device, and is significantly shorter than the slot length (for example, 1/3 or 1/2) can also be set.

Of the transmission start time and the transmission time of the downlink signal, the transmission start time is determined by the transmission control unit 23A of each roadside communication device 2 based on the start time of the slot i included in the slot information S6 of its own device. You may make it produce | generate autonomously.
The transmission control unit 23A of the roadside communication device 2 adds a time stamp Ts obtained by correcting the current time by the time correction unit 23C to the downlink signal including the slot information S6, and broadcasts it to the wireless communication unit 21. Send it.

When the in-vehicle communication device 3 receives the downlink signal including the slot information S6 and the time stamp Ts corrected by the time correction unit 23C, the value of the time stamp Ts (hereinafter, sometimes referred to as “stamp value”). Is used as a reference, and radio transmission is performed in a time zone (second slot T2 in FIG. 4) other than the first slot T1 of the slot number i described in the slot information S6.
If a main period Cm, which will be described later, is included in the slot information S6, the start time and the stamp value of the slot i can be expressed by a relative time within the main period Cm. In this case, the number of bits of the slot information S6 can be reduced as compared with the case where the stamp value is expressed in absolute time.

The slot information S6 only needs to include at least the time of the slot i used by the own device. However, the slot information S6 of the other roadside communication device 2 is obtained by inter-road communication or communication with the central device 4. If it is known, the slot information S6 of the other device may be transmitted from the own device.
In addition, the generation process of the time stamp Ts by the correction time unit 23C of the control unit 23 in the roadside communication device 3 will be described later.

[Contents of time slot]
FIG. 4 is a conceptual diagram illustrating an example of a time slot for road-to-vehicle communication.
As shown in FIG. 4, the time slot of road-to-vehicle communication includes a first slot T1 and a second slot T2, and the total period of these is repeated at a constant slot period Cs. The first slot T1 of each slot cycle Cs is a time slot for the roadside communication device 2, and wireless transmission by the roadside communication device 2 is permitted in this time zone.

A slot number i is assigned to the first slot T1, and the slot number i is periodically incremented (may be decremented).
The second slot T2 is a time slot for the in-vehicle communication device 3, and since this time period is opened for wireless transmission by the in-vehicle communication device 3, the transmission control unit 23A of the roadside communication device 2 has the second slot T2. Then, wireless transmission is not performed.

When the slot number i reaches a predetermined number n, it returns to the initial number (i = 1 in the figure). Therefore, if the slot period Cs for n times is the main period Cm, the first slot T1 of each slot number i to n is generated once for each main period Cm.
The system operator can appropriately set the time length of each cycle Cs, Cm and the total number n of slot cycles Cs. In this embodiment, as an example, Cs = 10 ms, Cm = 100 ms, and n = 10 is assumed.

In FIG. 4, the dot ● written in the first slot T1 of slot number i = 1 to 3 indicates the roadside communication device 2 wirelessly transmitting to the first slot T1 of the slot number i, and slot 1 having a plurality of dots ●. , 2 indicates that a plurality of roadside communication devices 2 are shared.
That is, in the example of FIG. 4, the slot 1 is shared by the two roadside communication devices 2 installed at the intersection J1 and the intersection J11, and the slot 2 is installed at the intersection J2, the intersection J9, and the intersection J10. Two roadside communication devices 2 are shared.

  The reason is that, for example, as shown in the intersection J1 and the intersection J11 in FIG. 1, the roadside communication devices 2 that are separated from each other do not overlap the downlink area A, and one in-vehicle communication device 3 includes a plurality of roadside communication devices. Since direct interference (see FIG. 6 (a)) receiving downlink signals simultaneously from 2 does not occur or the possibility thereof is extremely low, the same slot i is set in these and the transmission time of each roadside communication device 2 is set. This is because the in-vehicle communication device 3 can appropriately receive the downlink signal from each roadside communication device 2 even if the two are overlapped.

On the other hand, in the case of roadside communication devices 2 having a relatively close positional relationship in which the above-described direct interference can occur with respect to one in-vehicle communication device 3, it is not possible to set the same slot number i and overlap the transmission time .
However, the same slot number i can be shared between the roadside communication devices 2 that may cause direct interference by scheduling the transmission time in the first slot T1 in a time division manner.

[In-vehicle communication device]
Returning to FIG. 3, the in-vehicle communication device 3 includes a communication unit (transmission / reception unit) 31 connected to the antenna 30 for wireless communication, a control unit 32 including a processor that performs communication control on the communication unit 31, and the like. And a storage unit 33 including a storage device such as a ROM or a RAM connected to the control unit 32.
The storage unit 33 stores a computer program for communication control executed by the control unit 32, communication device IDs of the communication devices 2 and 3, and the like.

The control unit 32 of the in-vehicle communication device 3 causes the communication unit 31 to perform wireless communication based on the carrier sense method for inter-vehicle communication, and the communication control function in the time division multiplexing method like the roadside communication device 2 is I don't have it.
Accordingly, the communication unit 31 of the in-vehicle communication device 3 always senses the reception level of a predetermined carrier frequency, and when the value is equal to or greater than a certain threshold, wireless transmission is not performed, and when the value is less than the threshold Only intended to perform wireless transmission.

The control unit 32 of the in-vehicle communication device 3 includes a transmission control unit 32A that controls the wireless transmission timing of the communication unit 31 and a relay process of received data of the communication unit 31 as functional units achieved by executing the computer program. And a time correction unit 32C that corrects the received time stamp Ts to obtain a corrected time value Ta (see FIG. 5).
The transmission control unit 32A of the in-vehicle communication device 3 sets the wireless transmission time zone allowed for itself according to the start time of the slot information S6 acquired from the roadside communication device 2 and the slot information S6, and the communication unit only in this time zone 31 is made to perform wireless transmission.

  That is, the transmission control unit 32A extracts the slot information S6 of the roadside communication device 2 from the downlink signal from the road side and the received frame from the vehicle side, and the time correction unit 32C adds the time stamp Ts included in the downlink signal to the time correction unit 32C. Using the corrected time as a reference, wireless transmission by the carrier sense method is transmitted to the communication unit 31 only in a time zone other than the time slot T1 of the slot number i described in the slot information S6 (second slot T2 in FIG. 4). Let it be done.

Also, the transmission control unit 32A of the in-vehicle communication device 3 stores vehicle information S3 including the current position, direction, speed, and the like of the vehicle 5 (in-vehicle communication device 3) in a transmission frame, and the transmission frame is stored in the communication unit 31. Over the air via broadcast.
The data relay unit 32B of the in-vehicle communication device 3 can perform a relay process of extracting predetermined data from the reception frame received by the communication unit 31, and including the extracted data in the transmission frame and transmitting the data to the communication unit 31. .

For example, the data relay unit 32B extracts the traffic information S3 and the vehicle information S3 of the other vehicle 5 from the downlink signal of the roadside communication device 2, generates a transmission frame including the extracted data, and transmits the transmission frame to the communication unit 31. .
Further, when the slot information S6 is included in the downlink signal of the roadside communication device 2 or the received frame received from another vehicle 5, the data relay unit 32B extracts the slot information S6 and stores the slot information S6. The slot information S6 is stored in a transmission frame and transmitted to the communication unit 31.

In the present embodiment, the control unit 32 of the in-vehicle communication device 3 includes the vehicle information S3 received directly from the other vehicle 5 (in-vehicle communication device 3) and the vehicle information S3 of the other vehicle 5 received from the roadside communication device 2. Based on the included position, speed, and direction, it is possible to perform safe driving support control that avoids a right-handed collision or a head-on collision.
The time stamp Ts correction process by the time correction unit 32C of the control unit 32 in the in-vehicle communication device 3 will be described later.

[Time correction method]
As described above, in the wireless communication system of the present embodiment, the time correction unit 23C of the roadside communication device 2 generates the time stamp Ts obtained by correcting its own local time, and transmits a downlink signal including this to the in-vehicle communication device 3. The time correction unit 32C of the in-vehicle communication device 3 further corrects the acquired time stamp Ts to obtain a correction time value Ta, thereby obtaining a delay time generated in transmission / reception of the downlink signal of the roadside communication device 2 on the transmission side. And a time correction method in which the reception side shares and corrects the time.

FIG. 5 is a block diagram showing the configuration of the transmitter 41 and the receiver 51 of the wireless communication system in the protocol stack format to show the time correction method. Hereinafter, the time correction method of the present embodiment will be described with reference to FIG.
Since time synchronization between road vehicles is a problem here, FIG. 5 illustrates a case where the transmitter 41 is the roadside communication device 2 and the receiver 51 is the in-vehicle communication device 3.

As shown in FIG. 5, the transmitter 41 has an upper layer 42, a MAC (Media Access Control) unit 43, a PHY (Physical) unit 44, and an RF (Radio Frequency) unit 45 in order from the upper side. A transmission antenna 46 is connected to the RF unit 45. The control of the layers 42 to 44 is performed by the control unit 23 of the roadside communication device 2.
The receiver 51 includes an upper layer 52, a MAC (Media Access Control) unit 53, a PHY (Physical) unit 54, and an RF (Radio Frequency) unit 55 in order from the upper side. The transmission antenna 56 is connected. Protocol control of the layers 52 to 54 is performed by the control unit 32 of the in-vehicle communication device 3.

When receiving the SDU (Servise Data Unit) from the upper layer 42, the MAC unit (PDU generation unit) 43 of the transmitter 41 generates the PDU (Protocol Data Unit) from the SDU, and the time obtained by the time correction unit 23C. The PDU is generated with the stamp Ts, and the generated PDU is passed to the lower PHY unit 44.
The PHY unit 44 modulates a carrier wave (multi-carrier) using the generated PDU, performs digital modulation, and passes this modulated signal to the lower RF unit 45. The RF unit 45 converts the modulated signal into an analog signal, amplifies it, and transmits the radio wave from the transmitting antenna 46 to the outside.

Therefore, the PHY unit 44 and the RF unit 45 constitute a modulation transmission unit that modulates a carrier wave by a PDU and transmits a radio wave from a transmission antenna.
In the present embodiment, the modulation of the PHY unit 44 of the transmitter 41 includes an OFDM (Orthogonal Frequency-Division Multiplexing) modulation process. Therefore, demodulation of the PHY unit 54 of the receiver 51 includes demodulation processing using the OFDM method.

On the other hand, when the transmission radio wave reaches the reception antenna 56, the RF unit 55 of the receiver 51 amplifies the received signal and converts it into a digital signal, and passes this signal to the PHY unit 54. The PHY unit 54 performs predetermined digital demodulation on the digital signal, extracts the PDU from the carrier wave, and passes it to the MAC unit 53.
Therefore, the PHY unit 54 and the RF unit 55 constitute a demodulation receiving unit that extracts a PDU from a radio wave signal reaching the receiving antenna.

The MAC unit (SDU reproduction unit) 53 of the receiver 51 reproduces the SDU and the time stamp Ts from the extracted PDU, passes the reproduced SDU to the upper layer 52, and uses the reproduced time stamp Ts to the time correction unit 32C. To pass.
Here, the time correction unit (first time correction unit) 23C of the transmitter 41 obtains a time stamp Ts by adding a predetermined delay time to be corrected on the transmission side to its own local time, and this time stamp Ts. Is notified to the MAC unit (PDU generation unit) 43.

Further, the time generation unit 32C of the receiver 51 adds a predetermined delay time to be corrected on the reception side to the value of the time stamp Ts acquired from the MAC unit (SDU reproduction unit) 53 to obtain a correction time value Ta. .
In this case, if the delay time to be added on the transmission side and the reception side is appropriately set in the transmitter 41 and the receiver 51, the correction time value Ta is almost strictly set to the local time of the transmitter 41. Can be matched.

[Contents of delay time]
Therefore, with reference to FIG. 6, the content of the delay time generated when the communication frame is transmitted and received will be described.
Here, FIG. 6A is a time chart showing events that occur in transmission / reception of the time stamp Ts, and FIG. 6B is a table showing the contents of delay time between the events.
As shown in FIG. 6A, the events during the transmission / reception of the time stamp Ts include the following.

1) Time point A when the transmitter MAC sets the time stamp Ts
2) Time B when the transmitting antenna 46 transmits radio waves
3) Time C when the receiving antenna 56 receives radio waves
4) Time point D when the preamble reaches the receiver MAC
5) Time E when the receiver MAC acquires the time stamp Ts

  Among these, the “transmission delay time” from time A to time B is generated by the digital signal processing in the MAC unit 43 and the PHY unit 44 of the transmitter 41 and the delay characteristic of the analog filter in the RF unit 45 of the transmitter 41. On the transmitter 41 side, prediction can be made by calculation based on circuit characteristics or by performing measurement in advance. This time is, for example, about 5 μsec in an actual machine.

Since the “transmission delay time” from time B to time C depends on the distance between the transmitter 41 and the receiver 51, it cannot be predicted independently on either side of transmission and reception, but can be calculated if the distance is known. is there.
Therefore, if the position information including the coordinates (latitude / longitude) of the transmitter 41 is included in the SDU or PDU, the receiver 51 can detect the position of the transmitter 41 and the position of its own device (receiver 51). Based on the distance, the transmission delay time can be calculated.

Since this transmission delay time increases by about 1 μsec every 300 m, for example, when the communication distance in ITS is 600 m, it becomes about 2 μsec at the maximum.
Further, the information for specifying the position of the transmitter 41 is not limited to information for directly specifying the position such as the coordinates, but may be identification information such as a communication device ID of the roadside communication device 2, for example. In this case, if the receiver 51 stores position information corresponding to the communication device ID, the position of the transmitter 41 can be obtained from the communication device ID.

  “Reception delay time” from time C to time D is generated by analog filter delay characteristics in the RF unit 55 of the receiver 51 and digital signal processing in the MAC unit 53 and the PHY unit 54 of the receiver 51. On the 51st side, prediction can be made by performing calculation based on circuit characteristics or performing measurement in advance. Moreover, this time is 5 microseconds in an actual machine, for example.

The “reproduction delay time” from time D to time E is the time from when the preamble arrives at the MAC unit (SDU reproduction unit) 53 to when the time stamp Ts is acquired in the receiver 51. This time varies depending on which part of the communication frame stores the time stamp Ts and the communication speed.
FIG. 7 is an explanatory diagram of the reproduction delay time, FIG. 7 (a) is a diagram showing an example of the format of the communication frame, and FIG. 7 (b) is the time required for frame reproduction according to the modulation method and transmission speed. It is a table | surface which shows.

As shown in FIG. 7A, in the frame format conforming to IEEE 802.11, the playback time of the preamble and the signal is fixed length, and the data amount of the header is 30 bytes. In the illustrated example, time information (time stamp Ts) is stored in a 30-byte portion from the front of the data.
Also, in the MAC frame, the modulation method (BPSK, QPSK or 16QAM) of the frame is described in the signal in the part before the header (and therefore the part before the storage part of the time stamp Ts). Based on this, the transmission rate can be calculated.

Therefore, as shown in the table of FIG. 7B, each modulation method uses a reproduction time t1 from the beginning of the header portion to the beginning of the time stamp Ts and a reproduction time t2 from the beginning of the preamble to the beginning of the time stamp Ts. Can be determined in response to
Accordingly, the storage location of the time stamp Ts (the number of bytes from the beginning of the data, etc.) is determined in advance, and the reference table as shown in FIG. The reproduction delay time can be obtained from the speed.

  Note that the storage location of the time stamp Ts may be recorded in the vicinity of the beginning of the data in advance. In this case, since the required time from the head of the data to the time stamp Ts can be calculated, the reproduction delay time can be obtained by adding the time of the portion before the data to this time.

[Details of time correction]
In this embodiment, since each delay time is shared and corrected between the transmission side and the reception side, the time correction unit 32C of the receiver 51 adds the transmission delay time of the following (a) to its own local time. To obtain the time stamp Ts.
(A) Transmission delay time from the time of setting the time stamp Ts to the time of radio wave transmission (time from time A to time B in FIG. 6)

In this embodiment, the time correction unit 32C of the receiver 51 sets the total value of the reception delay time (b), the reproduction delay time (c), and the transmission delay time (d) in the time slot Ts. Add to find your local time.
(B) Reception delay time (time from time C to time D) from the time of radio wave reception until the arrival of the preamble at the MAC unit (SDU playback unit) 53

(C) Playback delay time from the time when the preamble reaches the MAC unit (SDU playback unit) 53 until the time stamp Ts is obtained (time from time D to time E in FIG. 6)
(D) Transmission delay time corresponding to the distance between the position of the transmitter 41 determined from the position specifying information and the position of the receiver 51 which is its own device (time from time B to time C in FIG. 6)

As described above, according to the wireless communication system of the present embodiment, the time correction unit 23C of the transmitter 41 adds the transmission delay time in its own device to its own local time to obtain the time stamp Ts. The time stamp Ts of the machine 41 almost exactly coincides with the time when the carrier wave of the PDU is transmitted from the transmission antenna 46.
On the other hand, the time correction unit 32C of the receiver 51 obtains a correction time value Ta by adding the reception delay time, reproduction delay time, and transmission delay time to the time stamp Ts. Matches the local time almost exactly.

For this reason, according to the wireless communication system of the present embodiment, for example, the time correction unit 32C of the receiver 51 adjusts its local time to the correction time value Ta, so that the local in-vehicle communication device (reception side) The time can match the local time of the roadside communication device 2 (transmission side) with high accuracy, and time synchronization between the two can be performed accurately.
In the road-to-vehicle communication between the roadside communication device 2 and the vehicle-mounted communication device 3, if the distance between the transmitter 41 and the receiver 51 is about 600 m, as shown in FIG. Since the value (2 μsec) is smaller than the other delay times, the transmission delay time may be excluded from the correction processing target.

[Time synchronization for inter-road communication]
In the example of FIG. 5, assuming that the transmitter 41 is in the roadside communication device 2 and the receiver 51 is in the vehicle-mounted communication device 41, the vehicle-mounted communication device 3 matches the local time of the roadside communication device 2. Although time synchronization (air synchronization) has been illustrated, the transmitter 41 and the receiver 51 may be mounted on different roadside communication devices 2 in the radio wave reachable range.
In this case, a specific roadside communication device 2 can perform time synchronization (air synchronization) that matches the local time of another roadside communication device 2.

In the air synchronization by the road-to-road communication, for example, one roadside communication device 2 performs GPS synchronization, but is particularly effective when the other roadside communication device 2 is not equipped with a GPS.
In this case, the transmitter 41 of the roadside communication device 2 that performs GPS synchronization with high local time accuracy transmits the corrected time stamp Ts, and receives this, the receiver of another roadside communication device 2 that does not perform GPS synchronization. In step 51, the local time is adjusted to the corrected time value Ta obtained by correcting the time stamp Ts.

[About accuracy of time slot and corrected time value]
By the way, in the OFDM system employed by the transceivers 41 and 51 of the present embodiment, the guard interval of the OFDM modulated wave is about 3.2 μsec. In contrast, as shown in FIG. 6B, the transmission delay time (5 μsec), the reception delay time (5 μsec), and the reproduction delay time (80 to 200 μsec) are all larger than the guard interval.

Therefore, for example, if the roadside communication devices 2 perform air synchronization without considering these delay times, a time error exceeding the guard interval may occur in the local time of both.
For this reason, for example, when the transmission times of a plurality of roadside communication devices 2 that are in a direct interference positional relationship are continuously assigned to one time slot T1, before and after the roadside communication devices 2 that are continuously transmitted. For downlink signals, there are cases where deterioration due to intersymbol interference cannot be avoided.

In this regard, in the wireless communication system according to the present embodiment, the transmitter 41 generates a highly accurate time slot Ts in consideration of the transmission delay time. Therefore, the accuracy of the time stamp Ts generated by the transmitter 41 (the time stamp Ts The time difference between the transmission time from the transmission antenna 46 and its stamp value) can be suppressed to a guard interval or less.
In addition, since the receiver 51 obtains the time correction value Ta by adding at least the reception delay time and the reproduction delay time to the time stamp Ts, the accuracy of the time correction value Ta can be suppressed to a guard interval or less.

  For this reason, according to the wireless communication system of the present embodiment, it is possible to effectively avoid deterioration due to intersymbol interference even when transmission times of a plurality of roadside communication devices 2 are continuously assigned.

[Air synchronization frequency]
On the other hand, when the time correction unit 32C of the receiver 51 synchronizes its own local time with the correction time value Ta, the timing of internal processing of the receiver 51 based on the local time is shifted discontinuously all at once. To do.
For this reason, if the frequency at which the receiver 51 performs time adjustment is too high, the signal processing on the receiver 51 side may become unstable.

Therefore, the time correction unit 32C of the receiver 51 determines whether or not the difference between the local time of the receiver 51 and the correction time value Ta is equal to or greater than a predetermined threshold, and if the determination result is affirmative, It is preferable to match the local time with the correction time value Ta.
In this way, by appropriately setting the threshold value, the receiver 51 can perform time synchronization only when a deviation that does not satisfy the predetermined communication condition occurs. The frequency at which the time shifts discontinuously and becomes unstable is minimized.

In the case of the present embodiment in which the transmitter / receiver 41 and the receiver 51 employ an OFDM modulation / demodulation method, the predetermined threshold is set to, for example, the guard interval (about 3.2 μsec) of the OFDM modulated wave. do it.
In this case, the time error between the transmitter 41 and the receiver 51 is suppressed to be equal to or less than the guard interval of the OFDM modulated wave. Therefore, assuming the case of road-to-road communication, the time error of the transmission timing of each roadside communication device 2 can be suppressed to be equal to or less than the guard interval, so that a radio communication system that can avoid deterioration due to intersymbol interference can be obtained.

[Other ways to use corrected time values]
In the wireless communication system according to the present embodiment, in the case of inter-road communication, the time correction unit 32C of the receiver 51 not only simply adjusts its own local time to the correction time value Ta but also the correction time value. It is also possible to determine whether or not the time between the transceivers 41 and 51 is accurate using Ta, and to perform a response process when an abnormality occurs based on the determination result.

For example, in the roadside communication device 2 on the receiving side that has obtained the correction time value Ta, if the local time of its own device is shifted from the correction time value Ta for the plurality of roadside communication devices 2 by more than the threshold value, When there is a possibility that an abnormality has occurred in its own internal clock, and only one of the correction time values Ta for the plurality of roadside communication devices 2 shows a deviation greater than the threshold, the roadside communication concerned There may be an abnormality in the internal clock of the machine 2.
In addition, the local time of the own device is once adjusted to the corrected time value Ta for one roadside communication device 2, but then the local time of the own device is again corrected within the predetermined time (for example, one hour). When it deviates from Ta by a threshold value or more, there is a possibility that an abnormality has occurred in the internal clock of at least one of the roadside communication devices 2 on the transmission side and the reception side.

Therefore, in the above case, the time correction unit 32C of the receiver 51 generates information indicating that there is an abnormality in the internal clock of either the transmission side or the reception side or both of the roadside communication devices 2, and this By transmitting information to the central device 4 through the wired communication unit 22 of the roadside communication device 2, it is possible to automatically notify the occurrence of an abnormality of the internal clock.
In this case, wireless transmission of the roadside communication device 2 in which an abnormality of the internal clock is estimated may be stopped together with or separately from the notification of the abnormality occurrence information.

[Other variations]
Each embodiment disclosed this time is an illustration of the present invention and is not restrictive. The scope of the present invention is shown not by the above-described embodiment but by the scope of claims for patent, and includes all modifications within the scope and meaning equivalent to the scope of claims for patent.

  For example, in the intelligent transportation system of the above embodiment, a communication device (portable communication terminal) carried by a pedestrian or the like can be used instead of or in addition to the in-vehicle communication device 3. However, in this case, the mobile communication terminal does not perform wireless transmission during the transmission time (first slot T1) of the roadside communication device 2 as in the case of the in-vehicle communication device 3 of the above embodiment. Need to follow.

2 Roadside communication device 3 In-vehicle communication device (mobile communication device)
21 wireless communication unit 23 control unit 23A transmission control unit 23B data relay unit 23C (first) time correction unit 31 communication unit 32 control unit 32A transmission control unit 32B data relay unit 32C (second) time correction unit 41 transmitter 42 Upper layer 43 MAC part (PDU generation part)
44 PHY unit (modulation transmission unit)
45 RF unit (modulation transmission unit)
46 Transmitting antenna 51 Receiver 52 Upper layer 53 MAC unit (SDU playback unit)
54 PHY unit (demodulation receiver)
55 RF unit (demodulation receiver)
56 Receiving antenna Ts Time stamp Ta Correction time value

Claims (14)

A wireless communication system comprising a transmitter and a receiver,
The transmitter is
A PDU generation unit that generates a PDU in which a time stamp is stored in a state where the position from the beginning of the frame is fixed ;
A modulation transmission unit that modulates a carrier wave by the PDU and transmits a radio wave from a transmission antenna;
A first time correction unit that obtains the time stamp by adding the transmission delay time of the next (a) preset in the transmitter to its local time, and
The receiver
A demodulation receiving unit that extracts the PDU from a radio wave signal reaching the receiving antenna;
An SDU playback unit for playing back the SDU and the time stamp from the PDU;
And a second time correction unit that calculates a correction time value by adding a total value of a reception delay time of (b) and a reproduction delay time of (c) to the time stamp. Communications system.
(A) Transmission delay time from the time of setting the time stamp to the time of radio transmission of the transmitting antenna (b) Reception delay time from the time of receiving the radio wave to the arrival of the preamble to the SDU playback unit (c) Preamble to the SDU playback unit Playback delay time from the time of arrival to the time of time stamp acquisition   The radio communication system according to claim 1, wherein the second time correction unit adjusts its own local time to the correction time value.   The second time correction unit determines whether or not the difference between its own local time and the correction time value is equal to or greater than a predetermined threshold, and on the condition that the determination result is affirmative, The wireless communication system according to claim 2, wherein a local time is adjusted to the corrected time value.   The second time correction unit determines whether or not a difference between its local time and the correction time value is equal to or greater than a predetermined threshold, and when the determination result is affirmative, The radio | wireless communications system of any one of Claims 1-3 which perform the response | compatibility process at the time of abnormality generation about the said receiver or both of these.   When the second time correction unit adjusts its own local time to the correction time value and the determination result is affirmative, an abnormality occurs in the transmitter or the receiver or both of them. The wireless communication system according to claim 4, wherein a time handling process is executed. The transmitter / receiver employs a modulation / demodulation scheme based on OFDM,
The wireless communication system according to any one of claims 3 to 5, wherein the predetermined threshold is set in a guard interval of an OFDM modulated wave. The PDU contains location information of the transmitter,
The wireless communication system according to any one of claims 1 to 6, wherein the second time correction unit further calculates the correction time value by adding a transmission delay time of (d) next to the time stamp.
(D) Transmission delay time according to the distance between the position of the transmitter determined from the position specifying information and the position of the receiver as its own device The transmitter / receiver employs a modulation / demodulation scheme based on OFDM,
The radio communication system according to any one of claims 1 to 7, wherein the accuracy of the time stamp obtained by the first time correction unit is equal to or less than a guard interval of an OFDM modulated wave. A plurality of roadside communication devices in which dedicated time slots are allocated in a time-sharing manner, and a plurality of mobile communication devices allowed to wirelessly transmit only in time zones other than the time slots,
The wireless communication system according to any one of claims 1 to 8, wherein the transmitter is mounted on the roadside communication device, and the receiver is mounted on the roadside communication device, the mobile communication device, or both of them. . A time correction method for a wireless communication system including a transmitter and a receiver,
The transmitter stores a PDU in which a time stamp obtained by adding the next transmission delay time (a) preset in the transmitter to its own local time is stored in a state where the position from the beginning of the frame is fixed. A first step of transmitting to the receiver;
A second step in which the receiver adds a total value of the next reception delay time (b) and the reproduction delay time (c) to the time stamp to obtain a corrected time value;
A time correction method for a wireless communication system.
(A) Transmission delay time from the time of setting the time stamp to the time of radio transmission of the transmitting antenna (b) Reception delay time from the time of receiving the radio wave to the arrival of the preamble to the SDU playback unit (c) Preamble to the SDU playback unit Playback delay time from the time of arrival to the time of time stamp acquisition The transmitter used in the time correction method according to claim 10,
A transmitter that performs the first step. It is the time correction part of the said transmitter used for the time correction method of Claim 10, Comprising:
A time correction unit for a transmitter, wherein the first step is executed. It is the receiver used for the time correction method according to claim 10,
The receiver that performs the second step. It is the time correction part of the said receiver used for the time correction method of Claim 10, Comprising:
A time correction unit of a receiver that performs the second step.

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