JP5479564B2 - Transmission method and terminal device - Google Patents

Description

The present invention relates to a transmission technique , and more particularly, to a transmission method and a terminal device that broadcast a signal including predetermined information.

  Road-to-vehicle communication is being studied to prevent collisions at intersections. In the road-to-vehicle communication, information on the situation of the intersection is communicated between the roadside device and the vehicle-mounted device. Road-to-vehicle communication requires the installation of roadside equipment, which increases labor and cost. On the other hand, if it is the form which communicates information between vehicle-to-vehicle communication, ie, onboard equipment, installation of a roadside machine will become unnecessary. In that case, for example, the current position information is detected in real time by GPS (Global Positioning System), etc., and the position information is exchanged between the vehicle-mounted devices so that the own vehicle and the other vehicle enter the intersection respectively. (See, for example, Patent Document 1).

JP 2005-202913 A

  In a wireless LAN (Local Area Network) compliant with a standard such as IEEE 802.11, an access control function called CSMA / CA (Carrier Sense Multiple Access Collision Aviation) is used. Therefore, in the wireless LAN, the same wireless channel is shared by a plurality of terminal devices. In such CSMA / CA, due to the distance between terminal devices and the influence of obstacles that attenuate radio waves, a situation occurs in which radio signals do not reach each other, that is, a situation where carrier sense does not function. When carrier sense does not function, packet signals transmitted from a plurality of terminal devices collide. In order to increase the communication speed, an OFDM modulation scheme is used in the wireless LAN.

  On the other hand, when a wireless LAN is applied to vehicle-to-vehicle communication, it is necessary to transmit information to an unspecified number of terminal devices, so it is desirable that the signal be transmitted by broadcast. However, at intersections and the like, an increase in the collision of packet signals is assumed due to an increase in the number of vehicles, that is, an increase in the number of terminal devices. As a result, data included in the packet signal is not transmitted to other terminal devices. If such a situation occurs in vehicle-to-vehicle communication, the purpose of preventing a collision accident at the intersection encounter will not be achieved.

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide a technique for reducing the collision probability of packet signals even when the amount of communication increases.

  In order to solve the above-described problem, a radio apparatus according to an aspect of the present invention includes a control unit that sets a time interval until a packet signal is broadcast using a carrier sense multiple access method, and a time interval set by the control unit. And a communication unit for informing the packet signal. The communication unit receives a control signal that is notified at a predetermined frequency from an access control device that controls communication between wireless devices, and the control unit determines a time interval when the control unit can receive the control signal as the remaining time interval. If shorter than the time interval.

  Another aspect of the present invention is also a wireless device. The apparatus is defined such that a frame including a first period formed by a plurality of slots and a second period having a predetermined length is repeated, and the first period and the second period are included. A detection unit that detects a slot that can be used for communication between a plurality of other wireless devices, and a notification unit that reports information on the slot detected by the detection unit. In the second period of the detection unit, a carrier sense multiple access scheme is performed by a plurality of other wireless devices, and the notification unit also reports parameters for deriving a time interval until transmission in the carrier sense multiple access scheme. In addition, the time interval derived from the notified parameter is set shorter than the time interval derived from the parameter stored in advance by another wireless device.

  Yet another aspect of the present invention is a notification method. This method includes a step of setting a time interval until a packet signal is broadcasted by the carrier sense multiple access method, and a step of broadcasting the packet signal at the set time interval. In the setting step, a time interval at which a control signal notified from an access control device that controls communication between wireless devices can be received at a predetermined frequency is made shorter than the remaining time intervals.

  Yet another embodiment of the present invention is also a notification method. This method is defined such that a frame including a first period formed by a plurality of slots and a second period having a predetermined length is repeated, and the first period and the second period are included. And a step of detecting a slot usable for communication between a plurality of other wireless devices, and a step of notifying information on the detected slot. In the second period in the detecting step, the carrier sense multiple access scheme is made by a plurality of other wireless devices, and the step of broadcasting also broadcasts parameters for deriving the time interval until transmission in the carrier sense multiple access scheme Thus, the time interval derived from the notified parameter is made shorter than the time interval derived from the parameter stored in advance by another wireless device.

  It should be noted that any combination of the above-described constituent elements and a conversion of the expression of the present invention between a method, an apparatus, a system, a recording medium, a computer program, etc. are also effective as an aspect of the present invention.

  According to the present invention, it is possible to reduce the collision probability of packet signals even when the amount of communication increases.

It is a figure which shows the structure of the communication system which concerns on the Example of this invention. It is a figure which shows the structure of the access control apparatus of FIG. FIGS. 3A to 3D are diagrams illustrating the format of the frame defined in the frame defining unit in FIG. FIGS. 4A to 4B are diagrams showing the format of the OFDM symbol used in the communication system of FIG. It is a figure which shows the structure of the terminal device mounted in the vehicle of FIG. It is a figure which shows the operation | movement outline | summary of the communication system of FIG. It is a figure which shows another operation | movement outline | summary of the communication system of FIG. 3 is a flowchart showing a procedure for notifying an empty slot in the access control apparatus of FIG. 3 is a flowchart showing a collision slot notification procedure in the access control apparatus of FIG. 2. It is a flowchart which shows the determination procedure of the 1st period or the 2nd period in the terminal device of FIG. It is a flowchart which shows the transmission procedure of the data in the terminal device of FIG. It is a flowchart which shows another transmission procedure of the data in the terminal device of FIG. It is a flowchart which shows another transmission procedure of the data in the terminal device of FIG.

  Before describing the present invention in detail, an outline will be described. An embodiment of the present invention relates to a communication system that performs data communication between terminal devices mounted on a vehicle. The terminal device broadcasts a packet signal that stores information such as the speed and position of the vehicle (hereinafter referred to as “data”). Further, the other terminal device receives the packet signal and recognizes the approach of the vehicle based on the data. Here, the terminal apparatus employs the OFDM modulation scheme for the purpose of increasing the communication speed. Under such circumstances, when the number of terminal devices increases at an intersection or the like, the generation probability of a packet signal increases. In order to cope with this, the communication system according to the present embodiment executes the following processing.

  The communication system according to the present embodiment includes an access control device in addition to a plurality of terminal devices, and the access control device is installed at an intersection, for example. The access control apparatus repeatedly defines a frame formed by the first period and the second period. Here, the first period includes a plurality of slots. The access control device specifies a slot (hereinafter referred to as “empty slot”) that is not used for communication between the plurality of terminal devices by measuring the reception power in each slot in the first period. In addition, the access control device includes frame configuration information and information regarding the specified slot in the control information, and a packet signal storing the control information (hereinafter, also referred to as “control information”) in one slot. Broadcast transmission. Here, it is assumed that one slot is predetermined. The terminal device selects one of the empty slots based on the control information, and broadcasts a packet signal storing data (hereinafter also referred to as “data”) in the selected slot. When transmitting data over a plurality of frames, the terminal device uses slots having the same relative timing in each frame.

  In each slot, the access control apparatus also determines whether a packet signal transmitted from a plurality of terminal apparatuses collides, thereby identifying a slot in which a collision occurs (hereinafter referred to as “collision slot”). The access control apparatus also includes information regarding the identified slot in the control information. Based on the control information, the terminal device confirms whether or not a collision has occurred in the already used slot. When a collision has occurred, the terminal device selects another free slot based on the control information. Here, the access control apparatus does not directly participate in data communication between terminal apparatuses, and does not directly specify a slot to be used for data communication. To the last, the access control device monitors the status of data communication between terminal devices and reports information on empty slots and collision slots. The access control device controls communication between terminal devices by such processing.

  Since the control information is also transmitted in one slot, there is a possibility that the data transmitted from the terminal device that cannot receive the control information collides with the control information. As a result, if other terminal devices cannot receive control information, it becomes difficult to execute the above processing. In order to cope with this, in an OFDM signal used to transmit data, data is not stored in some subcarriers and is set as a null carrier (hereinafter, such a subcarrier is referred to as an “identification carrier”. "). On the other hand, in the OFDM signal used for transmitting control information, a signal is also arranged on the identification carrier. Therefore, even if data and control information collide, the terminal device can detect the presence of control information by observing the signal component of the identification carrier.

  Furthermore, when the number of terminal devices increases compared to the number of slots included in the first period, the number of empty slots decreases, and there is a possibility that collisions are likely to occur. On the other hand, from the viewpoint of preventing traffic accidents, it can be said that the data transmitted from the terminal device close to the intersection is more important than the data transmitted from the terminal device far from the intersection. Considering the above points, the access control apparatus causes the terminal apparatus close to the intersection to use the slot in the first period, and the terminal apparatus far from the intersection to perform CSMA / CA in the second period. Let it run. As a result, the collision probability in the first period is lower than the collision probability in the second period, and an increase in the collision probability for important data is suppressed.

  As described above, a terminal device far from the intersection (hereinafter referred to as “CSMA terminal device”) executes CSMA / CA in the second period. In addition, a terminal device that cannot receive control information from the access control device, that is, a terminal device farther from the intersection (hereinafter referred to as “out-of-area terminal device”) executes CSMA / CA regardless of the second period. Therefore, although both the CSMA terminal device and the out-of-area terminal device are executing CSMA / CA, the transmission opportunity of the CSMA terminal device is less than the transmission opportunity of the out-of-area terminal device. As a result, the throughput of the CSMA terminal apparatus may be extremely low as compared with the throughput of the out-of-area terminal apparatus. Further, since the out-of-area terminal device transmits a packet signal regardless of the frame configuration, there is a possibility that communication not only of the CSMA terminal device but also of the terminal device that transmits data in the first period may be hindered. Even in the case where such an out-of-area terminal device exists, the communication system according to the present embodiment further executes the following process in order to reduce the deterioration of the throughput of the communication system.

  In CSMA / CA executed in a CSMA terminal device and an out-of-area terminal device, values of DIFS (Distributed Inter Frame Space) and CW (Contention Window) are defined. Note that IFS such as SIFS and PIFS may be used instead of DIFS, but here, in order to simplify the description, DIFS is the target of the description. Here, the CSMA terminal device that has detected the packet signal from the terminal device outside the area in the first period sets “first DIFS” and “first CW” as the values of DIFS and CW. Here, the out-of-area terminal device sets “second DIFS” and “second CW” as the values of DIFS and CW, and “first DIFS” and “first CW” are “second DIFS” and “second CW”. Is different from each other. In particular, the first DIFS and the first CW are defined as values smaller than the second DIFS and the second CW.

  Therefore, the waiting period until the transmission at the CSMA terminal apparatus tends to be shorter than the waiting period until the transmission at the terminal apparatus outside the area. As a result, in the second period, it becomes easier for the former to transmit data in preference to the latter in the second period, and a decrease in the throughput of the CSMA terminal device is suppressed. Here, the CSMA terminal apparatus notifies the out-of-area terminal apparatus of the second period, thereby causing the out-of-area terminal apparatus to perform transmission in the second period. As a result, the influence of the terminal device outside the area on the communication of the terminal device that transmits data in the first period is reduced.

  FIG. 1 shows a configuration of a communication system 100 according to an embodiment of the present invention. This corresponds to a case where one intersection is viewed from above. The communication system 100 includes a first vehicle 12a, a second vehicle 12b, a third vehicle 12c, a fourth vehicle 12d, a fifth vehicle 12e, a sixth vehicle 12f, and a seventh vehicle 12g, which are collectively referred to as the access control device 10 and the vehicle 12. , An eighth vehicle 12h, and a ninth vehicle 12i. Each vehicle 12 is equipped with a terminal device (not shown). In addition, a first area 200 and a second area 202 are formed by the access control device 10. The second area 202 is formed outside the first area 200 around the intersection.

  As shown in the drawing, the road that goes in the horizontal direction of the drawing, that is, the left and right direction, intersects the vertical direction of the drawing, that is, the road that goes in the up and down direction, at the central portion. Here, the upper side of the drawing corresponds to the direction “north”, the left side corresponds to the direction “west”, the lower side corresponds to the direction “south”, and the right side corresponds to the direction “east”. The intersection of the two roads is an “intersection”. The first vehicle 12a and the second vehicle 12b are traveling from left to right, and the third vehicle 12c and the fourth vehicle 12d are traveling from right to left. Further, the fifth vehicle 12e and the sixth vehicle 12f are traveling from top to bottom, and the seventh vehicle 12g, the eighth vehicle 12h, and the ninth vehicle 12i are traveling from bottom to top.

  The terminal device mounted on each vehicle 12 acquires data and broadcasts a packet signal in which the data is stored. Here, before explaining the embodiment of the present invention, the operation when the terminal device corresponds to a known wireless LAN, that is, the case corresponding to CSMA / CA will be described. Each terminal device broadcasts and transmits data when it is determined that transmission is possible by executing carrier sense. Therefore, data from a plurality of terminal devices may collide. Moreover, the probability of occurrence of a collision increases as the number of terminal devices increases. In particular, in a place such as an intersection, although a collision of the vehicle 12 is likely to occur, a data collision is also likely to occur, and data is not used in a place where data is required.

  Therefore, the communication system 100 arranges the access control device 10 at the intersection. The access control device 10 generates a frame repeatedly based on a signal received from a GPS satellite (not shown). Here, the frame is formed by a first period and a second period, and the first period includes a plurality of slots. Further, the access control device 10 identifies an empty slot and a collision slot from among a plurality of slots. A method for identifying the empty slot and the collision slot will be described later. The access control device 10 includes information on the identified empty slot and collision slot in the control information. Furthermore, the access control device 10 broadcasts control information in a predetermined slot. For example, in the first slot of each first period, the access control device 10 broadcasts control information.

  A plurality of terminal devices existing in the first area 200 formed by the access control device 10 receives the control information notified by the access control device 10 and selects one of the empty slots. The terminal device broadcasts data in the selected slot. Here, the terminal device broadcasts data in a slot corresponding to the selected slot over a plurality of frames. For example, when the 10th slot from the beginning of the first period is selected, the 10th slot from the beginning of the first period is also used in the next frame. Note that if the control information indicates that the slot used is a collision slot, the terminal device further selects another empty slot. The terminal device repeatedly executes the above process over a period existing in the first area 200.

  On the other hand, a plurality of terminal devices existing in the second area 202 formed by the access control device 10 perform communication by CSMA / CA in the second period. Such a terminal device corresponds to the aforementioned CSMA terminal device. Here, the terminal apparatus determines whether to execute communication in the first period or to execute communication in the second period, according to the received power of the control information from the access control apparatus 10. The terminal device repeatedly executes the above process over a period in which the control information notified by the access control device 10 can be received. That is, the terminal device monitors the control information, and detects that the terminal device has entered the second area 202 when the control information can be received. Moreover, the said terminal device detects having entered into the 1st area 200, when the reception power of control information becomes large. The terminal device that has received data from another terminal device recognizes the presence of the vehicle 12 on which the other terminal device is mounted based on the data.

  Further, like the ninth vehicle 12 i, the terminal device existing outside the second area 202 cannot receive control information from the access control device 10. Such a terminal device corresponds to the aforementioned out-of-area terminal device. For this reason, the out-of-area terminal apparatus cannot recognize the timings of the first period and the second period. At that time, the out-of-area terminal apparatus performs communication by CSMA / CA in all periods. In CSMA / CA, the waiting period until transmission is controlled by DIFS or CW.

  Here, both the control information broadcast from the access control apparatus 10 and the data broadcast from the terminal apparatus use OFDM signals. However, the subcarriers in which both are arranged are not the same. Data is not placed on the aforementioned identification carrier. On the other hand, the identification information is arranged on the identification carrier in addition to the subcarrier on which the data is arranged. As a result, even if data and identification information collide, the terminal device can detect the presence of control information by observing the signal component of the identification carrier. Note that the detection of entry into the second area 202 by the terminal device described above may be performed on the identification carrier.

  FIG. 2 shows the configuration of the access control apparatus 10. The access control device 10 includes an antenna 20, an RF unit 22, a modem unit 24, a processing unit 26, a GPS positioning unit 28, and a control unit 30. The processing unit 26 includes a detection unit 32, a frame definition unit 34, and a generation unit 36. The detection unit 32 includes a power measurement unit 38, a quality measurement unit 40, an empty slot identification unit 42, and a collision slot identification unit 44. .

  The GPS positioning unit 28 receives a signal from a GPS satellite (not shown), and acquires time information based on the received signal. In addition, since a well-known technique should just be used for acquisition of the information of time, description is abbreviate | omitted here. The GPS positioning unit 28 outputs time information to the frame defining unit 34. The frame defining unit 34 acquires time information from the GPS positioning unit 28. The frame defining unit 34 generates a plurality of frames based on the time information. For example, the frame defining unit 34 generates 10 frames of “100 msec” by dividing the period of “1 sec” into 10 with reference to the timing of “0 msec”. By repeating such processing, the frame is defined to be repeated.

  In addition, the frame defining unit 34 divides each frame into a first period and a second period. Here, the lengths of the first period and the second period will be described later. The frame definition unit 34 generates a plurality of slots by dividing the first period into a plurality of times. For example, when the first period is “50 msec” and the slot is “500 μsec”, 100 slots are generated. In such a case, the second period is defined as a period having a predetermined length, such as “50 msec”. As described above, since the communication system 100 adopts the OFDM modulation scheme, each slot is defined to be composed of a plurality of OFDM symbols. The OFDM symbol is composed of a guard interval (GI) and a valid symbol. A guard time may be provided in the front part or the rear part of each slot. Here, a group of a plurality of OFDM symbols included in the slot and a signal transmitted in the second period correspond to the aforementioned packet signal. In the second period, CSMA / CA is executed by a plurality of terminal devices (not shown).

  FIGS. 3A to 3D show the format of the frame defined by the frame defining unit 34. FIG. FIG. 3A shows the structure of the frame. As shown in the figure, it is defined that a plurality of frames are repeated such as the i-th frame to the (i + 2) -th frame. The period of each frame is “100 msec”, for example. FIG. 3B shows the configuration of one frame. As shown in the figure, one frame is composed of a first period and a second period, and the first period is composed of M slots. For example, the period of each slot is “500 μsec”. FIG. 3C shows the configuration of one slot. As shown in the figure, guard times are provided at the front portion and the rear portion of the slot. The remaining period of the slot is composed of N OFDM symbols. FIG. 3D shows the configuration of one OFDM symbol. As shown in the figure, one OFDM symbol is composed of a GI and an effective symbol. Returning to FIG.

  As reception processing, the RF unit 22 receives from the antenna 20 a packet signal transmitted in communication between other terminal devices (not shown) in each slot of the first period or in the second period. The RF unit 22 performs frequency conversion on a radio frequency packet signal received via the antenna 20 to generate a baseband packet signal. Further, the RF unit 22 outputs a baseband packet signal to the modem unit 24. In general, baseband packet signals are formed by in-phase and quadrature components, so two signal lines should be shown, but here only one signal line is shown for clarity. Shall be shown.

  The RF unit 22 also includes an LNA (Low Noise Amplifier), a mixer, an AGC, and an A / D conversion unit. As a transmission process, the RF unit 22 performs frequency conversion on the baseband packet signal input from the modem unit 24 in each slot or second period of the first period, and generates a radio frequency packet signal. Further, the RF unit 22 transmits a radio frequency packet signal from the antenna 20. The RF unit 22 also includes a PA (Power Amplifier), a mixer, and a D / A conversion unit.

  The modem unit 24 demodulates the baseband packet signal from the RF unit 22 as a reception process. Further, the modem unit 24 outputs the demodulated result to the processing unit 26. The modem unit 24 also modulates the data from the processing unit 26 as a transmission process. Further, the modem unit 24 outputs the modulated result to the RF unit 22 as a baseband packet signal. Here, since the communication system 100 corresponds to the OFDM modulation scheme, the modem unit 24 also executes FFT (Fast Fourier Transform) as reception processing and also executes IFFT (Inverse Fast Fourier Transform) as transmission processing.

  The power measuring unit 38 receives the received signal from the RF unit 22 or the modem unit 24 and measures the received power. Here, the received power is measured in slot units. Therefore, the power measuring unit 38 measures the received power for each of the plurality of slots. The power measuring unit 38 outputs the received power in slot units to the empty slot specifying unit 42 and the collision slot specifying unit 44. The quality measuring unit 40 receives the demodulation result from the modem unit 24 and measures the signal quality for each of the plurality of slots. Here, the error rate is measured as the signal quality. In addition, since a well-known technique should just be used for the measurement of an error rate, description is abbreviate | omitted here. Further, as the signal quality, instead of the error rate, EVM (Error Vector Magnet) or the like may be measured. The quality measuring unit 40 outputs the error rate to the collision slot specifying unit 44.

  The empty slot identifying unit 42 receives the received power in slot units from the power measuring unit 38. The empty slot specifying unit 42 compares each received power with a threshold (hereinafter referred to as “empty slot threshold”), and specifies a slot whose received power is smaller than the empty slot threshold. That is, the empty slot specifying unit 42 detects, as an empty slot, a slot that can be used for communication between a plurality of terminal devices from a plurality of slots in the first period of the first period and the second period. Here, when there are a plurality of empty slots, the empty slot specifying unit 42 specifies them. The empty slot specifying unit 42 outputs information regarding the specified empty slot to the generating unit 36.

  The collision slot specifying unit 44 receives the received power for each slot from the power measuring unit 38 and receives the error rate for each slot from the quality measuring unit 40. The collision slot specifying unit 44 associates the received power with the error rate for each slot. The collision slot identifying unit 44 compares the received power with the first threshold value and compares the error rate with the second threshold value for each slot. The collision slot identifying unit 44 identifies, as a collision slot, a slot whose received power is larger than the first threshold and whose error rate is worse than the second threshold. That is, the collision slot specifying unit 44 recognizes a slot having a large received power but having deteriorated communication quality as a collision slot. As described above, the collision slot specifying unit 44 generates a collision when a plurality of terminal apparatuses transmit signals in duplicate from a plurality of slots in the first period of the first period and the second period. The detected slot is detected as a collision slot. The collision slot identification unit 44 outputs information regarding the identified collision slot to the generation unit 36.

  The generation unit 36 receives information related to the empty slot from the empty slot specifying unit 42 and receives information related to the collision slot from the collision slot specifying unit 44. The generation unit 36 generates control information while including information regarding empty slots and the identification number of the access control device 10. The generation unit 36 also stores parameters for deriving a period until transmission by CSMA / CA in the second period. For example, the parameters include DIFS and CW. DIFS is one of IFS and is a period defined for determining whether or not a channel is in an idle state.

  Further, CW is a value used for deriving a back-off period, and the back-off period is derived by multiplying a random value by a certain period. Here, the random number value is a random integer generated from a range of 0 to CW. The sum of the DIFS and the back-off period corresponds to the period until transmission by CSMA / CA. Here, the DIFS stored in the generation unit 36 corresponds to the first DIFS, and the CW stored in the generation unit 36 corresponds to the first CW. Note that the terminal device (not shown) stores the second DIFS and the second CW in advance, but the first DIFS and the second CW are smaller values than the second DIFS and the second CW.

  Here, each of the plurality of slots included in the first period is assigned a number (hereinafter referred to as “slot number”) that is “1” and “2” in order from the front. The generation unit 36 includes the slot number of the empty slot included in the previous frame as information on the empty slot in the control information. In addition, the generation unit 36 includes the slot number of the collision slot included in the previous frame as information on the collision slot in the control information. In addition to these, the frame defining unit 34 includes the first DIFS and the first CW (hereinafter collectively referred to as “parameter”) in the control information. Further, the generation unit 36 receives information on frames and slots from the frame definition unit 34 and periodically assigns control information to one slot included in each first period. Here, it is predetermined that control information is assigned to the slot of slot number “1” in each first period, that is, the first slot, and the generation unit 36 assigns control information to the slot. The generation unit 36 outputs control information to the modem unit 24 in the assigned slot.

  As described above, since the communication system 100 supports the OFDM modulation scheme, the generation unit 36 generates control information as an OFDM signal. An OFDM signal is also used for data communication between a plurality of terminal devices (not shown). Here, an OFDM signal for arranging control information (hereinafter also referred to as “control information”) and an OFDM signal for arranging data (hereinafter also referred to as “data”) are compared. explain. FIGS. 4A and 4B show OFDM symbol formats used in the communication system 100. FIG. 4A corresponds to control information, and FIG. 4B corresponds to data.

  Here, in both, a vertical direction shows a frequency and a horizontal direction shows time. In the vertical direction, numbers “31”, “30”,... “−32” are shown in order from the top, but these numbers are given to identify subcarriers (hereinafter referred to as “subcarriers”). Carrier number). Further, in the OFDM signal, the frequency of the subcarrier of subcarrier number “31” is the highest, and the frequency of the subcarrier of subcarrier number “−32” is the lowest. In the figure, “D” corresponds to a data symbol, “P” corresponds to a pilot symbol, and “N” corresponds to null.

  In common with control information and data, subcarriers with subcarrier numbers “31” to “27”, “2”, “0”, “−2”, “−26” to “−32” are null. is there. In the control information, subcarriers with subcarrier numbers “26” to “3” and “−3” to “−25” are also used in data, and the symbols are used for the same purpose in both cases. . On the other hand, the subcarrier numbers “1” and “−1” in the control information are not used in the data. These correspond to the aforementioned identification carrier. That is, the identification carrier is arranged on a subcarrier near the center frequency in the OFDM signal. Furthermore, a guard band is provided in the control information between the subcarrier used in the data and the identification carrier, that is, in the subcarrier numbers “2” and “−2”. The subcarriers with subcarrier numbers “−2” to “2” may be collectively referred to as “identification carriers”.

  Here, the production | generation part 36 arrange | positions the information regarding an empty slot and the information regarding a collision slot in subcarriers other than the identification carrier of control information. Moreover, the production | generation part 36 arrange | positions the information regarding a flame | frame on an identification carrier. Moreover, the production | generation part 36 may arrange | position preferentially information with high importance not only to these information but an identification carrier. An example of information included in the control information is the latitude / longitude of the access control device 10 and the identification number of the access control device 10. A known signal is arranged in the OFDM symbol in front of the packet signal. Such a known signal is used for AGC in the terminal device and estimation of transmission path characteristics. The generation unit 36 may arrange the known signal on the identification carrier over a period of a part of the predetermined slot. Such a known signal is used like UW (Unique Word), for example. Returning to FIG.

  The modem unit 24 and the RF unit 22 notify the control information generated by the generation unit 36 from the antenna 20. In addition, the terminal device using the slot according to the information regarding the empty slot and the information regarding the collision slot included in the control information uses the slot corresponding to the slot over a plurality of frames. For example, the slot with the slot number “10” is continuously used. The control unit 30 controls processing of the entire access control apparatus 10.

  This configuration can be realized in terms of hardware by a CPU, memory, or other LSI of any computer, and in terms of software, it can be realized by a program loaded in the memory, but here it is realized by their cooperation. Draw functional blocks. Accordingly, those skilled in the art will understand that these functional blocks can be realized in various forms by hardware only, software only, or a combination thereof.

  FIG. 5 shows a configuration of the terminal device 14 mounted on the vehicle 12. The terminal device 14 includes an antenna 50, an RF unit 52, a modem unit 54, a processing unit 56, and a control unit 58. The processing unit 56 includes a timing specifying unit 60, an acquiring unit 62, a generating unit 64, and a notifying unit 70. The timing specifying unit 60 includes a control information extracting unit 66, a determining unit 80, a comparing unit 86, and a measuring unit 88. Including. Furthermore, the determination unit 80 includes a parameter reception unit 94 and a period derivation unit 96. The antenna 50, the RF unit 52, and the modem unit 54 execute the same processing as the antenna 20, the RF unit 22, and the modem unit 24 in FIG. Therefore, these descriptions are omitted here.

  The acquisition unit 62 includes a GPS receiver (not shown), a gyroscope, a vehicle speed sensor, and the like. Based on data supplied from the GPS receiver, a vehicle 12 (not shown), that is, a vehicle 12 on which the terminal device 14 is mounted, Get direction, speed, etc. The existence position is indicated by latitude and longitude. Since a known technique may be used for these acquisitions, description thereof is omitted here. The acquisition unit 62 outputs the acquired information to the generation unit 64.

  The control information extraction unit 66 receives the demodulation result from the modem unit 54. Further, the control information extraction unit 66 monitors the subcarrier portion corresponding to the identification carrier in the demodulation result. When valid data is included in the subcarrier portion corresponding to the identification carrier, the control information extraction unit 66 recognizes that the slot including the control information is received. Further, the control information extraction unit 66 establishes frame and slot synchronization with reference to the timing at which the slot including the control information is received. As a result, the control information extraction unit 66 receives control information notified at a predetermined frequency from the access control device 10 (not shown).

  Further, the control information extraction unit 66 acquires information on empty slots and information on collision slots from the control information. The control information extraction unit 66 also acquires parameters and an identification number of the access control apparatus 10 (not shown) from the control information. The control information extraction unit 66 outputs information on empty slots, information on collision slots, and determination unit 80, outputs parameters to parameter reception unit 94, and outputs identification numbers to generation unit 64 via determination unit 80.

  The measurement unit 88 receives a control information reception signal from the RF unit 52 or the modem unit 54 and measures received power. The measurement unit 88 outputs the received power to the comparison unit 86. The comparison unit 86 receives the received power from the measurement unit 88 and holds an area determination threshold value in advance. The comparison unit 86 compares the reception power with the area determination threshold value, and determines the use of the first period if the reception power is larger than the area determination threshold value. On the other hand, the comparison unit 86 determines to use the second period unless the received power is larger than the area determination threshold value. The comparison unit 86 outputs the determined content to the determination unit 80.

  The determination unit 80 receives the content determined by the comparison unit 86 from the comparison unit 86. In addition, the determination unit 80 receives information on empty slots and information on collision slots from the control information extraction unit 66. First, the determination unit 80 grasps whether to use the first period or the second period based on the information from the comparison unit 86. The determination unit 80 executes CSMA / CA (hereinafter referred to as “hereinafter referred to as“ CSMA / CA ”) regardless of the first period and the second period when the information from the comparison unit 86 is not received, that is, when the control information is not received. "Out-of-area operation"). Here, description will be made in the order of (1) using the first period, (2) using the second period, and (3) executing the operation outside the area.

(1) When using the first period The determination unit 80 selects one empty slot based on the information regarding the empty slot. It should be noted that an empty slot may be selected arbitrarily. The determination unit 80 outputs information regarding the selected empty slot to the generation unit 64. The generation unit 64 receives information from the acquisition unit 62. The generation unit 64 generates data based on the information. Here, the data is formed as shown in FIG. In addition, the generation unit 64 receives an instruction regarding an empty slot from the determination unit 80 and outputs data to the modem unit 54 in an empty slot corresponding to the instruction. Note that the processor 56 may perform carrier sense before outputting data. Further, the generation unit 64 outputs data in the slots having the same slot number even in the first period of the next frame. In that case, the production | generation part 64 may include the identification number contained in the received control information in data.

  Even during the continuation of such processing, the control information extraction unit 66 continues to acquire information on empty slots and information on collision slots from the control information for each frame. The determination unit 80 confirms whether the slot number corresponding to the currently used slot is not a collision slot based on the information on the collision slot. If not determined as a collision slot, the determination unit 80 continues to output the same slot number as before to the generation unit 64. On the other hand, if the slot is determined to be a collision slot, the determination unit 80 selects one empty slot again based on the information regarding the empty slot. That is, a slot different from the slot selected so far is selected. The determination unit 80 outputs information regarding the selected empty slot to the generation unit 64. Thereafter, the generation unit 64 performs the same processing.

(2) Case of Using Second Period Next, a case of using the second period will be described. The parameter receiving unit 94 receives parameters from the control information extracting unit 66. The parameter receiving unit 94 derives the parameters to the period deriving unit 96. The period deriving unit 96 stores the second DIFS and the second CW in advance, and derives a waiting period until the packet signal is transmitted based on the second DIFS and the second CW. Note that since a known technique may be used for deriving the waiting period, the description thereof is omitted here. In addition, when receiving a parameter from the parameter receiving unit 94, the period deriving unit 96 derives a waiting period until a packet signal is transmitted based on the first DIFS and the first CW corresponding to the parameter. The waiting period until the packet signal is transmitted is specified by the DIFS and the back-off period, but the description is omitted here.

  Here, the period deriving unit 96 preferentially uses the first DIFS and the first CW when the control information can be received, and uses the second DIFS and the second CW in the remaining cases. For this reason, when the second period is used, the first DIFS and the first CW are used. Also, the first DIFS and the first CW are set to values smaller than the second DIFS and the second CW, respectively. This corresponds to shortening the waiting period when control information can be received to be shorter than the remaining waiting period. The period deriving unit 96 outputs the derived period to the generating unit 64. That is, the period deriving unit 96 sets a waiting period until the packet signal is transmitted by CSMA in the generation unit 64.

  The processing unit 56 performs carrier sense in the second period. If the data can be transmitted as a result of the carrier sense, the generation unit 64 outputs the data to the modem unit 54 in the period set by the period derivation unit 96. Again, the generation unit 64 may include the identification number included in the received control information in the data. As a result, the modem unit 54, the RF unit 52, and the antenna 50 broadcast the packet signal in the period set in the period deriving unit 96 over the second period in which CSMA / CA is to be executed. The notification unit 70 acquires data from another terminal device 14 (not shown), and notifies the driver of the approach of another vehicle 12 (not shown) according to the contents of the data. The processing in the notification unit 70 is not limited to this.

  In the second period, processing different from the above description may be performed. For example, even when the second period is used, the period derivation unit 96 uses the second DIFS and the second CW to transmit the packet signal as in the case where the control information is not received. Deriving the waiting period. Moreover, the process part 56 detects reception of the packet signal from the other terminal device 14 alert | reported without following the information contained in control information in a 1st period. The packet signal corresponds to the packet signal from the terminal device outside the area. Although the operation of such an out-of-area terminal device will be described later, the out-of-area terminal device generally does not grasp the frame configuration.

  For this reason, the out-of-area terminal apparatus transmits a packet signal by performing carrier sense while using the second DIFS and the second CW regardless of whether the terminal period is the first period. The processing unit 56 obtains the transmission source of the packet signal when the reception timing of the packet signal is shifted from the slot timing or when the identification number of the access control device 10 is not included in the received packet signal. Authorize it as an out-of-area terminal device. When the processing unit 56 receives a packet signal from a terminal device outside the area, the processing unit 56 notifies the period deriving unit 96 to that effect.

  When receiving the notification from the processing unit 56, the period deriving unit 96 derives a waiting period until a packet signal is transmitted based on the first DIFS and the first CW, as described above. In addition, the generation unit 64, the modem unit 54, the RF unit 52, and the antenna 50 broadcast the packet signal in the period set in the period derivation unit 96 over the second period. At that time, the generation unit 64 includes information on the frame configuration in the packet signal. The information regarding the frame configuration includes, for example, the relative start time and end time of the first period relative to the current time, and the relative start time and end time of the second period. That is, when the generation unit 64 detects a packet signal from an out-of-area terminal device, the generation unit 64 or the like switches the second DIFS and the second CW to the first DIFS and the first CW, respectively, and transfers the second DIFS and the second CW to the out-of-area terminal device. A packet signal for informing at least two periods is transmitted. The period deriving unit 96 sets the second DIFS and the second CW again after transmitting the packet signal.

  Hereinafter, such an operation will be described with specific values as examples. When the second DIFS is 64 μsec, the second CW is 0 to 16 and the slot period is 8 μsec, the range between the DIFS period and the random backoff period is 64 to 192 μsec. On the other hand, if the first DIFS is 16 μsec and the first CW is defined to be 0 to 5, the range between the DIFS period and the random backoff period is 56 μsec or less. Therefore, the latter can transmit a packet signal before the former, and can notify the frame configuration.

(3) When executing an out-of-area operation Further, a case of executing an out-of-area operation will be described. When the control information extraction unit 66 cannot receive control information for a predetermined period, the determination unit 80 determines execution of the out-of-area operation. As described above, the period deriving unit 96 derives a waiting period until the packet signal is transmitted using the second DIFS and the second CW. The processing unit 56 performs carrier sense in a waiting period until a packet is transmitted, and the generation unit 64, the modem unit 54, the RF unit 52, and the antenna 50 transmit a packet signal. In addition, if the packet signal received via the antenna 50, the RF unit 52, and the modem unit 54 includes information on the frame configuration, the determining unit 80 extracts the information. Based on the information, the determination unit 80 specifies the second period and controls the generation unit 64 and the like so as to limit the transmission timing of the packet signal to the second period. The control unit 58 controls the operation of the entire terminal device 14.

  The operation of the communication system 100 configured as above will be described. FIG. 6 shows an operation outline of the communication system 100. In the drawing, the first terminal device 14a to the fourth terminal device 14d are shown, but these are not related to the positions of the first vehicle 12a to the fourth vehicle 12d in FIG. The second terminal device 14b is present in the second area 202 of FIG. The horizontal direction in the figure corresponds to time, and three frames from the i-th frame to the i + 2th frame are shown as described in the uppermost stage. For the sake of clarity, it is assumed that one frame includes 12 slots and a second period. As shown in the figure, the access control device 10 broadcasts control information in the first slot of each frame. “Control” in the figure corresponds to control information. In the lower part, information on empty slots and information on collision slots included in the control information are shown while being associated with the slots. “Empty” in the figure corresponds to an empty slot, and “attack” corresponds to a collision slot.

  Furthermore, the lower part shows the timing at which the first terminal device 14a to the fourth terminal device 14d notify the data. “De” in the figure corresponds to data. The first terminal device 14a, the third terminal device 14c, and the fourth terminal device 14d refer to the control information and select an empty slot. In the i-th frame, the first terminal device 14a, the third terminal device 14c, and the fourth terminal device 14d broadcast data in the selected empty slot. At this time, since the empty slots selected in the third terminal device 14c and the fourth terminal device 14d are the same, the data notified from both collide. The access control device 10 detects the occurrence of a collision in the slot. In the (i + 1) th frame, the control information broadcast from the access control device 10 indicates the slot in which the collision occurred as information on the collision slot. On the other hand, the 2nd terminal device 14b alert | reports data based on CSMA / CA in a 2nd period.

  The first terminal apparatus 14a uses the slot having the same slot number again because there is no collision in the already used slot. On the other hand, the third terminal device 14c and the fourth terminal device 14d select another empty slot again because a collision has occurred in the already used slot. The third terminal device 14c and the fourth terminal device 14d notify the data in the selected empty slot. Since all data does not collide, the collision slot is not indicated in the control information broadcast from the access control apparatus 10 in the (i + 2) th frame. Therefore, in the (i + 2) th frame, the first terminal device 14a, the third terminal device 14c, and the fourth terminal device 14d use again the slot having the same slot number as the already used slot.

  FIG. 7 shows another operation outline of the communication system 100. FIG. 7 is shown similarly to FIG. In the uppermost stage, a frame including a first period and a second period composed of a plurality of slots is shown. Here, control information is omitted. Moreover, the lower stage shows the timing which the 1st terminal device 14a alert | reports data. Here, the 1st terminal device 14a is the terminal device 14 which should operate | move in a 1st period. Moreover, the lower part shows the timing which the 2nd terminal device 14b alert | reports data. Here, the 2nd terminal device 14b is corresponded to the above-mentioned CSMA terminal device. Furthermore, the lowest level shows the timing at which the third terminal apparatus 14c notifies the data. Here, the third terminal device 14c is the aforementioned out-of-area terminal device.

  The first terminal device 14a transmits data in a predetermined slot. On the other hand, since the third terminal apparatus 14c does not grasp the frame configuration, the third terminal apparatus 14c transmits data at a timing that crosses a plurality of slots regardless of the slot timing. The second access control device 10b receives data from the third terminal device 14c. When the terminal device 14 (not shown) transmits data in the last slot of the first period, the second DIFS 302 is set starting from the start timing of the second period in the first terminal device 14a and the third terminal device 14c. .

  On the other hand, since the second terminal apparatus 14b has received the data from the third terminal apparatus 14c, the first DIFS 300 and the back-off 304 are set. Thereafter, the second terminal device 14b transmits data. Since the data includes information on the frame configuration and the current time, the third terminal device 14c recognizes the timing at which the second period is arranged. Thereafter, after the second DIFS 302 has elapsed from the first terminal device 14a to the third terminal device 14c, the third terminal device 14c waits for the backoff 306 before transmitting data. The data is transmitted in the second period.

  FIG. 8 is a flowchart showing a procedure for notifying an empty slot in the access control apparatus 10. Here, the object of explanation is the first period. The detection unit 32 sets the slot number m to 2 (S10). The power measuring unit 38 measures received power (S12). If the received power is smaller than the empty slot threshold (Y in S14), the empty slot specifying unit 42 specifies the slot with the slot number m as an empty slot (S16). If the received power is not smaller than the empty slot threshold (N in S14), the empty slot specifying unit 42 skips the process of step 16. If the slot number m is not the maximum number M (N in S18), the detection unit 32 adds 1 to the slot number m (S20), and returns to Step 12. On the other hand, if the slot number m is the maximum number M (Y in S18), the generation unit 36 includes the slot number of the empty slot in the control information (S22). The modem unit 24 and the RF unit 22 notify the control information (S24).

  FIG. 9 is a flowchart illustrating a collision slot notification procedure in the access control apparatus 10. Again, the subject of explanation is the first period. The detection unit 32 sets the slot number m to 2 (S40). The power measuring unit 38 measures the received power, and the quality measuring unit 40 measures the error rate (S42). If the received power is greater than the first threshold and the error rate is greater than the second threshold (Y in S44), the collision slot identifying unit 44 identifies the slot with the slot number m as a collision slot (S46). ). If the received power is not greater than the first threshold value or the error rate is not greater than the second threshold value (N in S44), the collision slot specifying unit 44 skips the process of step 46. If the slot number m is not the maximum number M (N in S48), the detection unit 32 adds 1 to the slot number m (S50), and returns to Step 42. On the other hand, if the slot number m is the maximum number M (Y in S48), the generation unit 36 includes the slot number of the collision slot in the control information (S52). The modem unit 24 and the RF unit 22 notify the control information (S54).

  FIG. 10 is a flowchart illustrating a procedure for determining the first period or the second period in the terminal device 14. The control information extraction unit 66 acquires control information (S160). The measuring unit 88 measures received power (S164). If the received power is larger than the area determining threshold value (Y in S166), the comparing unit 86 determines to use the first period (S168). The determination unit 80 executes communication by the slot (S172). If the received power is not greater than the area determining threshold value (N in S166), the comparing unit 86 determines to use the second period (S174). The determination unit 80 performs communication by CSMA / CA (S178). If control information is not acquired in step 160, the determination unit 80 determines to perform an out-of-area operation.

  FIG. 11 is a flowchart showing a data transmission procedure in the terminal device 14. The control information extraction unit 66 acquires control information (S70). If the slot to be used has already been identified (Y in S72), the determination unit 80 confirms whether or not a collision has occurred in the slot. If a collision has occurred (Y in S74), the determination unit 80 changes the slot (S76). If no collision has occurred (N in S74), step 76 is skipped. On the other hand, if the slot to be used is not already specified (N in S72), the determination unit 80 specifies an empty slot (S78). The generation unit 64 transmits data in the specified slot (S80).

  FIG. 12 is a flowchart illustrating another data transmission procedure in the terminal device 14. Here, a case where execution of an operation in the second period or execution of an out-of-area operation is selected will be described. If the control information extraction unit 66 cannot receive the control information (N in S200), the determination unit 80 sets the second DIFS (S202). The determination unit 80 sets the second back-off period from the second CW (S204). If the processing unit 56 does not detect a signal by carrier sense (N in S206), the generation unit 64 and the like notify the packet signal (S208). If the processing unit 56 detects a signal by carrier sense (Y in S206), step 208 is skipped.

  On the other hand, if the control information extraction unit 66 can receive the control information (Y in S200), the parameter reception unit 94 extracts the first DIFS and the first CW from the control information (S210). The determination unit 80 sets the first DIFS (S212). The determination unit 80 sets the first back-off period from the first CW (S214). If the processing unit 56 does not detect a signal by carrier sense (N in S216), the generation unit 64 and the like notify the packet signal (S218). If the processing unit 56 detects a signal by carrier sense (Y in S216), step 218 is skipped.

  FIG. 13 is a flowchart illustrating still another data transmission procedure in the terminal device 14. The determination unit 80 sets the second DIFS and the second CW (S230). If it is the first period (Y in S232) and data from outside the area is received (Y in S234), the determination unit 80 sets the first DIFS and the first CW (S236), and returns to Step 232. If data from outside the area is not received (N in S234), the process returns to step 232; If it is not the first period (N in S232) and there is no data to be transmitted (N in S238), the process returns to Step 232; On the other hand, if there is data to be transmitted (Y in S238) and the processing unit 56 does not detect the received power of the data within the IPS period (N in S240), the process returns to step S234.

  If the processing unit 56 detects the received power of data within the IPS period (Y in S240), the period deriving unit 96 generates a random backoff period (S242). If the processing unit 56 does not detect the received power of data during the random backoff period (N in S244), the process returns to step 234. If the processing unit 56 detects the received power of data during the random backoff period (Y in S244), the generation unit 64 and the like transmit data (S246). If the notification of the frame configuration is completed (Y in S248), the determination unit 80 sets the second DIFS and the second CW (S250). If the notification of the frame configuration is not completed (N in S248), step 250 is skipped.

  According to the embodiment of the present invention, since the period until the packet signal when the control signal can be received is made smaller than the period until the packet signal when the control signal cannot be received, When the control signal can be received, the packet can be transmitted quickly. Further, since the first DIFS and the first CW are included in the control information from the access control device, the first DIFS and the first CW can be freely set. In addition, since the packet is transmitted quickly when the control signal can be received, the opportunity for transmission can be increased even if the transmission period is limited. When the packet signal from the out-of-area terminal device is detected, the period until the packet signal is notified is shortened, so that the opportunity for transmitting the packet signal can be increased. In addition, since the opportunity for transmitting the packet signal is increased, the frame configuration can be notified to the terminal device outside the area.

  Moreover, since the slot which can be used for the communication between several terminal devices is alert | reported from the several slots contained in the 1st period, the occurrence probability of the collision in the communication between several terminal devices can be reduced. In addition, since the probability of collision in communication between a plurality of terminal devices is reduced, the collision probability of packet signals can be reduced even when the amount of communication increases. Further, since empty slots are identified based on the received power for each of the plurality of slots, the identification can be easily performed. In addition, since the number of the empty slot included in the previous frame is notified, the instruction to the terminal device can be surely executed. In addition, since the terminal device using the empty slot uses a slot corresponding to the slot over a plurality of frames, the processing can be simplified. Further, since the access control device does not participate in the data communication between the terminal devices, but only notifies the index relating to the empty slot, it can be easily applied to a communication system based on CSMA / CA.

  In addition, since a plurality of terminal devices transmit a signal redundantly to notify a slot in which a collision has occurred among a plurality of slots included in the first period, a collision in communication between the plurality of terminal devices is reported. The probability of occurrence can be reduced. Further, since the collision slot is specified based on the received power for each of the plurality of slots and the signal quality for each of the plurality of slots, the specification can be easily performed. In addition, since the number of the collision slot included in the previous frame is notified, the instruction to the terminal device can be reliably executed. In addition, since the access control apparatus does not participate in data communication between terminal apparatuses and only notifies an index related to a collision slot, the access control apparatus can be easily applied to a communication system based on CSMA / CA.

  Further, by including the second period in the frame, it is possible to separate communication with high importance from communication with low importance. Moreover, since communication with high importance and communication with low importance are isolate | separated, the increase in the collision probability in communication with high importance can be suppressed. Further, since the length of the first period is adjusted according to the number of terminal devices, the first period and the second period can be used effectively. In addition, since the number of terminal devices is estimated from the received power, the number of terminal devices can be easily estimated even when the terminal device is not in direct communication.

  In addition, since information on terminal devices that should use a plurality of slots in the first period of the first period and the second period is notified, the plurality of terminal apparatuses can be distributed to the first period or the second period. In addition, since a plurality of terminal devices are distributed to the first period or the second period, the probability of collision of packet signals in the first period can be reduced. Further, since the area determination threshold value is used as the information related to the terminal apparatus that should use a plurality of slots in the first period of the first period and the second period, the value can be easily adjusted. In addition, since the area determination threshold value is used, the use of the first period or the second period can be designated even if the terminal device is not provided with a position positioning function.

  Also, among the control information, the identification carrier is not used for data, and the remaining subcarriers are also used for data, so that even if the control information collides with the data signal, the signal component of the control information is observed. Thus, the presence of the control information can be detected. In addition, since a guard band is provided between the identification carrier and other subcarriers, interference between the two can be reduced, and the arrival probability of information transmitted on the identification carrier can be improved. Moreover, since important information is arranged on the identification carrier, the arrival probability of important information can be improved. Moreover, since UW is arrange | positioned at an identification carrier, the detection accuracy of an identification carrier can be improved.

  In the above, this invention was demonstrated based on the Example. This embodiment is an exemplification, and it will be understood by those skilled in the art that various modifications can be made to the combination of each component and each processing process, and such modifications are also within the scope of the present invention. .

  In the embodiment of the present invention, control information notified from the access control device 10 and data notified from one terminal device 14 are allocated to one slot. However, the present invention is not limited to this. For example, control information and data may be assigned to two or more slots. According to this modification, the communication speed of control information and data can be improved.

  In the embodiment of the present invention, the identification carrier corresponds to two subcarriers. Further, the identification carrier is arranged on a subcarrier near the center frequency of the OFDM symbol. However, the present invention is not limited to this. For example, the identification carrier may correspond to two or more subcarriers, and the identification carrier may be arranged on a subcarrier other than the vicinity of the center frequency of the OFDM symbol. According to this modification, the degree of freedom in designing the communication system 100 can be improved.

  DESCRIPTION OF SYMBOLS 10 Access control apparatus, 12 Vehicle, 14 Terminal apparatus, 20 Antenna, 22 RF part, 24 Modulation / demodulation part, 26 Processing part, 28 GPS positioning part, 30 Control part, 32 Detection part, 34 Frame definition part, 36 Generation part, 38 Power measurement unit, 40 quality measurement unit, 42 empty slot identification unit, 44 collision slot identification unit, 50 antenna, 52 RF unit, 54 modulation / demodulation unit, 56 processing unit, 58 control unit, 60 timing identification unit, 62 acquisition unit, 64 Generating unit, 66 control information extracting unit, 70 notifying unit, 80 determining unit, 86 comparing unit, 88 measuring unit, 94 parameter receiving unit, 96 period deriving unit, 100 communication system.

Claims (4)

Within the area formed around the location of the access control device, a first period in which the access control device can broadcast a packet signal of the OFDM modulation scheme and a second period that can be used for CSMA / CA communication between terminal devices. A first transmitter that broadcasts an OFDM modulated packet signal by CSMA / CA using DIFS and CW in a second period out of time-division multiplexed frames of two periods;
Outside the area formed around the installation position of the access control device, the second transmission for broadcasting the OFDM modulated packet signal by CSMA / CA using DIFS and CW regardless of the frame configuration And
A receiving unit that receives a packet signal of an OFDM modulation scheme for identifying the second period from the access control apparatus in the first period of the frame within an area formed around the installation position of the access control apparatus; With
The first transmission unit identifies a second period based on the packet signal received by the reception unit,
The receiving unit is a packet signal from another terminal device within an area formed around the installation position of the access control device outside the area formed around the installation position of the access control device, And also receives the packet signal of the OFDM modulation scheme for specifying the second period,
The second transmission unit specifies the second period outside the area formed around the installation position of the access control device based on the packet signal from the other terminal device received by the reception unit. In the second period, a terminal apparatus that broadcasts a packet signal of an OFDM modulation scheme by CSMA / CA using DIFS and CW . Within the area formed around the location of the access control device, a first period in which the access control device can broadcast a packet signal of the OFDM modulation scheme and a second period that can be used for CSMA / CA communication between terminal devices. A first transmitter that broadcasts an OFDM-modulated packet signal by CSMA / CA using DIFS and CW in a second period among 100 msec frames that are time-division multiplexed in two periods;
Outside the area formed around the installation position of the access control device, the OFDM modulation type packet signal is broadcasted by CSMA / CA using DIFS and CW regardless of the structure of the frame of 100 msec. Two transmitters;
Reception for receiving a packet signal of an OFDM modulation scheme for specifying the second period from the access control apparatus in the first period out of 100 msec frames within the area formed around the installation position of the access control apparatus With
The first transmission unit identifies a second period based on the packet signal received by the reception unit,
The receiving unit is a packet signal from another terminal device within an area formed around the installation position of the access control device outside the area formed around the installation position of the access control device, And also receives the packet signal of the OFDM modulation scheme for specifying the second period,
The second transmission unit specifies the second period outside the area formed around the installation position of the access control device based on the packet signal from the other terminal device received by the reception unit. In the second period, a terminal apparatus that broadcasts a packet signal of an OFDM modulation scheme by CSMA / CA using DIFS and CW . The terminal apparatus according to claim 1, wherein the reception unit performs timing synchronization based on the received packet signal of the OFDM modulation scheme. Within the area formed around the location of the access control device, a first period in which the access control device can broadcast a packet signal of the OFDM modulation scheme and a second period that can be used for CSMA / CA communication between terminal devices. Receiving a packet signal of an OFDM modulation scheme for specifying the second period from the access control apparatus in the first period among the frames in which the two periods are time-division multiplexed;
Identifying the second period based on the received packet signal;
Oite within the area formed around the installation position of the access control device, of the frame, in the second period, the CSMA / CA using a DIFS and CW, broadcasts a packet signal of the OFDM modulation scheme Steps,
Broadcasting an OFDM modulated packet signal by CSMA / CA using DIFS and CW outside the area formed around the installation position of the access control device, regardless of the frame configuration;
Outside the area formed around the installation position of the access control device, a packet signal from another terminal device within the area formed around the installation position of the access control device, and the second period Receiving an OFDM modulation scheme packet signal for identification;
A step of identifying the second period outside the area formed around the installation position of the access control device based on the received packet signal from another terminal device;
The step of broadcasting the packet signal of the OFDM modulation scheme by CSMA / CA using DIFS and CW in the second period of the frame even outside the area formed around the installation position of the access control device When,
A transmission method comprising:

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