JP5657036B2 - Transmission method and access control apparatus - Google Patents

Description

  The present invention relates to a transmission technique, and more particularly, to a transmission method and an access control apparatus 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 wireless device according to an aspect of the present invention includes a plurality of frames including at least a plurality of slots, and communication between a plurality of other wireless devices in a predetermined slot. A detecting unit for detecting a slot in which a collision has occurred due to another wireless device transmitting a signal redundantly, and an informing unit for informing information related to the slot detected by the detecting unit.

  Another aspect of the present invention is a notification method. In this method, a frame including at least a plurality of slots is repeated, and communication between a plurality of other wireless devices is performed in a predetermined slot, and a plurality of other wireless devices transmit signals in duplicate. And a step of detecting a slot in which a collision has occurred and a step of notifying information on the detected slot.

  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 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 including a plurality of slots. The access control device specifies a slot that is not used for communication between a plurality of terminal devices (hereinafter referred to as “empty slot”) by measuring the received power in each slot. 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.

  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.

  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. The 8th vehicle 12h is included. Each vehicle 12 is equipped with a terminal device (not shown). Further, an area 200 is formed by the access control device 10.

  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 the top to the bottom, and the seventh vehicle 12g and the eighth vehicle 12h are traveling from the bottom to the 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 including a plurality of slots so as to be repeated based on a signal received from a GPS satellite (not shown). 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 frame, the access control device 10 broadcasts control information.

  The plurality of terminal devices receive the control information notified by the access control device 10 and select 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 frame is selected, the 10th slot from the beginning of the frame is 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 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 it has entered the area 200 when it can receive the control information. In addition, even if it is a case where control information is not received, a terminal device may alert | report data. 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.

  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 area 200 by the terminal device 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.

  The frame defining unit 34 generates a plurality of slots by dividing each frame into a plurality of frames. For example, by dividing each frame into 200, 200 “500 μsec” slots are generated. 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 corresponds to the aforementioned packet signal.

  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 M slots. For example, M is “200”, and 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.

  The RF unit 22 receives, from the antenna 20, a packet signal transmitted in communication between other terminal devices (not shown) in each slot as reception processing. 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 the transmission processing, the RF unit 22 performs frequency conversion on the baseband packet signal input from the modem unit 24 in each slot to generate 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 a slot that can be used for communication between a plurality of terminal devices as an empty slot from the plurality of slots. 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 detects, as a collision slot, a slot in which a collision has occurred due to a plurality of terminal apparatuses transmitting signals in duplicate. 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 information regarding collision slots. Here, each of the plurality of slots included in the frame 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. 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 frame. Here, it is predetermined that control information is assigned to the slot of slot number “1” of each frame, 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 the control signal and data, subcarriers with subcarrier numbers “31” to “27”, “2”, “0”, “−2”, “−26” to “−32” are null. is there. Of the control signals, 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. . On the other hand, among the control signals, the subcarrier numbers “1” and “−1” 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. 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 notification unit 70, and the timing specifying unit 60 includes a control information extracting unit 66 and a slot determining unit 68. 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. 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 outputs the information regarding the empty slot and the information regarding the collision slot to the slot determination unit 68.

  The slot determination unit 68 receives information on empty slots and information on collision slots from the control information extraction unit 66. The slot determination unit 68 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 slot determination unit 68 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. The generation unit 64 receives an instruction regarding an empty slot from the slot determination unit 68 and outputs data to the modulation / demodulation unit 54 in an empty slot corresponding to the instruction. Note that the processor 56 may perform carrier sense before outputting data. In addition, the generation unit 64 outputs data in the slot having the same slot number in the next frame.

  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 slot determination unit 68 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 the slot is not a collision slot, the slot determination unit 68 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 slot determination unit 68 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 slot determination unit 68 outputs information regarding the selected empty slot to the generation unit 64. Thereafter, the generation unit 64 performs the same processing. 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. 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. 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 15 slots are included in one frame. As shown in the figure, the access control device 10 notifies the control signal in the first slot of each frame. “Control” in the figure corresponds to a control signal. 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 to 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 to 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.

  Since the first terminal device 14a and the second terminal device 14b do not collide in the slots that have already been used, the slots having the same slot number are used again. 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 to 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. Here, it is assumed that the second terminal apparatus 14b has not received control information from the access control apparatus 10 as a premise. Therefore, the second terminal apparatus 14b transmits data without grasping the frame configuration. The access control device 10 broadcasts a control signal in the first slot of the frame. On the other hand, the second terminal apparatus 14b broadcasts data in the first slot of the frame. As a result, the control signal and data collide in the slot. Here, the first terminal device 14a, the third terminal device 14c, and the fourth terminal device 14d can control signals by observing the signal component of the identification carrier in the control signal even when a collision occurs. The presence of can be detected.

  FIG. 8 is a flowchart showing a procedure for notifying an empty slot in the access control apparatus 10. 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. 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 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 specified (Y in S72), the slot determination unit 68 checks whether or not a collision has occurred in the slot. If a collision has occurred (Y in S74), the slot determining unit 68 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 slot determining unit 68 specifies an empty slot (S78). The generation unit 64 transmits data in the specified slot (S80).

  According to the embodiment of the present invention, since a slot usable for communication between a plurality of terminal devices is notified from among a plurality of slots, the probability of occurrence of a collision in communication between the plurality of 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 slot in which a collision has occurred due to a plurality of terminal apparatuses transmitting a signal redundantly is notified from among a plurality of slots, the probability of occurrence of a collision in communication between the plurality of terminal apparatuses 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.

  Also, among the control signals, the identification carrier is not used for data, and the remaining subcarriers are also used for data. Therefore, even if the control signal and the data signal collide, the signal component of the control signal is observed. Thus, the presence of the control signal 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, the frame defining portion 34 defines a frame formed by a plurality of slots. However, the present invention is not limited to this. For example, the frame defining unit 34 may provide periods other than a plurality of slots in the frame. Specifically, a plurality of slots may be arranged in a partial period of the frame, and CSMA / CA may be performed between the plurality of terminal apparatuses 14 in the remaining period. At this time, the access control apparatus 10 does not detect an empty slot or a collision slot during the CSMA / CA period. According to this modification, since the terminal device 14 can select communication by slot and communication by CSMA / CA, the degree of freedom of communication can be improved. That is, the frame only needs to include at least a plurality of slots.

  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. At this time, the identification carrier may include information on empty slots and information on collision slots. 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 Generator, 66 control information extraction unit, 68 slot determination unit, 70 notification unit, 100 communication system.

Claims (4)

An access control device that does not directly specify a period to be used for data communication with a terminal device,
A generating unit that generates a packet signal of an OFDM modulation method including information for informing a terminal device;
A transmitter that broadcasts the OFDM modulation scheme packet signal generated in the generator,
The transmission unit is configured so that only the access control apparatus can only transmit a period during which only the access control apparatus can broadcast the OFDM modulated packet signal and a period during which data transmission between terminal apparatuses not involving the access control apparatus can be used. Frames that are time-division-multiplexed in order of the period in which the OFDM modulation system packet signal can be broadcast and the period in which data communication between terminal devices not involving the access control device can be used are continuous. In the period in which the packet signal of the modulation method can be broadcasted, the packet signal of the OFDM modulation method is broadcasted,
The generating unit uses, as information for notifying the terminal device, information related to a period of data communication that does not involve the access control device and that should not be used for data communication between the terminal devices, as a packet signal of the OFDM modulation scheme. An access control device characterized by including. An access control device that does not directly specify a period to be used for data communication with a terminal device,
A generating unit that generates a packet signal of an OFDM modulation method including information for informing a terminal device;
A transmitter that broadcasts the OFDM modulation scheme packet signal generated in the generator,
The transmission unit is configured so that only the access control apparatus can only transmit a period during which only the access control apparatus can broadcast the OFDM modulated packet signal and a period during which data transmission between terminal apparatuses not involving the access control apparatus can be used. 100 msec frames that are time-division multiplexed in order of a period in which an OFDM modulation system packet signal can be broadcasted and a period in which data communication between terminal apparatuses not involving the access control apparatus can be used are continuous. Transmits a packet signal of the OFDM modulation scheme in a period in which the packet signal of the OFDM modulation scheme can be broadcast,
The generating unit uses, as information for notifying the terminal device, information related to a period of data communication that does not involve the access control device and that should not be used for data communication between the terminal devices, as a packet signal of the OFDM modulation scheme. An access control device characterized by including.   The access control device according to claim 1, wherein the transmission unit causes the terminal device to execute timing synchronization based on a broadcast-transmitted OFDM modulation scheme packet signal. A transmission method in an access control device that does not directly specify a period to be used for data communication with a terminal device,
Generating an OFDM modulation type packet signal including information for informing the terminal device;
Broadcasting the generated OFDM modulated packet signal, and
The transmitting step includes only a period in which only the access control apparatus can broadcast the OFDM modulated packet signal and a period in which data communication between terminal apparatuses not involving the access control apparatus can be used. Are time-division-multiplexed frames in the order of the period in which the OFDM modulation system packet signal can be broadcast and the period in which the access control apparatus is not involved in the data communication between terminal apparatuses. In the period in which the OFDM modulation system packet signal can be broadcast, the OFDM modulation system packet signal is broadcast,
Step, as information for informing the terminal device, the access control device is a data communication which is not involved, and a packet signal of the OFDM modulation method information regarding a time period should not be used for data communication between terminal devices the product The transmission method characterized by including in this.

Images