JP5842114B2 - Terminal device and communication method - Google Patents

Description

  The present invention relates to a notification technique, and more particularly to a terminal device and a communication method for reporting 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 problem, a wireless device according to an aspect of the present invention repeats a frame including a first period formed by a plurality of slots and a second period having a predetermined length. A detection unit that detects a slot that can be used for communication between a plurality of other wireless devices from a plurality of slots in the first period of the first period and the second period; and A notifying unit for notifying information on the detected slot.

  Another aspect of the present invention is 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 from a plurality of slots in the first period, 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. 4A and 4B are diagrams showing another format of the frame defined in the frame defining unit in FIG. FIGS. 5A and 5B 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. 3 is a flowchart showing a notification procedure of the length of a first period in the access control apparatus of FIG. 2. 3 is a flowchart showing a notification procedure of an area determination threshold value 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 figure which shows the structure of the terminal device which concerns on the modification of this invention.

  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.

  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. Further, the access control apparatus adjusts the length of the first period, that is, the number of slots included in the first period, according to the number of terminal apparatuses close to the intersection.

  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). 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 center. 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 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. Here, the use of the terminal apparatus that should perform communication in the first period and the terminal apparatus that should perform communication in the second period is transmitted from the access control apparatus 10. Further, the access control device 10 adjusts the length of the first period in the frame according to the number of terminal devices included in the first area 200. 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. In addition, the terminal device detects that it has entered the first area 200 based on the index from the access control device 10. 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 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, a collision slot identification unit 44, A threshold value determination unit 46 and a first period determination unit 48 are included.

  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.

  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, the collision slot specifying unit 44, and the first period determining unit 48. 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 first period determining unit 48 receives the received power in slot units from the power measuring unit 38. In addition, the first period determining unit 48 integrates the received power over the first period. That is, the first period determining unit 48 integrates the received power of signals in communication between a plurality of other wireless devices over at least a part of the period of the frame. The integrated value is assumed to be the number of terminal devices that are transmitting data in the first period and the terminal devices that are attempting to transmit data in the first period. That is, if the integrated value increases, the number of terminal devices is expected to increase. In addition, the first period determination unit 48 determines the length of the first period according to the integrated value.

  For example, the first period determination unit 48 determines a plurality of stage determination threshold values in advance, and compares them with the integrated value to determine the maximum of the period determination thresholds that exceed the integrated value. Select a value for. On the other hand, each of the plurality of stage determination threshold values is associated with the length of the first period in advance, and the first period determination unit 48 determines the length of the first period according to the selected maximum value. The length of the first period in one frame is determined. In the first period, a maximum value and a minimum value may be set. Further, it is assumed that the frame period is fixed. The first period determining unit 48 outputs the length of the first period to the generating unit 36, the threshold determining unit 46, and the frame defining unit 34.

  In response to the notification from the first period determining unit 48, the frame defining unit 34 adjusts the length of the first period in the frame, that is, the number of slots. 4A and 4B show another format of the frame defined by the frame defining unit 34. FIG. FIG. 4A is the same as FIG. 3B, and shows a case where M slots are included in the first period. On the other hand, FIG. 4B shows a frame format when the first period determining unit 48 recognizes that the number of terminal devices is larger than that in FIG. The first period in FIG. 4B is longer than the first period in FIG. 4A and includes many slots. Here, as illustrated, Z slots (Z> M) are included in the first period. Accordingly, the second period in FIG. 4B is shorter than the second period in FIG. Returning to FIG.

  Threshold determining unit 46 determines a threshold for received power (hereinafter referred to as “area determining threshold”) when a terminal device (not shown) receives control information. Here, in order to clarify the function of the area determination threshold value, the operation of a terminal device (not shown) will be briefly described. When receiving the control signal, the terminal device measures the received power of the control signal. Further, the terminal device extracts an area determination threshold value from the control signal, and compares the area determination threshold value with the received power. The terminal device determines use of any of the plurality of slots in the first period of the first period and the second period when the received power is greater than the area determination threshold value. On the other hand, the terminal device determines the use of the second period when the received power is not greater than the area determination threshold value.

  Here, the threshold value determination unit 46 determines a predetermined value as the area determination threshold value, and when the length of the first period reaches the maximum value or the minimum value, the threshold value for area determination Execute the process that changes. Here, this process will be described with reference to FIG. When the number of terminal devices existing in the first area 200 increases, the first period determination unit 48 extends the length of the first period. Even if the length of the first period reaches the maximum value, when the number of terminal devices further increases, the threshold value determination unit 46 increases the threshold value for area determination. This corresponds to reducing the first area 200. On the other hand, when the number of terminal devices existing in the first area 200 decreases, the same processing is executed, but the description is omitted here.

  In order to realize the above processing, the threshold value determination unit 46 may receive the integrated value from the first period determination unit 48. The threshold determination unit 46 preliminarily defines the correspondence between the integrated value and the area determination threshold, and determines the area determination threshold from the received integrated value based on the correspondence. . The threshold value determination unit 46 outputs the area determination threshold value 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. Further, the generation unit 36 receives the length of the first period from the first period determination unit 48 and the area determination threshold value from the threshold value determination unit 46. The generation unit 36 generates control information while including information regarding empty slots, information regarding collision slots, the length of the first period, and an area determination threshold value. The area determination threshold can be said to be an index for a terminal device that should use a plurality of slots in the first period of the first period and the second period. 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. Further, the generation unit 36 receives information on the frame, the first period, and the slot from the frame defining 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. 5A to 5B show OFDM symbol formats used in the communication system 100. FIG. FIG. 5A corresponds to control information, and FIG. 5B 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. Examples of information included in the control information are the latitude / longitude of the access control device 10, the size of the first period, and the threshold for area determination. 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. 6 shows the 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 first period specifying unit 82, and the like. A threshold value specifying unit 84, a comparison unit 86, and a measurement unit 88 are included. 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. In addition, the control information extraction unit 66 acquires information on the first period and an area determination threshold value from the control information. The control information extraction unit 66 outputs information on empty slots and information on collision slots to the determination unit 80, outputs information on the first period to the first period identification unit 82, and sets an area determination threshold as a threshold value. The data is output to the specifying unit 84.

  The first period specifying unit 82 receives information on the first period from the control information extracting unit 66 and specifies the first period and the second period in the frame. Specifying the first period in the frame may be specifying the number of slots included in the first period. The first period specifying unit 82 outputs the specified content to the determining unit 80. The threshold value specifying unit 84 receives the area determination threshold value from the control information extracting unit 66 and holds it. Further, threshold value specifying unit 84 outputs an area determination threshold value to comparing unit 86. The measurement unit 88 receives a control information reception signal from the RF unit 52 or 54 and measures the reception power. The measurement unit 88 outputs the received power to the comparison unit 86.

  The comparison unit 86 receives the area determination threshold value from the threshold value specifying unit 84 and the received power from the measurement unit 88. 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 determining unit 80 receives the content specified in the first period specifying unit 82 from the first period specifying unit 82, and receives the content determined in the comparing unit 86 from the comparing 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. In addition, the determination unit 80 specifies the first period to be used or the second period to be used based on the information from the first period specifying unit 82. Here, the case where the first period is used will be described first.

  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.

  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.

  Next, a case where the second period is used will be described. 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. 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. 7 shows an outline of the operation 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 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, 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 apparatus 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. 8 shows another operation outline of the communication system 100. FIG. 8 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. 9 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. 10 is a flowchart showing 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. 11 is a flowchart showing a procedure for notifying the length of the first period in the access control apparatus 10. The first period determination unit 48 measures received power (S100). The first period determining unit 48 compares the received power with the period determining threshold value (S102), and determines the length of the first period (S104). If the length of the first period is different from the current length (Y in S106), the first period determination unit 48 determines to change the length of the first period (S108). On the other hand, if the length of the first period is not different from the current length (N in S106), step 108 is skipped. The generation unit 36 includes the length of the first period in the control information (S110). The modem unit 24 and the RF unit 22 notify the control information (S112).

  FIG. 12 is a flowchart showing the notification procedure of the area determination threshold value in the access control apparatus 10. The threshold value determination unit 46 acquires the received power in the first period (S130). The threshold determining unit 46 determines an area determining threshold according to the received power (S132). If the area determination threshold value is different from the current value (Y in S134), the threshold value determination unit 46 determines to change the area determination threshold value (S136). On the other hand, if the area determination threshold value is not different from the current value (N in S134), step 136 is skipped. The generation unit 36 includes the area determination threshold value in the control information (S138). The modem unit 24 and the RF unit 22 notify the control information (S140).

  FIG. 13 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 threshold value specifying unit 84 and the first period specifying unit 82 extract information regarding the area determination threshold value and the length of the first period (S162). 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 first period specifying unit 82 specifies the first period (S170). 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 first period specifying unit 82 specifies the second period (S176). The determination unit 80 performs communication by CSMA / CA (S178).

  FIG. 14 is a flowchart illustrating a data transmission procedure in the terminal device 14. FIG. 14 corresponds to the processing of step 170 in FIG. 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).

  Next, a modified example will be described. In the embodiment, the threshold value determination unit 46 adjusts the size of the area determination threshold value in order to adjust the width of the first area 200 in FIG. In the modification, in order to adjust the width of the first area 200 in FIG. 1, the range of the first area 200 is included in the control information. The access control apparatus 10 according to the modification is of the same type as the access control apparatus 10 shown in FIG. Here, different points will be mainly described.

  The threshold value determination unit 46 determines an area (hereinafter referred to as “area information”) including a terminal device that should use a plurality of slots in the first period of the first period and the second period. For example, the area information is indicated by a radius of a circle centered on the access control device 10. Here, the threshold value determination unit 46 determines a predetermined value as the area information, and changes the area size when the length of the first period reaches the maximum value or the minimum value. Execute.

  In order to realize the above processing, the threshold value determination unit 46 may receive the integrated value from the first period determination unit 48. The threshold value determination unit 46 preliminarily defines a correspondence between the integrated value and the area radius value, and determines the area radius from the received integrated value based on the correspondence. The threshold value determination unit 46 outputs the area radius to the generation unit 36 as area information. The generation unit 36 includes the area information and the latitude / longitude of the position where the access control device 10 is installed (hereinafter referred to as “position information”) in the control information instead of the area determination threshold described above. .

  FIG. 15 is a diagram illustrating a configuration of the terminal device 14 according to a modification of the present invention. 15 includes an area specifying unit 90 instead of the threshold specifying unit 84 and a GPS positioning unit 92 instead of the measuring unit 88, as compared to the terminal device 14 of FIG. Here, the difference from FIG. 6 will be mainly described. The control information extraction unit 66 acquires area information and position information from the control information instead of the area determination threshold value, and outputs them to the area specifying unit 90. The area specifying unit 90 receives area information and position information from the control information extracting unit 66 and holds them.

  The GPS positioning unit 92 receives a signal from a GPS satellite (not shown), and acquires the location of the terminal device 14 based on the received signal. In addition, since a well-known technique should just be used for acquisition of an existing position, description is abbreviate | omitted here. The GPS positioning unit 92 outputs the presence position to the comparison unit 86. The comparison unit 86 receives area information and position information from the area specifying unit 90 and receives an existing position from the GPS positioning unit 92. Based on the area information and the position information, the comparison unit 86 specifies an area including a terminal device that should use a plurality of slots in the first period of the first period and the second period. Further, the comparison unit 86 determines whether or not the presence position is included in the specified area. The comparison unit 86 determines the use of the first period when it is included, and determines the use of the second period when it is not included. The comparison unit 86 outputs the determined content to the determination unit 80.

  According to the embodiment of the present invention, a slot that can be used for communication between a plurality of terminal devices is notified from among a plurality of slots included in the first period. The probability of occurrence 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. Further, 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 does not have a position positioning function.

  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, control information notified from the access control apparatus 10 and data notified from one terminal apparatus 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.

  In the embodiment of the present invention, the first period determination unit 48 determines the length of the first period according to the integrated value. However, the present invention is not limited to this. For example, the first period determination unit 48 determines the first period according to the ratio between the number of slots detected by the empty slot identification unit 42 and the collision slot identification unit 44 and the number of slots in the first period. The length may be determined. That is, the first period determination unit 48 shortens the first period if the ratio of the number of empty slots to the number of slots in the first period is larger than the threshold value. On the other hand, if the ratio of the number of empty slots to the number of slots in the first period is not larger than the threshold, or the ratio of the number of collision slots to the number of slots in the first period is larger than another threshold, The one period determination unit 48 lengthens the first period. Similarly to the first period determination unit 48, the threshold determination unit 46 also depends on the ratio between the number of slots detected by the empty slot identification unit 42 and the collision slot identification unit 44 and the number of slots in the first period. The area determination threshold value may be determined. According to this modification, since the available slots are directly confirmed, it is possible to improve the accuracy of adjusting the first period and the threshold for determining the area.

  In the modification of the present invention, the terminal device 14 is information about the terminal device that should use a plurality of slots in the first period of the first period and the second period, for example, an area determination threshold value and area information. And the first period or the second period is selected. However, the present invention is not limited to this. For example, the terminal device 14 may notify at least information related to prohibition of transmission in the first period (hereinafter referred to as “prohibition information”). When the other terminal device 14 that cannot receive the control information from the access control device 10 receives the prohibition information, the transmission is stopped for at least the first period. According to this modification, an increase in the collision probability of packet signals in the first period can be suppressed.

  In the embodiment of the present invention, the access control apparatus 10 preferentially adjusts the first period in the first period determining unit 48 and the threshold determining unit 46, and then adjusts the area determining threshold. However, the present invention is not limited to this. For example, the access control device 10 preferentially adjusts the area determination threshold in the first period determination unit 48 and the threshold determination unit 46, and then adjusts the first period. According to this modification, the degree of freedom in design 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 measuring unit, 40 quality measuring unit, 42 empty slot identifying unit, 44 collision slot identifying unit, 46 threshold value determining unit, 48 first period determining unit, 50 antenna, 52 RF unit, 54 modem unit, 56 processing unit, 58 control unit, 60 timing specifying unit, 62 acquiring unit, 64 generating unit, 66 control information extracting unit, 70 notifying unit, 80 determining unit, 82 first period specifying unit, 84 threshold specifying unit, 86 comparing unit, 88 Measuring unit, 100 communication system;

Claims (5)

Of the 100 msec frames in which the first period in which the access control apparatus can broadcast the OFDM modulated packet signal and the second period in which the access control apparatus can be used for CSMA / CA communication between the terminal apparatuses is time-division multiplexed, In the period, a receiving unit that receives the packet signal of the OFDM modulation scheme from the access control device;
Of the 100 msec frame in which the first period and the second period are time-division multiplexed, the transmitter includes a transmitter that broadcasts the OFDM modulated packet signal in the second period,
The packet signal of the OFDM modulation scheme received from the access control device in the receiving unit includes information on the length of the first period,
The transmission unit does not perform broadcast transmission in the first period based on information on the length of the first period ,
The receiving unit receives an OFDM-modulated packet signal broadcast from another terminal device in a second period among 100 msec frames in which the first period and the second period are time-division multiplexed ,
The transmission unit broadcast-transmits information related to transmission prohibition in the first period based on an OFDM modulation scheme packet signal received from the access control apparatus in the first period by the reception unit. . The packet signal of the OFDM modulation scheme received from the access control device in the receiving unit includes information on a period that should not be used for CSMA / CA communication between terminal devices,
The terminal apparatus according to claim 1, wherein the transmission unit stops broadcast transmission based on information related to a period that should not be used for CSMA / CA communication between terminal apparatuses. The reception unit, based on the packet signal of the OFDM modulation scheme received from the access control device according to claim 1 or 2, characterized in that to detect that it has entered the area formed in the periphery of the access control device The terminal device described in 1. 4. The terminal device according to claim 1, wherein the reception unit performs timing synchronization based on an OFDM modulation type packet signal received from the access control device. 5. Of the 100 msec frames in which the first period in which the access control apparatus can broadcast the OFDM modulated packet signal and the second period in which the access control apparatus can be used for CSMA / CA communication between the terminal apparatuses is time-division multiplexed, Receiving a packet signal of the OFDM modulation scheme from the access control device in the period;
Of the 100 msec frame in which the first period and the second period are time-division multiplexed, the step of broadcasting the packet signal of the OFDM modulation scheme in the second period,
The OFDM modulation system packet signal received from the access control device in the receiving step includes information on the length of the first period,
The transmitting step does not perform broadcast transmission in the first period based on information on the length of the first period ,
The receiving step receives a packet signal of an OFDM modulation scheme broadcast from another terminal device in a second period among 100 msec frames in which the first period and the second period are time-division multiplexed ,
The transmitting step broadcasts information related to transmission prohibition in the first period based on a packet signal of the OFDM modulation scheme received from the access control apparatus in the first period in the receiving step. Communication method.

Images