JP5830071B2 - RFID reader and control method - Google Patents

Description

  The present invention relates to an RFID reader and a control method.

  The use of RFID is being studied for the management of the state of goods in the logistics process and the visualization of the movement of goods. In RFID used for physical distribution management, RFID tags attached to objects regularly transmit beacon signals carrying their IDs and the like, and RFID readers receive these signals to manage the state and movement of objects. Yes.

  In the logistics management system using RFID, the RFID tag operation parameters such as the interval of transmitting the beacon signal of the RFID tag and the ID of the connected RFID reader depending on whether or not the item to which the RFID tag is attached is located at the logistics base. May change. Therefore, on the RFID tag side, it is necessary to detect its own position, that is, the position or state of the object to which the RFID tag is attached. As a method for detecting whether or not the RFID tag is present in the distribution base, a method in which an RFID reader installed at the distribution base returns a response signal in response to a beacon signal periodically transmitted by the RFID tag. There is.

  When a plurality of RFID readers are installed in the distribution base, a response signal collision occurs when a plurality of RFID readers return a response signal simultaneously in response to a beacon signal transmitted from the RFID tag. In some cases, the position and state cannot be detected on the RFID tag side. In order to avoid a collision of response signals, a technique is required in which only one RFID reader returns a response signal to the beacon signal. As one of such techniques, as shown in Non-Patent Document 1, there is a method combining CSMA / CA with random backoff that randomly determines a backoff time.

  FIG. 5 is a block diagram illustrating a configuration example of the RFID reader 9 to which a technique in which random backoff and CSMA / CA are combined is applied. As shown in the figure, the RFID reader 9 includes a demodulator 91, a back-off time calculator 92, a response signal generator 93, and a transmission controller 94.

  The demodulator 91 determines whether or not the beacon signal has been successfully received. When receiving the beacon signal, the demodulator 91 outputs the timing (reception end time) at which the reception of the beacon signal is terminated to the transmission controller 94. Further, the demodulation unit 91 outputs information included in the beacon signal that has been successfully received to the back-off time calculation unit 92 and the response signal generation unit 93. The back-off time calculation unit 92 randomly calculates a back-off time that is a waiting time from the reception end time to the transmission start time for starting transmission of the response signal to the beacon signal under a predetermined condition. The backoff time calculation unit 92 outputs the calculated backoff time to the transmission control unit 94.

  Based on the information output from the demodulator 91, the response signal generator 93 generates a response signal that is sent back to the RFID tag that transmitted the beacon signal, and outputs the response signal to the transmission controller 94. When the reception end time is output from the demodulation unit 91, the transmission control unit 94 transmits a response signal in the received signal in a period until the back-off time output from the back-off time calculation unit 92 elapses. The carrier level (reception level) is detected. The transmission control unit 94 transmits the response signal generated by the response signal generation unit 93 when the detected carrier level is equal to or lower than a predetermined threshold during the period until the back-off time elapses.

  FIG. 6 is a diagram showing an operation example of the RFID reader 9 shown in FIG. Here, as shown in the figure, the RFID tag is located within the communication range of three RFID readers 9 (RFID reader 9A, RFID reader 9B, and RFID reader 9C). The communication distance between the RFID tag and each RFID reader 9 is long in the order of the RFID reader 9A, the RFID reader 9B, and the RFID reader 9C, and the communication quality is low according to the communication distance. Further, it is assumed that the back-off time calculated by the back-off time calculating unit 92 of each RFID reader 9 is shorter in the order of the RFID reader 9C, the RFID reader 9A, and the RFID reader 9B.

  At this time, when the RFID tag finishes transmitting the beacon signal, the RFID reader 9C starts transmitting a response signal to the RFID tag earliest. At this time, since the RFID reader 9A and the RFID reader 9B have their own back-off time longer than the back-off time of the RFID reader 9C, the RFID reader 9C has already started returning a response signal when the back-off time has elapsed. ing. In the RFID reader 9A and the RFID reader 9B, the transmission control unit 94 detects the carrier level exceeding the threshold with respect to the reception signal including the response signal of the RFID reader 9C, and therefore transmission of the response signal to the beacon signal is stopped.

  As described above, by using the RFID reader 9, even when the RFID tag is located within the communication range of the plurality of RFID readers 9, only one RFID reader 9 returns a response signal. To avoid collisions.

Supervised by Morikura and Kubota, “Impress Standard Textbook Series Revised 802.11 High-Speed Wireless LAN Textbook”, Impress R & D Inc., August 2006, p. 86-88

  However, when the RFID reader 9 that calculates the back-off time as described above is used, when the RFID tag is located within the communication range of the plurality of RFID readers, the RFID reader that responds to the beacon signal of the RFID tag is randomly selected. Since it is selected, there is a possibility that the communication state between the RFID reader that has returned the response signal and the RFID tag is not good. For example, in the example shown in FIG. 6, the RFID reader 9C farthest from the RFID tag and having low communication quality returns a response signal. Therefore, the RFID tag communicates with the RFID reader 9C having the lowest communication quality among the RFID readers 9.

  The problem to be solved by the present invention is to provide an RFID reader and a control method capable of improving the communication quality with the RFID tag when the RFID tag is located in the communication range of a plurality of RFID readers.

  One aspect of the present invention is an RFID reader that receives a beacon signal transmitted from an RFID tag and transmits a response signal in response to the beacon signal, a level detection unit that detects a reception level of the beacon signal, and the reception A backoff time calculation unit that calculates a short backoff time as the level is high, and a signal level in a channel that transmits the response signal in a period from the end of reception of the beacon signal until the backoff time elapses in advance. An RFID reader comprising: a transmission control unit that transmits the response signal when the back-off time elapses when it is equal to or less than a predetermined threshold value.

  Further, according to one aspect of the present invention, in the RFID reader, the back-off time calculation unit may make a difference between the maximum value and the minimum value of the back-off time shorter than the time required for transmitting the response signal. Features.

  Further, according to one aspect of the present invention, in the RFID reader, the back-off time calculation unit calculates a first time that is shorter as the reception level is higher, and the second time is determined randomly at the first time. The time obtained by adding the times is used as the back-off time.

  In one embodiment of the present invention, in the above RFID reader, the threshold is a minimum value in a range in which it is possible to detect whether a signal transmitted from another RFID reader exists in the channel. And

  One embodiment of the present invention is the above RFID reader, wherein the transmission control unit is a signal in a channel that transmits the response signal in a period from when reception of the beacon signal ends until the back-off time elapses. Even when the level is equal to or lower than a predetermined threshold, transmission of the response signal is stopped with a certain probability.

  Further, according to one aspect of the present invention, in the RFID reader, the back-off time calculation unit is configured to generate the back level based on a cumulative probability distribution of reception levels calculated based on beacon signals transmitted from a plurality of RFID tags. An off time is calculated.

  Another embodiment of the present invention is a control method in an RFID reader that receives a beacon signal transmitted from an RFID tag and transmits a response signal according to the beacon signal, and detects a reception level of the beacon signal. The response signal is transmitted in a level detection step, a backoff time calculation step for calculating a shorter backoff time as the reception level is higher, and a period from the end of reception of the beacon signal to the elapse of the backoff time. A transmission control step of transmitting the response signal when the back-off time elapses when a signal level in a channel to be transmitted is equal to or lower than a predetermined threshold value.

  Further, according to one aspect of the present invention, in the control method described above, in the back-off time calculation step, the difference between the maximum value and the minimum value of the back-off time is made shorter than the time required for transmitting the response signal. Features.

  According to the present invention, the higher the reception level of the received beacon signal, the shorter the back-off time. Therefore, even when a plurality of RFID readers are within the communication range, the RFID reader that has received the beacon signal at a high reception level responds. Since the signal is returned, the communication quality with the RFID tag can be improved.

It is a block diagram which shows the structural example of the RFID reader | leader 1 in this embodiment. It is a figure showing an example of operation of RFID reader 1 in the embodiment. It is a graph which shows an example of the cumulative probability distribution of a reception level. It is a figure which shows an example which allocates back-off time based on the cumulative probability distribution of a reception level. It is a block diagram which shows the structural example of the RFID reader 9 to which the technique which combined random backoff and CSMA / CA is applied. FIG. 6 is a diagram illustrating an operation example of the RFID reader 9 illustrated in FIG. 5.

  Hereinafter, an RFID reader and a control method according to an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram illustrating a configuration example of an RFID reader 1 according to the present embodiment. The RFID reader 1 includes a demodulator 11, a level detector 12, a back-off time calculator 13, a response signal generator 14, and a transmission controller 15.

  The demodulator 11 determines whether or not the beacon signal transmitted from the RFID tag has been successfully received. The demodulation unit 11 outputs to the transmission control unit 15 the timing (reception end time) at which reception of the beacon signal ends after successful reception of the beacon signal. Further, the demodulation unit 11 outputs information included in the beacon signal that has been successfully received to the back-off time calculation unit 13 and the response signal generation unit 14. The level detection unit 12 detects the reception level of the beacon signal transmitted from the RFID tag and outputs the detected reception level to the back-off time calculation unit 13.

  Based on the reception level output from the level detection unit 12 and the information output from the demodulation unit 11, the back-off time calculation unit 13 starts from the reception end time to the transmission start time for starting transmission of the response signal corresponding to the beacon signal. The back-off time that is the waiting time is calculated. The back-off time calculated by the back-off time calculation unit 13 is shorter as the reception level is higher, and is longer as the reception level is lower. The back-off time calculation unit 13 calculates the back-off time so that the back-off time is inversely proportional to the reception level.

  The response signal generation unit 14 generates a response signal to be sent back to the RFID tag that has transmitted the beacon signal based on the information output from the demodulation unit 11, and outputs the response signal to the transmission control unit 15. When the reception end time is output from the demodulation unit 11, the transmission control unit 15 transmits the response signal in the received signal in the channel until the back-off time calculated from the back-off time calculation unit 13 elapses. The carrier level (reception level) is detected. The transmission control unit 15 responds to the RFID tag that transmitted the beacon signal when the back-off time has elapsed from the reception end time and the detected carrier level is equal to or lower than a predetermined threshold during the back-off time. Send a signal.

  FIG. 5 shows that the RFID reader 1 according to the present embodiment includes a level detection unit 12 and that the back-off time calculation unit 13 calculates the back-off time based on the reception level of the beacon signal. Different from the RFID reader 9. As the relationship between the reception level and the back-off time, an arbitrary relational expression in which the back-off time monotonously decreases with respect to the reception level can be used.

  FIG. 2 is a diagram illustrating an operation example of the RFID reader 1 in the present embodiment. Here, as shown in the figure, the RFID tag is located within the communication range of three RFID readers 1 (RFID reader 1A, RFID reader 1B, RFID reader 1C). The communication distance between the RFID tag and each RFID reader 1 is long in the order of the RFID reader 1A, the RFID reader 1B, and the RFID reader 1C, and the communication quality is low according to the communication distance. Further, it is assumed that the reception level of the beacon signal in each RFID reader 1 is good in the order of communication quality.

  Since the back-off time calculation unit 13 calculates the shorter back-off time as the reception level of the beacon signal is higher, the RFID reader having the highest reception level among the back-off times of the RFID reader 1A, RFID reader 1B, and RFID reader 1C. The back-off time a of 1A is the shortest, and the back-off time c of the RFID reader 1C having the lowest reception level is the longest. Further, the back-off time b of the RFID reader 1B having a reception level lower than that of the RFID reader 1A and higher than that of the RFID reader 1C is longer than the back-off time a and shorter than the back-off time c. Since the response of the RFID reader 1A with the shortest back-off time is started while the RFID reader 1B and the RFID reader 1C are waiting for the respective back-off times, transmission of the RFID reader 1B and the RFID reader 1C is started. The control unit 15 detects a carrier level that exceeds the threshold and stops transmitting the response signal.

  As described above, the RFID reader 1 according to the present embodiment assigns a shorter back-off time to its own device as the reception level of the beacon signal is higher. As a result, even when a plurality of RFID readers 1 exist within the communicable range of the RFID tag, the RFID reader 1 with the highest communication quality can respond to the beacon signal of the RFID tag. Communication quality can be improved.

  Note that the difference (time difference) between the minimum value and the maximum value of the back-off time calculated by the back-off time calculation unit 13 may be made shorter than the time required for transmitting the response signal returned to the beacon signal. . As a result, at the timing when the RFID reader 1 that has assigned the longest back-off time to the own device starts sending the response signal, the RFID reader 1 that has assigned the shortest back-off time to the own device is sending the response signal. It will be.

  As a result, in the RFID reader 1 that has assigned the longest back-off time to its own device, a carrier level exceeding the threshold is always detected during the period waiting for the back-off time to elapse, and a response signal is returned. The cancellation can be realized with higher accuracy. Similarly, in the RFID readers 1 other than the RFID reader 1 that has assigned the shortest back-off time to the own apparatus, the response signal can be stopped more accurately. As a result, occurrence of a state where the RFID tag receives a plurality of response signals can be suppressed.

  Further, the back-off time calculation unit 13 may add a random value within a predetermined range to the back-off time calculated based on the reception level of the beacon signal. Thereby, for example, even if a plurality of RFID readers 1 receive beacon signals at the same reception level, the back-off time in each RFID reader 1 is obtained by adding a random value. The back-off time can be different.

  As a result, even when a plurality of RFID readers 1 receive beacon signals at the same reception level, it is possible to prevent the plurality of RFID readers 1 from starting to transmit response signals at the same timing. That is, it is possible to avoid response signal collision by avoiding simultaneous transmission of response signals by a plurality of RFID readers 1 by assigning the same back-off time, which cannot be avoided only by controlling the back-off time based on the reception level of the beacon signal. Can be avoided.

  In addition, the threshold used by the transmission control unit 15 in detecting the carrier level in the received signal of the channel that transmits the response signal is set to the lowest level within a range in which it is possible to detect whether another device is transmitting a signal in the channel. It may be determined. As a result, it is possible to suppress the occurrence of a hidden terminal problem in which another RFID reader 1 starts transmitting a signal while a certain RFID reader 1 transmits a response signal or the like. The threshold of the lowest level is, for example, the lowest value that can be used to determine whether a signal other than white noise or thermal noise is included in the received signal of the RFID reader 1, and is based on simulations or measured values. This is the minimum value set. That is, the minimum value in a range in which it is possible to detect whether or not a signal transmitted by another RFID reader 1 is included in the received signal may be set as the threshold value.

  Further, even if the threshold value is set in the carrier level detection as described above, a hidden terminal problem may occur depending on the positional relationship between the RFID reader 1 and the RFID tag and the communication environment. No. 15 may return the response signal with a certain probability even if the carrier level (signal level) in the received signal is equal to or less than the threshold before the back-off time elapses from the reception end time. . As a result, even in a situation where a hidden terminal problem that cannot be avoided simply by controlling the back-off time according to the reception level, simultaneous transmission of response signals by a plurality of RFID readers 1 is suppressed, and a steady response is made. The simultaneous transmission of signals can be avoided, and the collision of response signals can be avoided.

  When the cumulative probability distribution of the reception level in the RFID reader 1 is known, the back-off time calculation unit 13 may calculate the back-off time based on the cumulative probability distribution of the reception level. As a result, the back-off times assigned to the apparatus by the plurality of RFID readers 1 are stochastically uniform, and the collision probability of response signals generated by assigning the same back-off time can be minimized. FIG. 3 is a graph illustrating an example of a cumulative probability distribution of reception levels. In the figure, the horizontal axis represents the reception level (dBm), and the vertical axis represents the cumulative probability (%). Graph A in the figure represents a case where there are many RFID tags in an area where the reception level is lower than −80 dBm. Graph B represents the case where RFID tags are present evenly in the range of the reception level from −110 dBm to −30 dBm.

  For example, in the case shown by the graph A, the back-off time may be allocated by finely dividing a reception level of −80 dBm or less. Further, in the case shown by the graph B, the back-off time may be equally allocated in the range of −110 dBm to −30 dBm. FIG. 4 is a diagram illustrating an example in which the back-off time is assigned based on the cumulative probability distribution of the reception level. FIG. 4A shows an example of allocation of back-off time corresponding to the graph A in FIG. FIG. 4B shows an example of allocation of back-off time corresponding to the graph B in FIG. 4A and 4B, the reception level is divided into four sections, and a back-off time is assigned to each section. Note that the back-off time becomes longer in the order of “back-off time 1”, “back-off time 2”, “back-off time 3”, and “back-off time 4”.

  By assigning the back-off time in this way, it is possible to disperse the back-off time for the RFID tag and suppress the probability that the response signals collide. In other words, the probability of collision of response signals can be suppressed without concentrating RFID tags at a specific backoff time.

  As described above, when the back-off time is assigned based on the cumulative probability distribution, the back-off time calculation unit 13 measures the reception level every time a beacon signal transmitted from the RFID tag is received, and the measurement result. Remember. The back-off time calculation unit 13 calculates a cumulative probability distribution based on either the measurement results stored in a certain period or a certain number of measurement results. Based on the calculated cumulative probability distribution, the back-off time calculation unit 13 divides the reception level into a predetermined number of sections so that the cumulative probability in each section becomes equal, and assigns a back-off time to each section. . In addition, after the RFID reader 1 is installed, the cumulative probability distribution may be calculated in an environment in which the RFID reader 1 is installed after a lapse of a specified time, or the cumulative probability distribution is updated every fixed period. Also good.

  The RFID reader 1 in the above-described embodiment may be realized by a computer. In that case, a program for realizing this function may be recorded on a computer-readable recording medium, and the program recorded on this recording medium may be read into a computer system and executed. Here, the “computer system” includes an OS and hardware such as peripheral devices. The “computer-readable recording medium” refers to a storage device such as a flexible medium, a magneto-optical disk, a portable medium such as a ROM and a CD-ROM, and a hard disk incorporated in a computer system. Furthermore, the “computer-readable recording medium” dynamically holds a program for a short time like a communication line when transmitting a program via a network such as the Internet or a communication line such as a telephone line. In this case, a volatile memory inside a computer system serving as a server or a client in that case may be included and a program held for a certain period of time. Further, the program may be for realizing a part of the functions described above, and may be a program capable of realizing the functions described above in combination with a program already recorded in the computer system. It may be realized using hardware such as PLD (Programmable Logic Device) or FPGA (Field Programmable Gate Array).

  The embodiment of the present invention has been described in detail with reference to the drawings. However, the specific configuration is not limited to this embodiment, and includes designs and the like that do not depart from the gist of the present invention.

DESCRIPTION OF SYMBOLS 1, 9 ... RFID reader 11, 91 ... Demodulation part 12 ... Level detection part 13, 92 ... Back-off time calculation part 14, 93 ... Response signal generation part 15, 94 ... Transmission control part

Claims (8)

In an RFID reader that receives a beacon signal transmitted from an RFID tag and transmits a response signal in response to the beacon signal,
A level detector for detecting the reception level of the beacon signal;
A back-off time calculating unit that calculates a shorter back-off time as the reception level is higher;
If the signal level in the channel for transmitting the response signal is equal to or lower than a predetermined threshold in the period from the end of reception of the beacon signal to the elapse of the back-off time, the response is received when the back-off time elapses. A transmission control unit for transmitting signals ,
The back-off time calculating unit
An RFID reader, wherein the maximum value and the minimum value of the calculated back-off time are set so that the time difference is shorter than the time required for transmitting the response signal . In an RFID reader that receives a beacon signal transmitted from an RFID tag and transmits a response signal in response to the beacon signal,
A level detector for detecting the reception level of the beacon signal;
A back-off time calculating unit that calculates a shorter back-off time as the reception level is higher;
If the signal level in the channel for transmitting the response signal is equal to or lower than a predetermined threshold in the period from the end of reception of the beacon signal to the elapse of the back-off time, the response is received when the back-off time elapses. A transmission control unit for transmitting signals;
With
The transmission control unit
In the period from completion of reception the beacon signal until the lapse of the backoff time, even if the signal level in the channel for transmitting the response signal is less than the threshold value, of the response signal with a certain probability An RFID reader characterized by stopping transmission. In an RFID reader that receives a beacon signal transmitted from an RFID tag and transmits a response signal in response to the beacon signal,
A level detector for detecting the reception level of the beacon signal;
A back-off time calculating unit that calculates a shorter back-off time as the reception level is higher;
If the signal level in the channel for transmitting the response signal is equal to or lower than a predetermined threshold in the period from the end of reception of the beacon signal to the elapse of the back-off time, the response is received when the back-off time elapses. A transmission control unit for transmitting signals;
With
The back-off time calculating unit
An RFID reader, wherein the back-off time is calculated based on a cumulative probability distribution of reception levels calculated based on beacon signals transmitted from a plurality of RFID tags. In RFID reader according to any one of claims 1 to 3,
The back-off time calculating unit
The RFID is characterized in that the higher the reception level is, the shorter the first time is calculated, and the time obtained by adding the second time randomly determined to the first time is used as the back-off time. leader. In the RFID reader according to any one of claims 1 to 4 ,
The threshold includes
An RFID reader using a minimum value in a range in which it can be detected whether or not a signal transmitted by another RFID reader exists in the channel. A control method in an RFID reader that receives a beacon signal transmitted from an RFID tag and transmits a response signal according to the beacon signal,
A level detection step of detecting a reception level of the beacon signal;
A backoff time calculating step of calculating a shorter backoff time as the reception level is higher;
If the signal level in the channel for transmitting the response signal is equal to or lower than a predetermined threshold in the period from the end of reception of the beacon signal to the elapse of the back-off time, the response is received when the back-off time elapses. It has a transmission control step of transmitting a signal,
In the back-off time calculating step,
A control method characterized in that the maximum value and the minimum value of the calculated back-off time are set so that the time difference is shorter than the time required for transmitting the response signal . A control method in an RFID reader that receives a beacon signal transmitted from an RFID tag and transmits a response signal according to the beacon signal,
A level detection step of detecting a reception level of the beacon signal;
A backoff time calculating step of calculating a shorter backoff time as the reception level is higher;
If the signal level in the channel for transmitting the response signal is equal to or lower than a predetermined threshold in the period from the end of reception of the beacon signal to the elapse of the backoff time, the response A transmission control step for transmitting a signal;
Have
In the transmission control step,
Even when the signal level in the channel transmitting the response signal is equal to or lower than the threshold in the period from the end of reception of the beacon signal to the lapse of the back-off time, the response signal has a certain probability. Stop sending
A control method characterized by that. A control method in an RFID reader that receives a beacon signal transmitted from an RFID tag and transmits a response signal according to the beacon signal,
A level detection step of detecting a reception level of the beacon signal;
A backoff time calculating step of calculating a shorter backoff time as the reception level is higher;
If the signal level in the channel transmitting the response signal is equal to or lower than a predetermined threshold in the period from the end of reception of the beacon signal to the elapse of the back-off time, the response is received when the back-off time elapses. A transmission control step for transmitting a signal;
Have
In the back-off time calculating step,
The back-off time is calculated based on a cumulative probability distribution of reception levels calculated based on beacon signals transmitted from a plurality of RFID tags.
A control method characterized by that.

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