JP4622969B2 - RF tag reader and RF tag system - Google Patents

Description

  The present invention relates to an RF tag reader and an RF tag system that communicate with an RF tag in a frequency band divided into a plurality of channels.

  In this type of RF tag system, when a plurality of reader / writers are installed and used within a range where radio waves can reach each other, the radio waves transmitted from the reader / writers interfere with each other and cannot communicate with the RF tag. Therefore, the LBT (Listen Before Talk) system is adopted in Japan as a technique for preventing interference. Each reader / writer detects a carrier for a predetermined carrier detection time for a predetermined frequency channel (hereinafter, referred to as a channel) for a predetermined carrier detection time or more, and starts communication on an available channel.

  FIG. 9 shows carrier transmission timings of two reader / writers RW1 and RW2 arranged close to each other. The reader / writers RW1 and RW2 each detect the carrier and then transmit an unmodulated carrier for power supply (ch3, ch6 / ch9). Perform data communication.

  For example, when the reader / writer RW1 is only supplying power to the RF tag and not communicating with the RF tag, the reader / writer RW2 detects the power supply frequency band (ch3) by the carrier detection process. Communication frequency bands (ch1, ch2, ch4, ch5) cannot be detected. As a result, when the reader / writer RW1 performs data communication at intervals equal to or longer than the carrier detection time, the reader / writer RW1 performs a carrier detection process, and the frequency bands for communication of the reader / writers RW1 and RW2 (in FIG. 9). ch4, ch5) may overlap, causing transmission / reception failure due to interference and lowering the throughput.

On the other hand, in Patent Document 1, when a channel is selected and wireless communication connection is established with an electronic device terminal in a wireless network system, the use state is determined by measuring the radio wave level at a predetermined cycle for each of the plurality of channels. In addition, a wireless communication control device is disclosed that obtains usage frequency statistical data for each channel and changes the channel based on the statistical data during wireless communication with the electronic device terminal.
JP 2005-333510 A

  The wireless communication control device described in Patent Document 1 changes the channel according to a substantial traffic volume, and reduces the occurrence of radio wave interference as much as possible to increase the throughput. However, this wireless communication control apparatus does not essentially prevent interference, but only reduces the occurrence of interference from a statistical point of view. Further, communication cannot be performed while the channel is changed. Such communication control can be applied to a wireless LAN that continuously transmits a large amount of data such as moving image data at a high speed, but has a relatively small capacity between a large number of RF tags that enter the communication range one after another. It is difficult to apply to an RF tag system that transmits / receives the data one after another.

  The present invention has been made in view of the above circumstances, and an object thereof is to provide an RF tag reader and an RF tag system capable of increasing the throughput of communication under the LBT method.

  According to the means described in claim 1, the RF tag reader detects a carrier for each channel in a predetermined frequency band before transmitting the carrier, so that a channel in which no carrier is detected for a predetermined carrier detection time or more is detected. A continuous empty channel region is obtained, and carrier transmission is started when the empty channel region is equal to or larger than a channel width necessary for communication at a predetermined speed with the RF tag.

  Then, when communicating with the RF tag during carrier transmission, based on the carrier detection result for the above empty channel region, the range of the maximum continuous channel within the empty channel region where no carrier is detected Communication with the RF tag is performed using a necessary channel width. Thereby, collision (interference) with a carrier transmitted by another RF tag reader can be prevented, and a decrease in throughput due to transmission / reception failure can be avoided. In this means, the channel width used for communication may be narrowed and the communication speed may be reduced. However, since retransmission due to collision can be avoided, substantial throughput can be increased.

  According to the means described in claim 2, when the carrier transmission is started, a channel necessary for communication at a predetermined speed with the RF tag is specified in the empty channel region, and the carrier communicates with the RF tag during the carrier transmission. At this time, based on the carrier detection result for the specified channel, communication with the RF tag is performed using the maximum continuous channel within the specified channel and no carrier detected. Thereby, during carrier transmission, carrier detection only needs to be performed for the channel specified at the start of carrier transmission, so the burden of carrier detection processing can be reduced.

  According to the means described in claim 3, the carrier detection means comprises storage means for storing carrier detection presence / absence information for each channel, and when the carrier detection level for each channel is equal to or higher than a predetermined communication level, Information indicating that the channel is detected is written in the storage means, and when the carrier detection level for each channel continues for a predetermined determination time or less and becomes less than the communication level, information indicating that the channel is not detected is stored in the storage means. Write.

  By determining whether or not a carrier has been detected for each channel based on the detection presence / absence information written in the storage means, the carrier is not detected continuously for a predetermined determination time or more even once the carrier is detected. Accordingly, the channel can be used again, and the channel provided in the frequency band used in the RF tag system can be used effectively.

  According to the means described in claim 4, the carrier detection means writes the detection time in the storage means along with the detection time, and the carrier detection level for the channel in which the detection presence information is written is communicated. When the level is less than the level, if the difference between the time at that time and the detection time written in the storage means is equal to or greater than the determination time, information indicating no detection is written to the storage means. Thereby, when the carrier has not been detected for the determination time or longer, information that no carrier is detected can be obtained immediately for the channel.

  According to the means described in claim 5, the first storage area for storing the carrier detection presence / absence information based on the communication level, and the carrier detection presence / absence information based on the power supply level set higher than the communication level Is stored in the second storage area. When the carrier detection level is equal to or higher than the power supply level, the carrier detection means writes information indicating that the channel is detected in the second storage area. When the carrier detection level is less than the power supply level, the carrier detection means stores the information in the second storage area. Writes no detection information about the channel. That is, the second storage area stores the presence / absence information of the carrier component for supplying power at that time.

  When the presence / absence information of detection of the second storage area transitions from detection to non-detection, that is, when the power supply carrier disappears, the first storage area for the surrounding data communication channels including that channel By rewriting the detection presence / absence information to non-detection, the surrounding channels can be used immediately without waiting for the determination time, and the communication throughput can be further increased.

  According to the means described in claim 6, when the presence / absence information of the detection of the second storage area transits from detection to non-detection, the carrier detection means detects the second storage area around the channel. The information in the first storage area is rewritten without detection for all the channels up to an intermediate position between adjacent channels in which the information is present. In the RF tag system, the channel width used by each RF tag reader for communication with the RF tag is often the same, and information on the presence or absence of detection of the first storage area for all channels used by the extinguished power supply carrier Can be rewritten almost without detection.

  According to the means described in claim 7, by configuring an RF tag system from each of the above-described RF tag readers and an RF tag that operates by receiving power supply from the RF tag reader, communication under the LBT system can be performed. Throughput can be increased.

(First embodiment)
Hereinafter, a first embodiment of the present invention will be described with reference to FIGS.
FIG. 2 is a block diagram showing an electrical configuration of a reader / writer that performs non-contact communication with an RF tag. The reader / writer includes a control unit 1, a transmission circuit 2, a transmission antenna 3, a reception antenna 4a, a sensing antenna 4b, a reception circuit 5, a frequency channel detection unit 6 and the like that are configured by a microcomputer.

  The transmission circuit 2 includes an oscillator 7 that generates a carrier signal (carrier signal) corresponding to a transmission frequency commanded from the control unit 1, a modulation unit 8 that modulates the carrier signal according to transmission data output from the control unit 1, And an amplifier 9 that amplifies the modulated signal and transmits it as a radio wave signal via the transmission antenna 3. The reception circuit 5 includes a demodulation unit 10 that demodulates a signal received via the reception antenna 4a, and a decoding unit 11 that decodes reception data from the demodulated reception signal.

  The frequency channel detection unit 6 receives a signal received via the sensing antenna 4b, detects the presence / absence of a carrier (carrier wave signal) in each frequency channel (hereinafter referred to as a channel), and outputs it to the control unit 1 The carrier detecting means is configured together with the control unit 1. A frequency channel detector 6 of the reader / writer RW1 described later detects a carrier not only before the carrier is transmitted but also during the carrier transmission.

  The control unit 1 (corresponding to a communication control unit) communicates with the RF tag while supplying power to the RF tag, and writes data to and reads data from the RF tag. Further, the control unit 1 includes a memory 12 as a clock function and a storage unit, and stores the carrier detection information on each channel input from the frequency channel detection unit 6 in a sense table of the memory 12. ing.

Next, the operation of this embodiment will be described with reference to FIGS. 1 and 3 to 7 as well.
FIG. 3 shows a configuration example of an RF tag system in which the reader / writers RW1, RW2, and RW3 are arranged close to each other and communicate with the RF tags TAG1 to TAG4, respectively. As described above, the reader / writer RW1 has a configuration in which carrier detection is always performed even during carrier transmission, and the reader / writer RW2 and RW3 have a configuration in which carrier detection is performed only before carrier transmission, as in the prior art. I have.

  A range indicated by a broken line is a range in which the transmission radio wave (carrier) of the reader / writer RW1 can be detected, and indicates a range in which radio wave interference occurs between the reader / writers RW2 and RW3 arranged inside the range. A range indicated by a solid line indicates a range in which communication and power supply can be performed from the reader / writer RW1 to the RF tag. In the arrangement shown in FIG. 3, the reader / writer RW1 can communicate and supply power to the RF tags TAG1 and TAG2.

  FIG. 1 shows carrier transmission timings of RW1 and RW2 among reader / writers RW1 to RW3, and FIG. 4 shows a DSB (Double Side Band) frequency channel used in this RF tag system. The vertical axis in FIG. 4 represents the signal level of the transmission radio wave. Channels that can be used for communication are ch1 to ch9, and the channel width (frequency band) used by the reader / writers RW1 to RW3 to supply power to the RF tag is 1, as shown in FIG.

  Further, the channel width (frequency band) used for signal transmission of commands and data from the reader / writers RW1 to RW3 to the RF tags TAG1 to TAG4 is 3, as shown in FIG. 4B, and the RF tags TAG1 to TAG4. As shown in FIG. 4C, the channel width (frequency band) used for signal reception in response to IDs and data from the reader / writers RW1 to RW3 is 5 as shown in FIG. Therefore, the channel width (frequency band) necessary for communication between the reader / writers RW1 to RW3 and the RF tags TAG1 to TAG4 is 5, as shown in FIG. However, this is an example, and the channel width (frequency band) used for signal transmission / reception may be appropriately set according to the communication speed. In general, a higher channel speed requires a wider channel width.

  The reader / writers RW1 to RW3 detect the presence / absence of carriers for ch1 to ch9 for a predetermined carrier detection time Tc or longer before transmitting the carriers. FIG. 5 is a flowchart showing the contents of the carrier transmission start process executed by the control unit 1 before carrier transmission. When communication with the RF tag is required, the control unit 1 inputs the detection result of the carrier of each channel from the frequency channel detection unit 6 at least during the carrier detection time Tc and stores it in the memory 12 (step S1, S2). In this case, each channel may be detected in order, or a plurality of channels may be detected collectively.

  When the carrier detection time Tc elapses (step S2; YES), the process proceeds to step S3, the number of channels in which carriers are detected is subtracted from the usable channel width (9 channels in this embodiment), and the maximum continuously present Calculate the free channel width of. Then, a lower limit channel and an upper limit channel (hereinafter referred to as a pre-transmission lower limit channel and a pre-transmission upper limit channel) of the empty channel width are obtained. If none of the reader / writers is transmitting and all the channels are free, the pre-transmission lower limit channel is 1 and the pre-transmission upper limit channel is 9. During the transmission of the carrier, the reader / writer does not use the channel beyond the range determined by the lower limit channel before transmission and the upper limit channel before transmission detected by the reader / writer.

  Subsequently, the process proceeds to step S4, and it is determined whether or not a channel width necessary for communication can be secured in an empty channel. The channel width necessary for communication refers to the channel width including the channel used for power supply, the channel used for signal transmission, and the channel used for signal reception as described above (in this embodiment, as shown in FIG. 4D). 5 channels) and increases as the communication speed increases as described above.

  If the channel width necessary for communication can be secured, a transmission frequency is set in step S5, and the frequency is instructed to the oscillator 7 in step S6 to start carrier transmission. On the other hand, when the channel width necessary for communication cannot be secured, the carrier transmission start process is terminated without executing steps S5 and S6 (that is, without performing carrier transmission).

  In the carrier transmission start process, the reader / writer RW1 shown in FIG. 1 calculates the pre-transmission lower limit channel as 1 and the pre-transmission upper limit channel as 9, and can secure the channel width (ch2 to ch6) necessary for communication. The carrier transmission is started using one of them, ch4. On the other hand, the reader / writer RW2 also executes carrier transmission start processing with a slight delay. When the reader / writer RW2 executes the above steps S1 and S2, the reader / writer RW1 transmits an unmodulated carrier only in ch4. Therefore, the reader / writer RW2 determines the maximum available free channel width. ch5 to ch9 are calculated, and the pre-transmission lower limit channel is determined to be 5, and the pre-transmission upper limit channel is determined to be 9. As a result, the reader / writer RW2 determines that the channel width (ch5 to ch9) necessary for communication can be secured, and starts carrier transmission using one of them, ch7.

  The control unit 1 of the reader / writer RW1 executes the sense table update process shown in FIG. 6 and the data communication speed change process shown in FIG. 7 during carrier transmission to prevent a collision between the reader / writer RW2 in ch5 and ch6. ing. The sense table update process is executed in parallel with a main process such as a data communication speed change process by a timer interrupt or the like. Hereinafter, these processes will be described in detail.

  In the sense table update process shown in FIG. 6, the control unit 1 uses the frequency channel detection unit 6 to sequentially change the carrier for the channel between the pre-transmission lower limit channel and the pre-transmission upper limit channel (empty channel region before carrier transmission). When the presence / absence is detected and a carrier is detected, information indicating that the channel is detected and its detection time are written in the memory 12, and when no carrier is detected continuously for a predetermined determination time Ta for each channel, the memory 12 is read. Information indicating no detection is written for the channel. The sense table is detection presence / absence information written and updated in the memory 12. The sense table used in this embodiment corresponds to the communication sense table (corresponding to the first storage area) in the second embodiment.

  The frequency channel detection unit 6 is a communication reference set lower than the detection reference level (power reference level) of an unmodulated carrier component for power supply in order to detect even sideband carrier components that are spread by modulation during data communication. Use levels. That is, when the detection level of the received signal for each channel is equal to or higher than the communication reference level, information that carrier detection is present is output to the control unit 1, and when the detection level of the received signal is less than the communication reference level, no carrier detection is performed. Is output to the control unit 1.

  The controller 1 sets a pre-transmission lower limit channel for the detection target channel N in step S11, and detects the presence or absence of a carrier for the channel N in step S12. Here, if detected (step S13; YES), the detection time and sense flag ON (information of presence of detection) are written in the area of channel N of the sense table of the memory 12 and not detected in step S14. In (Step S13; NO), writing to the sense table is not performed. Thereafter, the detection target channel N is incremented in step S15, and it is determined in step S16 whether or not the pre-transmission upper limit channel has been exceeded. If it is below the upper limit channel before transmission, the process returns to step S12.

  On the other hand, if the pre-transmission upper limit channel is exceeded, the process proceeds to step S17, where the sense table within the range of the pre-transmission upper limit channel from the pre-transmission lower limit channel is sequentially read, and the sense flag is off for each channel for which the sense flag is on. Execute reset processing to. In this reset process, the detection time written in the sense table is subtracted from the current time, and the carrier disappears when the difference (elapsed time since the previous carrier was detected) is equal to or greater than the predetermined determination time Ta. To turn the sense flag back from on to off. Through the sense table update process described above, the latest carrier detection presence / absence information is always stored in the sense table of the memory 12 for each channel from the pre-transmission lower limit channel to the pre-transmission upper limit channel.

  Next, the data communication speed change process shown in FIG. 7 will be described. The control unit 1 performs carrier transmission start processing shown in FIG. 5 to start carrier transmission, and then performs data communication with the RF tag in step S21. The channel used at this time is the transmission frequency (ch2 to ch6) set in step S5. However, when the elapsed time from the start of carrier transmission to data communication is long (for example, when the carrier detection time Tc or longer), another reader / writer may use the channel, so step S21 is executed. Instead, it is preferable to perform data communication in step S23 instead.

  In step S22, the control unit 1 reads the sense table in the memory 12, obtains a continuous channel width in which the sense flag is off, and temporarily sets the lower limit channel and the upper limit channel. In step S23, the RF tag is read / written using the channel width necessary for data communication within the temporarily set lower limit channel and upper limit channel. Thereafter, in step S24, it is determined whether or not the data communication is finished. If not finished, the process returns to step S22, and if finished, the carrier transmission is stopped in step S25. In order to give other readers / writers the opportunity to communicate, the carrier transmission time is limited to a predetermined value or less, and once transmission is stopped, transmission should not be started until a predetermined time has elapsed. I have control.

  In FIG. 1, the reader / writer RW2 performs data communication at ch6 to ch8 after the start of carrier transmission. Prior to the write operation, the reader / writer RW1 refers to the sense flag, and sets a temporary lower limit channel ch1 and an upper limit channel ch5 for the continuous channel width (ch1 to ch5) in which the sense flag is off. In this frequency range, data communication is performed using ch3 to ch5 that can be transmitted by the DSB method with ch4 as the center. This reduces the communication speed, but avoids transmission and reception failures due to interference.

  Thereafter, the reader / writer RW2 stops transmitting the carrier, and when the above-described determination time Ta elapses after the stop, the sense flag of the reader / writer RW1 is reset to sense-off for all channels. Therefore, the reader / writer RW1 sets the temporary lower limit channel ch1 and the upper limit channel ch9 for the continuous channel width (ch1 to ch9) in which the sense flag is off in the read operation after the determination time Ta has elapsed, Data communication can be performed at a desired speed using the initially set ch2 to ch6. Strictly speaking, if the determination time Ta elapses after the reader / writer RW2 finishes the last read operation, data communication using ch2 to ch6 becomes possible.

  As described above, the reader / writer used in the RF tag system of the present embodiment detects carriers for all channels before transmitting carriers, and an empty channel in which there are continuous channels in which no carriers are detected. An LBT (Listen Before Talk) method is adopted in which a region is obtained and carrier transmission is started when the empty channel region is equal to or larger than the channel width necessary for communication with the RF tag. Then, the carrier is detected during the carrier transmission and the sense table of the memory 12 is updated, and it is necessary within the range of the maximum continuous channel in the original empty channel region and the carrier not detected. Data communication is performed using the channel width.

  As a result, interference with carriers transmitted by other reader / writers can be prevented, and a decrease in throughput due to transmission / reception failure can be avoided. In this case, the channel width used for communication is narrowed and the communication speed of the reader / writer is reduced. However, since retransmission due to collision can be avoided, the substantial throughput can be increased. In addition, since the sense flag of the channel that has not detected the carrier continuously for the determination time Ta or longer is turned off and reused as an empty channel, the communication speed is temporarily lowered, and the communication throughput is further increased. be able to.

  In this RF tag system, it is not always necessary to configure all of the reader / writer as described above, and the above configuration may be adopted only for a part of the reader / writer (in this embodiment, reader / writer RW1) as shown in FIG. . When two or more reader / writers (reader / writers RW2 and RW3) placed in a region where radio wave interference occurs have a conventional configuration in which carrier detection is performed only before carrier transmission, interference may occur between them. There is. However, interference does not occur with the reader / writer RW1 having the above configuration, and an improvement in throughput as a whole can be expected.

(Second Embodiment)
Next, a second embodiment of the present invention will be described with reference to FIG.
FIG. 8 is a flowchart of the sense table update process executed by the control unit 1 of the reader / writer, and the same step numbers are assigned to the same process steps as in FIG. The memory 12 of the present embodiment is based on the power reference level in addition to the communication sense table (corresponding to the first storage area) that stores a communication sense flag that is carrier presence / absence information detected based on the communication reference level. A power sense table (corresponding to the second storage area) for storing a power sense flag, which is the presence / absence information of the detected carrier, is set. As described above, the power reference level is set higher than the communication reference level, the communication sense table indicates the presence / absence information of the carrier including the sideband carrier component at the time of communication with the RF tag, and the power sense table indicates the power. The presence / absence information of only the carrier component (carrier central component) for supply is shown.

  The control unit 1 updates the communication sense table in the memory 12 in steps S12 to S14 as in the first embodiment. In subsequent step S31, the presence or absence of a carrier based on the power reference level is detected for channel N. If detected (step S32; YES), the power sense flag ON is written in the area of channel N of the power sense table of the memory 12 in step S33, and if not detected (step S32; NO). In step S34, the power sense flag OFF is written in the channel N area of the power sense table of the memory 12. Thus, the power sense flag is immediately turned off when no carrier is detected based on the power reference level.

  After that, after executing the processing of steps S15 to S17, if there is a channel (carrier annihilation channel) in which the power sense flag transitions from on to off in the power sense table in step S35, the control unit 1 includes the channel. The communication sense flag in the communication sense table is rewritten from on to off for each channel.

  Specifically, in the communication sense table, the communication sense flag up to each intermediate position between the carrier disappearance channel and the power sense flag-on channel adjacent to both sides thereof is rewritten from on to off. If there is no channel with the power sense flag turned on, the communication sense flag is rewritten from on to off for all channels in that direction (side). Since this control requires the previous value and the current value of the power sense table, the current power sense flag of the power sense table is stored as the previous power sense flag in step S36.

  According to the present embodiment described above, when the power supply carrier component is reliably detected using the power reference level set higher than the communication reference level, and when the power supply carrier disappears, the channel Since the communication sense flag of the surrounding data communication channel including “OFF” is rewritten to OFF, the surrounding channel can be used immediately without waiting for the determination time Ta, and the communication throughput can be further increased. In addition, operations and effects similar to those of the first embodiment are achieved.

(Other embodiments)
The present invention is not limited to the embodiments described above and shown in the drawings, and can be modified or expanded as follows, for example.
In the sense table update process, the presence / absence of a carrier is detected for the channel between the pre-transmission lower limit channel and the pre-transmission upper limit channel (the empty channel region at the start of carrier transmission), but is determined in step S5 of the carrier transmission start process. The presence or absence of a carrier may be detected only for a specific channel. Thereby, during carrier transmission, carrier detection only needs to be performed for the channel specified at the start of carrier transmission, so that the execution time of the sense table update process of the control unit 1 can be reduced.

  In the second embodiment, when a carrier annihilation channel is generated, the surrounding channels in which the communication sense flag is rewritten from on to off are the carrier annihilation channel (carrier central component) and the sideband component when the carrier is modulated. Channel. To accurately determine these channels, when the power sense flag transitions from on to off, the communication sense flag is rewritten from on to off for the channel in which the same detection time as the carrier disappearance channel is written in the communication sense table. You may do it. Further, this communication sense flag update method may be used in combination with the update method described in the second embodiment.

The present invention can be applied not only to the DSB system but also to an SSB (Single Side Band) system.
In each embodiment, the reader / writer of the RF tag has been described as an example, but the present invention can be similarly applied to an RF tag reader.
The receiving antenna 4a and the sensing antenna 4b may be shared (integrated). Further, the transmission antenna 3, the reception antenna 4a, and the sensing antenna 4b may be shared (integrated).

The figure which is the 1st Embodiment of this invention and shows the carrier transmission timing of a reader / writer Block diagram showing electrical configuration of reader / writer The figure which shows the structural example of RF tag system The figure which shows the frequency channel and signal level which are used in the RF tag system Flow chart showing carrier transmission start processing Flow chart showing sense table update processing Flow chart showing data communication speed change processing FIG. 6 equivalent view showing the second embodiment of the present invention 1 equivalent diagram showing the prior art

Explanation of symbols

  In the drawings, 1 is a control unit (communication control unit, carrier detection unit), 6 is a frequency channel detection unit (carrier detection unit), 12 is a memory (storage unit), RW1 to RW3 are reader / writers (RF tag readers), TAG1 to TAG1. TAG4 indicates an RF tag.

Claims (7)

In an RF tag reader that communicates with an RF tag in a frequency band divided into a plurality of channels,
Carrier detection means for performing carrier detection processing for a predetermined channel before and during transmission of the carrier;
By detecting the carrier by the carrier detection means for each channel in a predetermined frequency band before transmitting the carrier, an empty channel region in which a channel in which no carrier is detected for a predetermined carrier detection time or more continuously exists is obtained. When the empty channel region is equal to or larger than the channel width necessary for communication with the RF tag at a predetermined speed, carrier transmission is started, and when communicating with the RF tag during carrier transmission, the empty channel region Based on the detection result of the carrier by the carrier detection means, the RF tag is used with the necessary channel width within the empty channel region and within the maximum continuous channel where no carrier is detected. An RF tag reader comprising communication control means for performing communication.   The communication control means specifies a channel required for communication at a predetermined speed with the RF tag in the empty channel region when starting transmission of a carrier, and communicates with the RF tag during transmission of a carrier. Based on the carrier detection result by the carrier detection means for the specified channel, communication with the RF tag is performed using the maximum continuous channel within the specified channel and no carrier detected. The RF tag reader according to claim 1. The carrier detection means includes storage means for storing carrier detection presence / absence information for each channel. When the carrier detection level for each channel is equal to or higher than a predetermined communication level, the storage means detects that the channel is detected. And when the carrier detection level for each channel continues for a predetermined determination time or less and becomes less than the communication level, the storage means is configured to write information indicating no detection for the channel,
3. The RF tag reader according to claim 1, wherein the communication control unit determines whether or not a carrier is detected for each channel based on the detection presence / absence information written in the storage unit.   The carrier detection means writes the detection time in the storage means together with the information that the detection is present, and when the carrier detection level for the channel in which the detection presence information is written is less than the communication level, the detection time 4. The RF according to claim 3, wherein when the difference between the detection time written in the storage unit and the detection time is equal to or greater than the determination time, information indicating no detection for the channel is written in the storage unit. Tag reader. Configured to supply power to the RF tag by transmitting a carrier;
The storage means stores carrier detection presence / absence information based on a power supply level set higher than the communication level, in addition to a first storage area for storing carrier detection presence / absence information based on the communication level. A second storage area,
When the carrier detection level is equal to or higher than the power supply level, the carrier detection means writes information indicating that the channel is detected in the second storage area, and the carrier detection level is lower than the power supply level. When the information indicating that the channel is not detected is written in the second storage area, and the presence / absence information of the detection in the second storage area is changed from detected to not detected, the surrounding channels including the channel are included. 5. The RF tag reader according to claim 3, wherein the presence / absence information of detection of the first storage area is rewritten to non-detection.   When the presence / absence information of the detection of the second storage area transits from detection to non-detection, the carrier detection means writes information indicating detection to the second storage area centering on the channel. 6. The RF tag reader according to claim 5, wherein the information in the first storage area is rewritten without detection for all channels up to an intermediate position between adjacent channels. 7. An RF tag system comprising: the RF tag reader according to claim 1; and an RF tag that communicates with the RF tag reader.

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