Patent Publication Number: US-2023137527-A1

Title: Communication device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2021-178511, filed on Nov. 1, 2021, the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate to a communication device. 
     BACKGROUND 
     In a communication device that shares an antenna for transmission and reception, a part of a transmission signal may be superimposed on a reception signal and flow into a reception system. A component of the transmission signal superimposed on the reception signal becomes a self-jammer signal, which may cause a decrease in communication quality due to saturation of the reception system and an increase in noise. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a block diagram showing a circuit configuration of main parts of a reading device according to an embodiment; 
         FIG.  2    is a block diagram showing a configuration of a circuit built in an FPGA for control of an SJC process; 
         FIG.  3    is a diagram showing frequency characteristics of a first LPF and a second LPF; 
         FIG.  4    is a flowchart of a control process executed by a control circuit; and 
         FIG.  5    is a diagram showing an example of a change in a residual level measured by a measurement circuit. 
     
    
    
     DETAILED DESCRIPTION 
     In a typical communication device, a cancellation signal whose phase is opposite to a phase of the self-jammer signal is generated from the transmission signal, and the self-jammer signal is canceled by using the cancellation signal. 
     Typically, a gain and a phase shift amount or value for generating the cancellation signal are appropriately set before communication starts. During the communication, the self-jammer signal is canceled by a cancellation signal generated based on the gain and the phase shift amount or value set before the communication starts. Such a process is referred to as a self-jammer cancellation process, and is hereinafter abbreviated as SJC control. 
     During the communication, however, the self-jammer signal may fluctuate due to a change in a surrounding environment of the antenna. The self-jammer signal remaining after the cancellation may become louder when the self-jammer signal fluctuates significantly. 
     It is desired that an influence of the self-jammer signal can be reduced even when the self-jammer signal fluctuates during the communication. 
     In general, according to one embodiment, a communication device capable of reducing an influence of a self-jammer signal even when the self-jammer signal fluctuates during communication is provided. 
     A communication device according to an embodiment includes a first generation unit or first generator, a shared unit or signal sharer, a second generation unit or second generator, a combining unit or combiner, a first measurement unit or first measurement circuit, a first control unit or first control circuit, a second measurement unit or second measurement circuit, a second control unit or second control circuit, and a third control unit or third control circuit. The communication device includes one or more control circuits that may include one processor or multiple processors. The first generation unit or first generator is configured to generate a transmission signal for wireless transmission. The shared unit or signal sharer is configured to receive the transmission signal generated by the first generation unit from an input terminal, output the transmission signal from an input and output terminal, and output, from an output terminal, a signal input from the input and output terminal. The second generation unit or second generator is configured to generate a cancellation signal by changing an amplitude and a phase of the transmission signal generated by the first generation unit. The combining unit or combiner is configured to combine the cancellation signal generated by the second generation unit with the signal output from the output terminal. The first measurement unit or first measurement circuit is configured to measure a level of a self-jammer signal extracted in a first bandwidth from a combined signal or a signal output from the combining unit. The first control unit or first control circuit is configured to set change amounts or values of an amplitude and a phase in the second generation unit so as to reduce the level measured by the first measurement unit to a level satisfying a predetermined condition during a period of a predetermined first time width. The second measurement unit or second measurement circuit is configured to measure a level of a self-jammer signal extracted in a second bandwidth wider than the first bandwidth from the combined signal or signal output from the combining unit. The second control unit or second control circuit is configured to control change amounts or values of the amplitude and the phase in the second generation unit so as to reduce the level measured by the second measurement unit during a period of a second time width shorter than the first time width, in response to an increase in the level measured by the second measurement unit. The third control unit or third control circuit is configured to cause the first control unit to set the change amounts or values of the amplitude and the phase in the second generation unit before start of reception of a response signal transmitted from a partner device, and cause the second control unit to set the change amounts or values of the amplitude and the phase in the second generation unit during the reception of the response signal transmitted from the partner device. 
     Hereinafter, embodiments will be described with reference to the drawings. In the following, a reading device that reads data stored in a radio frequency identification (RFID) tag will be described as an example. The reading device is an example of a communication device that wirelessly communicates with the RFID tag as a partner device when reading the data. 
       FIG.  1    is a block diagram showing a circuit configuration of main parts of a reading device or reader  100  according to the present embodiment. 
     The reading device  100  includes an oscillator  11 , a phase shifter  12 , a digital to analog (DA) converter  13 , a quadrature modulator  14 , a band-pass filter (BPF)  15 , a power amplifier  16 , a low-pass filter (LPF)  17 , an antenna sharer  18 , a feed line  19 , an antenna  20 , a vector modulator  21 , a DA converter  22 , a power combiner  23 , a quadrature detector  24 , a baseband filter (BBF)  25 , a baseband amplifier  26 , an analog to digital (AD) converter  27 , an LPF  28 , an AD converter  29 , and a control unit or control circuit  30 . The control unit  30  includes a CPU  301 , a field-programmable gate array (FPGA)  302 , and a memory  303 . The antenna  20  may not be included in the reading device  100 , and any antenna may be connected to the feed line  19 . In addition, the antenna  20  and the feed line  19  may not be included in the reading device  100 , and any feed line to which any antenna is connected may be connected to the antenna sharer  18 . 
     The oscillator  11  generates a sine wave having a predetermined frequency as a carrier wave. 
     The phase shifter  12  shifts a phase of the carrier wave generated by the oscillator  11  by 90 degrees to output a cosine wave as another carrier wave. 
     The DA converter  13  converts two groups of transmission baseband signals output from the CPU  301  in a digital state into analog signals. 
     The quadrature modulator  14  receives, as modulated waves, the two groups of transmission baseband signals converted into the analog signals by the DA converter  13 . The quadrature modulator  14  receives the carrier wave generated by the oscillator  11  and the carrier wave output from the phase shifter  12 . Then, the quadrature modulator  14  obtains a transmission signal by quadrature modulation using the two groups of modulated waves and the two groups of carrier waves. 
     The BPF  15  removes a low frequency component and a high frequency component due to band limitation from the transmission signal obtained by the quadrature modulator  14 . 
     The power amplifier  16  amplifies the transmission signal that passes through the BPF  15  to a level suitable for wireless transmission. 
     The LPF  17  removes a harmonic component from the transmission signal amplified by the power amplifier  16 . 
     By the processes in the BPF  15 , the power amplifier  16 , and the LPF  17 , the transmission signal becomes a signal for the wireless transmission. That is, the BPF  15 , the power amplifier  16 , and the LPF  17  constitute a first generation unit or first generator that generates the transmission signal for the wireless transmission. 
     The antenna sharer  18  includes an input terminal TI, an input and output terminal TIO, an output terminal TOA, and an output terminal TOB. The transmission signal that passes through the LPF  17  is input to the input terminal TI. The antenna sharer  18  outputs, from the input and output terminal TIO and the output terminal TOB, the transmission signal input to the input terminal TI. The antenna sharer  18  outputs, from the output terminal TOA, a signal input to the input and output terminal TIO. The signal output from the output terminal TOA of the antenna sharer  18  is a signal obtained by combining a reception signal generated in the antenna  20  and a self-jammer signal to be described later. This signal is simply referred to as a reception signal below. The antenna sharer  18  is an example of a shared unit or signal sharer. 
     The feed line  19  supplies, to the antenna  20 , the transmission signal output from the input and output terminal TIO of the antenna sharer  18 . The feed line  19  transmits the reception signal generated in the antenna  20  to the input and output terminal TIO of the antenna sharer  18 . 
     The antenna  20  radiates a radio wave corresponding to the transmission signal supplied from the feed line  19 . The antenna  20  generates an electric signal corresponding to an incoming radio wave as the reception signal. 
     The vector modulator  21  modulates the transmission signal output from the output terminal TOB of the antenna sharer  18  such that the transmission signal becomes a signal having an amplitude and a gain corresponding to a vector represented by two groups of control signals (hereinafter referred to as an I control signal and a Q control signal) supplied from the DA converter  22 . The transmission signal modulated by the vector modulator  21  is hereinafter referred to as a cancellation signal. That is, the vector modulator  21  generates the cancellation signal by changing the amplitude and the phase of the transmission signal, and corresponds to a second generation unit or second generator. 
     The DA converter  22  converts, into analog signals, the I control signal and the Q control signal output from the control unit  30  in a digital state to obtain the I control signal and the Q control signal to be supplied to the vector modulator  21 . 
     The power combiner  23  combines the cancellation signal output from the vector modulator  21  with the reception signal output from the output terminal TOA of the antenna sharer  18 . As a result, the power combiner  23  reduces the self-jammer signal included in the reception signal. The power combiner  23  is an example of a combining unit or combiner. 
     The quadrature detector  24  executes quadrature detection on the reception signal output from the power combiner  23  using the two carrier waves respectively output from the oscillator  11  and the phase shifter  12 . The quadrature detector  24  outputs, in parallel, two groups of analog-state reception baseband signals obtained by the quadrature detection. That is, the quadrature detector  24  is an example of a wave detector. 
     The BBF  25  extracts a predetermined baseband component from each of the two groups of reception baseband signals output from the quadrature detector  24 . A pass band of the BPF  25  includes a band including a response signal from an RFID tag  200 . 
     The baseband amplifier  26  amplifies each of the two groups of reception baseband signals that pass through the BBF  25  to a level suitable for digitization by the AD converter  27 . 
     The AD converter  27  digitizes the two groups of reception baseband signals amplified by the baseband amplifier  26 . 
     The LPF  28  extracts low-frequency band signals corresponding to components of the self-jammer signals that are respectively included in the two groups of reception baseband signals output from the quadrature detector  24 . Each of the signals extracted by the LPF  28  includes the component of the self-jammer signal remaining in the output of the power combiner  23 . The two groups of signals extracted by the LPF  28  are hereinafter referred to as an I residual signal and a Q residual signal. 
     The AD converter  29  digitizes the I residual signal and the Q residual signal output from the LPF  28 . 
     In communication with the RFID tag  200 , the CPU  301  outputs the two groups of transmission baseband signals in accordance with a predetermined sequence. The CPU  301  reconstructs data sent from the RFID tag  200  based on the two groups of reception signals digitized by the AD converter  27 . 
     The FPGA  302  executes signal processing programmed in advance to execute various high-speed calculation associated with information processing executed by the CPU  301 . One of functions of the FPGA  302  is a function of controlling the vector modulator  21  based on the I residual signal and the Q residual signal that are digitized by the AD converter  29 . 
     The memory  303  stores an information processing program described for information processing to be executed by the CPU  301 . The memory  303  stores various types of data required for the CPU  301  to execute various types of information processing. The memory  303  stores various types of data generated or acquired when the CPU  301  executes various types of information processing. 
       FIG.  2    is a block diagram showing a configuration of a circuit built in the FPGA  302  for control of an SJC process. 
     As shown in  FIG.  2   , the circuit built in the FPGA  302  includes a control circuit  3021 , a generation circuit  3022 , a conversion circuit  3023 , selectors  3024  and  3025 , a first LPF  3026 , a second LPF  3027 , a measurement circuit  3028 , and a detection circuit  3029 . 
     When the CPU  301  gives a start signal for instructing execution of the SJC process, the control circuit  3021  executes a process for realizing the SJC process (hereinafter, referred to as a control process), which will be described later. The control circuit  3021  instructs the generation circuit  3022  to start scanning in the control process. The control circuit  3021  controls the selectors  3024  and  3025  in the control process. In the control process, the control circuit  3021  sets the amplitude and the phase of the cancellation signal according to an amplitude minimum point detected by the detection circuit  3029 . The control circuit  3021  outputs a state signal for notifying the CPU  301  of an execution state of the SJC process. 
     The generation circuit  3022  generates, in a digital state, a phase signal and an amplitude signal respectively indicating the amplitude and the phase of the cancellation signal. The generation circuit  3022  sequentially changes the phase signal and the amplitude signal such that amplitudes and phases in a scan period are swept. The generation circuit  3022  fixes the phase signal and the amplitude signal so that the amplitude and the phase are constant in a period other than the scan period. 
     The conversion circuit  3023  converts the phase signal and the amplitude signal supplied from the generation circuit  3022  into the I control signal and the Q control signal. The I control signal and the Q control signal represent, in a digital state, the vector corresponding to the phase and the amplitude that are represented by the phase signal and the amplitude signal. The I control signal and the Q control signal obtained by the conversion circuit  3023  are supplied to the DA converter  22 . 
     The selectors  3024  and  3025  alternatively validate the first LPF  3026  and the second LPF  3027  under control of the control circuit  3021 . Then, the selector  3024  supplies the I residual signal and the Q residual signal output from the AD converter  29  to the valid one of the first LPF  3026  and the second LPF  3027 . The selector  3025  supplies the I residual signal and the Q residual signal that pass through the valid one of the first LPF  3026  and the second LPF  3027  to the measurement circuit  3028 . 
     The first LPF  3026  and the second LPF  3027  all pass low frequency components of the I residual signal and the Q residual signal. As a result, the first LPF  3026  and the second LPF  3027  attenuate an unnecessary signal component that lowers measurement accuracy of the measurement circuit  3028 . The characteristics of the first LPF  3026  and the second LPF  3027  may be appropriately determined by, for example, a designer of the reading device  100 . However, a cutoff frequency of the first LPF  3026  is set smaller than that of the second LPF  3027 . As a result, the first LPF  3026  has a capability of attenuating the unnecessary signal component higher than that of the second LPF  3027 . For example, a finite impulse response (FIR) filter can be used as the first LPF  3026  and the second LPF  3027 . 
     The first LPF  3026  is an example of a first filter, and the second LPF  3027  is an example of a second filter. 
       FIG.  3    is a diagram showing frequency characteristics of the first LPF  3026  and the second LPF  3027 . 
     As shown in  FIG.  3   , the cutoff frequency of the first LPF  3026  is smaller than that of the second LPF  3027 . The first LPF  3026  has an order larger than that of the second LPF  3027 . As a result, as shown in  FIG.  3   , the first LPF  3026  has an inclination of an attenuation amount or value larger than an inclination of an attenuation amount or value of the second LPF  3027 . 
     The measurement circuit  3028  measures an amplitude of the residual signal (hereinafter referred to as a residual level) based on the I residual signal and the Q residual signal supplied from the selector  3025 . 
     The detection circuit  3029  detects a timing at which the residual level measured by the measurement circuit  3028  is minimized (hereinafter referred to as the amplitude minimum point). Then, the detection circuit  3029  notifies the control circuit  3021  of the detected amplitude minimum point. 
     Next, an operation of the reading device  100  including the above configurations will be described. The operation for reading the RFID tag  200  may be, for example, another known operation using a quadrature modulation scheme as a modulation scheme for wireless communication. Therefore, here, the description of the operation is omitted, and an operation relating to the SJC process will be described. 
     The self-jammer signal will be described before describing the operation. 
     The antenna sharer  18  is designed such that the transmission signal input to the input terminal TI is not output from the output terminal TOA. In an actual circuit configuration, however, it is difficult to completely prevent the transmission signal input to the input terminal TI from leaking from the output terminal TOA. Therefore, a part of the transmission signal input to the input terminal TI is output from the output terminal TOA as it is. A part of the transmission signal output from the input and output terminal TIO of the antenna sharer  18  is reflected at a feed point of the antenna  20  and transmitted to the antenna sharer  18  by the feed line  19 . Such a reflected signal is output from the output terminal TOA by a function of the antenna sharer  18 . Thus, a signal output from the output terminal TOA of the antenna sharer  18  includes a component of the transmission signal leaked without being output from the input and output terminal TIO and a component of the transmission signal input as the reflected signal to the input and output terminal TIO. The self-jammer signal is a signal obtained by combining the components of the transmission signal. A reflection characteristic of the transmission signal at the feed point of the antenna  20  changes depending on an environment around the antenna  20 , such as how close the RFID tag  200  and other objects are to the antenna  20 . Therefore, an amplitude and a phase of the signal reflected at the feed point of the antenna  20  also fluctuate depending on the environment around the antenna  20 . Due to this influence, an amplitude and a phase of the self-jammer signal also fluctuate depending on the environment around the antenna  20 . 
     The environment around the antenna  20  is mainly influenced by an article attached with the RFID tag  200 . The environment around the antenna  20  is influenced by a table on which the article attached with the RFID tag  200  is placed. Therefore, as a separation distance between the antenna  20  and the RFID tag  200 , that is, a communication distance becomes smaller, a stability of the environment around the antenna  20  tends to be lower. 
     The self-jammer signal is a signal derived from the transmission signal. Therefore, by changing the amplitude and the phase of a signal branched from the transmission signal, it is possible to generate a signal having the same frequency and amplitude but having an opposite phase with respect to the self-jammer signal. Then, by combining such a signal with the reception signal output from the output terminal TOA of the antenna sharer  18 , the self-jammer signal included in the reception signal can be canceled out. In the reading device  100 , the cancellation signal obtained by changing the amplitude and the phase by the vector modulator  21  is combined, by the power combiner  23 , with the reception signal output from the output terminal TOA of the antenna sharer  18 , thereby reducing the self-jammer signal included in the reception signal. 
     The reading device  100  is used, for example, to read the data from the RFID tag  200  attached to an article placed at a predetermined position in a warehouse. Therefore, an operator carrying the reading device  100  moves to the front of a shelf, and causes the reading device  100  to read the RFID tag  200  attached to the article placed on the shelf. One of operation modes at this time is a mode in which the operator is in the front of the shelf and holds the reading device  100  toward the shelf to read the RFID tag  200  located in a relatively wide range. Another mode of the operation modes is a mode in which the operator holds the reading device  100  close to the RFID tag  200  placed on the shelf to read the RFID tag  200  in the vicinity. A communication distance between the reading device  100  and the RFID tag  200  in the former operation mode is larger than that in the latter operation mode. Therefore, the operation mode of the reading device  100  adapted to a long communication distance as the former operation mode is referred to as a “long-distance mode”. The operation mode of the reading device  100  adapted to a short communication distance such as the latter operation mode is referred to as a “short-distance mode”. The long-distance mode corresponds to a first communication mode, and the short-distance mode corresponds to a second communication mode. 
     When the reading of the RFID tag  200  should be started, the CPU  301  starts outputting a predetermined transmission baseband signal such that a radio wave radiated from the antenna  20  becomes a non-modulated carrier wave. In addition, the CPU  301  starts supplying the start signal to the control circuit  3021 . 
     Upon receiving the start signal, the control circuit  3021  starts the control process for the SJC process. 
       FIG.  4    is a flowchart of the control process. 
     In ACT  1 , the control circuit  3021  sets the state signal to a state indicating “scanning”. 
     In ACT  2 , the control circuit  3021  sets the long-distance mode. At this time, the control circuit  3021  causes the selectors  3024  and  3025  to select the first LPF  3026 , for example. 
     In ACT  3 , the control circuit  3021  starts scanning an entire range. For example, the control circuit  3021  instructs the generation circuit  3022  to start sweeping. In response to this instruction, the generation circuit  3022  starts generating and outputting the amplitude signal and the phase signal. After that, the generation circuit  3022  sequentially changes states of the amplitude signal and the phase signal such that the amplitude and the phase indicated by the amplitude signal and the phase signal are sequentially changed over an entire range in which the amplitude and the phase are changeable in the vector modulator  21 . As a result, the gain and the phase of the cancellation signal generated by the vector modulator  21  are gradually changed. A period from the start of the output of the amplitude signal and the phase signal by the generation circuit  3022  to the end of the change over the entire range in which the amplitude and the phase are changeable is the scan period for scanning the entire range. 
     In the scan period, a reduction amount or value of the self-jammer signal is sequentially changed by combining the cancellation signal by the power combiner  23 . Therefore, the I residual signal and the Q residual signal extracted by the LPF  28  are also sequentially changed. The I residual signal and the Q residual signal are input to the measurement circuit  3028  through the first LPF  3026 . The measurement circuit  3028  measures the residual level based on the I residual signal and the Q residual signal that are changed as described above. The detection circuit  3029  detects the amplitude minimum point, that is, the timing at which the residual level measured by the measurement circuit  3028  is minimized in the scan period. However, the amplitude minimum point is not necessarily the strict timing at which the residual level is minimized, and may be a timing at which the residual level becomes sufficiently low to a value close to the minimum value. The degree of the value of the residual level when the amplitude minimum point is detected may be determined freely by, for example, a designer of the reading device  100 . 
     At this time, the level of the self-jammer signal extracted by the first LPF  3026  as the first filter is measured by the measurement circuit  3028 , and thus the first LPF  3026  and the measurement circuit  3028  constitute a first measurement unit or first measurement circuit. 
     In ACT  4 , the control circuit  3021  sets, as the amplitude and the phase in a period other than the scan period, an amplitude and a phase for reproducing an operation state of the vector modulator  21  at the amplitude minimum point detected by the detection circuit  3029 . Then, the control circuit  3021  instructs the generation circuit  3022  to fix the amplitude and the phase. Then, the generation circuit  3022  fixes the amplitude signal and the phase signal to be generated and output to a state in which the amplitude signal and the phase signal represent the instructed amplitude and the phase. In response to this, the vector modulator  21  is set to a state of generating the cancellation signal capable of favorably canceling the self-jammer signal in a current surrounding environment. Through this process, the control circuit  3021  functions as a first control unit or first control circuit. 
     In ACT  5 , the control circuit  3021  sets the state signal to a state indicating “canceling”. Then, the CPU  301  starts outputting, in a predetermined sequence, the transmission baseband signal for radiating the radio wave from the antenna  20  in order to read the RFID tag  200 . 
     In ACT  6 , the control circuit  3021  confirms whether the surrounding environment is stable. For example, the control circuit  3021  calculates a stability of the residual level measured by the measurement circuit  3028  within a predetermined period, and confirms whether the surrounding environment is stable based on the stability. The stability can be calculated, for example, as a difference between a maximum value and a minimum value. In this case, a smaller value of the stability indicates a more stable surrounding environment. Therefore, it is considered that the control circuit  3021  may determine that the surrounding environment is stable, for example, when the stability is equal to or smaller than a predetermined first threshold value. If the surrounding environment is stable, the control circuit  3021  determines YES and proceeds to ACT  7 . The first threshold value may be appropriately determined by, for example, the designer of the reading device or reader  100 . 
     In ACT  7 , the control circuit  3021  waits for completion of the reading. 
     When the reading of all the RFID tags  200  that should be read is completed, the CPU  301  notifies the control circuit  3021  of the completion of the reading. For example, the CPU  301  notifies the control circuit  3021  of the completion of the reading by stopping the supply of the start signal to the control circuit  3021 . In this case, the CPU  301  continues supplying the start signal started when the reading should be started as described above. Then, for example, the control circuit  3021  determines YES in ACT  7  when the completion is notified as described above, and ends the control process. 
     Meanwhile, for example, when the stability is not equal to or smaller than the first threshold value, the control circuit  3021  determines NO in ACT  6 , which indicates that the surrounding environment is not stable, and proceeds to ACT  8 . 
     In ACT  8 , the control circuit  3021  sets the short-distance mode. At this time, the control circuit  3021  causes the selectors  3024  and  3025  to select the second LPF  3027 , for example. 
     Thus, by executing the processes of ACT  6  and ACT  8 , the control circuit  3021  functions as a setting unit or setting circuit. 
     In ACT  9 , the control circuit  3021  confirms whether the surrounding environment changes. If the control circuit  3021  cannot confirm the corresponding event, the control circuit  3021  determines NO and proceeds to ACT  10 . 
     In ACT  10 , the control circuit  3021  confirms whether the reading is completed. If the control circuit  3021  cannot confirm the corresponding event, the control circuit  3021  determines NO and returns to ACT  9 . 
     Thus, in ACT  9  and ACT  10 , the control circuit  3021  waits for the change in environment or the completion of the reading. 
     At this time, since the operation of the vector modulator  21  is fixed, the residual level fluctuates according to the fluctuation of the self-jammer signal. Therefore, for example, when the residual level becomes equal to or larger than a predetermined second threshold value, the control circuit  3021  determines YES, which indicates that the environment changes, and proceeds to ACT  11 . The second threshold value may be appropriately determined by, for example, the designer of the reading device  100 . 
     In ACT  11 , the control circuit  3021  starts scanning the entire range similar to that in ACT  3 . Here, the short-distance mode is set, and the second LPF  3027  is selected by the selectors  3024  and  3025 . Since the cutoff frequency of the second LPF  3027  is larger than that of the first LPF  3026 , a time constant of the second LPF  3027  is smaller than that of the first LPF  3026 . That is, a response speed related to the measurement of the residual level in the entire range scan in ACT  11  is higher than that in the entire range scan in ACT  3 . Therefore, in ACT  11 , the control circuit  3021  changes the amplitude and the phase of the cancellation signal at a time interval shorter than that in ACT  3 . The time interval for changing the amplitude and the phase in each of ACT  3  and ACT  11  may be appropriately determined by, for example, the designer of the reading device  100 , such that the residual level can be measured normally by the measurement circuit  3028  in consideration of the response speeds of the first LPF  3026  and the second LPF  3027 . 
     At this time, the level of the self-jammer signal extracted by the second LPF  3027  as the second filter is measured by the measurement circuit  3028 , and thus the second LPF  3027  and the measurement circuit  3028  constitute a second measurement unit or second measurement circuit. 
     Since the cutoff frequency of the second LPF  3027  is larger than that of the first LPF  3026 , an SN ratio of the I residual signal and the Q residual signal supplied to the measurement circuit  3028  in the entire range scan in ACT  11  is larger than that in the entire range scan in ACT  3 . As a result, an error of the residual level measured by the measurement circuit  3028  in the entire range scan in ACT  11  is larger than that in the entire range scan in ACT  3 . That is, an error of the amplitude minimum point detected by the detection circuit  3029  in the entire range scan in ACT  11  is larger than that in the entire range scan in ACT  3 . 
     In ACT  12 , the control circuit  3021  sets an amplitude and a phase of the vector modulator  21  in the same manner as in ACT  4 . Through this process, the control circuit  3021  functions as a second control unit or second control circuit. 
     After that, the control circuit  3021  returns to a standby state of ACT  9  and ACT  10 . Thus, the control circuit  3021  repeats the entire range scan in ACT  11  and the setting of the amplitude and the phase in ACT  12  each time the surrounding environment changes, until the reading is completed. The control circuit  3021  determines YES in ACT  10  when the completion is notified from the CPU  301 , and ends the control process. 
       FIG.  5    is a diagram showing an example of a change in the residual level measured by the measurement circuit  3028 . 
     As shown in  FIG.  5   , the residual level is reduced to a sufficiently small value over a relatively long time by the control to the vector modulator  21  in ACT  3  and ACT  4  (hereinafter, referred to as initial control). 
     In the example of  FIG.  5   , after completion of the initial control, the residual level gradually increases depending on the change in the surrounding environment. The control to the vector modulator  21  in ACT  11  and ACT  12  (hereinafter, referred to as in-reception control) is executed in response to the residual level becoming equal to or larger than the second threshold value. By the in-reception control, the residual level is reduced to smaller than the second threshold value again. However, due to an influence of the detection error of the amplitude minimum point as described above, the residual level after the in-reception control is larger than the residual level after the initial control. 
     As described above, the control circuit  3021  executes the initial control by the function as the first control unit before the start of the reception, and executes the in-reception control by the function as the second control unit during the reception, and thus functions as a third control unit or third control circuit. 
     According to the above operation, in the reading device  100 , the residual level is also monitored during reception. When the residual level increases, the vector modulator  21  is readjusted while the reception operation is continued. Therefore, the influence of the self-jammer signal can be reduced even when the self-jammer signal fluctuates during communication due to the influence of the change in the surrounding environment or the like. 
     The reading device  100  executes the entire range scan in the in-reception control in a short period of time with reduced accuracy. Therefore, a state in which the residual level is large can be eliminated in the short period of time, and the influence of the self-jammer signal can be reduced. 
     Further, the reading device  100  executes the in-reception control only when the residual level after completion of the initial control is not stable. In a case where the residual level after the completion of the initial control is stable, it is considered that an object that causes the change in the surrounding environment is not close to the antenna  20 . In this case, it is known that the fluctuation of the self-jammer signal is small. Therefore, in such a situation, an increase in a process load due to the in-reception control can be prevented by not executing the in-reception control. 
     In addition, the reading device  100  switches between the long-distance mode using the first LPF  3026  to adapt to a relatively large communication distance and the short-distance mode using the second LPF  3027  to adapt to a relatively small communication distance, according to the stability of the residual level after the completion of the initial control. The reading device  100  executes the initial control in the long-distance mode, and executes the in-reception control only in the short-distance mode. Thus, it is possible to switch operation states in which the initial control and the in-reception control are respectively executed by switching operation modes adapted to the change in the communication distance. 
     This embodiment can be variously modified as follows. 
     The disclosure can be realized as a communication device that communicates with the RFID tag  200  in order to write in the RFID tag  200 . The disclosure can also be realized as a communication device that communicates with a communication device different from the RFID tag  200 . 
     The in-reception control may be executed when the residual level after the completion of the initial control is stable, that is, when the communication in the long-distance mode is executed. 
     The cancellation signal may be generated without using the vector modulator  21 . For example, a variable attenuator and a variable phase shifter may be connected in series instead of the vector modulator  21 . 
     An LPF capable of changing the cut-off frequency may be used instead of the selectors  3024  and  3025 , the first LPF  3026 , and the second LPF  3027 . 
     Although several embodiments are described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, substitutions, and changes may be made without departing from the spirit of the invention. These embodiments and the modifications thereof are included in the scope and the spirit of the disclosure and are also included in the disclosure described in the claims and an equivalent scope thereof.