JP5643256B2 - Signal detection apparatus, communication system, and signal detection method - Google Patents

Description

  The present invention relates to a signal detection device, a communication system, and a signal detection method, and in particular, a signal detection device used in a two-dimensional communication system that performs communication between communication devices using a communication sheet having an electromagnetic wave propagation function, a communication system, and The present invention relates to a signal detection method.

  A two-dimensional communication system using a communication device to which a proximity coupler is connected and a communication sheet is known. The communication sheet is a sheet-like transmission medium having an electromagnetic wave propagation function. The proximity coupler corresponds to an interface of a communication device with respect to the communication sheet. The proximity coupler is disposed such that the electromagnetic wave radiation surface is in contact with the communication sheet. The proximity coupler transmits (radiates) the high frequency signal input from the communication device to the communication sheet as an electromagnetic wave. Further, the proximity coupler receives an electromagnetic wave from another communication device that has propagated through the communication sheet, and inputs the received electromagnetic wave as a high-frequency signal to the communication device. As described above, the two-dimensional communication using the communication sheet is executed in a state where the proximity coupler and the communication sheet are in contact with each other.

  In relation to the present invention, Patent Document 1 discloses a configuration of a two-dimensional communication system in which a plurality of communication devices communicate via a communication sheet. In the two-dimensional communication system described in Patent Document 1, a communication device brings a connector (proximity coupler) that radiates electromagnetic waves into contact with a communication sheet, and propagates electromagnetic waves modulated by digital signals through the communication sheet. The communication device of the communication destination detects the electromagnetic wave propagated through the communication sheet and demodulates the digital signal.

  Patent Document 2 describes a configuration of a power feeding device in which both a cradle and a portable terminal are provided with a non-contact power feeding module and a non-contact communication circuit. When the cradle detects that the mobile terminal is installed in the cradle, the cradle starts non-contact power feeding to the mobile terminal. When the contactless communication circuit provided in the cradle can receive a response signal from the contactless communication circuit of the mobile terminal, the cradle maintains the contactless power supply, and when the response signal cannot be received, the cradle performs the contactless power supply. Stop.

JP2009-055379 JP2011-160547

  In a two-dimensional communication system, when the proximity coupler is in contact with the communication sheet, most of the electromagnetic waves radiated by the proximity coupler propagate through the communication sheet. However, if a high frequency signal is input to the proximity coupler in a state where the proximity coupler is not in contact with the communication sheet for some reason, electromagnetic waves are also radiated to the space around the proximity coupler. Since the energy density of the electromagnetic wave radiated into the space is small, communication cannot be performed by receiving the electromagnetic wave radiated into the space with another proximity coupler. That is, the currently known two-dimensional communication system has a problem that the electromagnetic wave energy may not be effectively used because the electromagnetic wave is radiated even when the proximity coupler is not in contact with the communication sheet. Furthermore, the currently known two-dimensional communication system has a problem that electromagnetic waves radiated to the space may affect other two-dimensional communication and electronic devices in the surrounding area. Therefore, the two-dimensional communication system is required to suppress the emission of electromagnetic waves to the surroundings when the proximity coupler and the two-dimensional communication sheet are not in contact.

  Patent Document 1 discloses a configuration of a two-dimensional communication system. However, there is no description about a configuration for solving the problem of suppressing emission of electromagnetic waves in a state where the proximity coupler is not in contact with the two-dimensional communication sheet.

  On the other hand, in Patent Document 2, when non-contact communication is not possible between the mobile terminal and the cradle, the non-contact power supply using electromagnetic waves is stopped to prevent the cradle from feeding power that is not a power supply target. The configuration is disclosed. However, Patent Document 2 does not describe a configuration for solving the problem of suppressing the emission of electromagnetic waves in a state where the proximity coupler is not in contact with the two-dimensional communication sheet in the two-dimensional communication system.

  An object of the present invention is to provide a technique for suppressing unnecessary electromagnetic radiation in a communication system.

  The signal detection device of the present invention attenuates an input transmission signal and radiates it as an electromagnetic wave to an external medium, a second interface that transmits and receives the electromagnetic wave between the external medium, and a transmission signal. When the transmission signal radiated from the first interface is received as an electromagnetic wave by the second interface via the external medium when connected to be input to the first interface, the transmission signal is second. And a switch connected so as to be input to the interface.

  The signal detection method of the present invention attenuates an input transmission signal and radiates it as an electromagnetic wave from the first interface to an external medium, connects the transmission signal to be input to the first interface, and connects the first interface. When the transmission signal radiated from is received as an electromagnetic wave by the second interface that transmits and receives the electromagnetic wave to and from the external medium via the external medium, the transmission signal is input to the second interface.

  The present invention has an effect of making it possible to reduce unnecessary electromagnetic radiation with a simple configuration in a communication system.

It is a figure which shows the structure of the two-dimensional communication system of 1st Embodiment. It is a figure which shows the voltage waveform of the signal of each part of the two-dimensional communication system of 1st Embodiment. It is a figure which shows the structure of the signal detection apparatus of 2nd Embodiment.

(First embodiment)
Embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a diagram illustrating a configuration of a two-dimensional communication system 400 according to the first embodiment of this invention. The two-dimensional communication system 400 includes a communication device 451, a signal detection device 401, and a communication sheet 441.

  The signal detection device 401 includes a combined signal switch 411, an attenuator 412, a combiner 413, a rectifier circuit 414, an amplifier circuit 415, a comparison circuit 416, a push switch 417, a high-frequency signal switch 421, and a proximity coupler 431.

  The coupling signal changeover switch 411 is configured so that the high-frequency signal RFin from the communication device 451 is input to the attenuator 412 if the amplitude of the comparison result signal (detection signal) TRG of the comparison circuit 416 is low level (TRG = Low). Connecting. The coupling signal changeover switch 411 connects the circuit so that the termination resistor R is connected to the attenuator 412 if the amplitude of the detection signal TRG is Hi level (TRG = Hi).

  The attenuator 412 is an attenuator and attenuates the amplitude of the high frequency signal RFin input from the communication device 451 when TRG = Low. Due to the attenuator 412, the intensity of the electromagnetic wave when emitted from the subsequent coupling device 413 into the air is reduced. A signal after the high-frequency signal RFin is attenuated is defined as a combined signal RFcp1. The coupled signal RFcp1 is radiated as an electromagnetic wave from the coupling device 413 to the outside. The high frequency signal RFin is attenuated by the attenuator 412 within a range in which the combined signal RFcp1 can be received by the proximity coupler via the communication sheet 431.

  The coupling device 413 transmits the coupling signal RFcp1 to the communication sheet 441 by AC coupling. As the coupling device 413, for example, a coupler similar to the proximity coupler 431, a coaxial cable without a termination resistor, or the like can be used.

  The proximity coupler 431 is an interface for the communication device 451 to perform two-dimensional communication with another communication device via the communication sheet 441. Proximity couplers are generally composed of a dielectric and a metal. The proximity coupler is used in close contact with the communication sheet, and detects an electromagnetic wave propagating in the communication sheet and outputs it as an electric signal, or propagates the inputted electric signal as an electromagnetic wave in the communication sheet.

  The coupled signal RFcp1 radiated from the coupling device 413 propagates through the communication sheet 441 and is received by the proximity coupler 431. By disposing the coupling device 413 and the proximity coupler 431 at relatively close positions, the coupling signal RFcp1 attenuated by the attenuator 412 can be detected by the proximity coupler 431.

  The high-frequency signal RFout indicates a signal input / output with the proximity coupler 431. When the proximity coupler 431 receives a signal from the communication sheet 441, the high-frequency signal RFout indicates a reception signal output by the proximity coupler 431.

  The rectifier circuit 414 rectifies the high-frequency signal RFout received by the proximity coupler 431 using a rectifier diode and a capacitor and converts it into direct current. The signal after rectification is defined as a rectified signal DCcp1. The initial value of the amplitude of the rectified signal DCcp1 is proportional to the reception intensity of the proximity coupler 431. Further, the capacitor of the rectifier circuit 414 is arranged so that the amplitude of DCcpl gradually increases due to the capacitor charging function when the coupling signal RFcpl is continuously input from the proximity coupler 431 to the rectifier circuit 414.

  The amplifier circuit 415 amplifies the voltage of the rectified signal DCcp1 to a level comparable to the reference voltage Vref of the comparison circuit 416 at the subsequent stage. The rectified signal amplified by the amplifier circuit 415 is referred to as an amplified signal DCamp.

  The adder circuit 418 outputs a voltage obtained by adding the voltages of the input signals. That is, the addition circuit 418 outputs a voltage obtained by adding the voltage of the amplified signal DCamp and the voltage of the detection signal TRG that is the output of the comparison circuit 416 fed back via the push switch 417 to the comparison circuit 416. The signal after addition output from the adder circuit 418 is referred to as an addition signal DCadd.

  The comparison circuit 416 compares the voltage of the addition signal DCadd with the reference voltage Vref, and outputs a detection signal TRG that is a binary signal according to the result. If DCadd> Vref, the TRG amplitude is Hi (high level), and if DCadd ≦ Vref, the TRG amplitude is Low (low level). The detection signal TRG is used as a switching trigger for the combined signal changeover switch 411 and the high frequency signal changeover switch 421.

  The push switch 417 is a switch provided in the middle of a path connecting the output of the comparison circuit 416 and the addition circuit 417. The button of the push switch 417 is configured to be pressed when the proximity coupler 431 and the coupling device 413 contact the object. For example, the proximity coupler 431 and the coupling device 413 are fixedly arranged on the same plane, and when the proximity coupler 431 and the coupling device 413 contact an object such as a communication sheet, the button of the push switch 417 protruding from the plane is pressed. Such a structure may be used.

  When the button of the push switch 417 is not pressed, the output of the comparison circuit 416 is grounded. In this state, the amplitude of the detection signal TRG is fixed to Low regardless of the amplitude of the input signal of the comparison circuit 416. In the state where the button of the push switch 417 is pressed, the output of the comparison circuit 416 is input to the addition circuit 418.

  The high-frequency signal selector switch 421 switches the connection destination of the proximity coupler 431 in conjunction with the coupling signal selector switch 411 described above. Specifically, the high-frequency signal selector switch 421 connects the proximity coupler 431 to the rectifier circuit 414 if the detection signal TRG is Low (TRG = Low), and communicates with the proximity coupler 431 if TRG is Hi (TRG = Hi). The device 451 is connected. Here, the voltage when the detection signal TRG is Hi is assumed to be larger than Vref.

  When TRG = Low, the high-frequency signal RFout received from the communication sheet 441 by the proximity coupler 431 is converted into the rectified signal DCcp1 by the rectifier circuit 414. In this case, the high frequency signal RFout is a signal obtained by the proximity coupler 431 receiving the combined signal RFcp1. The rectified signal DCcp1 is amplified by the amplifier circuit 415 and is input to the comparison circuit 416 as the addition signal DCadd via the addition circuit 418.

  When TRG = Hi, the high-frequency signal RFin output from the communication device 451 is input to the proximity coupler 431, and communication with other communication devices is performed via the communication sheet 441. In this case, the high frequency signal RFout is the high frequency signal RFin output from the communication device 451.

  First, an outline of the operation of the two-dimensional communication system 400 will be described. In the two-dimensional communication system 400, a state in which electromagnetic waves for communication propagate through the communication sheet 441 is considered. In this state, the reception intensity of the electromagnetic wave received by the proximity coupler 431 when the proximity coupler 431 is in contact with the communication sheet 441 is greater than the reception intensity when the proximity coupler 431 is not in contact with the communication sheet 441. The reception intensity of the electromagnetic wave received by the proximity coupler 431 is greater than the reception intensity when the proximity coupler 431 is in contact with an object other than the communication sheet 441. This is because when the proximity coupler 431 is not in contact with the communication sheet, the proximity coupler cannot detect a signal propagating through the communication sheet, and the propagation efficiency of electromagnetic waves of a medium other than the communication sheet is generally that of the communication sheet. This is because it is lower than the propagation efficiency.

  In the two-dimensional communication system 400 of the first embodiment, whether or not the proximity coupler 431 is in contact with the communication sheet 441 using the difference in the received signal strength of the signal radiated from the coupling device 413 in the proximity coupler 431. Is determined. Then, based on the determination result, it is controlled whether the communication device 451 is connected to the proximity coupler 431 to perform communication. For example, when the proximity coupler 431 is in contact with an object other than the communication sheet 441, the reception power of the proximity coupler 431 is smaller than when the electromagnetic wave is received in contact with the communication sheet 441. Therefore, when the reception power of the proximity coupler 431 is small, it is determined that the proximity coupler 431 is not in contact with the communication sheet 441, and it is possible to control so that the output of the communication device 451 is not radiated from the proximity coupler 431. .

  Detailed operations of the two-dimensional communication system 400 shown in FIG. 1 will be described with reference to FIG. FIG. 2 is a diagram illustrating voltage waveforms of signals at various parts of the two-dimensional communication system 400.

  (1) of FIG. 2 shows the waveform of the high-frequency signal RFin output from the communication device 451. 2 shows the waveform of the detection signal TRG that is the output of the comparison circuit 416. FIG. (3) of FIG. 2 shows the waveform of the combined signal RFcp1 which is the output of the attenuator 412. (4) of FIG. 2 shows the waveform of the high-frequency signal RFout input / output to / from the proximity coupler 431. (5) of FIG. 2 shows the waveform of the rectified signal DCcp1 which is the output of the rectifier circuit 414. (6) of FIG. 2 shows the waveform of the amplified signal DCamp which is the output signal of the amplifier circuit 415. (7) of FIG. 2 shows the waveform of the addition signal DCadd which is an output signal of the addition circuit 418.

  In FIG. 2, it is assumed that the proximity coupler 431 and the coupling device 413 are not in contact with the communication sheet 441 at the time before the time t = t0. In this case, since the button of the push switch 417 is not pressed by the communication sheet 441, the output of the comparison circuit 416 is fixed to the ground potential by the push switch 417. That is, the detection signal TRG which is the output of the comparison circuit 416 is TRG = Low. As a result, the high frequency signal output from the communication device 451 is radiated from the coupling device 413 as the coupling signal RFcp1 via the attenuator 412. However, since the coupling device 413 and the proximity coupler 431 are separated from the communication sheet at this time, the proximity coupler 431 cannot receive the coupling signal RFcp1 from the communication sheet.

  Next, at time t = t0, the proximity coupler 431 and the coupling device 413 come into contact with the communication sheet 441. At this time, the coupling device 413 couples with the communication sheet 441 in an alternating manner. That is, the coupling signal RFcp1 is transmitted from the coupling device 413 to the communication sheet 441 as an electromagnetic wave. Further, when the button of the push switch 417 is pressed by contact between the communication sheet 441 and the push switch 417, the connection destination of the output of the comparison circuit 416 is switched from the ground potential to the addition circuit 418.

  While the time t is t0 <t <t1, the coupling signal RFcp1 propagated from the coupling device 413 through the communication sheet 441 is received by the proximity coupler 431. At this time, the high-frequency signal RFout output from the proximity coupler 431 is a combined signal RFcp1 that is output from the attenuator 412 and attenuated by the sum of the passing losses of the coupling device 413, the communication sheet 441, and the proximity coupler 431. The proximity coupler 431 is connected to the rectifier circuit 414 side by a high frequency signal changeover switch 421. Therefore, the rectified signal DCcpl is charged in the capacitor in the rectifier circuit 414, and the voltage of the rectified signal DCcpl increases with time. As the voltage of the rectified signal DCcp1 increases, the voltage amplitude of the amplified signal DCamp, which is the output of the amplifier circuit 415, also increases. The adder circuit 418 adds the voltage of the detection signal TRG, which is an output signal of the comparison circuit 416, to the amplified signal DCamp, and outputs the result as an added signal DCadd. During this period, the detection signal TRG is connected to the addition circuit 418. However, since DCadd ≦ Vref, the detection signal TRG is TRG = Low, and the voltage of the addition signal DCadd is substantially equal to the voltage of the amplified signal DCamp.

  Next, assume that DCadd> Vref at the input of the comparison circuit 416 at time t = t1. At this time, the detection signal TRG changes from TRG = Low to TRG = Hi. As a result, in the high-frequency signal changeover switch 421, the rectifier circuit 414 when the connection destination of the proximity coupler 431 is TRG = Low is switched to the communication device 451 when TRG = Hi, and the communication device 451 and the proximity coupler 431 are connected. Is done. At this time, since the button of the push switch 417 is kept pressed, a voltage of TRG = Hi is input to the adding circuit 418. As a result, a signal having an amplitude obtained by adding the Hi voltage of the amplified signal DCamp and the detection signal TRG is output from the addition circuit 418 as the addition signal DCadd. At the moment when the detection signal TRG changes from Low to Hi, the amplitude of DCadd increases by the potential difference between Hi and Low of the detection signal TRG.

  When the time t is between t1 <t <t2, the proximity coupler 431 is connected to the communication device 451 side by the high frequency signal switch 421. On the other hand, since the rectifier circuit 414 is disconnected from the proximity coupler 431, the voltage of the signal input to the rectifier circuit 414 is 0V. As a result, the voltage of the amplified signal DCamp gradually decreases toward 0 V as the capacitor of the rectifier circuit 414 is discharged. Here, the detection signal TRG is continuously TRG = Hi, and its amplitude is input to the addition circuit 418 to become the addition signal DCadd. For this reason, the voltage of the addition signal DCadd decreases to the voltage when the detection signal TRG is Hi (time t = t2). However, since the voltage when the detection signal TRG is Hi is larger than Vref, the output logic of the comparison circuit 416 does not change even at time t = t2, and the amplitude of the detection signal TRG is maintained as Hi.

  Next, at time t = t2, since TRG = Hi as described above, the state after time t = t2 is maintained until t = t3.

  It is assumed that the proximity coupler 431 and the coupling device 413 are separated from the communication sheet 441 at time t = t3. At this time, the pressing state of the button of the push switch 417 by the communication sheet 441 is also released, so that the connection between the adder circuit 418 and the output of the comparison circuit 416 (detection signal TRG) in the push switch 417 is disconnected. As a result, the voltage of the detection signal TRG is not applied to the adding circuit 418, and the voltage of the adding signal DCadd is reduced to 0V. As the voltage of the addition signal DCadd decreases, DCadd <Vref, so that the output of the comparison circuit 416 changes and the detection signal TRG changes from TRG = Hi to TRG = Low.

  Due to the change in the detection signal TRG, the connection of the high-frequency signal changeover switch 421 is switched from the communication device 451 side on the TRG = Hi side to the rectifier circuit 414 side on the TRG = Low side. By such an operation, when the proximity coupler 431 is separated from the communication sheet 441, the communication device 451 and the proximity coupler 431 are disconnected. That is, the proximity coupler 431 can radiate the high-frequency signal RFin from the communication device 451 as an electromagnetic wave only when it is in contact with the communication sheet 441 (t1 <t <t3 in FIG. 2). On the other hand, when the proximity coupler 431 is not in contact with the communication sheet (t <t0, t> t3 in FIG. 2), the proximity coupler cannot radiate electromagnetic waves, so that unnecessary electromagnetic waves can be prevented from being emitted. Therefore, the two-dimensional communication system 400 of the first embodiment has an effect of making it possible to suppress the emission of unnecessary electromagnetic waves.

  Further, when the proximity coupler comes into contact with an object different from the communication sheet 441, electromagnetic waves cannot propagate between the coupling device 413 and the proximity coupler 431. Therefore, the proximity coupler 431 is not changed even after time t = t0 in FIG. The coupled signal RFcpl cannot be detected, and the voltage of the rectified signal DCcpl remains 0V. As a result, the voltage of DCadd remains 0V and the detection signal TRG is fixed at TRG = Low. Accordingly, since the high frequency signal changeover switch 421 does not connect the proximity coupler 431 and the communication device 451, the proximity coupler 431 does not emit the high frequency signal RFin.

  Note that when TRG = Low at time t = t3, the coupling signal changeover switch 411 switches the connection destination of the coupling device 413 from the terminating resistor side to the communication device 451 side. This state is the same as the state before time t = t0 in FIG. 2. After time t = t3, the proximity coupler 431 and the coupling device 413 come into contact with the communication sheet again, so that the time t = t0 and later described above. Can be repeated.

Further, in the two-dimensional communication system 400 of the first embodiment, the signal detection device 401 detects the presence / absence of contact between the proximity coupler 431 and the communication sheet 441 using electromagnetic waves radiated from the coupling device 413. That is, the contact state between the proximity coupler 431 and the communication sheet 441 can be detected by the signal detection device 401 itself. Therefore, the communication sheet 441 and the facing communication apparatus do not need to have a function of transmitting and receiving signals in order for the signal detection apparatus 401 to detect the contact state between the proximity coupler 431 and the communication sheet 441. Furthermore, in the two-dimensional communication system 400, the electromagnetic wave input to the coupling device 413 is attenuated by the attenuator 412. Therefore, the intensity of the electromagnetic wave radiated from the coupling device 413 is higher than the strength of the electromagnetic wave in normal two-dimensional communication. small. For this reason, even if the signal detection device 401 is separated from the communication sheet 441, the influence of the electromagnetic waves radiated to the space on the surroundings is small.
(Second Embodiment)
FIG. 3 is a diagram showing the configuration of the signal detection apparatus according to the second embodiment of the present invention. A signal detection device 300 illustrated in FIG. 3 includes a first interface 301, a second interface 302, and a switch 303. The first interface 301 includes an attenuator 305, attenuates an input transmission signal 304 and radiates it as an electromagnetic wave to an external medium (not shown). The second interface 302 transmits / receives electromagnetic waves to / from an external medium. When the transmission signal 304 radiated from the first interface 301 is received as an electromagnetic wave by the second interface 302 via the external medium, the switch 303 transmits the transmission signal 304 to the second interface 302. Connect as input. The signal detection apparatus 300 radiates the transmission signal 304 input to the second interface 302 to the external medium and starts communication with the communication destination.

  When the transmission signal 304 radiated from the first interface 301 is received by the second interface 302 via the external medium, the signal detection apparatus 300 uses the first and second interfaces 301 and 302 via the external medium. To determine that communication is possible. Then, the signal detection apparatus 300 changes the connection destination of the transmission signal 304 in the switch 303 from the first interface 301 to the second interface 302. As a result, the transmission signal 304 input to the second interface 302 is radiated toward the external medium.

  As described above, the second interface 302 emits the transmission signal 304 only when the electromagnetic wave can be propagated by the external medium. That is, the second interface 302 does not emit electromagnetic waves when there is no external medium around it. Further, since the transmission signal 304 radiated from the first interface 301 is attenuated by the attenuator 305, the radiated power of the electromagnetic wave is suppressed as compared with the case where the transmission signal 304 is not attenuated. Therefore, the signal detection device 300 has an effect that it is possible to suppress unnecessary electromagnetic radiation.

  While the present invention has been described with reference to the embodiments, the present invention is not limited to the above embodiments. Various changes that can be understood by those skilled in the art can be made to the configuration and details of the present invention within the scope of the present invention.

DESCRIPTION OF SYMBOLS 300 Signal detection apparatus 301 1st interface 302 2nd interface 303 Switch 304 Transmission signal 305 Attenuator 400 Two-dimensional communication system 401 Signal detection apparatus 411 Combination signal changeover switch 412 Attenuator 413 Combination apparatus 414 Rectifier circuit 415 Amplifier circuit 416 Comparison circuit 417 Push switch 421 High frequency signal changeover switch 431 Proximity coupler 441 Communication sheet 451 Communication device

Claims (5)

A signal detection device used in a communication system in which communication is performed between an external medium and a communication device, the signal detection device,
A first interface for radiating to the external medium as an electromagnetic wave attenuates the transmission signal input from the communication device,
A second interface for transmitting and receiving electromagnetic waves to and from the external medium;
When the transmission signal is connected to be input to the first interface, the transmission signal radiated from the first interface is received as an electromagnetic wave by the second interface via the external medium. A switch connected so that the transmission signal is input to the second interface;
Detecting means for detecting a state corresponding to a contact state between the second interface and the external medium;
If the detection means detects that the second interface and the external medium are not in contact, the switch does not input the transmission signal to the second interface;
A signal detection device characterized by that . A rectifier circuit that rectifies a signal output from the second interface that has received the electromagnetic wave, an amplifier circuit that amplifies the output of the rectifier circuit to an amplitude comparable to a reference amplitude, an output of the amplifier circuit, and the reference amplitude A comparison circuit for controlling the switch based on the comparison result with
2. The signal detection according to claim 1, wherein the comparison circuit connects the switch so that the transmission signal is input to the second interface when the output of the amplification circuit exceeds the reference amplitude. 3. apparatus. A communication system comprising: a communication device that outputs a transmission signal; an external medium that propagates an electromagnetic wave including the transmission signal; and the signal detection device according to claim 1. The communication system according to claim 3, wherein the external medium is a communication sheet used in two-dimensional communication. A signal detection method used in a communication system in which communication is performed between an external medium and a communication device, wherein the signal detection method includes:
Attenuating a transmission signal input from the communication device and radiating it as an electromagnetic wave from the first interface to the external medium,
When the transmission signal radiated from the first interface is received as an electromagnetic wave by the second interface that transmits / receives an electromagnetic wave to / from the external medium via the external medium, the second interface The transmission signal is input to
When a state corresponding to the contact state between the second interface and the external medium is detected, and the state corresponding to the contact state indicates a state where the second interface and the external medium are not in contact with each other Do not input the transmission signal to the second interface;
And a signal detection method.

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