Patent Publication Number: US-2013252549-A1

Title: Radio apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This is a Continuation Application of PCT Application No. PCT/JP2010/002459, filed on Apr. 2, 2010, which was published under PCT Article 21 (2) in Japanese, the entire contents of which are incorporated herein by reference. 
    
    
     FIELD 
     This disclosure relates to a radio apparatus. 
     BACKGROUND ART 
     In apparatus which perform short-range wireless communication in manners that are standardized by such communication standards as IEEE 802.15.1 and IEEE 802.15.4, the power consumption is reduced by shortening the operation time of a reception circuit by performing intermittent receiving operations. However, where, for example, a call is made by establishing a connection between a cell phone (controlling side) and a headset (controlled side) according to the IEEE 802.15.1 standard, the standby time is very long relative to the call time during which a communication is made between the cell phone and the headset. In this case, there is a problem that the standby power consumption is large even if intermittent receiving operations are performed. In particular, there is demand that the standby power consumption be reduced in controlled-side radio apparatus because it is difficult for them to be equipped with a large battery. 
     In this connection, a method in which a standby-dedicated wave detector which operates at a much lower power consumption than, for example, radio units that comply with the IEEE 802.15.1 standard is provided in a controlled-side radio apparatus and a controlling-side radio apparatus causes power-on of an IEEE 802.15.1-compliant radio unit of the controlled-side radio apparatus was proposed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       A general architecture that implements the various features of the present invention will now be described with reference to the drawings. The drawings and the associated descriptions are provided to illustrate embodiments and not to limit the scope of the present invention. 
         FIG. 1  shows a wireless communication system according to a first embodiment. 
         FIG. 2  shows example first signals (A), an example wave detection signal (B), and an example detection signal (C). 
         FIG. 3  shows a second radio apparatus  215  according to a second embodiment. 
         FIG. 4  shows an example fourth signal. 
         FIG. 5  shows a second radio apparatus  315  according to a third embodiment. 
         FIG. 6  shows a second radio apparatus  415  according to a fourth embodiment. 
         FIG. 7  shows a third bandpass filter  305  used in the fourth embodiment. 
         FIG. 8  shows a second radio apparatus  515  according to a fifth embodiment. 
         FIG. 9A  shows a frequency characteristic of first signals, and  FIG. 9B  shows a frequency characteristic of fourth signals. 
         FIG. 10  shows a second radio apparatus  615  according to a sixth embodiment. 
         FIG. 11  shows a fifth bandpass filter  505  used in the sixth embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention will be hereinafter described with reference to the drawings. In the following embodiments, it is assumed that items that are given the same numeral operate in the same manner or similar manners and they will not be described redundantly. 
     According to one embodiment, a radio apparatus configured to communicate with a second radio apparatus after receiving plural first signals transmitted from the second radio apparatus in a constant cycle, the radio apparatus includes: a communication circuit configured to communicate with the second radio apparatus; a wave detection circuit configured to generate a wave detection signal by envelope-detecting the plural first signals; a bandpass filter having an IIR filter, configured to generate a detection signal from the wave detection signal, the detection signal having amplitude that is increased at a frequency corresponding to the constant cycle; and a control unit configured to cause supply of power to the communication circuit if the amplitude is larger than a first threshold value. 
     A wireless communication system according to a first embodiment of the invention will be described.  FIG. 1  shows an example wireless communication system according to this embodiment. This wireless communication system has a first radio apparatus  114  such as a cell phone (controlling side) and a second radio apparatus  115  such as a headset (controlled side). The second radio apparatus  115  is powered on when receiving a signal that is transmitted from the first radio apparatus  114 , and starts communicating with the first radio apparatus  114 . In the embodiment, the first radio apparatus  114  which sends out a signal for powering on the second radio apparatus  115  is also called a controlling radio apparatus  114 . The second radio apparatus  115  which is powered on by a signal transmitted from the first radio apparatus (controlling radio apparatus)  114  is also called a controlled radio apparatus. 
     The first radio apparatus  114  also belongs to a cellular system and has a cellular circuit  112  for communicating with a base station  111 , for example. The first radio apparatus  114  has a communication circuit  113  for performing, for example, an IEEE 802.15.1-compliant communication with the second radio apparatus  115 . 
     The cellular circuit  112 , which has a circuit (not shown) such as a frequency converter for communicating with the base station  111 , receives audio data, image data, or the like from the base station  111  via an antenna  117 . The cellular circuit  112  passes, to a communication circuit  113 , the audio data received from the base station  111 . The cellular circuit  112  sends audio data passed from the communication circuit  113  to the base station  111  via the antenna  117 . The cellular circuit  112  also sends, to the base station  111 , data (e.g., image data) other than audio data passed from the communication circuit  113 . The cellular circuit  112  is also called a second communication circuit. 
     The communication circuit  113 , which has a circuit (not shown) such as a frequency converter for communicating with the second radio apparatus  115 , receives audio data from second radio apparatus  115  via an antenna  118 . The communication circuit  113  passes, to the cellular circuit  112 , audio data received from the second radio apparatus  115 . The communication circuit  113  sends audio data received from the cellular circuit  112  to the second radio apparatus  115  via the antenna  118 . 
     The communication circuit  113  sends out first signals in a constant cycle. Where the communication circuit  113  is a circuit which performs an IEEE 802.15.1-compliant communication, the communication circuit  113  sends out first signals in a burst-like manner in the cycle corresponding to 800 Hz. The communication circuit  113  sends out plural first signals while doing transmission frequency hopping in order of f 0 , f 6 , f 2 , f 9 , . . . . 
     Next, the configuration of the second radio apparatus  115  will be described. The second radio apparatus  115  is equipped with a receiving circuit  120 , a communication circuit  108  which performs, for example, an IEEE 802.15.1-compliant communication, a power circuit  109  which supplies power to the communication circuit  108 , and a switch  110  which makes switching as to whether to allow supply of power to the communication circuit  108  from the power unit  109  according to an instruction from the receiving circuit  120 . 
     The receiving circuit  120  is equipped with a wave detection circuit  104  which envelope-detects a first signal and thereby generates a wave detection signal, a bandpass filter  105  which generates a detection signal from the wave detection signal, the detection signal having amplitude that is increased at the frequency corresponding to the constant cycle, a comparator  106  which compares the amplitude of the detection signal with a threshold value, and a control unit  109  which causes supply of power to the communication circuit  108  if the amplitude of the detection signal is larger than the threshold value. The receiving circuit  120  is also equipped with an antenna  101 , a matching circuit  102 , a first bandpass filter  103 , and a wave detection circuit  104 . 
     The receiving circuit  120  will be described in detail. 
     The antenna  101  receives a first signal transmitted from the communication circuit  113 . The matching circuit  102  is a circuit for matching between the output impedance of the antenna  101  and the input impedance of the first bandpass filter  103 . The first bandpass filter  103  suppresses out-of-band signals contained in the first signal received by the antenna  101  and thereby generates a first signal in a desired band. The first bandpass filter  103  is a filter having, as pass-bands, plural bands of frequency hopping. For example, where first signals are transmitted with frequency hopping between frequencies f 0  and f 9  in order of f 0 , f 6 , f 2 , f 9 , the first bandpass filter  103  has, as pass-bands, frequency bands which are in the range of f 0  to f 9 . 
     The wave detection circuit  104  wave-detects the first detects the first signal in the desired band generated by the first bandpass filter  103 . For example, the wave detection circuit  104  has a rectifier and a comparator (not shown). The wave detection circuit  104  envelope-detects the signal detected by the antenna  101  and thereby generates a wave detection signal. 
     The bandpass filter  105 , which has a second-order IIR filter (not shown), generates a detection signal from the wave detection signal, the detection signal having amplitude that is increased at the frequency corresponding to the constant cycle. The pass-band of the bandpass filter  105  includes the frequency corresponding to the constant cycle. Where the communication circuit  113  is a circuit which performs an IEEE 802.15.1-compliant communication, since the communication circuit  113  sends out first signals in a burst-like manner in the cycle corresponding to 800 Hz, the bandpass filter  105  extracts an 800-Hz signal. The bandpass filter  105  is a filter having a narrower band than the first bandpass filter  103 . 
     The comparator  106  compares the detection signal with a threshold value. If the detection signal is larger than the threshold value, the comparator  106  generates a power control signal. When receiving the power control signal, the control unit  119  controls the switch  110  so that power is supplied to the communication circuit  108  from the power unit  109 . 
     Next, how the receiving circuit  120  operates will be described with reference to  FIG. 2 . 
       FIG. 2(A)  show example signals which are received by the antenna  101 . As shown in  FIG. 2(A) , the first radio apparatus  114  sends out first signals while doing frequency hopping in order of f 0 , f 6 , f 2 , f 9 , . . . in the cycle corresponding to 800 Hz. For example, if the frequency bands (f 0 -f 9 ) in which the first radio apparatus  114  sends out first signals are parts of an ISM band, wireless LAN signals may be received by the antenna  101  as interference signals. The antenna  101  passes the received first signals and interference signals to the wave detection circuit  104  via the matching circuit  102  and the first bandpass filter  103 . 
     The wave detection circuit  104  generates a wave detection signal containing low signals and high signals (see  FIG. 2(B) ) on the basis of the first signals and the interference signals. The wave detection circuit  104  outputs a high signal when receiving a first signal or an interference signal. When receiving neither a first signal nor an interference signal, the wave detection circuit  104  outputs a low signal. 
     The wave detection signal containing the low signals and the high signals is input to the bandpass filter  105 . In the bandpass filter  105 , the wave detection signal is converted into a detection signal in which signals other than the 800-Hz signal are suppressed.  FIG. 3(C)  shows an example detection signal. The amplitude of the detection signal is increased at 800 Hz. Since the wave detection circuit  104  generates a wave detection signal by performing envelope detection without discriminating between first signals and interference signals, the wave detection signal becomes a high signal if an interference signal exists. However, in the embodiment, the bandpass filter  105  averages first signals having the desired frequency (800 Hz) over a prescribed period and outputs a resulting signal. Since interference signals which have no periodicity are suppressed by the bandpass filter  105 , the amplitude of the detection signal is increased at the desired frequency. Even if interference signals are periodic, if their cycle is different from the desired cycle (corresponds to 800 Hz), they are out of the pass-band of the bandpass filter  105  and hence are suppressed by the bandpass filter  105 . This is because the bandpass filter  105  is a narrow-band filter and stops signals whose frequencies are different from the desired frequency. In this manner, the detection signal is made a signal having amplitude that is increased at the desired frequency. 
     If the amplitude of the detection signal becomes larger than the threshold value, the comparator outputs a power control signal. Receiving the power control signal, the control unit  119  controls the switch  110 . An alternative configuration is possible in which the control unit  119  is integral with the comparator  106  and the switch receives a power control signal directly. In this case, the switch  110  operates so as to supply power to the communication circuit  108  when receiving a power control signal. 
     As described above, in the wireless communication system according to the embodiment, the communication circuit  108  is activated when first signals having a constant cycle which are transmitted from the communication circuit  113  are detected by the receiving circuit  120 . Therefore, the first radio apparatus  114  need not be equipped with a circuit that is dedicated to activation of the communication circuit  108 . Furthermore, the communication circuit  108  can operate while the first radio apparatus  114  is sending out first signals. Therefore, the power consumption of the second radio apparatus  115  can be reduced without increasing the circuit scale of the first radio apparatus  114  (controlled side). 
     Although in the embodiment the second-order IIF filter is used in the bandpass filter  105 , the same advantages can also be obtained by using an even higher order IIF filter. However, the use of the second-order IIR filter can suppress increase in circuit scale because it is smaller in circuit scale than even higher order IIF filters. 
     (Modification 1) 
     Whereas the above-described first embodiment is directed to the case that the communication circuits  113  and  108  of the first radio apparatus  114  and the second radio apparatus  115  perform an IEEE 802.15.1-compliant communication, they may perform a wireless LAN communication. A description will be made of a case that the first radio apparatus  114  is an access point and the second radio apparatus  115  is a station. In this case, beacon signals, for example, can be used as first signals. 
     General access points sends out beacon signals in a cycle of 102.4 ms. In this case, the bandpass filter  105  generates a detection signal having amplitude that is increased in a cycle of 102.4 ms. As a result, the second radio apparatus  115  can activate the communication circuit  108  only in the case where it is located within the communication range of the first radio apparatus  114  which is the access point. 
     The transmission interval of beacon signals can be switched between plural values. Where the communication ranges of plural access points overlap with each other, the beacon signal transmission intervals of adjoining access points are made different from each other. This allows the communication circuit  108  to be activated only in the case where the second radio apparatus  115  is located in the communication range of a particular access point. For example, even in a case that an access point which is sending out beacon signals in a cycle of 102.4 ms exists near a home, it is possible to activate the communication circuit  108  of the second radio apparatus  115  only when it is located in the operation range of an access point installed in the home by setting the beacon signal transmission interval of the home access point to 70 ms. 
     (Modification 2) 
     Whereas in the first modification the second radio apparatus  115  is a station, the first radio apparatus  114  and the second radio apparatus  115  can be made a station and an access point, respectively. Whereas access points send out a beacon signal regularly, stations do not. In view of this, in this modification, the first radio apparatus  114  sends out first signals with such timing that the communication circuit  113  is activated. For example, active scan probe signals can be used as first signals. 
     When receiving probe signals, the second radio apparatus  115  activates the communication circuit  108  and starts a communication with the communication circuit  113  of the first radio apparatus  114 . On the other hand, if a communication with the communication circuit  108  is not started even if probe signals have been sent out for a prescribed period, the first radio apparatus  114  suspends the transmission of probe signals. The first radio apparatus  114  may restart transmission of probe signals after a lapse of a prescribed period from the suspension of transmission of probe signals. 
     A case that operation of the first radio apparatus  114  has been stopped will be considered. Where the first radio apparatus  114  is a notebook PC, this is a case that the notebook PC has been powered off. When instructed to power itself off, the first radio apparatus  114  instructs the second radio apparatus  115  to stop operation of the communication circuit  108 . This may be done by the communication circuit  113  by accessing the communication circuit  108  using a wireless LAN communication. After instructing the second radio apparatus  115  to stop operation of the communication circuit  108 , the first radio apparatus  114  powers itself off. The second radio apparatus  115  controls the switch  110  so that the supply of power to the communication circuit  108  is stopped. 
     Next, a wireless communication system according to a second embodiment of the invention will be described with reference to  FIG. 3 . In the wireless communication system according to this embodiment, the configuration of the first radio apparatus  114  is the same as shown in  FIG. 1  and hence will not be described. The first radio apparatus  114  sends out a fourth signal which contains a second signal and a third signal which is sent so as to have a constant interval from the second signal. For example, where the communication circuit  113  is a circuit that performs an IEEE 802.15.1-compliant communication, the fourth signal corresponds to an inquiry scan signal or a page scan signal. In this case, the constant interval is 312.5 μs and the constant cycle is 1.250 ms. The fourth signal will be described later in detail. A second radio apparatus  215  activates the communication circuit  108  when receiving a fourth signal. 
     The second radio apparatus  215  has a second bandpass filter  205  and a second comparator  206  in place of the bandpass filter  105  and the comparator  106  of the receiving circuit  120  of the second radio apparatus  115  shown in  FIG. 1 . 
     The second bandpass filter  205 , which has a second-order IIR filter (not shown), generates a second detection signal from a wave detection signal, the second detection signal having amplitude that is increased at a frequency corresponding to the constant interval. The pass-band of the second bandpass filter  205  includes the frequency corresponding to the constant interval. Where the communication circuit is a circuit that performs an IEEE 802.15.1-compliant communication, the frequency corresponding to the constant cycle (312.5 μs) between the second signal and the third signal is 3.2 kHz. Therefore, the second bandpass filter  205  extracts a 3.2-kHz signal. The second bandpass filter  205  is a filter having a narrower band than the first bandpass filter  103 . 
     The second comparator  206  compares the second detection signal with a second threshold value. If the second detection signal is larger than the second threshold value, the second comparator  206  generates a second power control signal. 
     When receiving the second power control signal, the control unit  119  controls the switch  110  so that power is supplied to the communication circuit  108  from the power unit  109 . 
     Next, how a receiving circuit  220  operates will be described with reference to  FIG. 4 . 
       FIG. 4  shows an example signal which is received by the antenna  101 . The first radio apparatus  114  sends out a fourth signal in a burst-like manner. The fourth signal contains a second signal and a third signal which are separated from each other by a constant interval. In the case of an IEEE 802.15.1-compliant communication, the second signal and the third signal are separated from each other by 312.5 μs which corresponds to 3.2 kHz. The first radio apparatus  114  sends out the fourth signal plural times in a cycle of 1.250 ms which corresponds to 800 Hz. The fourth signal may be sent out with frequency hopping in the same manner as the second signals shown in  FIG. 2  (A). The antenna  101  passes the received fourth signals to the wave detection circuit  104  via the matching circuit and the first bandpass filter  103 . The wave detection circuit  104  generates, on the basis of the fourth signals, a wave detection signal which contains low signals and high signals. The description of this embodiment is directed to a case without interference signals. Therefore, the wave detection signal generated by the wave detection circuit  104  has the same waveform as shown in  FIG. 4 . The wave detection circuit  104  passes the generated wave detection signal to the second bandpass filter  205 . 
     In the second bandpass filter  205 , the detection signal is converted into a second detection signal in which signals other than the 3.2-kHz signal are suppressed. The second detection signal is a signal having amplitude that is increased at 3.2 kHz. The second detection signal is input to the second comparator  206 . If the amplitude of the second detection signal becomes larger than the second threshold value, the second comparator  206  passes a second power control signal to the control unit  119 . 
     Receiving the second power control signal, the control unit  119  controls the switch  110 . An alternative configuration is possible in which the control unit  119  is omitted and the switch  110  receives a second power control signal directly. In this case, the switch  110  operates so as to supply power to the communication circuit  108  when receiving a second power control signal. 
     As described above, in the wireless communication system according to the embodiment, the same advantages as provided by the first embodiment are provided. Furthermore, the communication circuit  108  can be activated when such a signal as an inquiry scan signal or a page scan signal of the IEEE 802.15.1 standard is received. 
     Next, a wireless communication system according to a third embodiment of the invention will be described with reference to  FIG. 5 . In the wireless communication system according to this embodiment, the configuration of the first radio apparatus  114  is the same as shown in  FIG. 1  and hence will not be described. The first radio apparatus  114  sends out a fourth signal which contains a second signal and a third signal which is sent so as to have a constant interval from the second signal. For example, where the communication circuit  113  is a circuit that performs an IEEE 802.15.1-compliant communication, the fourth signal corresponds to an inquiry scan signal or a page scan signal. In this case, the constant interval is 312.5 μs and the constant cycle is 1.250 ms. The fourth signal will be described later in detail. A second radio apparatus  215  activates the communication circuit  108  when receiving a fourth signal. 
     The second radio apparatus  315  has the bandpass filter  105  and the comparator  106  of the receiving circuit  120  of the second radio apparatus  115  shown in  FIG. 1  and the second bandpass filter  205  and the second comparator  206  shown in  FIG. 3 . The individual constituent elements are the same as shown in  FIGS. 1 and 3  and hence will not be described. 
     How a receiving circuit  320  of the second radio apparatus  315  operates will be described. 
     The first radio apparatus  114  sends out a fourth signal in a burst-like manner. The fourth signal contains a second signal and a third signal which are separated from each other by a constant interval. In the case of an IEEE 802.15.1-compliant communication, the second signal and the third signal are separated from each other by 312.5 μs which corresponds to 3.2 kHz. The first radio apparatus  114  sends out the fourth signal plural times in a cycle of 1.250 ms which corresponds to 800 Hz. The fourth signal may be sent out with frequency hopping in the same manner as the second signals shown in  FIG. 2  (A). The antenna  101  passes the received fourth signals to the wave detection circuit  104  via the matching circuit and the first bandpass filter  103 . The wave detection circuit  104  generates, on the basis of the fourth signals, a wave detection signal which contains low signals and high signals. The description of this embodiment is directed to a case without interference signals. Therefore, the wave detection signal generated by the wave detection circuit  104  has the same waveform as shown in  FIG. 4 . The wave detection circuit  104  passes the generated wave detection signal to the band-pass filter  104  and the second bandpass filter  205 . 
     In the bandpass filter  105 , the wave detection signal is converted into a detection signal in which signals other than the 800-Hz signal are suppressed. The detection signal is a signal having amplitude that is increased—at 800 Hz. The detection signal is input to the comparator  106 . If the amplitude of the detection signal becomes larger than the threshold value, the comparator  106  passes a power control signal to the control unit  119 . 
     In the second bandpass filter  205 , the detection signal is converted into a second detection signal in which signals other than the 3.2-kHz signal are suppressed. The second detection signal is a signal having amplitude that is increased at 3.2 kHz. The second detection signal is input to the second comparator  206 . If the amplitude of the second detection signal becomes larger than the second threshold value, the second comparator  206  passes a second power control signal to the control unit  119 . 
     Receiving the power control signal and the second power control signal, the control unit  119  controls the switch  110 . An alternative configuration is possible in which the control unit  119  is omitted and the switch  110  receives a power control signal and a second power control signal directly. In this case, the switch  110  operates so as to supply power to the communication circuit  108  when receiving a power control signal or a second power control signal. 
     As described above, in the wireless communication system according to the embodiment, the same advantages as provided by the first embodiment are provided. Furthermore, the communication circuit  108  can be activated when such a signal as an inquiry scan signal or a page scan signal of the IEEE 802.15.1 standard is received. 
     Since the control unit  119  controls the switch  110  when receiving both of a power control signal and a second power control signal, an event that the communication circuit  108  of the second radio apparatus  315  is activated undesirably can be prevented. Where an apparatus which performs an IEEE 802.15.1-compliant communication exists near the second radio apparatus  115  in addition to the first radio apparatus  114 , the second radio apparatus  115  may receive a signal having a cycle corresponding to 800 Hz that is transmitted from that apparatus. 
     In view of the above, the control unit  119  controls the switch  110  when receiving both of an activation signal and a second power control signal. This makes it possible to prevent activation of the communication circuit  108  of the second radio apparatus  115  unless fourth signals are received even if, for example, an apparatus which is making a communication using first signals as used in the first embodiment exists near the second radio apparatus  115 . The fourth signal is a signal that is sent from the first radio apparatus  114  at the start of a communication, such as an inquiry scan signal or a page scan signal. Therefore, the communication circuit  108  of the second radio apparatus  315  can be activated only when the first radio apparatus  115  starts a communication. 
     A wireless communication system according to a fourth embodiment of the invention will be described with reference to  FIG. 6 . In the wireless communication system according to this embodiment, the first radio apparatus  114  is the same in configuration and operates in the same manner as the first radio apparatus  114  according to the third embodiment. A second radio apparatus  415  has a third bandpass filter  305  in place of the bandpass filter  105  and the second bandpass filter  205  shown in  FIG. 5 . The second radio apparatus  415  has a third comparator  306  in place of the comparator  106 . 
     The third bandpass filter  305 , which has a fourth-order IIR filter, generates a second detection signal having amplitude that is increased at 3.2 kHz and a third detection signal having amplitude that is increased at 800 Hz and 3.2 kHz when receiving a wave detection signal. 
     An example bandpass filter  305  will be described with reference to  FIG. 7 . Having two, cascade-connected second-order IIR filters, the third bandpass filter  305  generates a second detection signal and a third detection signal. More specifically, the third bandpass filter  305  has first to fifth adders  311 - 315 , first to fourth registers  321 - 324 , and first to fifth amplifiers  331 - 335 . 
     The first adder  311  generates a first addition signal by adding a wave detection signal and a second addition signal (described later) together. The first register  321  holds a one-clock portion of the first addition signal according to a clock signal (not shown) and outputs the currently held one-clock portion of the first addition signal as a first delay signal when receiving the next clock pulse. The first amplifier  331  generates a first amplification signal by amplifying the first delay signal by a factor of a. 
     The second register  322  holds a one-clock portion of the first delay signal according to the clock signal (not shown) and outputs the currently held one-clock portion of the first delay signal as a second delay signal when receiving the next clock pulse. The second amplifier  331  generates a second amplification signal by amplifying the second delay signal by a factor of b. 
     The second adder  312  generates a second addition signal by adding first amplification signal and the second amplification signal together. 
     The third amplifier  333  generates a third amplification signal by multiplying the second delay signal by −1. The third adder  313  generates a third addition signal by adding the first addition signal and the third amplification signal together. The third addition signal is a signal having amplitude that is increased at 3.2 kHz, and is output from the third bandpass filter  305  as a second detection signal. 
     The fourth adder  314  generates a fourth addition signal by adding the third addition signal and a fifth addition signal together. The fourth register  324  holds a one-clock portion of the fourth addition signal according to the clock signal (not shown) and outputs the currently held one-clock portion of the fourth addition signal as a fourth delay signal when receiving the next clock pulse. The fourth amplifier  334  generates a fourth amplification signal by amplifying the fourth delay signal by a factor of c. 
     The fifth register  325  holds a one-clock portion of the fourth delay signal according to the clock signal (not shown) and outputs the currently held one-clock portion of the fourth delay signal as a fifth delay signal when receiving the next clock pulse. The fifth amplifier  335  generates a fifth amplification signal by amplifying the fifth delay signal by a factor of d. The fifth adder  315  generates a fifth addition signal by adding the fourth amplification signal and the fifth amplification signal together. The fourth addition signal which is generating the third addition signal and the fifth addition signal together is a signal having amplitude that is increased at 800 Hz and 3.2 kHz, and is output from the third bandpass filter  305  as a third detection signal. 
     The second comparator  206  compares the second detection signal with a second threshold value, and generates a second power control signal if the second detection signal is larger than the second threshold value. The third comparator  306  compares the third detection signal with a third threshold value, and generates a third power control signal if the third detection signal is larger than the third threshold value. When receiving the second power control signal and the third power control signal, the control unit  119  controls the switch  110  so that power is supplied to the communication circuit  108 . Since the third detection signal has energy that is an addition of the energy of the 800-Hz signal and that of the 3.2-kHz signal, it is made higher in noise resistance than an output of a second-order IIR filter. Therefore, the third threshold value may be set larger than the second threshold value. 
     As described above, in the wireless communication system according to the embodiment, the same advantages as provided by the third embodiment are provided. Furthermore, using an output of a fourth-order IIR filer as a detection signal increases the noise resistance and hence the detection accuracy of fourth signals. 
     A wireless communication system according to a fifth embodiment of the invention will be described with reference to  FIG. 8 . In the wireless communication system according to this embodiment, the first radio apparatus  114  is the same in configuration and operates in the same manner as the first radio apparatus  114  according to the third embodiment. A second radio apparatus  515  is equipped with a fourth bandpass filter  405  and a fourth comparator  406  in addition to the configuration of the second radio apparatus  415  shown in  FIG. 6 . 
     The fourth bandpass filter  405 , which has a second-order IIR filter, generates a fourth detection signal having amplitude that is increased at 1.6 kHz. The fourth comparator  406  compares the fourth detection signal with a fourth threshold value, and generates a stop signal if the fourth detection signal is larger than the fourth threshold value. 
     The control unit  119  controls the switch  110  so that power is supplied to the communication circuit  108  if it has received a second power control signal and a third control signal and has not received a stop signal. The control unit  119  does not control the switch  110  if it receives a stop signal in addition to a second power control signal and a third power control signal. That is, the control unit  119  controls the switch  110  if the second detection signal is larger than the second threshold value, the third detection signal is larger than the third threshold value, and the fourth detection signal is smaller than or equal to the third threshold value. 
       FIG. 9A  shows a frequency characteristic of first signals and  FIG. 9B  shows a frequency characteristic of fourth signals. As shown in  FIG. 9A , although the relative power of the main frequency component (0.8 kHz) of the first signals is high, they include frequency components of 1.6 kHz (second harmonic of 0.8 kHz) and 3.2 kHz (fourth harmonic of 0.8 kHz). 
     On the other hand, as shown in  FIG. 9B , the fourth signals have large main frequency components (0.8 kHz and 3.2 kHz) and also have large frequency components of 2.4 kHz and 4.0 kHz. However, the fourth signals include almost no frequency component of 1.6 kHz. 
     Therefore, the communication circuit  108  is not activated if a 1.6-kHz signal is detected even if a 0.8-kHz signal and a 3.2-kHz are detected. 
     As described above, in the wireless communication system according to the embodiment, the same advantages as provided by the third embodiment are provided. Furthermore, since the switch  110  is not controlled if a fourth detection signal having amplitude that is increased in a cycle corresponding to 1.6 kHz is detected, the detection accuracy of fourth signals can be increased. Thus, the communication circuit  108  of the second radio apparatus  515  can be prevented from being activated undesirably even if a radio apparatus which is making a communication using first signals exists near the second radio apparatus  515 . 
     Although in the embodiment the fourth bandpass filter  405  and the fourth comparator  406  are additionally provided in the second radio apparatus  415  according to the fourth embodiment, the fourth bandpass filter  405  and the fourth comparator  406  may be provided additionally in the second radio apparatus  215  according to the second embodiment or the second radio apparatus  315  according to the third embodiment. 
     A wireless communication system according to a sixth embodiment of the invention will be described with reference to  FIG. 10 . In the wireless communication system according to this embodiment, the first radio apparatus  114  is the same in configuration and operates in the same manner as the first radio apparatus  114  according to the third embodiment. A second radio apparatus  615  has a fifth bandwidth filter  505  in place of the third bandpass filter  305  and the fourth bandpass filter  405  of the second radio apparatus  515  shown in  FIG. 8 . The fifth bandwidth filter  505  operates according to the clock signal and a second clock signal. The fifth bandwidth filter  505  generates a second detection signal having amplitude that is increased at 3.2 kHz and a third detection signal having amplitude that is increased at 800 Hz and 3.2 kHz, or a fourth detection signal having amplitude that is increased at 1.6 kHz. 
     An example fifth bandpass filter  505  will be described with reference to  FIG. 11 . The fifth bandpass filter  505  is configured in such a manner that a switch  510  is added to the third bandpass filter  305  shown in  FIG. 7 . The fifth bandpass filter  505  has first to third adders  511 - 513  in place of the first to third adders  311 - 313 , has first and second registers  521  and  522  in place of the first and second registers  321  and  322 , and has first to third amplifiers  531 - 533  in place of the first to third amplifiers  331 - 333 . 
     The first and second registers  521  and  522  operate according to the clock signal or the second clock signal whose rate is two times the rate of the clock signal. Where the first and second registers  521  and  522  operate according to the clock signal, the first to third adders  511 - 513 , the first and second registers  521  and  522 , and the first to third amplifiers  531 - 533  are the same in configuration and operate in the same manners as the first to third adders  311 - 313 , the first and second registers  321  and  322 , and the first to third amplifiers  331 - 333  and hence will not be described. Where the first and second registers  521  and  522  operate according to the second clock signal, the switch  510  operates so as to connect the third adder  313  to the second comparator  206 . As a result, the fifth bandpass filter  505  generates a second detection signal and a third detection signal as in the case of  FIG. 7 . 
     A description will be made of how the fifth bandpass filter  505  operates in the case where the first and second registers  521  and  522  operate according to the second clock signal. 
     The first adder  511  generates a (1-2) th addition signal by adding a wave detection signal and a (2-2)th addition signal (described later) together. The first register  521  holds a one-clock portion of the (1-2)th addition signal according to the second clock signal (not shown) and outputs the currently held one-clock portion of the (1-2) th addition signal as a (1-2) th delay signal when receiving the next clock pulse. The first amplifier  531  generates a (1-2) th amplification signal by amplifying the (1-2)th delay signal by a factor of a. 
     The second register  522  holds a one-clock portion of the (1-2)th delay signal according to the second clock signal (not shown) and outputs the currently held one-clock portion of the (1-2)th delay signal as a (2-2)th delay signal when receiving the next clock pulse. The (2-2) th amplifier  532  generates a (2-2)th amplification signal by amplifying the (2-2)th delay signal by a factor of b. 
     The second adder  512  generates a (2-2) th addition signal by adding (1-2)th amplification signal and the (2-2)th amplification signal together. 
     The third amplifier  533  generates a (3-2) th amplification signal by multiplying the (2-2) th delay signal by −1. The third adder  513  generates a (3-2) th addition signal by adding the (1-2) th addition signal and the (3-2)th amplification signal together. The (3-2)th addition signal is a signal having amplitude that is increased at 1.6 kHz, and is output from the fifth bandpass filter  505  as a fourth detection signal. 
     The switch  510  connects the third adder  513  to one of the second comparator  206  and the fourth comparator  406  according to an instruction from the control unit  119 , for example. 
     The control unit  119  controls the switch  510  so that the third adder  513  is connected to the second comparator  206  or the fourth comparator  406 . 
     A description will be made of how a receiving circuit  620  of the second radio apparatus  515  according to the embodiment operates. 
     First, in a standby state, the control unit  119  of the receiving circuit  620  controls the switch  510  so that the third adder  513  is connected to the second comparator  206 . An operation that is performed to generation of a second power control signal and a third power control signal by the second comparator  206  and the third comparator  306 , respectively, is the same as in the receiving circuit  420  shown in  FIG. 6 , and hence will not be described. 
     When receiving the second power control signal and the third power control signal, the control unit  119  controls the switch  510  so that the third adder  513  is connected to the fourth comparator  406 . The control unit  119  performs controls so that the first and second registers  521  and  522  of the fifth bandpass filter  505  operate according to the second clock signal. The control unit  119  resets (clears to zero) the values being held by the first to fourth registers of the fifth bandpass filter  505 . As a result, if the wave detection signal contains a 1.6-kHz component, the fifth bandpass filter  505  generates a fourth detection signal having amplitude that is increased in a cycle corresponding to 1.6 kHz. The fourth comparator  406  generates a stop signal if the fourth detection signal is larger than the fourth threshold value. 
     When receiving the stop signal, the control unit  119  judges that the signals received by the antenna  101  are not fourth signals and returns to a standby state instead of activating the communication circuit  108 . More specifically, the control unit  119  performs controls so that the first and second registers  521  and  522  of the fifth bandpass filter  505  operate according to the clock signal. The control unit  119  resets (clears to zero) the values being held by the first to fourth registers of the fifth bandpass filter  505 . As a result, the receiving circuit  620  comes to detect whether or not the signals received by the antenna  101  contain an 800-Hz component and a 3.2-kHz component. 
     On the other hand, if the control unit  119  has not received a stop signal even after a lapse of a prescribed time from reception of a second power control signal and a third power control signal, the control unit  119  judges that the signals received by the antenna  101  are fourth signals and controls the switch  110 . The switch  110  connects the power unit  109  to the communication circuit  108  so that power is supplied to the communication circuit  108 . 
     As described above, in the wireless communication system according to the embodiment, the same advantages as provided by the fourth embodiment are provided. Furthermore, since the first and second registers  521  and  522  of the fifth bandpass filter  505  operate according to the clock signal or the second clock signal whose rate is two times the rate of the clock signal, the single circuit can generate a second detection signal having amplitude that is increased at 3.2 kHz and a fourth detection signal having amplitude that is increased at 1.6 kHz. The circuit scale can thus be reduced. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel methods and systems described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.