Patent Publication Number: US-2007110126-A1

Title: Radio communication system and communication apparatus

Description:
BACKGROUND OF THE INVENTION  
      1) Field of the Invention  
      The present invention relates to an art for controlling the impulse amplitude and the impulse repetition frequency in an UWB (Ultra WideBand)-impulse radio apparatus (system) which uses extremely short impulse signals.  
      2) Description of the Related Art  
      Recently, studies for making a UWB (Ultra WideBand)-impulse radio system, in which an impulse signal is used in place of a carrier wave, suitable for practical use have been performed. This UWB-impulse radio communication uses an extremely short (approximately 1 ns) impulse signal (hereinafter also simply called impulses), without using a carrier wave, to perform communication.  
      This UWB-impulse radio communication has the following characteristic features (1) through (5):  
      (1) use of impulses makes the spectrum considerably wide, but it hardly interferes with other systems (radio communication) because of its low spectrum density;  
      (2) power transmission only performed at the time of impulse transmission reduces power consumption;  
      (3) use of a high band increases a transfer rate;  
      (4) impulses make it easy to separate multipath, and is thus highly resistant to multipath and fazing;  
      (5) detection of extremely short impulses realizes high distance measuring resolution.  
      The reception methods in UWB-impulse radio apparatuses employing the UWB-impulse radio system, as disclosed in the following patent document 1, include a coherent scheme (see  FIG. 24  described below) and a non-coherent scheme (see  FIG. 25  described below). In the coherent scheme, a pulse train is generated in synchronism with a transmission pulse train and a correlation operation is performed, thereby receiving data. In the non-coherent scheme, pulses are detected asynchronously with the transmission pulse, thereby receiving data.  
      Now, a description will be made hereinbelow of a reception apparatus  100  employing a coherent scheme with reference to  FIG. 24 . The reception apparatus  100  is capable of repeating (feedback) a correlation operation (integration processing) by a mixer  104 , an integrator  105 , a comparator  106 , a baseband  107 , and a PG (Pulse Generator)  108 , onto an impulse signal which has been received through an antenna  101  and has passed through a BPF (Band Pass Filter)  102  and an LNA (Low Noise Amplifier)  103 .  
      In this communication system employing the coherent method, if the reception apparatus lengthens the integration time by adjusting the time (integration time) during which the reception apparatus performs a correlation operation, or by adjusting the number of pulses per bit (that is, by increasing the number of repetition times of integration processing), the sensitivity (this is called a spread gain) is improved, so that long distance communication becomes available while the transfer rate is decreased.  
      Further, in such a communication system, if the reception apparatus shortens the integration time (that is, the number of repetition times integration processing is reduced), the sensitivity is decreased (a spread gain is not obtained), so that the transfer rate is increased while the communication distance is shortened.  
      However, the coherent scheme requiring a Phase Locked Loop (PLL) with respect to transmission pulses on the receiver end, will increase the size of the reception circuit. Further, a long preamble data is required until the synchronization is established, so that electric power unnecessary for data transmission is generated.  
      Next, referring to  FIG. 25 , a description will be made hereinbelow of a reception apparatus  110  employing the non-coherent scheme. The reception apparatus  110  includes: an operation  111  which performs energy detection by obtaining the square of an impulse signal which has been received through an antenna  101  and has passed through a BPF  102  and an LNA  103 ; a LPF (Low Pass Filter)  112 ; a comparator  106 ; and a baseband  107 .  
      This non-coherent scheme detects impulses asynchronously with transmission pulses. Hence, the construction of a reception circuit of the reception apparatus  110  is simpler than that of the reception apparatus  100  employing the coherent scheme.  
      Further, since preamble data can be short in the non-coherent scheme, the scheme is suitable for use in small-sized, inexpensive, low power-consumption equipment.  
      In the coherent scheme, however, since only the presence or the absence of reception pulses is detected, it is necessary to enlarge the pulse amplitude for realizing long-distance communication.  
      In this instance, there has been an art for decreasing the pulse amplitude when the distance between communication apparatuses is short, to reduce unnecessary effects on other peripheral equipment (see the following patent document 1).  
      Generally speaking, radio laws define that the peak radiation power and the average radiation power of an impulse signal should be maintained equal to or lower than a specified power.  
      For example, the FCC (Federal Communications Commission) defines that in the UWB, as shown in  FIG. 26 , the average radiation power of impulses is smaller than −41.3 dBm/MHz inclusive in a range of 3.1 GHz through 10.6 GHz. In addition, the peak radiation power is defined to be −33.98 dBm/MHz (+0 dBm/MHz at a resolution of 50 MHz, and this value is obtained by a conversion formula at a resolution of 1 MHz). This is called an FCC mask.  
      When there is a regulation for the power of impulses such as the FCC mask, increase in pulse amplitude for long-distance communication can make the average radiation power exceed the reference value.  
      In addition, in the above-described non-coherent scheme, impulses are detected by energy detection without synchronization with transmission pulses. Thus, when the communication distance is long, the pulse amplitude needs to be sufficiently enlarged, so that the average radiation power can exceed its reference value.  
      Therefore, in the communication system employing the non-coherent scheme, when an impulse generation method is set on an assumption of long-distance communication, increase in pulse amplitude will necessitate decrease in pulse repetition frequency [pulse rate; PRF (Pulse Repetition Frequency)] to observe the reference value of the average radiation power. As a result, in such a communication system, the communication speed remains low even when the communication distance becomes short, so that the communication speed cannot be increased. In other words, if the pulse repetition frequency is decreased in order to observe the reference value of the average radiation power, the communication speed becomes slow irrespectively of the communication distance.  
      [Non-patent Document 1] Rick Roberts; “Harris TG4a CFP Proposal Response” [online], January 2005, IEEE (the Institute of Electrical and Electronic Engineers); [searched on Sep. 15, 2005], the Internet &lt;URL:http://grouper.ieee.org/groups/802/15/pub/05/15-05-0006-01-004a-harris-cfp-response.ppt&gt; 
      [Patent Document 1] Published Japanese Translation of a PCT application No. 2004-510388  
     SUMMARY OF THE INVENTION  
      With the foregoing problems in view, one object of the present invention is to make it possible for communication apparatuses which employ the non-coherent scheme and which are used in communication systems employing the UWB-impulse radio communication method, to reliably realize long-distance communication while observing the reference values (the upper limit values) of the average radiation power and the peak radiation power. Another object of the invention is to realize high-speed communication in short-distance communication.  
      In order to accomplish the above objects, according to the present invention, there is provided a radio communication system including a plurality of communication apparatuses which are communicably connected with each other by radio under the UWB (Ultra WideBand)-impulse radio system, the radio communication system comprising: a distance detecting unit which detects the distance between two communication apparatuses, of the plurality of communication apparatuses, the two communication apparatuses being communicably connected by radio; and an impulse adjusting unit which adjusts the amplitude and the repetition frequency of impulses used in radio communication between the two communication apparatuses according to the distance detected by the distance detecting unit.  
      As a preferred feature, the impulse adjusting unit (i) reduces the repetition frequency when increasing the amplitude of the impulses according to the distance, and (ii) increases the repetition frequency of the impulses when reducing the amplitude of the impulses according to the distance.  
      As a generic feature, there is provided a radio communication system including a plurality of communication apparatuses which are communicably connected with each other by radio under the UWB (Ultra WideBand)-impulse radio system, the radio communication system comprising: an electric power detecting unit which detects electric power of impulses which are sent from one of the two communication apparatuses to be connected with each other, of the plurality of communication apparatuses, and which are received by the other of the two communication apparatuses; an impulse adjusting unit which adjusts the amplitude and the repetition frequency of impulses used in radio communication between the two communication apparatuses, according to the electric power detected by the power detecting unit.  
      As another generic feature, there is provided a radio communication system including a plurality of communication apparatuses which are communicably connected with each other by radio under the UWB (Ultra WideBand)-impulse radio system, the radio communication system comprising: a minimum amplitude detecting unit which detects the minimum amplitude of impulses which can be received by one of the two communication apparatuses to be communicably connected with each other by radio, of the plurality of communication apparatuses, the impulses being sent from the other of the two communication apparatuses; an impulse adjusting unit which adjusts the amplitude and the repetition frequency of impulses used in radio communication between the two communication apparatuses according to the minimum amplitude of impulses detected by the minimum amplitude detecting unit.  
      As yet another generic feature, there is provided a communication apparatus for use in a radio communication system in which communication is carried out under the UWB (Ultra WideBand)-impulse radio system, the apparatus comprising: a distance detecting unit which detects the distance from another communication apparatus with which communication is to be performed; and an impulse adjusting unit which adjusts the amplitude and the repetition frequency of impulses used in radio communication with the other communication apparatus according to the distance detected by the distance detecting unit.  
      In this manner, according to the present invention, the impulse adjusting unit adjusts the amplitude and the repetition frequency of impulses used in radio communication in accordance with the distance between the two communication apparatuses. Thus, even when the reception scheme used in these communication apparatuses is the non-coherent scheme, the repetition frequency of impulses is reduced when the amplitude of impulses is increased, so that the long-distance communication is reliably realized while observing the reference values (upper limits) of the average radiation power and the peak radiation power.  
      Further, since the impulse adjusting unit adjusts the amplitude and the repetition frequency of impulses, the amplitude of impulses can be reduced when the repetition frequency of impulses is increased, so that high-speed communication can be realized while the reference values are observed.  
      Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a block diagram showing a construction of a radio communication system according to a first embodiment of the present invention;  
       FIG. 2 ( a ) through  FIG. 2 ( c ) are diagrams for describing adjustment processing of the amplitude and the repetition frequency of impulses according to the communication distance, which adjustment is carried out by an impulse adjusting unit of the radio communication system according to the first embodiment;  FIG. 2 ( a ) shows adjustment processing in a case of long distance communication;  FIG. 2 ( b ) shows adjustment processing in a case of intermediate distance communication;  FIG. 2 ( c ) shows adjustment processing in a case of short distance communication;  
       FIG. 3  is a diagram for describing communication processing using impulse signals in the radio communication system of the first embodiment of the present invention;  
       FIG. 4  is a block diagram showing a construction of one of the two communication apparatuses in the radio communication system of the first embodiment of the present invention;  
       FIG. 5  is a diagram showing a table held by a pulse determining unit of the communication apparatus of  FIG. 4 ;  
       FIG. 6  is a diagram for describing a relationship between the repetition frequency of impulse signals for use in the present invention and radiation power;  
       FIG. 7  is a block diagram showing a construction of the other one of the two communication apparatuses in the radio communication system of the first embodiment of the present invention;  
       FIG. 8  is a diagram for describing processing procedures in the radio communication system of the first embodiment;  
       FIG. 9  is a diagram for describing a distance calculation method performed by a distance calculating unit (as a distance detecting unit) of the radio communication system of the first embodiment;  
       FIG. 10  is a block diagram showing a construction of a radio communication system according to a second embodiment of the present invention;  
       FIG. 11  is a block diagram showing a construction of one of the two communication apparatuses in the radio communication system of the second embodiment of the present invention;  
       FIG. 12  is a block diagram showing a construction of the other one of the two communication apparatuses in the radio communication system of the second embodiment of the present invention;  
       FIG. 13  is a table held by the communication apparatus shown in  FIG. 12 ;  
       FIG. 14  is a diagram for describing processing procedures in the radio communication system of the second embodiment;  
       FIG. 15  is a block diagram showing a construction of a radio communication system according to a third embodiment of the present invention;  
       FIG. 16  is a block diagram showing a construction of one of the two communication apparatuses in the radio communication system of the third embodiment of the present invention;  
       FIG. 17  is a table held by the communication apparatus shown in  FIG. 16 ;  
       FIG. 18  is a diagram for describing processing procedures in the radio communication system of the third embodiment;  
       FIG. 19  is a block diagram showing a construction of a radio communication system according to a fourth embodiment of the present invention;  
       FIG. 20  is a block diagram showing a construction of one of the two communication apparatuses in the radio communication system of the fourth embodiment of the present invention;  
       FIG. 21  is a table held by a pulse determining unit of the communication apparatus shown in  FIG. 20 ;  
       FIG. 22  is a block diagram showing a construction of the other one of the two communication apparatuses in the radio communication system of the fourth embodiment of the present invention;  
       FIG. 23  is a diagram for describing processing procedures in the radio communication system of the fourth embodiment;  
       FIG. 24  is a block diagram showing a construction of a receiving apparatus which employs a previous coherent scheme;  
       FIG. 25  is a block diagram showing a construction of a receiving apparatus which employs a previous non-coherent scheme; and  
       FIG. 26  is a diagram for describing the requirements (FCC mask) of average radiation power and peak radiation power of impulses in the UWB (Ultra WideBand) according to the FCC (Federal Communications Commission). 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT(S)  
      Embodiments of the present invention will now be described with reference to the accompanying relevant drawings.  
     [1] First Embodiment  
      First of all, referring to the block diagram of  FIG. 1 , a description will be made hereinbelow of a construction of a radio communication system according to a first embodiment of the present invention. As shown in  FIG. 1 , the present radio communication system  1  includes multiple (here, two) communication apparatuses [UWB (Ultra WideBand)-impulse radio apparatuses]  10   a - 1  and  10   b - 1 , which are communicably connected with one another by radio under the UWB-impulse radio communication scheme.  
      The communication apparatus  10   a - 1  includes: a pulse generator  11   a  which generates impulses (impulse signal) based on transmission data; a PA (Power Amplifier)  12   a  which amplifies the impulses generated by the pulse generator  11   a ; and an antenna  13   a  which sends out the impulses having been amplified by the PA  12   a.    
      Further, the communication apparatus  10   a - 1  also includes: an LNA (Low Noise Amplifier)  16   a  which amplifies impulses received from a communication apparatus  10   b - 1  through the antenna  13   a , the communication apparatus  10   a - 1 ; and a pulse detecting unit  17   a  which detects the impulses amplified by the LNA  16   a l as reception data.    
      The communication apparatus  10   b - 1  has a construction similar to the communication apparatus  10   a - 1 . That is, a pulse generating unit  11   b , a PA  12   b , an antenna  13   b , an LNA  16   b , and a pulse detecting unit  17   b , of a communication apparatus  10   b - 1  correspond to the pulse generator  11   a , the PA  12   a , the antenna  13   a , the LNA  16   a , and the pulse detecting unit  17   a , respectively, of the communication apparatus  10   a - 1 , and have functions similar to those of the corresponding elements.  
      The radio communication system  1  includes: a distance detecting unit (distance measuring unit)  14  which detects (measures) the distance between the communication apparatuses  10   a - 1  and  10   b - 1 ; and an impulse adjusting unit  15 - 1  which adjusts (i) impulse amplitude (hereafter also simply called amplitude or pulse amplitude) and (ii) impulse repetition frequency [pulse rate; hereafter also called repetition frequency, pulse repetition frequency, and PRF (Pulse Repetition Frequency)] of impulses used in radio communication between the communication apparatuses  10   a - 1  and  10   b - 1 , based on the distance between the communication apparatus  10   a - 1  and communication apparatus  10   b - 1  detected by the distance detecting unit  14 .  
      The distance detecting unit  14 - 1  has a first distance detecting unit  14   a  of the communication apparatus  10   a - 1  and a second distance detecting unit  14   b - 1  of the communication apparatus  10   b - 1 . The distance detecting unit  14 - 1  detects the distance between the communication apparatuses  10   a - 1  and  10   b - 1  based on a propagation time which is required for impulses to travel therebetween. The method for detecting this distance will be detailed later with reference to  FIG. 9 .  
      The impulse adjusting unit  15 - 1  has a first impulse adjusting unit  15   a - 1  of the communication apparatus  10   a - 1  and a second impulse adjusting unit  15   b - 1 . As shown in  FIG. 2 ( a ), for example, if the distance detected by the distance detecting unit  14  is long, the impulse adjusting unit  15 - 1  performs adjustment so that the amplitude of impulses is large and that the repetition frequency of impulses is low.  
      Further, as shown in  FIG. 2 ( b ), if the distance detected by the distance detecting unit  14  is intermediate, the impulse adjusting unit  15 - 1  performs adjustment so that the amplitude of impulses is smaller than that when the distance is long and that the repetition frequency of impulses is higher than that when the distance is long.  
      In addition, as shown in  FIG. 2 ( c ), if the distance measured by the distance detecting unit  14  is short, the impulse adjusting unit  15 - 1  performs adjustment so that the amplitude of impulses is smaller than that when the distance is intermediate and that the repetition frequency of impulses is higher than that when the distance is intermediate.  
      In this manner, the impulse adjusting unit  15 - 1  adjusts the amplitude and the repetition frequency of impulses according to the distance measured by the distance detecting unit  14 . That is, as shown  FIG. 2 ( a ) through  FIG. 2 ( c ), the impulse adjusting unit  15 - 1  reduces the repetition frequency when increasing the amplitude of impulses according to the distance. On the other hand, the impulse adjusting unit  15 - 1  increases the repetition frequency when reducing the amplitude of impulses according to the distance. As a result, it becomes possible to reliably realize communication regardless of the distance, while suppressing a peak radiation power and an average radiation power under a specified value (for example, the FCC mask). Further, in the case of a short distance, the communication speed can be increased.  
      A concrete construction of the impulse adjusting unit  15 - 1  will be detailed later with reference to  FIG. 4 ,  FIG. 5 , and  FIG. 7 .  
      In this instance, in the radio communication system  1 , the distance detecting unit  14  and a part [a pulse determining unit  34  (will be detailed later) of  FIG. 4 ] of the impulse adjusting unit  15 - 1  which determines the amplitude and the repetition frequency is provided for the communication apparatus  10   a - 1 , but they can be provided for the communication apparatus  10   b - 1 , or for both of the communication apparatus  10   a - 1  and the communication apparatus  10   b - 1 .  
      Here, referring to  FIG. 3 , a description will be made hereinbelow of the data transceiving method (the UWB-impulse radio system) between the communication apparatuses  10   a - 1  and  10   b - 1  of the radio communication system  1 , in which method impulses are used for transceiving data.  
      As shown in  FIG. 3 , transmission data in the radio communication system  1  is time-hopped by 8-value (that is, eight values from 0 through 7) RS (Reed-Solomon) sequence, which is a kind of PN (Pseudo Noise) sequence, and also is data-modulated by Pulse Position Modulation (PPM).  
      In cases where the minimum time unit for changing the pulse position, 1 chip, is 100 ns, if “5763421” is used as the RS sequence, in seven pulse divisions [equal to 1 symbol (7, s)] (1 pulse division is 1,s) of a preamble portion (data is not modulated) for synchronization of transmission data (TH data), the initial pulse is time-hopped at the position of 500 ns; the next pulse, at 700 ns; the next pulse, at 600 ns; the next pulse, at 300 ns; the next pulse, at 400 ns; the next pulse, at 200 ns; the next pulse, at 100 ns.  
      Here, in  FIG. 3 , for simplification of illustration, the 4th to the 6th symbols from the leading end (the left end in the drawing) of the preamble portion not shown. Further, in  FIG. 3 , the vertical thick solid line indicates that pulses are time-hopped.  
      Likewise, the data portion (communication data) following the preamble portion is time-hopped by the RS sequence. When “1” is indicated in each pulse division of the data portion, a pulse is shifted backwards by one chip in comparison with the position time-hopped in the preamble portion. Thus, so-called pulse position modulation is performed.  
      For example, the data portion is “0110000”, the RS sequence of “5763421” indicated by the preamble portion is modulated into “5873421” in the data portion. In the seven pulse divisions, the initial pulse is time-hopped at the position of 500 ns; the next pulse, at 800 ns; the next pulse, 700 ns; the next pulse, at 300 ns; the next pulse, 400 ns; the next pulse, at 200 ns; the next pulse, at 100 ns.  
      That is, in the second pulse division indicating “1”, the pulse is hopped at the position of 700 ns in the preamble portion, while the pulse is hopped at the position of 800 ns (backwards by one chip) in the data portion. Likewise, in the third pulse division indicating “1”, the pulse is hopped at the position of 600 ns in the preamble portion, while the pulse is hopped at the position of 700 ns (backwards by one chip) in the data portion. In  FIG. 3 , for the simplification of illustration, the 5th through the 7th symbols from the leading end of the data portion are omitted.  
      Next, referring to the block diagram of  FIG. 4 , a description will be made more in detail of a construction of the communication apparatus  10   a - 1  of the radio communication system  1 . In  FIG. 4 , like reference characters designate the same or the similar elements already described.  
      As shown in  FIG. 4 , the communication apparatus  10   a - 1  has the above-described PA  12   a , antenna  13   a , LNA  16   a , and pulse detecting unit  17   a . In addition, the communication apparatus  10   a - 1  includes: a pulse frequency source  20   a ; a PN sequence generating unit  21   a ; a PPM data modulating unit  22   a ; an impulse generating unit  23   a ; a BPF (Band Pass Filter)  24   a ; an ATT (attenuator)  25   a ; a correlator  26   a ; a PPM data modulating unit  27   a ; a timer  30   a ; a transmission time holding unit  31   a ; a reception time holding unit  32   a ; a distance calculating unit  33   a ; a pulse determining unit  34 - 1 ; and a pulse controlling unit  36   a - 1 .  
      On the communication apparatus  10   a - 1 , a clock of 10 MHz (100 ns cycle) is generated by the pulse frequency source  20   a . On the basis of the clock, the PN sequence generating unit  21   a  generates the above-described RS sequence.  
      The PPM data modulating unit  22   a  performs PPM modulation according to whether transmission data is “1” or “0”, and pulses are sent to the impulse generating unit  23   a.    
      The impulse generating unit  23   a , for example, generates extremely fine impulses at the rise of pulses using a step recovery diode.  
      The BPF  24   a  removes unnecessary spectrum of the impulses generated by the impulse generating unit  23   a.    
      That is, the impulses generated by the impulse generating unit  23   a  have a significantly wide band. However, for the purpose of making the impulse adapt to the FCC mask, the impulses generated by the impulse generating unit  23   a  are made to pass through the BPF  24   a  of 3.1 GHz through 10.6 GHz, so that unnecessary spectrum lower than 3.1 GHz or higher than 10.6 GHz are removed.  
      Then, impulses which have passed through the BPF  24   a  are amplified by the PA  12   a , and are attenuated by the ATT  25   a  as necessary, and are then radiated from the antenna  13   a.    
      In this manner, in the communication apparatus  10   a - 1 , the PN sequence generating unit  21   a , the PPM data modulating unit  22   a , the impulse generating unit  23   a , and the BPF  24   a  function as a pulse generator  11   a.    
      On the receiver end of the communication apparatus  10   a - 1 , the BPF  24   a  removes unnecessary spectrum from impulse waves (that is, impulses sent from the communication apparatus  10   b - 1 ) received through the antenna  13   a , and then the LNA  16   a  amplifies the impulse waves.  
      The pulse detecting unit  17   a  detects pulses from the impulses waves amplified by the LNA  16   a.    
      This pulse detecting unit  17   a  has an envelop detecting circuit (not illustrated) formed by a diode and a comparator (not illustrated). The communication apparatuses  10   a - 1  and  10   b - 1  employ the non-coherent scheme as a reception scheme.  
      Further, the pulse detected by the pulse detecting unit  17   a  is input to the correlator  26   a.    
      The correlator  26   a  compares the pulses detected by the pulse detecting unit  17   a  with the RS sequence generated by the PN sequence generating unit  21   a , thereby detecting a preamble portion from the pulses.  
      The correlator  26   a , which includes, for example, a digital matched filter (not illustrated), monitors matching (correlation) between the pulses and the RS sequence, to extract a preamble portion from the pulses.  
      When the preamble portion is detected by the correlator  26   a , it is regarded that synchronization is established, and the PPM data modulating unit  27   a  demodulates the PPM of the data portion following the preamble, thereby generating reception data.  
      As shown in  FIG. 4 , the communication apparatus  10   a - 1  includes a timer  30   a . At the time (initial setting time) the amplitude and the repetition frequency of impulses used in radio communication between the communication apparatus  10   a - 1  and the communication apparatus  10   b - 1  are initially determined and set [a distance measuring command (will be described later) is sent to the communication apparatus  10   b - 1 ], the transmission time holding unit  31   a  holds the time when the PN sequence generating unit  21   a  generates the first pulse of the data portion following the preamble portion transmitted to the communication apparatus  10   b - 1 , based on the timer  30   a.    
      Further, at the initial setting time, when the communication apparatus  10   a - 1  receives data (Tb; described later) relating to a time from the communication apparatus  10   b - 1 , the reception time holding unit  32   a  of the communication apparatus  10   a - 1  holds the time when the pulse detecting unit  17   a  detects the initial pulse of the data portion of the data, based on the timer  30   a.    
      After that, the distance calculating unit  33   a  calculates the distance between the communication apparatus  10   a - 1  and the communication apparatus  10   b - 1 , based on the time held in the transmission time holding unit  31   a , the time held in the reception time holding unit  32   a , and the data (Tb; described later) relating to the time received from the communication apparatus  10   b - 1 . The method of calculation of the distance by the distance calculating unit  33   a  will be detailed later with reference to  FIG. 9 .  
      In this manner, in the communication apparatus  10   a - 1 , the timer  30   a , the transmission time holding unit  31   a , the reception time holding unit  32   a , and the distance calculating unit  33   a , function as a first distance detecting unit  14   a.    
      Further, as shown in  FIG. 4 , in the communication apparatus  10   a - 1 , after calculation of the distance by the distance calculating unit  33   a , the pulse determining unit  34 - 1  determines the amplitude and the repetition frequency used in radio communication between the communication apparatus  10   a - 1  and the communication apparatus  10   b - 1 .  
      The pulse determining unit  34 - 1  has a table  35 - 1 , as shown in  FIG. 5 , in which the amplitude and the repetition frequency of impulses are associated. Based on the table  35 - 1 , the amplitude and the repetition frequency are determined.  
      In this instance, the table  35 - 1  holds set values [power attenuation amount (rate); here, 9 stages of set values of “+0 dB”, “−3 dB”, “−6 dB”, “−9 dB”, “−15 dB”, “−18 dB”, “−21 dB”, and “−24 dB”] of ATT  25   a  as the amplitude of impulses, and also holds the maximum PRF as the repetition frequency of impulses. In addition, the table  35 - 1  also holds the 1-chip time (the value of 1 chip) in the pulse frequency source  20   a  corresponding to the maximum PRF.  
      Here, a description will be made hereinbelow of a relationship between the pulse repetition frequency and the radiation power.  FIG. 6  shows the result of measurement, by a spectrum analyzer, of an average radiation power (solid line designated by “RMS” in the note field) and a peak radiation power (two-dotted line designated by “Pk” in the note field), when the pulse repetition frequency (PRF) is changed according to nine stages of ATT  25   a . In this instance, the average radiation power is measured by the RMS detecting function of the spectrum analyzer; the peak radiation power is measured by the peak detection function of the spectrum analyzer.  
      When requirements such as the FCC mask are taken into consideration in communication between the communication apparatuses  10   a - 1  and  10   b - 1 , it is ideal that increase in pulse frequency will not cause the peak radiation power to increase. However, in the practical spectrum analyzer, when the frequency exceeds a certain level, the peak radiation power increases as the pulse repetition frequency increases.  
      Further, the average radiation power increases as the pulse repetition frequency increases.  
      With the FCC mask, the upper limit of the average radiation power (RMS Mask) is −41.3 dB/MHz, and the upper limit of the peak radiation power (Pk Mask) is −33.98 dBm/MHz. Thus, in the radio communication system  1 , to suppress such powers under the above-mentioned upper limits, the impulse repetition frequency due to the pulse frequency source  20   a  and the power attenuation amount (here, nine stages of set values) due to the ATT  25   a  must be subjected to adjustment.  
      Accordingly, the table  35 - 1  is set so that it satisfies the reference values of the average radiation power and the reference values of the peak radiation power shown in  FIG. 6 .  
      That is, as shown in  FIG. 5 , when the attenuation of ATT  25   a  is not present (“+0 dB”) in the table  35 - 1 , the amplitude of impulses is set so that the peak radiation power is the upper limit of the reference values. Here, on the basis of  FIG. 6 , when the pulse repetition frequency (PRF) exceeds 0.68 MHz, the average radiation power exceeds the reference value of the average radiation power. Thus, the maximum PRF is set to 0.68 MHz, and the 1-chip time due to the pulse frequency source  20   a  is set to 148 ns. With such setting, the communication apparatuses  10   a - 1  and  10   b - 1  are capable of communicating with each other at a distance of 84.9 m therebetween.  
      When the power attenuation amount of ATT  25   a  is “−3 dB”, the maximum PRF is 1.04 MHz, and the 1-chip time due to the pulse frequency source  20   a  is 100 ns. At this time, the maximum communication distance between the communication apparatuses  10   a - 1  and  10   b - 1  is 60 m.  
      Further, when the power attenuation amount of ATT  25   a  is “−6 dB”, the maximum PRF is 1.5 MHz, and the 1-chip time due to the pulse frequency source  20   a  is 67 ns. At this time, the maximum communication distance between the communication apparatuses  10   a - 1  and  10   b - 1  is 42.4 m.  
      Still further, when the power attenuation amount of ATT  25   a  is “−9 dB”, the maximum PRF is 2.2 MHz, and the 1-chip time due to the pulse frequency source  20   a  is 46 ns. At this time, the maximum communication distance between the communication apparatuses  10   a - 1  and  10   b - 1  is 30 m.  
      Furthermore, when the power attenuation amount of ATT  25   a  is “−12 dB”, the maximum PRF is 2.9 MHz, and the 1-chip time due to the pulse frequency source  20   a  is 35 ns. At this time, the maximum communication distance between the communication apparatuses  10   a - 1  and  10   b - 1  is 21.2 m.  
      Further, when the power attenuation amount of ATT  25   a  is “−15 dB”, the maximum PRF is 4.1 MHz, and the 1-chip time due to the pulse frequency source  20   a  is 25 ns. At this time, the maximum communication distance between the communication apparatuses  10   a - 1  and  10   b - 1  is 15 m.  
      Still further, when the power attenuation amount of ATT  25   a  is “−18 dB”, the maximum PRF is 6.0 MHz, and the 1-chip time due to the pulse frequency source  20   a  is 17 ns. At this time, the maximum communication distance between the communication apparatuses  10   a - 1  and  10   b - 1  is 10.6 m.  
      Furthermore, when the power attenuation amount of ATT  25   a  is “−21 dB”, the maximum PRF is 8.8 MHz, and the 1-chip time due to the pulse frequency source  20   a  is 12 ns. At this time, the maximum communication distance between the communication apparatuses  10   a - 1  and  10   b - 1  is 7.5 m.  
      Further, when the power attenuation amount of ATT  25   a  is “−24 dB”, the maximum PRF is 13.5 MHz, and the 1-chip time due to the pulse frequency source  20   a  is 8 ns. At this time, the maximum communication distance between the communication apparatuses  10   a - 1  and  10   b - 1  is 5.3 m.  
      Here, a description will be made hereinbelow of a concrete method of determining the amplitude and the repetition frequency of impulses by the pulse determining unit  34 - 1  using the table  35 - 1 . For example, if the distance between the communication apparatuses  10   a - 1  and  10   b - 1  measured by the distance calculating unit  33   a  is 60 m, the pulse determining unit  34 - 1  sets the power attenuation amount of ATT  25   a  as the amplitude of impulses to “+0 dB”, and sets the pulse repetition frequency to a value equal to or smaller than 0.68 MHz. Concretely, since the pulse cycle is 1/0.68 MHz=1.47, s, the 1-chip time due to a clock generated by the pulse frequency source  20   a  corresponding to the pulse repetition frequency of 0.68 MHz is setto 148 ns. As a result, the 1-pulse division becomes 1.48, s, and the PRF becomes 1/1.48, s=0.68 MHz.  
      Likewise, when the distance calculated by the distance calculating unit  33   a  is equal to or greater than 42.4 m, the pulse determining unit  34 - 1  sets the power attenuation amount of ATT  25   a  to “−3 dB” based on the table  35 - 1 . Further, to make the pulse repetition frequency equal to or lower than 1.04 MHz, the pulse determining unit  34 - 1  sets the 1-chip time which is based on the clock generated by the pulse frequency source  20   a  to 100 ns.  
      Further, when the distance calculated by the distance calculating unit  33   a  is shorter than 7.5 m, the pulse determining unit  34 - 1  sets the power attenuation amount of ATT  25   a  to “−24 dB” based on the table  35 - 1 . Further, the pulse determining unit  34 - 1  also sets the 1-chip time to 8 ns.  
      Here, in the present invention, values in the table  35 - 1  of  FIG. 5  are not limited to the present example. For example, the maximum PRF of the table  35 - 1  merely shows the maximum value of PRF. Thus, if the pulse frequency source  20   a  is only capable of changing the chip-time in 5 ns units, the 1-chip time when the maximum PRF is 0.68 MHz is changed from 148 ns to 150 ns. Likewise, the 1-chip time can be changed into 100 ns, 70 ns, 50 ns, 35 ns, 25 ns, 20 ns, 15 ns, and 10 ns.  
      As described so far, the radio communication system  1  sets the amplitude and the repetition frequency of impulses based on the table  35 - 1 , thereby realizing a communication distance of 84.9 m at maximum and also realizing the high-speed communication at a rate of 13.5 MHz at maximum.  
      In addition, as shown in  FIG. 4 , a pulse controlling unit  36   a - 1  sets the attenuation amount of the ATT  25   a  based on the amplitude (here, the set value of the ATT  25   a ) determined by the pulse determining unit  34 - 1 .  
      Further, the pulse controlling unit  36   a - 1  sets the PRF of the pulse frequency source  20   a  based on the repetition frequency of impulses determined by the pulse determining unit  34 - 1  so that the above-mentioned repetition frequency is realized.  
      That is, the pulse controlling unit  36   a - 1  controls the pulse frequency source  20   a  and the ATT  25   a  so that the amplitude and the repetition frequency of impulses determined by the pulse determining unit  34 - 1  are transmitted.  
      In this manner, in the communication apparatus  10   a - 1 , the pulse determining unit  34 - 1 , the pulse controlling unit  36   a - 1 , the pulse frequency source  20   a , and the ATT  25   a  function as a first impulse adjusting unit  15   a - 1 .  
      Next, referring to the block diagram of  FIG. 7 , a description will be made in more detail hereinbelow of a construction of the communication apparatus  10   b - 1  of the radio communication system  1 . Here, in  FIG. 7 , like reference characters designate the same or similar elements already described.  
      Further, in  FIG. 7 , elements designated by reference characters whose two numeric characters on the left are the same as those of the reference characters in  FIG. 4  are constituents having the same or approximately the same functions.  
      As shown in  FIG. 7 , the communication apparatus  10   b - 1  includes the PA  12   b , the antenna  13   b , the LNA  16   b , and the pulse detecting unit  17   b , shown in  FIG. 1 . In addition, the communication apparatus  10   b - 1  includes: a pulse frequency source  20   b ; a PN sequence generator  21   b ; a PPM data modulating unit  22   b ; an impulse generating unit  23   b ; a BPF  24   b ; an ATT  25   b ; a correlator  26   b ; a PPM data demodulating unit  27   b ; a timer  30   b ; a transmission time holding unit  31   b ; a reception time holding unit  32   b ; and a pulse controlling unit  36   b - 1 .  
      Here, the communication apparatus  10   b - 1  does not include elements equivalent to the distance calculating unit  33   a  and the pulse determining unit  34 - 1  of the communication apparatus  10   a - 1 .  
      Further, the pulse frequency source  20   b , the PN sequence generator  21   b , the PPM data modulating unit  22   b , the impulse generating unit  23   b , the BPF  24   b , the ATT  25   b , the correlator  26   b  the PPM data demodulating unit  27   b , and the timer  30   b , have functions similar to those of the pulse frequency source  20   a , the PN sequence generating unit  21   a , the PPM data modulating unit  22   a , the impulse generating unit  23   a , the BPF  24   a , the ATT  25   a , the correlator  26   a , the PPM data modulating unit  27   a , and the timer  30   a , respectively, of the communication apparatus  10   a - 1 . Thus, a detailed description of the above elements is omitted.  
      Here, a description will be made of constituents (the transmission time holding unit  31   b , the reception time holding unit  32   b , and the pulse controlling unit  36   b - 1 ) of the communication apparatus  10   b - 1  which carry out operations different from those of the communication apparatus  10   a - 1 .  
      Upon receipt of a distance measurement command sent from the communication apparatus  10   a - 1  at the initial setting time, on the communication apparatus  10   b - 1 , the reception time holding unit  32   b  holds the time when the pulse detecting unit  17   b  detects the initial pulse of the data portion of the received data, based on the timer  30   b.    
      Further, on the communication apparatus  10   b - 1 , when sending back a response, meaning that such received data (the distance measurement command) has been received, to the communication apparatus  10   a - 1 , the transmission time holding unit  31   b  holds the time when the PN sequence generator  21   b  generates the initial pulse of the data portion of transmission data as the response, based on timer  30   b.    
      In this manner, in the communication apparatus  10   b - 1 , the timer  30   b , the transmission time holding unit  31   b , and the reception time holding unit  32   b  function as a second distance detecting unit  14   b - 1 .  
      The communication apparatus  10   b - 1  transmits the time held in the transmission time holding unit  31   b  and the time held in the reception time holding unit  32   b  or the difference therebetween calculated, to the communication apparatus  10   a - 1  as transmission data.  
      The pulse controlling unit  36   b - 1  of the communication apparatus  10   b - 1  controls the pulse frequency source  20   b  and the ATT  25   b  based on the amplitude and the repetition frequency of impulses which are determined by the pulse determining unit  34 - 1  of the communication apparatus  10   a - 1  and received from the communication apparatus  10   a - 1  as received data.  
      In this manner, in the communication apparatus  10   b - 1 , the pulse frequency source  20   b , the ATT  25   b , and the pulse controlling unit  36   b - 1 , function as a second impulse adjusting unit  15   b - 1 .  
      Next, referring to  FIG. 8 , a description will be made hereinbelow of a processing procedure (that is, communication procedures between the communication apparatuses  10   a - 1  and  10   b - 1 ).  
      When the communication apparatus  10   a - 1  communicates with the communication apparatus  10   b - 1 , an initial setting operation [see (a) through (o) in  FIG. 8 ] for determining the amplitude and the repetition frequency used in radio communication therebetween is executed.  
      The communication apparatus  10   a - 1  sets the amplitude of impulses used at the time of initial setting to a maximum value, and sets the repetition frequency of impulses to a minimum value [see (a) of  FIG. 8 ]. That is, the pulse controlling unit  36   a - 1  controls the ATT  25   a  to have a power attenuation amount of “+0 dB”, thereby realizing the maximum pulse amplitude which can be generated by the communication apparatus  10   a - 1 . Further, the pulse controlling unit  36   a - 1  controls the pulse frequency source  20   a  to realize the minimum repetition frequency (here, 1-chip is 148 ns) which can be generated by the pulse frequency source  20   a.    
      Next, the communication apparatus  10   a - 1  sends a distance measurement command for measuring the distance from the communication apparatus  10   b - 1  with which communication is to be performed using the maximum amplitude and the minimum repetition frequency [see  FIG. 8 ( b )].  
      At this time, the transmission time holding unit  31   a  of the communication apparatus  10   a - 1  holds the time (Tat) when the PN sequence generating unit  21   a  generates the initial pulse immediately after the preamble portion of the distance measuring command (that is, the initial pulse of the data portion), based on the timer  30   a  [see (c) of  FIG. 8 ].  
      Then, when the communication apparatus  10   b - 1  receives the distance measuring command, the reception time holding unit  32   b  of the communication apparatus  10   b - 1  holds the time (the arrival time of a distance measuring time; Tbr) when the pulse detecting unit  17   b  detects the initial pulse after the preamble portion of the distance measuring command, based on the timer  30   b  [see (d) of  FIG. 8 ].  
      After that, as in the case of procedure (a) in the communication apparatus  10   a - 1 , the pulse controlling unit  36   b - 1  of the communication apparatus  10   b - 1  sets the amplitude of impulses to the maximum value (sets ATT  25   b  to “+0 dB”), and also sets the repetition frequency of impulses to the minimum value (1 chip is 148 ns) by controlling the pulse frequency source  20   b  [see (e) of  FIG. 8 ].  
      The communication apparatus  10   b - 1  transmits the response (distance measurement command response) to the distance measuring command to the communication apparatus  10   a - 1  [see (f) of  FIG. 8 ].  
      In this instance, the transmission time holding unit  31   b  of the communication apparatus  10   b - 1  holds the time (Tbt) when the PN sequence generator  21   b  generates the initial pulse (of the data portion) immediately after the preamble portion of the distance measurement command response, based on the timer  30   b.    
      After that, when the communication apparatus  10   a - 1  receives the distance measurement command response from the communication apparatus  10   b - 1  as received data, the reception time holding unit  32   a  of the communication apparatus  10   a - 1  holds the time when the pulse detecting unit  17   a  detects the initial pulse immediately after the preamble portion of the distance measuring command (arrival time of the distance measuring command; Tar), based on the timer  30   a  [see (h) of  FIG. 8 ].  
      In parallel with this processing (h), in the communication apparatus  10   b - 1 , an operation unit (not illustrated) subtracts Tbr held in the reception time holding unit  32   b  from Tbt held in the transmission time holding unit  31   b , thereby calculating the difference Tb [see (i) of  FIG. 8 ].  
      Then, the communication apparatus  10   b - 1  transmits the difference Tb to the communication apparatus  10   a - 1  [see (j) of  FIG. 8 ].  
      When the communication apparatus  10   a - 1  receives the difference Tb from the communication apparatus  10   b - 1 , the distance calculating unit  33   a  calculates the distance between the communication apparatuses  10   a - 1  and  10   b - 1  [see (k) of  FIG. 8 ].  
      The distance calculating unit  33   a  measures the distance between the communication apparatuses  10   a - 1  and  10   b - 1  with the TWR (Two Way Ranging) scheme. Here, referring to  FIG. 9 , a description will be made here in below of a concrete distance calculation method by the distance calculating unit  33   a . First of all, it is assumed that the timer  30   a  held in the communication apparatus  10   a - 1  differs in time from the timer  30   b  of the communication apparatus  10   b - 1  by to.  
      This is because complete synchronization between the timer  30   a  and the timer  30   b  is practically unavailable, because such complete synchronization can be realized by super-accurate atomic clocks.  
      Assuming that the propagation time of impulses (radio wave) is given as tp, the following equations (1) and (2) are held based on (i) the time Tat when the communication apparatus  10   a - 1  transmits a distance measurement command (that is, the time held by the transmission time holding unit  31   a ), (ii) the time Tbr when the communication apparatus  10   b - 1  receives the distance measurement command (that is, the time held by the reception time holding unit  32   b ), (iii) the time Tbt when the communication apparatus  10   b - 1  transmits a distance measurement command response (that is, the time held by the transmission time holding unit  31   b ), and (iv) the time Tar when the communication apparatus  10   a - 1  receives the distance measurement command response (that is, the time held by the reception time holding unit  32   a ).
 
 Tbr=Tat+to+tp   (1)
 
 Tat=Tbr+to−tp   (2)
 
      When these equations are solved for tp, the following equation (3) is obtained.
 
 tp={ ( Tar−Tat )−( Tbt−Tbr )}/2=( Ta−Tb )/2  (3)
 
      In the above equation (3), Ta=Tar−Tat and Tb=Tbt−Tbr.  
      Accordingly, the distance calculating unit  33   a  calculates the propagation time tp of impulses between the communication apparatuses  10   a - 1  and  10   b - 1  from the above equation (3) based on the Ta, which is the difference between Tar held by the reception time holding unit  32   a  and Tat held by the transmission time holding unit  31   a , and on Tb, which is received from the communication apparatus  10   b - 1 . Further, on the basis of the following equation (4), the distance calculating unit  33   a  calculates the distance Lab between the communication apparatus  10   a - 1  and the communication apparatus  10   b - 1 .
 
Lab= c·tp   (4)
 
 where c is the speed of light. 
 
      After the distance calculating unit  33   a  calculates the distance, as shown in  FIG. 8 , the pulse determining unit  34 - 1  determines the amplitude and the repetition frequency of impulses according to the distance calculated by the distance calculating unit  33   a  based on the table  35 - 1  [see (l) in  FIG. 8 ].  
      Here, the pulse determining unit  34 - 1  determines the power attenuation amount of the ATT  25   a  as the amplitude of impulses based on the table  35 - 1 , and also determines the 1-chip time due to the pulse frequency source  20   a  from the maximum PRF as the repetition frequency of impulses.  
      Subsequently, the communication apparatus  10   a - 1  transmits the determined amplitude and the repetition frequency of impulses to the communication apparatus  10   b - 1  as transmission data [see (m) of  FIG. 8 ]. The pulse controlling unit  36   a - 1  controls the ATT  25   a  and the pulse frequency source  20   a , thereby setting the amplitude and the repetition frequency of the impulses [see (n) of  FIG. 8 ].  
      When the communication apparatus  10   b - 1  receives the amplitude and the repetition frequency of impulses from the communication apparatus  10   a - 1 , the pulse controlling unit  36   b - 1  controls the ATT  25   b  and the pulse frequency source  20   b , thereby setting the amplitude and the repetition frequency of the received impulses (see (o) of  FIG. 8 ). The initial setting is thus completed.  
      The communication apparatus  10   a - 1  and the communication apparatus  10   b - 1  then transceive data therebetween using impulses with the amplitude and the repetition frequency having been set at the initial setting [see (p) and (q) of  FIG. 8 ].  
      After that, upon completion of transceiving data, the pulse controlling units  36   a - 1  and  36   b - 1  of the communication apparatuses  10   a - 1  and  10   b - 1 , respectively, set the amplitude of impulses to the maximum value and also set the repetition frequency to the minimum value, in preparation for initial setting for transceiving processing of the next data [see (r) and (s) of  FIG. 8 ].  
      In this manner, according to the radio communication system  1  (communication apparatuses  10   a - 1  and  10   b - 1 ) of the first embodiment of the present invention, the impulse adjusting unit  15 - 1  adjusts the amplitude and the repetition frequency of impulses used in radio communication in accordance with the distance between the communication apparatuses  10   a - 1  and  10   b - 1  detected by the distance detecting unit  10   d . Thus, in cases where the distance between the communication apparatuses  10   a - 1  and  10   b - 1  is large, even when the amplitude of impulses is increased because the reception scheme is a non-coherent scheme, the repetition frequency can also be adjusted, so that long distance communication is reliably realized while strictly observing the reference values such as the FCC mask.  
      In cases where the distance between the communication apparatuses  10   a - 1  and  10   b - 1  is short, the impulse adjusting unit  15 - 1  reduces the amplitude of impulses while increasing the repetition frequency of impulses, thereby realizing high-speed communication.  
      In this instance, the pulse determining unit  34 - 1  of the impulse adjusting unit  15 - 1  determines the amplitude and the repetition frequency of impulses used in communication based on the contents of the table  35 - 1 . By setting the table  35 - 1  with consideration paid to the reference values of the peak radiation power and the average radiation power, the impulse adjusting unit  15 - 1  is capable of adjusting the amplitude of impulses so that the peak radiation power becomes equal to or smaller than a specified value, and is also capable of setting the repetition frequency of impulses so that the average radiation power becomes equal to or smaller than a specified value.  
      Further, since the distance calculating unit  33   a  of the distance detecting unit  14  detects the distance between the communication apparatuses  10   a - 1  and  10   b - 1  based on a propagation time which is required for impulses to travel therebetween, it is possible to reliably calculate the distance between the communication apparatuses  10   a - 1  and  10   b - 1  even if the timer  30   a  of the communication apparatus  10   a - 1  is not completely synchronized with the timer  30   b  of the communication apparatus  10   b - 1 .  
      Furthermore, in the radio communication system  1 , at the initial setting time, since the distance detecting unit  14  uses impulses with the maximum amplitude and the minimum repetition frequency as impulses for detecting the distance between the communication apparatuses  10   a - 1  and  10   b - 1 . Thus, even when the distance therebetween is the maximum distance the radio communication system  1  can support, it is still possible to reliably execute the initial setting, thereby setting the amplitude and the repetition frequency appropriately.  
     [2] Second Embodiment  
      Next, referring to the block diagram of  FIG. 10 , a description will be made hereinbelow of a construction of a radio communication system according to a second embodiment of the present invention. In  FIG. 10 , like reference characters designate elements the same as or similar to elements already described, so their detailed description is omitted here.  
      As shown in  FIG. 10 , the present radio communication system  2  includes multiple (here, two) communication apparatuses ( 10   a - 2  and  10   b - 2 ). The system construction is similar to that of the radio communication system  1  of the above-described first embodiment except for the distance detecting unit  14   b - 2  and the impulse adjusting unit  15 - 2 . In this instance, in the radio communication system  1  of the first embodiment already described, the distance detecting unit  14  includes the first distance detecting unit  14   a  of the communication apparatus  10   a - 1  and the second distance detecting unit  14   b - 1  of the communication apparatus  10   b - 1 . However, in the radio communication system  2 , a distance detecting unit  14   b - 2  is provided for a communication apparatus  10   b - 2 .  
      Further, in the radio communication system  2 , the impulse adjusting unit  15 - 2  includes a first impulse adjusting unit  15   a - 2  of the communication apparatus  10   a - 2  and a second impulse adjusting unit  15   b - 2  of the communication apparatus  10   b - 2 .  
      Hereafter, a description will be made in detail of the distance detecting unit  14   b - 2  and the impulse adjusting unit  15 - 2 .  
      That is, the distance detecting unit  14   b - 2  of the radio communication system  2  measures the distance between the communication apparatuses  10   a - 2  and  10   b - 2  by the RSS (Receive Single Strength) scheme. More specifically, on the basis of the electric power which is sent from one (communication apparatus  10   a - 2 ) of the two communication apparatuses and is then received by the other one (communication apparatus  10   b - 2 ), the distance between the communication apparatuses  10   a - 2  and  10   b - 2  is detected.  
      Here, referring to the block diagram of  FIG. 11 , a description will be made hereinbelow of a construction of the communication apparatus  10   a - 2  of the radio communication system  2 . In addition, referring to the block diagram of  FIG. 12 , a description will be made hereinbelow of a construction of the communication apparatus  10   b - 2  of the radio communication system  2 . In  FIG. 11  and  FIG. 12 , like reference characters designate elements the same as or the similar to those already described, so their detailed description will be omitted here.  
      As shown in  FIG. 11 , the communication apparatus  10   a - 2  includes: a PA  12   a ; an antenna  13   a ; an LNA  16   a ; a pulse detecting unit  17   a ; a pulse frequency source  20   a ; a PN sequence generating unit  21   a ; a PPM data modulating unit  22   a ; an impulse generating unit  23   a ; a BPF  24   a ; an ATT  25   a ; a correlator  26   a ; a PPM data modulating unit  27   a ; and a pulse controlling unit  36   a - 2 .  
      To transmit impulses with the amplitude and the frequency used in radio communication, which are received from the communication apparatus  10   b - 2 , the pulse controlling unit  36   a - 2  controls the pulse frequency source  20   a  and the ATT  25   a . Here, the pulse controlling unit  36   a - 2 , and the pulse frequency source  20   a , the ATT  25   a , function as a first impulse adjusting unit  15   a - 2 .  
      As shown in  FIG. 12 , the communication apparatus  10   b - 2  includes a PA  12   b ; an antenna  13   b ; an LNA  16   b ; a pulse detecting unit  17   b ; a pulse frequency source  20   b ; a PN sequence generator  21   b ; a PPM data modulating unit  22   b ; an impulse generating unit  23   b ; a BPF  24 ; an ATT  25   b ; a correlator  26   b ; a PPM data demodulating unit  27   b ; a reception power detecting unit (power detecting unit)  37 ; a distance calculating unit  33   b ; a table  35 - 2 ; a pulse determining unit  34 - 2 ; and a pulse controlling unit  36   b - 2 .  
      The reception power detecting unit  37  detects impulses (here, a distance measurement command) transmitted from the communication apparatus  10   a - 2 , and detects the reception power when the preamble portion of data is received from the communication apparatus  10   a - 2  by the correlator  26   b.    
      Here, the reception power detecting unit  37  detects the power of impulses after the impulses passes through the LNA  16   b  as a reception power.  
      Then, the distance calculating unit  33   b  detects the distance between the communication apparatuses  10   a - 2  and  10   b - 2  according to the power detected by the reception detecting unit  37 , based on the table  35 - 2  constructed as shown in  FIG. 13 . Here, as shown in  FIG. 13 , the table  35 - 2  holds 9-stage distances (“84.9 m”, “60 m”, “42.4 m”, “30 m”, “21.2 m”, “15 m”, “10.6 m”, “7.5 m”, and “5.3 m”) corresponding to 9-stage reception power (“−85 dBm”, “−82 dBm”, “−79 dBm”, “−76 dBm”, “−73 dBm”, “−70 dBm”, “−67 dBm”, “−64 dBm”, and “−61 dBm”), respectively.  
      Further, as in the case of the above-described pulse determining unit  34 - 1 , the table  35 - 2  holds the power attenuation amount of the ATT  25   a  corresponding to each distance, the maximum PRF, and 1-chip time.  
      Here, the reception power detected by the reception power detecting unit  37  is greater than −61 dBm, the distance calculating unit  33   b  decides that the distance between the communication apparatuses  10   a - 2  and  10   b - 2  is shorter than 5.3 m, based on the table  35 - 2 .  
      When the distance calculating unit  33   b  detects the distance between the communication apparatus  10   a - 2  and communication apparatus  10   b - 2  is shorter than 5.3 m, the pulse determining unit  34 - 2  determines the amplitude and the repetition frequency of impulses based on the table  35 - 2 . In this case, the pulse determining unit  34 - 2  sets the power attenuation amount of the ATT  25   b , as the amplitude of impulses, to −24 dB, and sets the 1-chip time due to the pulse frequency source  20   b  to 8ns, regarding the maximum PRF as the repetition frequency of impulses to be 13.5 MHz.  
      When the power detected by the reception power detecting unit  37  is lower than −61 dBm inclusive and also larger than 64 dBm inclusive, the distance calculating unit  33   b  determines that the distance is 7.5 m, based on table  35 - 2 . The pulse determining unit  34 - 2  then determines the amplitude (the ATT  25   b  is −21 dB) and the repetition frequency of impulses (the maximum PRF is 8.8 MHz; 1-chip time of the pulse frequency source  20   b  is 12 ns) corresponding to a distance of 7.5 m.  
      In this manner, in the communication apparatus  10   b - 2 , the reception power detecting unit  37  and the distance calculating unit  33   b  function as a distance detecting unit  14   b - 2 . The pulse determining unit  34 - 2 , the pulse controlling unit  36   b - 2 , the pulse frequency source  20   b , the ATT  25   b  function as a second impulse adjusting unit  15   b - 2 .  
      Next, referring to  FIG. 14 , a description will be made hereinbelow of the procedures (that is, communication procedures between the communication apparatuses  10   a - 2  and  10   b - 2 ) of processing carried out by the present radio communication system  2 .  
      First of all, the communication apparatus  10   a - 2  sets the amplitude of impulses used at the initial setting time, to the maximum value, and the repetition frequency of impulses to the minimum value [see (a) in  FIG. 14 ].  
      Subsequently, the communication apparatus  10   a - 2  uses impulses with the maximum amplitude and the minimum repetition frequency to transmit a distance measurement command for measuring the distance between the communication apparatuses  10   a - 2  and  10   b - 2 , between which communication is to be performed, to the communication apparatus  10   b - 2  [see (b) of  FIG. 14 ].  
      When the communication apparatus  10   b - 2  receives this distance measurement command, the reception power detecting unit  37  detects the power of impulses that have been received [see (c) of  FIG. 14 ]. (Concretely, the reception power detecting unit  37  detects the reception power when the preamble portion of the distance measuring command is detected by the correlator  26   b ).  
      Then, the distance calculating unit  33   b  detects the distance corresponding to the reception power detected by the reception power detecting unit  37  [see (d) of  FIG. 14 ]. On the basis of the table  35 - 2 , the pulse determining unit  34 - 2  determines the power attenuation amount of the ATT  25   b  as the amplitude of impulses corresponding to the distance detected by the distance calculating unit  33   b  and the 1-chip time due to the pulse frequency source  20   b  as the repetition frequency of impulses [see (e) of  FIG. 14 ].  
      After that, as in the case of the communication apparatus  10   a - 2 , the communication apparatus  10   b - 2  sets the amplitude of impulses to the maximum value, and also sets the repetition frequency of impulses to the minimum value [see (f) of  FIG. 14 ], and sends the power attenuation amount of the ATT  25   a  and the 1-chip time of the pulse frequency source  20   b  to the communication apparatus  10   a - 2  [see (g) of  FIG. 14 ].  
      In this instance, in the communication apparatus  10   b - 2 , the pulse controlling unit  36   b - 2  controls the ATT  25   b  and the pulse frequency source  20   b  to perform setting of the amplitude and the repetition frequency determined by the pulse determining unit  34 - 2  [see (h) of  FIG. 14 ].  
      When the communication apparatus  10   a - 2  receives the amplitude and the repetition frequency of impulses from the communication apparatus  10   b - 2 , the pulse controlling unit  36   a - 2  controls the ATT  25   a  and pulse frequency source  20   a  to perform setting of the amplitude and the repetition frequency [see (i) of  FIG. 14 ], thereby completing initial setting.  
      After that, the communication apparatuses  10   a - 2  and  10   b - 2  use the amplitude and the repetition frequency of impulses, which have been set at initial setting, to transceive data therebetween [see (j) and (k) of  FIG. 14 ].  
      Upon completion of data transceiving, the pulse controlling units  35  and  35   b - 2  of the communication apparatuses  10   a - 2  and  10   b - 2 , respectively, set the amplitude of impulses to the maximum value in order to restore the initial setting value of impulses, and also sets the repetition frequency of impulses to the minimum value [see (l) and (m) of  FIG. 14 ].  
      In this manner, according to the radio communication system  2  of the second embodiment of the present invention, the above described effects are obtained. In addition, although the reception power detected by the reception power detecting unit  37  cannot sometimes be correctly measured because of circumstances such as multipath or blocking, the pulse determining unit  34 - 2  sets the amplitude and the repetition frequency of impulses including such circumstances. Thus, it is possible to set the amplitude and the repetition frequency in conformity with such circumstances. As a result, communication between the communication apparatus  10   a - 2  and the communication apparatus  10   b - 2  can be reliably realized.  
      Further, according to the radio communication system  2 , when the communication apparatus  10   b - 2  receives impulses as a distance measurement command sent from the communication apparatus  10   a - 2 , the communication apparatus  10   b - 2  determines the amplitude and the repetition frequency of impulses to be used in radio communication based on the reception power of the impulses. Thus, the processing procedures required at the initial setting are reduced in comparison with those in the above-described first embodiment, so that the time required for performing the initial setting is decreased.  
     [3] Third Embodiment:  
      Next, a description will be made hereinbelow of a radio communication system according to a third embodiment of the present invention. As shown in  FIG. 15 , the present radio communication system  3  includes communication apparatuses  10   a - 2  and  10   b - 3 , and its construction is similar to that of the second embodiment except for a reception detecting unit (power detecting unit)  37  and an impulse adjusting unit  15 - 3 . In  FIG. 15 , like reference characters designate elements the same as or similar to elements already described, so their detailed description is omitted here.  
      That is, in the radio communication system  2  of the second embodiment, the distance detecting unit  14   b - 2  detects the distance between the communication apparatuses  10   a - 2  and  10   b - 2 . On the basis of the detected distance, the impulse adjusting unit  15 - 2  determines the amplitude and the repetition frequency of impulses. According to the third embodiment, however, the communication apparatus  10   b - 3  does not detect the distance between the communication apparatuses  10   a - 2  and  10   b - 3 , and the impulse adjusting unit  15 - 3  directly determines the amplitude and the repetition frequency of impulses used in radio communication between the communication apparatuses  10   a - 2  and  10   b - 3  according to the power detected by the reception power detecting unit (power detecting unit)  37  and performs controlling (adjustment).  
      In this instance, the impulse adjusting unit  15 - 3  includes a first impulse adjusting unit  15   a - 2  and a second impulse adjusting unit  15   b - 3  of the communication apparatus  10   a - 2  and the first impulse adjusting unit  15   a - 2 , respectively.  
      Further, in the radio communication system  3 , the communication apparatus  10   a - 2  has a construction similar to that of the communication apparatus  10   a - 2  of the second embodiment as shown in  FIG. 11 , and thus, its detailed description is omitted here.  
      Now, referring to the block diagram of  FIG. 16 , a description will be made hereinbelow of a construction of the communication apparatus  10   b - 3 . As shown in  FIG. 16 , the communication apparatus  10   b - 3  detects the power of impulses as the reception power when the preamble portion of data received from communication apparatus  10   a - 2  is detected by the correlator  26   b.    
      Then, a pulse determining unit  34 - 3  determines the amplitude and the repetition frequency of impulses according to the reception power detected by the reception power detecting unit  37  based on a table  35 - 3  constructed as shown in  FIG. 17 .  
      Here, the table  35 - 3  of  FIG. 17  does not have an item of distance in comparison with the table  35 - 2  held by the communication apparatus  10   b - 2  of the second embodiment as shown in  FIG. 13 . Except for this point, the table  35 - 3  has a construction similar to that of the table  35 - 2 . Hence, the table  35 - 3  is used in a way similar to the above-described table  35 - 2 .  
      Further, in the communication apparatus  10   b - 3 , a pulse controlling unit  36   b - 3  controls the pulse frequency source  20   b  and the ATT  25   b  in such a manner that the amplitude and the repetition frequency of impulses output from the communication apparatus  10   a - 2  become those that are determined by the table  35 - 3  is obtained.  
      In this manner, in the communication apparatus  10   b - 3 , the pulse frequency source  20   b , the ATT  25   b , the pulse determining unit  34 - 3 , and the pulse controlling unit  36   b - 3  function as a second impulse adjusting unit  15   b - 3 .  
      Accordingly, as shown in  FIG. 18 , the processing procedures (that is, the communication procedures between the communication apparatus  10   a - 2  and the communication apparatus  10   b - 3 ) performed by the radio communication system  3  does not include the procedure of detecting the distance between the communication apparatus  10   a - 2  and the communication apparatus  10   b - 2  as shown in  FIG. 14  [see (d) of  FIG. 14 ]. When the reception power detecting unit  37  detects the power of impulses as a power measurement command from the communication apparatus  10   a - 2  [see (c) of  FIG. 18 ], the pulse determining unit  34 - 3  directly determines the amplitude and the repetition frequency of impulses according to the detected power of impulses based on the table  35 - 3  [see (e) of  FIG. 18 ].  
      In this manner, according to the radio communication system  3  of the third embodiment of the present invention, like effects and benefits to those of the second embodiment will be realized, and also, the efficiency of an initial operation is improved because the process of detecting the distance between the communication apparatuses  10   a - 2  and  10   b - 3  is omitted in comparison with the above-described second embodiment.  
     [4] Fourth Embodiment  
      Next, a description will be made of a radio communication system of the fourth embodiment of the present invention. As shown in  FIG. 19 , the present radio communication system  4  includes communication apparatuses  10   a - 4  and  10   b - 4 , and its construction is similar to that of the radio communication system  1  of the first embodiment except for an impulse adjusting unit  15 - 4  and a minimum amplitude detecting unit  18 . In  FIG. 19 , like reference characters designate elements the same as or similar to elements already described, so their detailed description is omitted here.  
      Now, a detailed description will be made hereinbelow of the minimum amplitude detecting unit  18  and the impulse adjusting unit  15 - 4 .  
      The minimum amplitude detecting unit  18  includes a first minimum amplitude detecting unit  18   a  of the communication apparatus  10   a - 4  and a second minimum amplitude detecting unit  18   b . The minimum amplitude detecting unit  18  detects the minimum amplitude of impulses sent from one (here, the communication apparatus  10   a - 4 ) of the communication apparatuses  10   a - 4  and  10   b - 4  which can be received by the other one (here, the communication apparatus  10   b - 4 ) of the communication apparatuses  10   a - 4  and  10   b - 4 .  
      More specifically, the minimum amplitude detecting unit  18  attenuates the amplitude level (radio wave intensity) of impulses sent from the communication apparatus  10   a - 4  in stages, and detects the amplitude of an impulse which has been sent from the communication apparatus  10   a - 4  immediately before the communication apparatus  10   b - 4  becomes unable to correctly receive the impulses, as the minimum amplitude.  
      The impulse adjusting unit  15 - 4  includes a first impulse adjusting unit  15   a - 4  of the communication apparatus  10   a - 4  and a second impulse adjusting unit  15   b - 4  of the communication apparatus  10   b - 4 . In accordance with the minimum amplitude of impulses detected by the minimum amplitude detecting unit  18 , the impulse adjusting unit  15 - 4  adjusts the amplitude and the repetition frequency of impulses used in radio communication between the communication apparatuses  10   a - 4  and  10   b - 4 .  
      More concretely, the impulse adjusting unit  15 - 4  sets the amplitude of impulses used in communication to a value greater than the minimum amplitude detected by the minimum amplitude detecting unit (an amplitude greater by one stage than the amplitude detected as the minimum amplitude, when the minimum amplitude detecting unit  18  attenuates the amplitude level in stages), and also sets the repetition frequency corresponding to the amplitude having been set).  
      Here, referring to  FIG. 20 , a description will be made hereinbelow of a construction of communication apparatus  10   a - 4  of the present radio communication system  4 . In this instance, in  FIG. 20 , like reference characters designate elements the same as or similar to elements already described, so their detailed description is omitted here.  
      As shown in  FIG. 20 , the communication apparatus  10   a - 4  includes: a PA  12   a ; an antenna  13   a ; an LNA  16   a , a pulse detecting unit  17   a ; a pulse frequency source  20   a ; a PN sequence generating unit  21   a ; a PPM data modulating unit  22   a ; an impulse generating unit  23   a ; a BPF  24   a ; an ATT  25   a ; a correlator  26   a ; a PPM data modulating unit  27   a ; a judging unit  38 ; a pulse determining unit  34 - 4 ; and a pulse controlling unit  36   a - 4 .  
      The judging unit  38  evaluates whether or not the communication apparatuses  10   b - 4 -correctly receives a reception confirmation command for confirming correct reception of a reception confirmation command sent from the communication apparatus  10   a - 4  at the initial setting time, and has a timer  39 .  
      The timer  39  detects an elapse of a specified time after the transmission of a reception confirmation command by the communication apparatus  10   a - 4 .  
      Now, a description will be made hereinbelow of a decision-making method of the judging unit  38 . After sending out a reception confirmation command to the communication apparatuses  10   b - 4 , if the judging unit  38  receives a reception confirmation command (success response) indicating a successful reception of the reception confirmation command from the communication apparatus  10   b - 4  before the timer  39  detects the elapse of a specific time, the judging unit  38  decides that the communication apparatus  10   b - 4  has correctly received the reception confirmation command. In this instance, when receiving a successful response from the communication apparatus  10   b - 4 , the judging unit  38  resets the timer  39 .  
      On the other hand, after transmission of a reception confirmation command, if the timer  39  detects an elapse of a specified time (that is, if a reception confirmation command is not received from the communication apparatus  10   b - 4  within a specified time period after transmission of a reception confirmation command), it is decided that the communication apparatus  10   b - 4  has not been correctly received.  
      Since the judging unit  38  makes a judgment in this manner, the specified time period measured by the timer  39  is set to a sufficiently long time, with consideration paid to the time period required for the communication apparatus  10   b - 4  to generate a successful response after the communication apparatus  10   b - 4  receives a reception confirmation command and to the time required for the successful response to arrive at the communication apparatus  10   a - 4 .  
      Here, cases where the communication apparatus  10   b - 4  is not capable of receiving a reception confirmation command correctly mean a case where the communication apparatus  10   b - 4  can only receive a part of a reception confirmation command, or a case where an error rate becomes greater than a predetermined specific value, or a case where a reception confirmation command is not received at all and the correlator  26   b  (see  FIG. 22 ) cannot perform synchronization.  
      While the judging unit  38  keeps deciding that a reception confirmation command has been correctly received by the communication apparatus  10   b - 4 , the pulse determining unit  34 - 4  determines the amplitude level of impulses as a reception confirmation command so that the amplitude level is attenuated in stages every time such a judgment is made.  
      That is, when the judging unit  38  decides that the communication apparatuses  10   b - 4  has correctly received a reception confirmation command, the pulse determining unit  34 - 4  sets the amplitude level lower than the amplitude level of impulses as a reception confirmation command as an amplitude level of the reception confirmation command which is to be subsequently transmitted.  
      More specifically, the pulse determining unit  34 - 4  has a table  35 - 4  as shown in  FIG. 21 . The pulse determining unit  34 - 4  sets impulses, as an initial reception confirmation command at the initial setting time, to the maximum amplitude (that is, the power attenuation amount of the ATT  25   a  is “0 dB”) and to the minimum repetition frequency (that is, the maximum PRF is “0.68 MHz”). After that, every time the judging unit  38  decides that a reception confirmation command has been correctly received, the pulse determining unit  34 - 4  attenuates the amplitude level of impulses as a reception response command to be subsequently transmitted in stages (here, the power attenuation amount of the ATT  25   a  is attenuated by −3 dB) based on the table  35 - 4 .  
      When the judging unit  38  decides that the communication apparatuses  10   b - 4  has not received a reception confirmation command correctly, the pulse-determining unit  34 - 4  detects the minimum amplitude which can be correctly received by the communication apparatus  10   b - 4  as the amplitude level (here, the amplitude level larger than the reception confirmation command by one stage) of the reception confirmation command transmitted to the communication apparatus  10   b - 4  immediately before the reception confirmation command.  
      That is, in this example, the pulse determining unit  34 - 4  detects any one of the nine stages of setting values of the ATT  25   a  as the minimum amplitude of impulses.  
      Further, the pulse determining unit  34 - 4  determines the detected minimum amplitude or an amplitude further greater than the minimum amplitude level (for example, the amplitude level greater than the minimum amplitude level by one or more stages) as the amplitude of impulses used in radio communication.  
      For example, when the detected minimum amplitude of the ATT  25   a  is a power attenuation amount of “−12 dB”, the pulse determining unit  34 - 4  determines the amplitude of impulses used in radio communication as a power attenuation amount of the ATT  25   a  of “−12 dB”, “−9 dB”, “−6 dB”, or “−3 dB”. In this manner, by setting the amplitude of impulses to a value greater than the minimum amplitude (giving a margin), communication between the communication apparatuses  10   a - 4  and  10   b - 4  is reliably performed.  
      Then, on the basis of the table  35 - 4 , the pulse determining unit  34 - 4  determines the repetition frequency (here, the 1-chip time due to the pulse frequency source  20   a  corresponding to the maximum PRF) of impulses corresponding to the determined amplitude (here, the power attenuation amount of the ATT  25   a ).  
      The pulse controlling unit  36   a - 4  controls the pulse frequency source  20   a  and the ATT  25   a  based on the power attenuation amount of the ATT  25   a  and the maximum PRF (1-chip time) determined by the pulse determining unit  34 - 4 .  
      The pulse controlling unit  36   a - 4  controls impulses as a reception confirmation command at the amplitude level determined by the pulse determining unit  34 - 4  at the initial setting.  
      In this manner, in the communication apparatus  10   a - 4 , the judging unit  38 , the pulse determining unit  34 - 4 , the pulse controlling unit  36   a - 4 , the pulse frequency source  20   a , and ATT  25   a , function as a first minimum amplitude detecting unit  18   a . Further, the pulse determining unit  34 - 4 , the pulse controlling unit  36   a - 4 , the pulse frequency source  20   a , and the ATT  25   a  also function as a first impulse impulse-adjusting unit  15   a - 4 .  
      Next, referring to  FIG. 22 , a description will be made hereinbelow of a construction of a communication apparatus  10   b - 4  in the present communication system  4 . In  FIG. 22 , like reference characters designate elements the same as or similar to elements already described, so their detailed description is omitted here.  
      As shown in  FIG. 22 , the communication apparatus  10   b - 4  includes: a PA  12   b ; an antenna  13   b ; an LNA  16   b ; a pulse detecting unit  17   b ; a pulse frequency source  20   b ; a PN sequence generator  21   b ; a PPM data modulating unit  22   b ; an impulse generating unit  23   b ; a BPF  24   b ; an ATT  25   b ; a correlator  26   b ; a PPM data demodulating unit  27   b ; and a pulse controlling unit  36   b - 4 .  
      The pulse controlling unit  36   b - 4  controls the pulse frequency source  20   b  and the ATT  25   b  based on the amplitude and the repetition frequency of impulses determined by the pulse determining unit  34 - 4  of the communication apparatus  10   a - 4 , which impulses have been received from the communication apparatus  10   a - 4  as reception data.  
      Accordingly, in the communication apparatus  10   b - 4 , the pulse controlling unit  36   b - 4  functions as a second minimum amplitude detecting unit  18   b , and the pulse controlling unit  36   b - 4 , the pulse frequency source  20   b , and the ATT  25   b  function as a second impulse adjusting unit  15   b - 4 .  
      In this instance, when receiving a reception confirmation command from the communication apparatus  10   b - 4  at the initial setting time, the communication apparatus  10   b - 4  transmits a reception confirmation command response to the communication apparatus  10   a - 4  in response to the reception confirmation command.  
      Next, referring to  FIG. 23 , a description will be made hereinbelow of the processing procedures (that is, communication procedures between the communication apparatuses  10   a - 4  and  10   b - 4 ) of the present radio communication system  4 .  
      First of all, the pulse controlling unit  36   a - 4  of the communication apparatus  10   a - 4  sets the amplitude of impulses used in the initial setting to the maximum value, and also sets the repetition frequency of the impulses to the minimum value [see (a) of  FIG. 23 ].  
      Subsequently, the communication apparatus  10   a - 4  uses the maximum amplitude and the minimum repetition frequency of impulses to transmit a reception confirmation command for confirming the reception to the communication apparatus  10   b - 4  [see (b) of  FIG. 23 ]. In this instance, the timer  39  of the judging unit  38  starts counting a specific time period elapsed.  
      When the communication apparatus  10   b - 4  is capable of normally receiving the reception confirmation command (impulses) sent from the communication apparatus  10   a - 4  because the reception confirmation command has a sufficient radio wave intensity (amplitude) (that is, the correlator  26   b  is capable of obtaining correlation) [see (c) of  FIG. 23 ], the pulse controlling unit  36   b - 4  of the communication apparatus  10   b - 4  sets the amplitude of impulses to the maximum value (sets the ATT  25   b  to “+0 dB”), as in the case of the processing (a) of the communication apparatus  10   a - 4 , and also controls the pulse frequency source  20   b  so that the repetition frequency of impulses to the minimum value (1-chip is equal to 148 ns) [see (d) of  FIG. 23 ].  
      After that, the communication apparatus  10   b - 4  sends a successful response (reception confirmation command response) to the communication apparatus  10   a - 4  in response to the reception confirmation command [see (e) of  FIG. 23 ].  
      If the communication apparatus  10   a - 4  receives a successful response from the communication apparatus  10   b - 4  before the timer  39  detects an elapse of a specified time period, the judging unit  38  decides that the communication apparatus  10   b - 4  has correctly received the successful response, and resets the timer  39 . In addition, the pulse determining unit  34 - 4  attenuates the amplitude level of impulses (the power attenuation amount of the ATT  25   a ) by one stage based on the table  35 - 4  [amplitude level down; see (f) of  FIG. 23 ]. In this instance, the repetition frequency of impulses maintains the level of the minimum repetition frequency having been set in the above-mentioned process (a), and the repetition frequency will not be changed during the initial setting.  
      Next, the communication apparatus  10   a - 4  transmits a reception confirmation command once again to the communication apparatus  10   b - 4  at the amplitude level of impulses having been set at the procedure (f) [see (g) of  FIG. 23 ].  
      After that, the processing corresponding to the above processes (b), (c), (e), and (f), is repeated [see (h) (i), (j), and (k) of  FIG. 23 ].  
      Here, if the amplitude level of impulses, as a reception confirmation command, from the communication apparatus  10   a - 4  is decreased too much to be correctly received by the communication apparatus  10   b - 4  [see ( 1 ) of  FIG. 23 ], and the timer  39  of the judging unit  38  thus detects elapse of the specified time period after transmission of the reception confirmation command [when time-out for the reception confirmation command occurs; see (m) of  FIG. 23 ], the judging unit  38  decides that the communication apparatus  10   b - 4  can not receive the reception confirmation command.  
      Then, when the pulse determining unit  34 - 4  detects the amplitude level of the reception confirmation command transmitted immediately before the reception of the confirmation command (here, any of the power attenuation amounts of the ATT  25   a  shown in the table  35 - 4 ), as the minimum amplitude, and also determines the power attenuation amount of the ATT  25   a  to be greater than the detected minimum amplitude by one stage as the amplitude level of impulses used in radio communication between the communication apparatus  10   a - 4  and the communication apparatus  10   b - 4 .  
      Further, the pulse determining unit  34 - 4  determines the repetition frequency (1-chip unit time corresponding to the PRF) of impulses corresponding to the determined amplitude level (the power attenuation amount of the ATT  25   a ), based on the table  35 - 4  [see (n) of  FIG. 23 ].  
      Then, the pulse controlling unit  36   a - 4  sets the power attenuation amount of the ATT  25   a  to the attenuation amount determined by the pulse determining unit  34 - 4  [see (o) of  FIG. 23 ]. Further, the communication apparatus  10   a - 4  transmits the amplitude and the repetition frequency determined by the pulse determining unit  34 - 4  to the communication apparatus  10   b - 4  [see (p) of  FIG. 23 ].  
      At this time, in the communication apparatus  10   a - 4 , the pulse controlling unit  36   a - 4  controls the pulse frequency source  20   a , thereby setting the repetition frequency (1-chip unit time) [see (q) of  FIG. 23 ].  
      When the communication apparatus  10   b - 4  receives the amplitude and the repetition frequency of impulses from the communication apparatus  10   a - 4 , the pulse controlling unit  36   b - 4  controls the ATT  25   b  and the pulse frequency source  20   b , thereby setting the amplitude and the repetition frequency of the received impulses [see (r) of  FIG. 23 ]. Then, the initial setting is completed.  
      Then, the communication apparatuses  10   a - 4  and  10   b - 4  transceive data with each other using the amplitude and the repetition frequency set at the initial setting [see (s) and (t) of  FIG. 23 ].  
      After completion of transceiving of the data, the pulse controlling units  36   a - 4  and  36   b - 4  of the communication apparatuses  10   a - 4  and  10   b - 4 , respectively, sets the amplitude of impulses to the maximum value in preparation for initial setting, and also sets the repetition frequency of impulses to the minimum value [see (u) and (v) of  FIG. 23 ].  
      As described so far, in accordance with the radio communication system  4  of the fourth embodiment of the present invention, like effects to those of the above-described first embodiment are realized. In addition, it is possible to realize the radio communication system  4  with a construction which is more simple than that of the radio communication system  1  of the first embodiment and of the radio communication system  2  of the second embodiment.  
      Further, as in the case of the second embodiment, the amplitude and the repetition frequency of impulses eventually used is determined while the communication apparatus  10   a - 4  is transmitting impulses to the communication apparatus  10   b - 4 , so that setting of the amplitude and the repetition frequency of impulses is available with consideration paid to multipath and blocking.  
      [5] Other Modifications:  
      Further, the present invention should by no means be limited to the above-illustrated embodiment, and various changes or modifications may be suggested without departing from the gist of the invention.  
      For example, in the above-describe embodiments, the two communication apparatuses differ from each other in construction. The present invention, however, should by no means be limited to this. In the first embodiment, for example, the above-described radio communication system  1  can have the communication apparatus  10   a - 1  and the communication apparatus  10   b - 1  with similar construction. Further, in the second embodiment, the radio communication system  2  can have the communication apparatus  10   a - 2  and the communication apparatus  10   b - 2  with a similar construction. Furthermore, in the third embodiment, the radio communication system  3  can have the communication apparatus  10   a - 2  and the communication apparatus  10   b - 3  with a similar construction. Moreover, in the fourth embodiment, the radio communication system  4  can have the communication apparatus  10   a - 4  and the communication apparatus  10   b - 4  with a similar construction.  
      Further, according to the first and the second embodiments, the impulse adjusting units  15 - 1  and  15 - 2  adjust the amplitude of impulses based on the tables  35 - 1  and  35 - 2 . The present invention, however, should by no means be limited to this. For example, the impulse adjusting units  15 - 1  and  15 - 2  can adjust the amplitude of impulses to the reciprocal square root of the distance detected by the distance detecting units  14  and  14   b - 2 . With this arrangement, like effects to those of the first and the second embodiment will be realized.  
      Further, in the above-described second embodiment, the distance calculating unit  33   b  and the pulse determining unit  34 - 2  of the communication apparatus  10   b - 2  commonly use the table  35 - 2 . The present invention, however, should by no means be limited to this. For example, the table  35 - 2  can be divided so that the distance calculating unit  33   b  is constructed so as to execute processing based on the table indicating only the reception power and the distance, and that the pulse determining unit  34 - 2  is constructed so as to execute processing based on the table  35 - 1  of the first embodiment indicated in  FIG. 5 .  
      Further, in the above-described radio communication system  4 , the description was made, taking an example in which the pulse determining unit  34 - 4 , as the minimum amplitude detecting unit  18 , attenuates the amplitude of the reception confirmation command in stages based on the table  35 - 4 . The present invention, however, should by no means be limited to this in the method of changing the amplitude of impulses, as a reception confirmation command, at the initial setting time (that is, the minimum amplitude detection time) For example, the amplitude can be changed by two-divisional searching, thereby efficiently detecting the minimum amplitude.