Abstract:
A method of stabilizing frequency of an output signal of a data-relay apparatus provided in a radio communication system that includes a relay station and a radio terminal device, wherein the data-relay apparatus receives a signal as an input signal from the relay station, and transmits the signal as an output signal to the radio terminal device. The method includes the steps of demodulating the output signal to generate a demodulated signal by use of a first signal, detecting frequency deviation of the output signal from the demodulated signal, generating the first signal according to the detected frequency deviation of the output signal, and generating a second signal to mix with the input signal to generate the output signal according to the generation of the first signal and the second signal being carried out such that the detected frequency deviation of the output signal becomes substantially zero.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method and an apparatus for stabilizing frequency of a signal outputted from a relay station in a communication system, an more particularly to a method and an apparatus for stabilizing frequency of a signal outputted from a relay station in a telecommunication system and in a broadcasting system by use of a satellite. 
     2. Description of the Related Art 
     A telecommunication system that performs digital communication for mobile terminal devices and a broadcasting system that broadcasts programs for digital television by use of a relay station located at a high altitude such as a communication satellite or a balloon in the stratosphere have been suggested recently. 
     FIG. 1 is a block diagram showing a radio communication system using a communication satellite for digital communication between a base station and a radio terminal device. The radio communication system shown in FIG. 1 includes a base station  11 , a relay station  12 , a radio terminal device  13 , and data-relay apparatuses  14  and  15 . 
     The base station initially  11  modulates communication information, and transmits a code-division-multiplexed signal having a frequency f 1  to the relay station  12  such as a communication satellite. The relay station  12  coverts the signal having the frequency f 1  received from the base station  11  to a signal having a frequency f 2  and a signal having a frequency f 3 , and transmits those signals in a code-division-multiplex format to earth stations. 
     The radio terminal device  13  receives the signal having the frequency f 3  from the relay station  12 , and demodulates the communication information included therein. The data-relay apparatus  14  receives the signal having the frequency f 2  from the relay station  12 , and converts the received signal to a code-division-multiplexed signal having the frequency f 3 , then transmitting the converted signal to other devices such as the radio terminal device  13 . The data-relay apparatus  14  amplifies the received signal if necessary. The data-relay apparatus  15  receives the signal having the frequency f 3  from the relay station  12 , and amplifies the received signal in addition to attending to other signal processing, followed by transmitting the processed signal to other devices without converting the frequency of the received signal. 
     Accordingly, the radio terminal device  13  can receive the signal having the frequency f 3  from the relay station  12  through the apparatuses  14  and  15  even if the radio terminal device  13  is located in a blind zone. The above-described system may also be applied to digital-television broadcasting. In this case, the system substitutes a broadcasting station for the base station  11 . Additionally, a broadcasting satellite is used as the relay station  12 . 
     FIG. 2 is a block diagram showing a conventional data-relay apparatus  14  that converts frequency of a signal to another frequency, and transmits the signal to other devices. It should be noted that the above-described conventional data-relay apparatus  14  is, hereinafter, referred to as a conventional frequency-converting-data-relay apparatus  14 . 
     The conventional frequency-converting-data-relay apparatus  14  includes a data-receiving antenna  21 , an amplifier  22 , a mixer  23 , a generator  24 , a band-pass filter  25 , an amplifier  26  and a data-transmitting antenna  27 . In FIG. 2, frequency f 2 ′ is the frequency f 2  with an error Δf 2 . Similarly, frequency f 3 ′ is the frequency f 3  with an error Δf 3 . 
     The conventional frequency-converting-data-relay apparatus  14  initially receives a signal having the frequency f 2 ′ from the data-receiving antenna  21 , and amplifies the signal by use of the amplifier  22 . Subsequently, the conventional frequency-converting-data-relay apparatus  14  converts the frequency f 2 ′ of the signal amplified by the amplifier  22  to the frequency f 3 ′ by mixing the signal amplified by the amplifier  22  with a signal generated by the generator  24  by use of the mixer  23 . The frequency f 3 ′ includes the error Δf 3  of the frequency f 3  that is caused by the error Δf 2  of the frequency f 2  and performance of the generator  24 . 
     The conventional frequency-converting-data-relay apparatus  14  then filters the frequency f 3 ′ of the signal by use of the band-pass filter  25 . Additionally, the conventional frequency-converting-data-relay apparatus  14  amplifies the signal by use of the amplifier  26 , and then outputs the signal having the frequency f 3 ′ from the data-transmitting antenna  27 . 
     FIG. 3 is a block diagram showing a conventional data-relay apparatus  15  that transmits the signal to other devices without converting frequency of a signal to another frequency. The above-described conventional data-relay apparatus  15  includes a data-receiving antenna  31 , an amplifier  32 , a band-pass filter  33 , an amplifier  34  and a data-transmitting antenna  35 . 
     The conventional data-relay apparatus  15  initially receives a signal having the frequency f 3 ′ from the data-receiving antenna  31 , and amplifies the signal by use of the amplifier  32 . The conventional data-relay apparatus  15  then filters the frequency f 3 ′ of the signal by use of the band-pass filter  33 . Additionally, The conventional data-relay apparatus  15  amplifies the signal by use of the amplifier  34 , and then outputs the signal having the frequency f 3 ′ from the data-transmitting antenna  35 . 
     In the communication system described with reference to FIG. 1, frequency deviation occurs in a signal received by the radio terminal device  13  according to performance of generators provided in the base station  11 , the relay station  12  such as a satellite, and the data-relay apparatuses  14  and  15 . Additionally, the frequency of the signal received by the radio terminal device  13  is shifted by the Doppler shift that is caused by movement of the radio terminal device  13  and speed of the satellite relative to the earth. 
     Additionally, when the radio terminal device  13  simultaneously receives a code-division-multiplexed signal having the frequency f 3  from the relay station  12  and the signal having the frequency f 3 ′ from the data-relay apparatuses  14  and  15 , the radio terminal device  13  cannot receive the signal effectively if the frequency deviation of the signals outputted from the data-relay apparatuses  14  and  15  is large. Consequently, quality of the signal received by the radio terminal device  13  decreases. 
     In order to solve the above-described problems, the radio terminal device  13  may include functions to search frequency of the received signal and execute an automatic frequency control (AFC) on the frequency, thereby reducing the frequency deviation of the received signal before demodulating the received signal. However, the number of the radio terminal devices  13  in the above-described communication system is very large, especially in the broadcasting system. Thus, it is necessary to minimize production cost of the radio terminal device  13 . If the radio terminal device  13  is to execute a frequency search and the AFC, a circuit arrangement of the device  13  becomes complex, and the circuit scale becomes large. As a result, the production cost of the radio terminal device  13  increases. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a general object of the present invention to provide a method and an apparatus for stabilizing frequency of an output signal of a data-relay apparatus by minimizing frequency deviation of the output signal, thereby minimizing the size of a radio terminal device that receives the output signal from the data-relay apparatus. 
     The above-described objects of the present invention is achieved by a method of stabilizing frequency of an output signal of a data-relay apparatus provided in a radio communication system that includes a relay station and a radio terminal device, wherein the data-relay apparatus receives a signal as an input signal from the relay station, and transmits the signal as an output signal to the radio terminal device, the method including the steps of demodulating the output signal by use of a first signal to generate a demodulated signal, detecting frequency deviation of the output signal from the demodulated signal, generating the first signal according to the frequency deviation of the output signal, and generating a second signal to mix with the input signal to generate the output signal, the generation of the first signal and the second signal being carried out such that the detected frequency deviation of the output signal becomes substantially zero. 
     Accordingly, the data-relay apparatus stabilizes frequency of a signal inputted thereto, and transmits the signal having the stabilized frequency to the radio terminal device in the communication system, and thus the radio terminal device does not need to execute a frequency search and AFC control so that circuit structure of the radio terminal device can be simplified and its production cost can decrease. 
    
    
     Other objects, features and advantages of the present invention will become more 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 communication system using a communication satellite for digital communication between a base station and a radio terminal device; 
     FIG. 2 is a block diagram showing a conventional data-relay apparatus that relays a received signal to other devices after converting frequency of the received signal to another frequency; 
     FIG. 3 is a block diagram showing a conventional data-relay apparatus that relays a received signal to other devices without converting frequency of the received signal to another frequency; 
     FIG. 4 is a block diagram showing a data-relay apparatus  14  that relays a received signal to other devices after converting frequency of the received signal to another frequency, according to a first embodiment of the present invention; 
     FIG. 5 is a block diagram showing an embodiment of a frequency-deviation-detecting unit  49  provided in the data-relay apparatus  14 ; 
     FIG. 6 is a graph showing characteristic of frequency discrimination; 
     FIG. 7 is a data-relay apparatus  15  that relays a received signal to other devices without converting frequency of the received signal to another frequency, according to a second embodiment of the present invention; and 
     FIG. 8 is the data-relay apparatus  14  according to a third embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A description will now be given of preferred embodiments of the present invention, with reference to the accompanying drawings. 
     In a communication or broadcasting system using a satellite such as a geostationary satellite, as shown in FIG. 1 wherein a code division multiplex (CDM) is adopted to, a base station  11  or a broadcasting station  11  transmits various information for telecommunication or for broadcasting as a signal having a frequency f 1  to a relay station  12  such as a satellite, after separating the information and modulating the separated information digitally by using a phase shift keying (PSK), for example. The relay station  12  coverts the signal having the frequency f 1  received from the base station  11  or the broadcasting station  11  to the signal having a frequency f 2  and the signal having a frequency f 3 . The frequency f 2  and the frequency f 3  are respectively set to 12 GHz and 2.6 GHz, for instance. 
     A radio terminal device  13  receives the signal having the frequency f 3  from the relay station  12 , and demodulates the information for the communication or for the broadcasting. A data-relay apparatus  14 , as a relay station, receives the signal having the frequency f 2  from the relay station  12 , and converts the signal having the frequency f 2  to the signal having the frequency f 3  therein, followed by transmitting the converted signal to the radio terminal device  13 . If amplification of the signal received from the relay station  12  is necessary, the data-relay apparatus  14  amplifies the received signal, and then transmits the amplified signal therefrom. A data-relay apparatus  15  receives the signal having the frequency f 3  from the relay station  12 , and amplifies the received signal, followed by transmitting the amplified signal therefrom to the radio terminal device  13 . 
     FIG. 4 is a block diagram showing the data-relay apparatus  14  that relays a signal received from the relay station  12  to other devices such as the radio terminal device  13  after converting frequency of the received signal to another frequency, according to a first embodiment of the present invention. 
     The data-relay apparatus  14  includes a data-receiving antenna  41 , an amplifier  42 , a generator  43 , a mixer  44 , a band-pass filter  45 , a splitter  46 , a generator  47 , a demodulator  48 , a frequency-deviation-detecting unit  49 , a voltage-controlled oscillator  50 , an amplifier  51  and a data-transmitting antenna  52 . The generator  47 , the demodulator  48 , the frequency-deviation-detecting unit  49  and the voltage-controlled oscillator  50  together compose a receiver  53 . The generators  43  and  47 , a combination of the generator  43  and the mixer  44 , the demodulator  48  and the frequency-deviation-detecting unit  49  correspond respectively to frequency-multiplication units, a frequency-conversion unit, a demodulation unit and a control unit in the claims. 
     The data-relay apparatus  14  initially receives a signal having the frequency f 2  from the data-receiving antenna  41 , and supplies the signal to the amplifier  42 , where the signal is amplified. The device  14  then supplies the signal from the amplifier  42  to the mixer  44 . The mixer  44  mixes the signal having the frequency f 2  and a signal having a frequency fs 1  generated by the generator  43 , and outputs the signal having a frequency approximately equal to the frequency f 3  (f 3 =f 2 −fs 1 ). This frequency approximately equal to the frequency f 3  is referred to as a frequency f 3 ′. Subsequently, the data-relay apparatus  14  removes unnecessary parts of the frequency f 3 ′ by filtering the signal through the band-pass filter  45 . The device  14  then supplies the signal having the frequency f 3 ′ to the splitter  46 , where the signal is supplied to the amplifier  51  and the demodulator  48 . The device  14  amplifies the signal having the frequency f 3 ′ by use of the amplifier  51 , and outputs the amplified signal from the data-transmitting antenna  52  to other devices such as the radio terminal device  13 . 
     The receiver  53  carries out AFC control as described below. The demodulator  48  receives the signal having the frequency f 3 ′ from the splitter  46 , and demodulates the signal to a base-band signal by use of a signal generated by the generator  47 . The base-band signal is then supplied to the frequency-deviation-detecting unit  49 . Subsequently, the frequency-deviation-detecting unit  49  detects frequency deviation of the base-band signal, and transmits a detection signal notifying of the detected frequency deviation. A frequency fvco generated by the voltage-controlled oscillator  50  is altered according to the value of the detection signal supplied from the frequency-deviation-detecting unit  49 . 
     Additionally, the voltage-controlled oscillator  50  supplies a signal having the frequency fvco to the generators  43  and  47 . The generator  47  generates a signal having a frequency fs 2 , and supplies the signal to the demodulator  48  such that the frequency deviation of the signal detected by the frequency-deviation-detecting unit  49  is removed by the AFC control. The generator  43  outputs a signal having the frequency fs 1  to the mixer  44  so that the frequency f 2  is mixed with the frequency fs 1  for converting the frequency f 2  to the frequency f 3 ′. The frequency fs 1  is controlled by the receiver  53 , and more particularly by the voltage-controlled oscillator  50  so that the frequency f 3 ′ becomes close to the frequency f 3 . 
     FIG. 5 is a block diagram showing an embodiment of the frequency-deviation-detecting unit  49 , whereto a cross-product AFC circuit is adopted. The frequency-deviation-detecting unit  49  includes orthogonal detectors  101  and  102 , low-pass filters (LPF)  103  and  104 , delay circuits  105  and  106 , multipliers  107  and  108 , a subtractor  109 , a loop filter  10 , a voltage-controlled oscillator (VCO)  111  and a phase shifter  112 . 
     The orthogonal detectors  101  and  102  initially receive the base-band signal as a signal S(t) from the demodulator  48 . The orthogonal detector  101  is supplied additionally with a signal Si(t) generated by the voltage-controlled oscillator  111 . The orthogonal detector  102  is supplied additionally with a signal Sq(t) that is the signal Si(t) with its phase being shifted by π/2 by the phase shifter  112 . The orthogonal detector  101  multiplies the signal S(t) and the signal Si(t), executes orthogonal detection of the multiplied signal and then supplies the detected signal to the low-pass filters  103 . Similarly, The orthogonal detector  102  multiplies the signal S(t) and the signal Sq(t), executes orthogonal detection of the multiplied signal, and then supplies the detected signal to the low-pass filters  104 . 
     The low-pass filter  103  eliminates unnecessary parts of the frequency Si(t) supplied from the orthogonal detector  101 , in other words, cuts off a high frequency part of the frequency Si(t), and supplies the signal as a signal Bi(t) to the multiplier  108  and the delay circuit  105 . The delay circuit  105  holds the signal Bi(t) for a certain period τ, and then supplies the signal Bi(t) to the multiplier  107 . The low-pass filter  104  eliminates unnecessary parts of the frequency Sq(t) supplied from the orthogonal detector  102 , in other words, cuts off a high frequency part of the frequency Sq(t), and supplies the signal as a signal Bq(t) to the multiplier  107  and the delay circuit  106 . The delay circuit  106  holds the signal Bq(t) for the certain period τ, and then supplies the signal Bi(t) to the multiplier  108 . 
     The multiplier  107  multiplies the delayed signal Bi(t) from the delay circuit  105  and the signal Bq(t) from the low-pass filter  104 , and then supplies the result to the subtractor  109 . Similarly, the multiplier  108  multiplies the signal Bi(t) from the low-pass filter  103  and the delayed signal Bq(t) from the delay circuit  106 , and then supplies the result to the subtractor  109 . Subsequently, the subtractor  109  subtracts the signal that is supplied from the multiplier  107 , from the signal that is supplied from the multiplier  108 , and outputs the result as a frequency-discriminated signal Rc(t) to the loop filter  110 . The loop filter  110  extracts a low part of the signal Rc(t) as error voltage, and supplies the error voltage to the voltage-controlled oscillator  111 . The voltage-controlled oscillator  111  changes the value of the frequency of the signal Si(t) according to the value of the supplied error voltage. As previously described, the signal Si(t) is then supplied to the orthogonal detector  101  and the phase shifter  112 . 
     The signal S(t) supplied to the orthogonal detectors  101  and  102  is expressed as: 
     
       
           S ( t )=cos(ω 0   t +θ) 
       
     
     The signal Si(t) supplied to the orthogonal detector  101  is expressed as: 
     
       
         Si(t)=cosω 1 t 
       
     
     Additionally, the signal Sq(t) supplied to the orthogonal detector  102  is expressed as: 
     
       
           Sq ( t )=−sinω 1   t   
       
     
     It should be noted that “ω 0 ” is carrier frequency of the signal S(t), “θ” is a modulation element of the signal S(t), and “ω 1 ” is freerunning frequency generated by the voltage-controlled oscillator  111 . Accordingly, the signal Bi(t) and the signal Bq(t) are expressed as: 
     
       
           Bi ( t )=cos(Δω t +θ)/2 
       
     
     
       
           Bq ( t )=sin(Δω t +θ)/2 
       
     
     It should be noted that Δω=ω 0 −ω 1 =2 πΔf . Consequently, the frequency-discriminated signal Rc(τ) is obtained as:                Rc                   (   τ   )       =                  Bi                   (   t   )     ×   Bq                   (     t   +   τ     )       -     Bq                   (   t   )     ×   Bi                   (     t   +   τ     )                     =                sin                     (     Δ                 ω                 t     )     /   4                   =                sin                     (     2                 π                 Δ                 f                 τ     )     /   4                                    
     It is ascertained from the above equation that an output of the cross-product AFC circuit, that is, the frequency-discriminated signal Rc(τ), is a function of frequency deviation. FIG. 6 shows characteristic of frequency discrimination. As seen from FIG. 6, when the value of frequency error Δfτ is between −1/4 and 1/4, Δf can be set to “0” by altering frequency generated by the voltage-controlled oscillator  111 , by amount of the frequency-discriminated signal Rc(τ). Additionally, the signal outputted from the loop filter  110  is supplied to the voltage-controlled oscillator  50  in the receiver  53 . 
     A description will now be given of equations to obtain frequency deviation of a signal transmitted by the data-relay apparatus  14 . The frequency of a signal received by the data-receiving antenna  41 , the frequency of the signal transmitted by the data-transmitting antenna  52 , the output frequency of the generator  43 , the output frequency of the generator  47  and the output frequency of the voltage-controlled oscillator  50  are respectively expressed as f 2 +Δf 2 , f 3 +Δf 3 , fs 1 +Δs 1 , fs 2 +Δs 2  and fvco+Δvco. It should be noted that “Δ” means frequency deviation. Frequency deviation ΔBB of the base-band signal is derived from the following equations: 
     
       
         ΔBB=(f 2 +Δf 2 )−(fs 1 +Δs 1 )−(fs 2 +Δs 2 ) 
       
     
     The output frequency of the generator  43  is obtained by multiplying the output frequency of the voltage-controlled oscillator  50  by (f 2 -f 3 )/fvco. The output frequency of the generator  43  is then expressed as: 
     
       
         fs 1 +Δs 1 =(f 2 -f 3 )×(fvco+Δvco)/fvco 
       
     
     Similarly, the output frequency of the generator  47  is obtained by multiplying the output frequency of the voltage-controlled oscillator  50  by f 3 /fvco. The output frequency of the generator  47  is then expressed as: 
     
       
         fs 2 +Δs 2 =f 3 ×(fvco+Δvco)/fvco 
       
     
     Accordingly, the frequency deviation ΔBB of the base-band signal is expressed as: 
     
       
         
           
             
               
                 
                   
                     Δ 
                      
                     
                         
                     
                      
                     BB 
                   
                   = 
                   
                       
                   
                    
                   
                     
                       ( 
                       
                         f2 
                         + 
                         
                           Δ 
                            
                           
                               
                           
                            
                           f2 
                         
                       
                       ) 
                     
                     - 
                     
                       ( 
                       
                         fs1 
                         + 
                         
                           Δ 
                            
                           
                               
                           
                            
                           s1 
                         
                       
                       ) 
                     
                     - 
                     
                       ( 
                       
                         fs2 
                         + 
                         
                           Δ 
                            
                           
                               
                           
                            
                           s2 
                         
                       
                       ) 
                     
                   
                 
               
             
             
               
                 
                   = 
                   
                       
                   
                    
                   
                     
                       ( 
                       
                         f2 
                         + 
                         
                           Δ 
                            
                           
                               
                           
                            
                           f2 
                         
                       
                       ) 
                     
                     - 
                     
                       
                         ( 
                         
                           f2 
                           - 
                           f3 
                         
                         ) 
                       
                       × 
                       
                         
                           ( 
                           
                             fvco 
                             + 
                             
                               Δ 
                                
                               
                                   
                               
                                
                               vco 
                             
                           
                           ) 
                         
                         / 
                         fvco 
                       
                     
                     - 
                     
                       f3 
                       × 
                     
                   
                 
               
             
             
               
                 
                   
                       
                   
                    
                   
                     
                       ( 
                       
                         fvco 
                         + 
                         
                           Δ 
                            
                           
                               
                           
                            
                           vco 
                         
                       
                       ) 
                     
                     / 
                     fvco 
                   
                 
               
             
             
               
                 
                   = 
                   
                       
                   
                    
                   
                     
                       Δ 
                        
                       
                           
                       
                        
                       f2 
                     
                     - 
                     
                       f2 
                       × 
                       Δ 
                        
                       
                           
                       
                        
                       
                         vco 
                         / 
                         fvco 
                       
                     
                   
                 
               
             
           
         
                 
         
             
         
      
     
     The value of ΔBB becomes “0” by the above-described AFC circuit so that Δvco is expressed as: 
     
       
         Δvco=fvco×Δf 2 /f 2   
       
     
     Accordingly, the frequency f 3 +Δf 3  of the signal transmitted by the data-transmitting antenna  52  is expressed as:                f3   +     Δ                 f3       =                  (     f2   +     Δ                 f2       )     -     (     fs1   +     Δ                 s1       )                   =                  (     f2   +     Δ                 f2       )     -       (     f2   -   f3     )     ×       (     fvco   +     Δ                 vco       )     /   fvco                     =                  (     f2   +     Δ                 f2       )     -         (     f2   -   f3     )     /   fvco     ×     (     fvco   +     Δ                 fvco   ×                                      Δ                   f2   /   f2       )               =                  (     f2   +     Δ                 f2       )     -     (     f2   -   f3     )     -       (     f2   -   f3     )     ×   Δ                   f2   /   f2                     =                f3   +     f3   ×   Δ                   f2   /   f2                                      
     Consequently, the frequency deviation Δf 3  of the signal transmitted from the data-relay apparatus  14  is expressed as: 
     
       
         Δf 3 =f 3 ×Δf 2 /f 2   
       
     
     As described above, the frequency deviation Δs 1  of the generator  43  does not affect the frequency deviation Δf 3 . 
     Additionally the frequency f 2  of a signal transmitted from the relay station  12  in FIG. 1 is generally set higher than the frequency f 3  of the signal transmitted from the data-relay apparatus  14 . For instance, the frequencies f 2  and f 3  are respectively set to 12 GHz and 2.6 GHz, and thus the frequency deviation Δf 3  of the output signal can be set to a value comparatively smaller than the frequency deviation Δf 2  of the input signal at the data-relay apparatus  14 . 
     A description will now be given of the data-relay apparatus  15  that relays a signal received from the relay station  12  to other devices such as the radio terminal device  13  without converting frequency of the received signal to another frequency, according to a second embodiment of the present invention with reference to FIG.  7 . 
     The data-relay apparatus  15  includes an amplifier  42 , a generator  43 , a mixer  44 , a band-pass filter  45 , a splitter  46 , a generator  47 , a demodulator  48 , a frequency-deviation-detecting unit  49 , a voltage-controlled oscillator  50 , an amplifier  51 , a data-receiving antenna  61 , amplifier  62 , a generator  63 , a mixer  64 , a band-pass filter  65  and a data-transmitting antenna  66 . The generator  47 , the demodulator  48 , the frequency-deviation-detecting unit  49  and the voltage-controlled oscillator  50  together compose a receiver  53 . A unit in FIG. 7 corresponding to a unit in FIG. 4 has the same unit number as the unit in FIG. 4. A combination of the generator  63  and the mixer  64  corresponds to a frequency-conversion unit in the claims. 
     The data-relay apparatus  15  initially receives a signal having the frequency f 3  from the relay station  12  by use of the data-receiving antenna  61 , and supplies the received signal to the amplifier  62 . The amplifier  62  then amplifies the signal received from the data-receiving antenna  61 , and supplies the amplified signal to the mixer  64 . The mixer  64  mixes the signal having the frequency f 3  and a signal having a frequency fs 3  (=fs 1 ) generated by the generator  63 , and thus frequency approximately equal to the frequency f 2  is obtained. This step is expressed as an equation f 3 +fs 3 =f 2  (up-convert). The frequency approximately equal to the frequency f 2  is referred to as frequency f 2 ′. The band-pass filter then filters the signal having the frequency f 2 ′, and supplies the signal to the amplifier  42 . The amplifier  42  amplifies the signal supplied thereto, and outputs the amplified signal to the mixer  44 . The mixer  44  mixes the signal having the frequency f 2 ′ and a signal having the frequency fs 1  generated by the generator  43 , and supplies the mixed signal having the frequency f 3 ′ to the band-pass filter  45 , where the mixed signal is filtered. This step is expressed as an equation f 2 −fs 1 =f 3  (down-convert). The signal having the frequency f 3 ′ is then supplied to the amplifier  51 , where the signal is amplified, and subsequently the amplified signal is supplied from the amplifier  51  to the data-transmitting antenna  66 . 
     The receiver  53  executes AFC control as described below. The splitter  46  is provided between the band-pass filter  45  and the amplifier  51 , supplying the signal from the band-pass filter  45  to the demodulator  48 . The demodulator  48  demodulates the signal to a base-band signal by use of a signal generated by the generator  47 . The base-band signal demodulated by the demodulator  47  is then supplied to the frequency-deviation-detecting unit  49 . Subsequently, the frequency-deviation-detecting unit  49  detects frequency deviation of the base-band signal, and transmits a detection signal notifying of the detected frequency deviation to the voltage-controlled oscillator  50 . The frequency fvco generated by the voltage-controlled oscillator  50  is altered according to the detection signal supplied from the frequency-deviation-detecting unit  49 . 
     Each of the generators  43  and  47  outputs a signal with the multiplied frequency as described in the first embodiment. Since the frequency fs 2  of the signal generated by the generator  47  is adjusted by the AFC control such that the frequency deviation detected by the frequency-deviation-detecting unit  49  becomes “0”, the frequency fs 1  of the signal generated by the generator  43  is controlled by the voltage-controlled oscillator  50  such that the frequency of the signal outputted from the mixer becomes close to the frequency f 3 . 
     A description will now be given of the data-relay apparatus  14  according to a third embodiment of the present invention with reference to FIG.  8 . 
     The data-relay apparatus  14  includes an amplifier  42 , a generator  43 , a mixer  44 , a band-pass filter  45 , a splitter  46 , a generator  47 , a demodulator  48 , a frequency-deviation-detecting unit  49 , a voltage-controlled oscillator  50 , an amplifier  51 , a data-receiving antenna  52  and an error-rate-measuring unit  54 . A unit in FIG. 8 corresponding to a unit in FIG. 4 has the same unit number as the unit in FIG.  4 . The generator  47 , the demodulator  48 , the frequency-deviation-detecting unit  49 , the voltage-controlled oscillator  50  and the error-rate-measuring unit  54  together compose a receiver  53 . The error-rate-measuring unit  54  corresponds to an error-monitoring unit in the claims. 
     The demodulator  48  supplies the base-band signal described in the first embodiment to the frequency-deviation-detecting unit  49  as well as supplying the signal to the error-rate-measuring unit  54 . Subsequently, the error-rate-measuring unit  54  calculates error rate of communication or broadcasting information obtained from the base-band signal by use of an error-correcting sign attached to the communication or broadcasting information. If the error rate exceeds a predetermined value, the error-rate-measuring unit  54  determines that an error has occurred in the data-relay apparatus  14  or in an input signal to the data-relay apparatus  14 , and may output an alarm signal to a monitoring device that monitors condition of the data-relay apparatus  14 . This error-rate-measuring unit  54  may be provided similarly in the receiver  53  of the data-relay apparatus  15  according to the second embodiment of the present invention. 
     According to the present invention as described above, the frequency deviation of a signal outputted from each of the data-relay apparatuses  14  and  15  decreases compared to that of an input signal thereto. Accordingly, the radio terminal device  13  shown in FIG. 1 can receive a clear signal from the data-relay apparatuses  14  and  15 , and thus does not need to execute either a frequency search or AFC control for detecting frequency deviation of the received signal. As a result, the size of the radio terminal device  13  can be minimized. Additionally, each of the data-relay apparatuses  14  and  15  supplies a signal from its output signal to the receiver thereof so that the device can also detect any error occurred in the output signal. 
     The above description is provided in order to enable any person skilled in the art to make and use the invention and sets forth the best mode contemplated by the inventors of carrying out the invention. 
     The present invention is not limited to the specially disclosed embodiments and variations, and modifications may be made without departing from the scope and spirit of the invention. 
     The present application is based on Japanese Priority Application No. 2000-056465, filed on Mar. 1, 2000, the entire contents of which are hereby incorporated by reference.