Abstract:
Disclosed is a communication system, having: a transmitter including: a first frequency converter converting a frequency of a provided signal to a first frequency band to generate a first signal and converting the frequency of the provided signal to a second frequency band to generate a second signal, the second frequency band having a given frequency difference against the first frequency band; and a combiner combining the first signal and the second signal to output a transmission signal, and a receiver including: a second frequency converter converting the transmission signal to a frequency-shifting signal shifted by the given frequency difference; a first multiplier multiplying the transmission signal and the frequency-shifting signal together to output a multiplied signal; a filter extracting a baseband signal from the multiplied signal; and a detector detecting significant information from the baseband signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2007-10373, filed on Jan. 19, 2007; the entire contents of which are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a radio communication system and particularly relates to a transmission method of a broadcast signal. 
         [0004]    2. Description of the Related Art 
         [0005]    In general, a receiver in a radio communication system is required to know in advance concrete information (information necessary for communication such as a transmission frequency and synchronization information, for example) of a corresponding transmitter. It is suggested a technology in which transmission of a broadcast signal and the like by a transmitter enables communication even if a receiver does not know part of such concrete information in advance. As a method in which a receiver detects existence of a transmitter of an existing radio communication system and further establishes time synchronization with the transmitter in the system, there is known a method of transmitting a transmission sequence superior in an autocorrelation characteristic continuously in terms of time (for example, see JP-A 2000-341240 (KOKAI)). 
         [0006]    According to this method, the transmitter transmits a signal sequence of a given sequence length continuously and the receiver multiplies together the received signal and a signal made by delaying the received signal by the sequence length and subjecting the delayed signal to a complex conjugate computation, whereby a high correlation value can be obtained. Further, a time at which the correlation value is obtained by this method can be regarded as a time when the transmitter transmits the signal, and therefore it is possible to establish time synchronization between the transmitter and the receiver. If the high correlation value cannot be obtained by this method, it can be regarded that such a signal sequence is not transmitted, and it is possible to judge that the transmitter does not exist. 
         [0007]    As a means for detecting a transmission signal by such a method, there is known a method in which a matched filter is used. In this method, a signal sequence with high autocorrelation capability is decided in advance between a transmitter and a receiver and the transmitter transmits this signal sequence with a given frequency. The corresponding receiver operates in advance a matched filter whose tap coefficient is this signal sequence and obtains a high signal output if the signal sequence is received, whereby detects existence of the transmitter. 
         [0008]    However, in the method in which the same signal sequence is repeatedly transmitted, since the received signal is delayed, the receiver must have a relatively large capacity memory. This is ineffective in terms of design, and becomes a cause of a large scale device configuration, increased power consumption and the like. 
         [0009]    In a method in which the matched filter is used, it is required to negotiate the signal sequence in advance, and in addition, the frequency with which the signal sequence is transmitted must be known. Therefore, in a circumstance in which the transmission frequency of the signal sequence is not known, using the method in which the matched filter is used is difficult. 
       SUMMARY OF THE INVENTION 
       [0010]    A communication system being an aspect of the present invention has, a transmitter including: a first frequency converter converting a frequency of a provided signal to a first frequency band to generate a first signal and converting the frequency of the provided signal to a second frequency band to generate a second signal, the second frequency band having a given frequency difference against the first frequency band; and a combiner combining the first and second signals to output a transmission signal, and a receiver including: a second frequency converter converting the transmission signal to a frequency-shifting signal shifted by the given frequency difference; a first multiplier multiplying the transmission signal and the frequency-shifting signal together to output a multiplied signal; a filter extracting a baseband signal from the multiplied signal; and a detector detecting significant information from the baseband signal. 
         [0011]    A transmitter being another aspect of the present invention includes: a frequency converter converting a frequency of a provided signal to a first frequency band to generate a first signal and converting the frequency of the provided signal to a second frequency band to generate a second signal, the second frequency band having a given frequency difference against the first frequency band; and a combiner combining the first and second signals to output a transmission signal. 
         [0012]    A communication method being still another aspect of the present invention includes: transmitting, by a transmitter, a first signal generated by converting a frequency of a provided signal to a first frequency band, and a second signal generated by converting the frequency of the provide signal to a second frequency band having a given frequency difference against the first frequency band collectively as a transmission signal; converting, by a frequency converter of a receiver, the transmission signal to a frequency-shifting signal shifted by the given frequency difference; multiplying, by a multiplier of the receiver, the transmission signal and the frequency-shifting signal together to output a multiplied signal; extracting, by a filter of the receiver, a baseband signal from the multiplied signal; and detecting, by a detector of the receiver, significant information from the baseband signal. 
         [0013]    A transmitter detecting method being yet another aspect of the present invention includes: converting, by a frequency converter, a transmission signal having a first signal generated by converting a frequency of a provided signal to a first frequency band and a second signal generated by converting the frequency of the provided signal to a second frequency band having a given frequency difference against the first frequency band, to a frequency-shifting signal shifted by the given frequency difference; multiplying, by a multiplier, the transmission signal and the frequency-shifting signal together to output a multiplied signal; extracting, by a filter, a baseband signal from the multiplied signal; and detecting, by a detector, significant information from the baseband signal. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0014]      FIG. 1  is a block diagram showing a configuration of a communication system according to a first embodiment. 
           [0015]      FIG. 2  is a chart showing a waveform example of a transmission sequence of the communication system according to the first embodiment. 
           [0016]      FIG. 3  is a chart showing output examples of a first frequency converter and a second frequency converter of the communication system according to the first embodiment. 
           [0017]      FIG. 4  is a chart showing an example of broadcast signals transmitted by the transmitter of the communication system according to the first embodiment. 
           [0018]      FIG. 5  is a chart explaining an operation of the communication system according to the first embodiment. 
           [0019]      FIG. 6  is a flowchart explaining the operation of the communication system according to the first embodiment. 
           [0020]      FIG. 7  is a block diagram showing a modification example of the transmitter of the communication system according to the first embodiment. 
           [0021]      FIG. 8  is a block diagram showing a configuration of a communication system according to a second embodiment. 
           [0022]      FIG. 9  is a diagram showing a structure example of a transmission sequence included in a broadcast signal in the communication system according to the second embodiment. 
           [0023]      FIG. 10  is a chart showing a waveform example of the transmission sequence of the communication system according to the second embodiment. 
           [0024]      FIG. 11  is a chart showing output examples of a first frequency converter and a second frequency converter of the communication system according to the second embodiment. 
           [0025]      FIG. 12  is a chart explaining an operation of the communication system according to the second embodiment. 
           [0026]      FIG. 13  is a block diagram showing a configuration of a communication system according to a third embodiment. 
           [0027]      FIG. 14  is a diagram showing an example of a transmission sequence generated by a transmitter of the communication system according to the third embodiment. 
           [0028]      FIG. 15  is a block diagram showing a configuration of a transmitter of a communication system according to a fourth embodiment. 
           [0029]      FIG. 16  is a chart explaining an operation of the communication system according to the fourth embodiment. 
           [0030]      FIG. 17  is a chart showing an application example of each embodiment. 
           [0031]      FIG. 18  is a chart showing another application example of each embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Description of Embodiments 
       [0032]    Embodiments of the present invention will be described with reference to the drawings, but those drawings are provided for the purpose of illustration only and are not intended to define the limits of the invention. 
         [0033]    Hereinafter, the embodiments will be described in detail with reference to the drawings.  FIG. 1  shows a configuration of a communication system according to a first embodiment,  FIG. 2  shows a waveform example of a transmission sequence of the communication system according to this embodiment, and  FIG. 3  shows waveform examples of a first frequency converter and a second frequency converter as well.  FIG. 4  shows an example of a broadcast signal transmitted by a transmitter of the communication system according to this embodiment,  FIG. 5  explains an operation of the communication system of this embodiment, and  FIG. 6  explains the operation as well. As shown in  FIG. 1 , the communication system of this embodiment includes a transmitter  10  and a receiver  15 . In the communication system of this embodiment, the transmitter  10  transmits a signal (hereinafter, referred to as “broadcast signal”) to indicate existence of the transmitter  10  to the receiver  15 . 
         [0034]    The broadcast signal includes detailed information (concrete information) with regard to a communication method used by the transmitter  10  in communication with the receiver  15 . As the detailed information of the communication method, there can be cited, for example, a frequency (not always the same as a transmission frequency of a broadcast signal) used by a transmitter in communication, a modulation method, an error correction encoding method, an error correction encoding rate, a frame format and the like. A synchronizing signal for establishing synchronization between the transmitter  10  and the receiver  15  may be included. 
         [0035]    The receiver  15  has a function of searching a broadcast signal transmitted by the transmitter  10 , in order to investigate whether or not the corresponding transmitter  10  exists within a communication range. The receiver  15  first receives the broadcast signal and detects existence of the transmitter  10 , and performs synchronous processing with the transmitter  10 . Further, the receiver  15  demodulates data included in the received broadcast signal and obtains concrete information to perform communication with the transmitter  10  in subsequent communication, the concrete information being the frequency, the modulation method, the error correction encoding method, the error correction encoding rate, the frame format, and the like, for example, and becomes ready for reception of the subsequent communication. 
         [0036]    The receiver  15  receives not only the broadcast signal transmitted by the transmitter  10  but also an interference wave transmitted from another transmitter. In the communication system according to this embodiment, based on the broadcast signal transmitted by the corresponding transmitter  10 , the receiver  15  searches the transmitter  10  and obtains concrete information used for the subsequent communication, so that setting of a communication procedure can be realized even under a circumstance in which an interference wave exists. 
         [0037]    Subsequently, the transmitter  10  will be described in detail. The transmitter  10  includes a transmission sequence generator  100 , a local oscillator  101 , a first frequency converter  102 , a second frequency converter  104 , a combiner  106  and a transmitting antenna  108 . By using given concrete information for communicating with the receiver  15 , the transmission sequence generator  100  generates a transmission sequence including the concrete information. The transmission sequence is a baseband signal to which a bit-modulation by a data column to be transmitted is applied. In this embodiment, a modulation method is OOK (On-Off Keying). A length of the sequence is τ, and a modulation speed is Fs. In other words, the transmission sequence generator  100  generates a signal shown in  FIG. 2 . 
         [0038]    The local oscillator  101  generates local signals for the first frequency converter  102  and the second frequency converter  104  each performing frequency conversion, and provides the local signals to each thereof. The first frequency converter  102  and the second frequency converter  104  convert the transmission sequence of the baseband generated by the transmission sequence generator  100  to given transmission frequencies f 1  and f 2  respectively. The first frequency converter  102  and the second frequency converter  104  are synchronous to each other, and a given frequency difference Δf (=f 2 −f 1 ) is set between the converted frequencies f 1  and f 2 . 
         [0039]    The first frequency converter  102  and the second frequency converter  104  can be realized by a mixer and the like. In this case, the local oscillator  101  supplies local signals of the frequency difference Δf to the first frequency converter  102  and the second frequency converter  104  respectively. It can also be constituted that the second frequency converter  104  is provided thereinside with a signal generator of a frequency Δf and a mixer in advance, and that the local oscillator  101  supplies common local signals to the first frequency converter  102  and the second frequency converter  104  respectively. Examples of output signals of the first frequency converter  102  and the second frequency converter  104  are shown in  FIG. 3 . 
         [0040]    The combiner  106  combines an output of the first frequency converter  102  and an output of the second frequency converter  104 . In other words, the combiner  106  superimposes a transmission signal of a frequency f 1  on a transmission signal of a frequency f 2  to generate a transmission signal having two frequency spectra. The transmitting antenna  108  radiates the transmission signal generated by the combiner  106  as a radio wave. 
         [0041]    The signals transmitted by the transmitter  10  are represented on a time frequency plane as signals indicated by solid lines in  FIG. 4 . In other words, the OOK signals whose frequencies are apart by Δf form a line in a frequency direction. The signals are transmitted simultaneously (synchronously) in a time direction. It should be noted that a signal indicated by a broken line in  FIG. 4  is an example of an existing broadcast signal made by the same transmission sequence being transmitted at the same frequency with the time difference T being set. 
         [0042]    Next, the receiver  15  will be described in detail. The receiver  15  includes a receiving antenna  110 , a first mixer  112 , a local oscillator  114 , a complex conjugate processor  116 , a second mixer  118 , a low pass filter (LPF)  120  and a signal detector  122 . 
         [0043]    The receiving antenna  110  receives broadband high frequency signals including the broadcast signal of the frequencies f 1  and f 2  transmitted by the transmitter  10 . In other words, an interference wave transmitted simultaneously by an interference source is inputted together to the receiving antenna  110 . A high frequency amplifier such as an LNA, for example, can be provided for output of the receiving antenna  110 . 
         [0044]    The first mixer  112  is a frequency converter converting a frequency of a high frequency signal received by the receiving antenna  110 . The local oscillator  114  oscillates a local signal for the first mixer  112  performing frequency conversion. This local signal corresponds to the frequency difference Δf between the frequencies f 1  and f 2  of the transmission signal transmitted by the transmitter  10 , and is a sinusoidal wave of the frequency Δf indicated by e j2πΔf , for example. In other words, the first mixer  112  multiplies the received broadcast signal of the frequencies f 1  and f 2  and the local signal of the frequency Δf together, to convert the signals to a signal of frequencies f 2  and f 2 +Δf. 
         [0045]    The complex conjugate processor  116  converts an inputted high frequency signal to a complex conjugate signal (signal made by reversing a sign of an imaginary part). For example, the complex conjugate processor  116  converts a signal indicated by e j2πf2  to a signal indicated by e −j2πf2 . 
         [0046]    The second mixer  118  multiplies the high frequency signal received by the receiving antenna  110  and a high frequency signal passed through the first mixer  112  and the complex conjugate processor  116  together. The LPF  120  passes a signal (a baseband signal in this embodiment) of frequency equal to or smaller than a given frequency among the signals outputted by the second mixer  118 . 
         [0047]    If a pass bandwidth of the LPF  120  is equal to a signal bandwidth of the broadcast signal transmitted by the transmitter  10 , the broadcast signal itself is obtained as an output of the LPF  120 . In a case that the broadcast signal is an OOK-modulated signal, if the pass bandwidth of the LPF  120  is narrower than the bandwidth of the broadcast signal, demodulation of a transmitted bit sequence is difficult. However, even in such a case, since a comparatively large output can be obtained from the LPF  120 , detection of existence of the broadcast signal (furthermore, existence of the transmitter  10 ) itself is possible. 
         [0048]    The signal detector  122  has a function of detecting presence/absence of a signal outputted from the LPF  120  and retrieving the concrete information from the signal in a case that the signal is detected. More specifically, the signal detector  122  performs threshold value judgment on the output signal of the LPF  120 , and if a value larger than a threshold value is obtained, the signal detector  122  judges that the transmitter  10  transmits the broadcast signal (or that the transmitter  10  exists). Further, the signal detector  122  retrieves the OOK-modulated concrete information from the output signal of the LPF  120 , to be ready for reception of subsequent communication. 
         [0049]    Here, an operation of the communication system according to the first embodiment will be described with reference to  FIG. 5  and  FIG. 6 . When the transmission sequence generator  100  generates the transmission sequence of the baseband (a step  130  in  FIG. 6 . Hereinafter referred to as “S 130 ”), and the transmission sequence is distributed (S 132 ), the first frequency converter  102  converts the transmission sequence to the frequency f 1  (b in  FIG. 5 ) and the second frequency converter  104  converts the transmission sequence to the frequency f 2  (c in  FIG. 5 ) (S 134 ). Here, the frequency difference Δf=f 2 −f 1 . A spectrum a in  FIG. 5  positioned at a frequency f 0  indicates an interference wave radiated from other than the transmitter  10 . 
         [0050]    The combiner  106  combines the converted transmission sequence of the frequency f 1  and the transmission sequence of the frequency f 2  (S 136 ), and outputs the high frequency signal having spectra of the frequencies f 1  and f 2 . The high frequency signal outputted from the combiner  106  is radiated to a space as a radio wave from the transmitting antenna  108  (S 138 ). 
         [0051]    When the receiving antenna  110  receives the high frequency signal from the transmitter  10  (S 140 ), the high frequency signal is distributed into two, one being inputted to the first mixer  112  and the other being inputted to the second mixer  118  respectively (S 142 ). The first mixer  112  multiplies the high frequency signal from the receiving antenna  110  and the local signal of the local oscillator  114  together (S 144 ). 
         [0052]    Here, a frequency of the interference wave is f 0 , and frequencies of the broadcast signal are f 1  and f 2 . The interference wave of the frequency f 0 , the broadcast signal of the frequencies f 1  and f 2  and the local signal of the frequency Δf are presumed to be unmodulated sinusoidal waves for the sake of simplification, and they are indicated by e j2πf0  (a in  FIG. 5 ), e j2πf1  (b in  FIG. 5 ), e j2πf2  (c in  FIG. 5 ), and e j2πΔf  respectively. Then, the signals outputted from the first mixer  112  are as follows. 
         [0000]        e   j2πf0   ×e   j2πΔf   =e   j2π(f0+Δf)    (d in FIG. 5) 
         [0000]        e   j2πf1   ×e   j2πΔf   =e   j2π(f1+f2−f1)   =e   j2πf2    (e in FIG. 5) 
         [0000]        e   j2πf2   ×e   j2πΔf   =e   j2π(f2+Δf)    (f in FIG. 5) 
         [0053]    Subsequently, the complex conjugate processor  116  outputs the complex conjugate signal of the signal from the first mixer  112  (S 146 ). In other words, the output signals (signals at a point B in  FIG. 1 ) of the complex conjugate processor  116  are as follows. 
         [0000]      e j2π(f0+Δf) →e −j2π(f0+Δf)    
         [0000]      e j2πf2 →e −j2πf2    
         [0000]      e j2π(f2+Δf) →e −j2π(f2+Δf)    
         [0054]    Meanwhile, the signal received by the receiving antenna  110  is distributed and inputted also to the second mixer  118 . The second mixer  118  multiplies the signal received by the receiving antenna  110  and the output signal of the complex conjugate processor  116  together (S 148 ). The high frequency signals received by the receiving antenna  110 , that is, signals at a point A in  FIG. 1  are e j2πf0  (a in  FIG. 5 ), e j2πf1  (b in  FIG. 5 ), e j2πf2  (c in  FIG. 5 ), while the high frequency signals at the point B in  FIG. 1  are, as stated above, e −j2π(f0+Δf) , e −j2πf2 , and e −jπ2(f2+Δf) . When the both are multiplied, a formula becomes as follows. 
         [0000]    
       
         
           
             
               
                 
                    
                   
                     j2π 
                      
                     
                         
                     
                      
                     f 
                      
                     
                         
                     
                      
                     0 
                   
                 
                 · 
                 
                    
                   
                     - 
                     
                       j2π 
                        
                       
                         ( 
                         
                           
                             f 
                              
                             
                                 
                             
                              
                             0 
                           
                           + 
                           
                             Δ 
                              
                             
                                 
                             
                              
                             f 
                           
                         
                         ) 
                       
                     
                   
                 
               
               + 
               
                 
                    
                   
                     j2π 
                      
                     
                         
                     
                      
                     f 
                      
                     
                         
                     
                      
                     1 
                   
                 
                 · 
                 
                    
                   
                     - 
                     
                       j2π 
                        
                       
                         ( 
                         
                           
                             f 
                              
                             
                                 
                             
                              
                             0 
                           
                           + 
                           
                             Δ 
                              
                             
                                 
                             
                              
                             f 
                           
                         
                         ) 
                       
                     
                   
                 
               
               + 
               
                 
                    
                   
                     j2π 
                      
                     
                         
                     
                      
                     f 
                      
                     
                         
                     
                      
                     2 
                   
                 
                 · 
                 
                    
                   
                     - 
                     
                       j2π 
                        
                       
                         ( 
                         
                           
                             f 
                              
                             
                                 
                             
                              
                             0 
                           
                           + 
                           
                             Δ 
                              
                             
                                 
                             
                              
                             f 
                           
                         
                         ) 
                       
                     
                   
                 
               
               + 
               
                 
                    
                   
                     j2π 
                      
                     
                         
                     
                      
                     f 
                      
                     
                         
                     
                      
                     0 
                   
                 
                 · 
                 
                    
                   
                     
                       - 
                       j2π 
                     
                      
                     
                         
                     
                      
                     f 
                      
                     
                         
                     
                      
                     2 
                   
                 
               
               + 
               
                 
                    
                   
                     j2π 
                      
                     
                         
                     
                      
                     f 
                      
                     
                         
                     
                      
                     1 
                   
                 
                 · 
                 
                    
                   
                     
                       - 
                       j2π 
                     
                      
                     
                         
                     
                      
                     f 
                      
                     
                         
                     
                      
                     2 
                   
                 
               
               + 
               
                 
                    
                   
                     j2π 
                      
                     
                         
                     
                      
                     f 
                      
                     
                         
                     
                      
                     2 
                   
                 
                 · 
                 
                    
                   
                     
                       - 
                       j2π 
                     
                      
                     
                         
                     
                      
                     f 
                      
                     
                         
                     
                      
                     2 
                   
                 
               
               + 
               
                 
                    
                   
                     j2π 
                      
                     
                         
                     
                      
                     f 
                      
                     
                         
                     
                      
                     0 
                   
                 
                 · 
                 
                    
                   
                     - 
                     
                       j2π 
                        
                       
                         ( 
                         
                           
                             f 
                              
                             
                                 
                             
                              
                             2 
                           
                           + 
                           
                             Δ 
                              
                             
                                 
                             
                              
                             f 
                           
                         
                         ) 
                       
                     
                   
                 
               
               + 
               
                 
                    
                   
                     j2π 
                      
                     
                         
                     
                      
                     f 
                      
                     
                         
                     
                      
                     1 
                   
                 
                 · 
                 
                    
                   
                     - 
                     
                       j2π 
                        
                       
                         ( 
                         
                           
                             f 
                              
                             
                                 
                             
                              
                             2 
                           
                           + 
                           
                             Δ 
                              
                             
                                 
                             
                              
                             f 
                           
                         
                         ) 
                       
                     
                   
                 
               
               + 
               
                 
                    
                   
                     j2π 
                      
                     
                         
                     
                      
                     f 
                      
                     
                         
                     
                      
                     2 
                   
                 
                 · 
                 
                    
                   
                     - 
                     
                       j2π 
                        
                       
                         ( 
                         
                           
                             f 
                              
                             
                                 
                             
                              
                             2 
                           
                           + 
                           
                             Δ 
                              
                             
                                 
                             
                              
                             f 
                           
                         
                         ) 
                       
                     
                   
                 
               
             
             = 
             
               
                  
                 
                   
                     - 
                     j2πΔ 
                   
                    
                   
                       
                   
                    
                   f 
                 
               
               + 
               
                  
                 
                   - 
                   
                     j2π 
                      
                     
                       ( 
                       
                         
                           f 
                            
                           
                               
                           
                            
                           0 
                         
                         + 
                         
                           Δ 
                            
                           
                               
                           
                            
                           f 
                         
                         - 
                         
                           f 
                            
                           
                               
                           
                            
                           1 
                         
                       
                       ) 
                     
                   
                 
               
               + 
               
                  
                 
                   - 
                   
                     j2π 
                      
                     
                       ( 
                       
                         
                           f 
                            
                           
                               
                           
                            
                           0 
                         
                         - 
                         
                           f 
                            
                           
                               
                           
                            
                           1 
                         
                       
                       ) 
                     
                   
                 
               
               + 
               
                  
                 
                   j2π 
                    
                   
                     ( 
                     
                       
                         f 
                          
                         
                             
                         
                          
                         0 
                       
                       - 
                       
                         f 
                          
                         
                             
                         
                          
                         2 
                       
                     
                     ) 
                   
                 
               
               + 
               
                  
                 
                   
                     - 
                     j2πΔ 
                   
                    
                   
                       
                   
                    
                   f 
                 
               
               + 
               
                  
                 0 
               
               + 
               
                  
                 
                   - 
                   
                     j2π 
                      
                     
                       ( 
                       
                         
                           f 
                            
                           
                               
                           
                            
                           2 
                         
                         + 
                         
                           Δ 
                            
                           
                               
                           
                            
                           f 
                         
                         - 
                         
                           f 
                            
                           
                               
                           
                            
                           0 
                         
                       
                       ) 
                     
                   
                 
               
               + 
               
                  
                 
                   
                     
                       
                         - 
                         j2π 
                       
                       · 
                       2 
                     
                      
                     Δ 
                      
                     
                         
                     
                      
                     f 
                   
                   + 
                   e 
                   - 
                   
                     j2πΔ 
                      
                     
                         
                     
                      
                     f 
                   
                 
               
             
           
         
       
     
         [0000]    In other words, a broadcast signal component of the frequency f 2  is converted to the baseband signal, while the interference wave of the frequency f 0  and the broadcast signal component of flare converted to the high frequency signal of frequencies other the frequencies f 0 , f 1 , and f 2 . 
         [0055]    The LPF  120  mainly passes e 0  being the baseband signal among signals outputted from the second mixer  118  (S 159 ) . Therefore, the signal components other than e 0  are attenuated by the LPF  120 . The signal filtered by the LPF  120  is transmitted to the signal detector  122 . 
         [0056]    Here, when attention is paid to the interference wave in  FIG. 5 , an interference wave a and an interference wave d in  FIG. 5  are located at different frequencies f 0  and f 0 +Δf, respectively. In a case that signal of the spectrum of the interference wave a and signal of the spectrum of the interference wave d are multiplied together, since there is no other signal corresponding to the frequency f 0  or f 0 +Δf by which the interference waves are to be multiplied, as a result of the multiplication, the interference wave signal of the frequency f 0  disappears. Similarly, in a noise circumstance, if an interference wave and a noise are multiplied together, only a signal much smaller than a multiplication result of desired waves (c and e) is outputted. Therefore, the signal detector  122  is able to detect only a desired broadcast signal (in this example, the component of the frequency f 2 ) carried by signals having the frequency difference Δf (S 152 ). 
         [0057]    As stated above, according to the communication system of this embodiment, the desired wave is converted to the baseband signal and the interference wave is converted to the very small signal. In other words, even under existence of the interference wave, it is possible to detect existence of the broadcast signal being the desired wave and of the transmitter. Further, according to the communication system of this embodiment, the receiver is not required to know the frequencies f 1  or f 2 . If the receiver knows only the difference of the f 1  and f 2 , that is, Δf, it is possible to detect the transmission sequence transmitted by a transmitter side. 
         [0058]    Here, a modification example of the transmitter  10  will be described with reference to  FIG. 7 . In the transmitter  10  shown in  FIG. 1 , the output of the transmission sequence generator  100  is distributed into two and inputted to the two frequency converters. A transmitter  11  shown in  FIG. 7  is different in that an output of a transmission sequence generator  100  is distributed into two, and a frequency of one of the outputs is Δf converted. In description below, components common to  FIG. 1  are denoted with common symbols and numerals and redundant description will be omitted. 
         [0059]    As shown in  FIG. 7 , the transmitter  11  includes the transmission sequence generator  100 , a third frequency converter  103 , a fourth frequency converter  105 , a combiner  106 , a first local oscillator  107 , a second local oscillator  109 , and a transmitting antenna  108 . The third frequency converter  103  corresponds to the first frequency converter  102  shown in  FIG. 1 , and converts an inputted signal to a signal of a frequency f 1 . The first local oscillator  107  corresponds to the local oscillator  101  shown in  FIG. 1 , and generates a local signal necessary for a frequency conversion processing of the third frequency converter  103 . The fourth frequency converter  105  converts an inputted baseband signal to a signal of a frequency Δf. The second local oscillator  109  generates a local signal necessary for a frequency conversion processing of the fourth frequency converter  105 . 
         [0060]    A transmission sequence outputted by the transmission sequence generator  100  is distributed into two, one being inputted to the fourth frequency converter  105  and the other being inputted to the combiner  106 . The fourth frequency converter  105  converts the inputted transmission sequence of the baseband to the signal of the frequency Δf. The combiner  106  combines the transmission sequence of the baseband and the transmission sequence of the frequency Δf and inputs the combined transmission sequence to the third frequency converter  103 . The third frequency converter  103  converts the inputted transmission sequence of the baseband and the transmission sequence of the frequency Δf to a transmission sequence of a frequency f 1  and a transmission sequence of a frequency f 2  (=f 2 −f 1 +f 1 ). The converted high frequency signal is radiated as a radio wave from the transmitting antenna  108 . 
         [0061]    The transmitter shown in  FIG. 7  can transmit a similar broadcast signal to that of the transmitter shown in  FIG. 1 . 
         [0062]    Next, a communication system according to a second embodiment will be described in detail with reference to  FIG. 8  to  FIG. 12 .  FIG. 8  shows a configuration of the communication system according to the second embodiment,  FIG. 9  shows a structure of a transmission sequence included in a broadcast signal,  FIG. 10  shows a waveform example of the transmission sequence,  FIG. 11  shows output waveform examples of the first frequency converter and the second frequency converter, and  FIG. 12  explains an operation of the communication system according to the second embodiment. In description below, the configuration common to the communication system according to the first embodiment are denoted with common symbols and numerals and redundant description will be omitted. 
         [0063]    As shown in  FIG. 8 , the communication system of this embodiment includes a transmitter  20  and a receiver  25 . Compared with the transmitter  10  of the first embodiment, the transmitter  20  is different in that the transmitter  20  includes a direct-current signal generator  200  and a transmission sequence generator  202  instead of the transmission sequence generator  100 . Also, compared with the receiver  15  of the first embodiment, the receiver  25  is different in that the receiver  25  further includes a demodulator  223 . 
         [0064]    The direct-current signal generator  200  generates a direct-current signal of a given level. The transmission sequence generator  202  corresponds to the transmission sequence generator  100  of the first embodiment and generates a transmission sequence. However, the transmission sequence generator  202  of the second embodiment generates a BPSK-modulated transmission sequence. 
         [0065]    In the transmitter  10  of the first embodiment, the transmission sequence generated by the transmission sequence generator  100  is distributed into two and each is inputted to the first frequency converter  102  and the second frequency converter  104 , while in the transmitter  20  of the second embodiment, an output signal of the direct-current signal generator  200  is inputted to the first frequency converter  102  and an output signal of the transmission sequence generator  202  is inputted to the second frequency converter  104 . The demodulator  223  in the receiver  25  demodulates a transmission sequence of a baseband to regenerate the concrete information. 
         [0066]    Here, an operation of the communication system according to the second embodiment will be described with reference to  FIG. 9  to  FIG. 12 . The transmission sequence generator  202  generates the BPSK-modulated transmission sequence of the baseband. The transmission sequence generated by the transmission sequence generator  202  has two main blocks as shown in  FIG. 9 , containing in a first block a known sequence decided in advance between the transmitter  20  and the receiver  25 , and containing in a second block a broadcast information sequence in which the concrete information such as a modulation method, an error correction encoding method, an encode rate, a frame configuration and the like are written. As a result of the BPSK-modulation, the transmission sequence has a waveform shown in  FIG. 10 . 
         [0067]    The first frequency converter  102  converts the direct-current signal outputted from the direct-current signal generator  200  to a sinusoidal wave signal of a frequency f 1  (i in  FIG. 12 ) and inputs the signal to the combiner  106 . On the other hand, the second frequency converter  104  converts the transmission sequence generated by the transmission sequence generator  202  to a frequency f 2  (j in  FIG. 12 ) and inputs it to the combiner  16 . In other words, the combiner  106  combines the sinusoidal wave signal of the frequency f 1  and a high frequency signal of the frequency f 2  modulated by the transmission sequence, shown in  FIG. 11 , and outputs a high frequency signal having spectra of frequencies f 1  and f 2 . The combined high frequency signal is radiated to a space from the transmitting antenna  108  as a radio wave. Incidentally, h in  FIG. 12  indicates an interference wave radiated from other than the transmitter  20 . 
         [0068]    When the receiving antenna  110  receives the high frequency signal from the transmitter  20 , the high frequency signal is distributed into two, one being inputted to a first mixer  112  and the other being inputted to a second mixer  118 , respectively. The first mixer  112  multiplies the high frequency signal from the receiving antenna  110  and an oscillation signal of a local oscillator  114  together. Here, if a frequency of the interference wave is f 0  and frequencies of the broadcast signal are f 1  and f 2 , a spectrum indicated by k in  FIG. 12 , a spectrum indicated by 1 as well, and a spectrum indicated by m as well are outputted from the first mixer  112 , similarly to in the first embodiment. 
         [0069]    A complex conjugate processor  116  outputs a complex conjugate signal of an output signal from the first mixer  112 . On the other hand, the signal received by the receiving antenna  110  is distributed and inputted also to the second mixer  118 . The second mixer  118  multiplies the signal received by the receiving antenna  110  and an output signal of the complex conjugate processor  116  together. The high frequency signal received by the receiving antenna  110 , that is, the high frequency signal at a point D in  FIG. 8 , has a spectrum indicated by h in  FIG. 12 , a spectrum indicated by i in  FIG. 12 , and a spectrum indicated by j in  FIG. 12 . When the signal at the point D and a signal at a point E in  FIG. 8  are multiplied together, a broadcast signal component of the frequency f 2  is converted to a baseband signal, while an interference wave of a frequency f 0  and a broadcast signal component of the frequency f 1  are converted to high frequency signals with frequencies other than the frequencies f 0 , f 1 , f 2 . 
         [0070]    The second embodiment is different from the first embodiment in that the baseband signal is obtained in the second embodiment by multiplying the transmission sequence modulation signal of the frequency f 2  and the sinusoidal wave signal of the frequency f 2  together, while the baseband signal in the first embodiment is obtained by multiplying the transmission sequence modulation signals of frequency f 2  each other. In other words, in the second embodiment, the transmission sequence generated by the transmission sequence generator  202  of the transmitter  20  is precisely reproduced in the end. This means that the original transmission sequence can be reproduced even if a modulation method of the transmission sequence generated by the transmission sequence generator  202  is a phase modulation method such as a BPSK method. 
         [0071]    An LPF  120  is set in a manner to pass mainly the baseband signal among the signals obtained in the multiplication. The signal filtered by the LPF  120  is transmitted to a signal detector  122  and the demodulator  223 . The demodulator  223  demodulates the baseband signal from the LPF  120  to reproduce the broadcast signal sequence. 
         [0072]    The signal detector  122  detects the known sequence included in the transmission sequence, and the demodulator  223  reproduces the broadcast signal sequence included in the transmission sequence. Since the known sequence is already known between the transmitter and the receiver, it is possible to judge reception of a known signal by providing the signal detector  122  with a matched filter, for example. In other words, even a signal whose frequency is modulated to an unknown frequency at a transmitter side can be converted to a baseband signal as long as a receiver side knows a frequency difference Δf. Further, timing when the broadcast signal is transmitted can be obtained by referring to an output of this matched filter. Therefore, it is possible to establish time synchronization between the transmitter and the receiver. 
         [0073]    As stated above, according to the communication system of this embodiment, the desired wave is converted to the baseband signal, while the interference wave is converted to a very small signal. In other words, even under existence of the interference wave, it is possible to detect existence of the broadcast signal being the desired wave and a transmitter. Further, according to the communication system of this embodiment, the receiver is not required to know the frequency f 1  or f 2  of the broadcast signal, and the transmission sequence transmitted by the transmission side can be detected as long as the difference between the frequencies f 1  and f 2 , that is, Δf, is known. Additionally, in the communication system of this embodiment, since the transmission sequence transmitted by the transmitter can be precisely reproduced at the receiver side, the concrete information included in the transmission sequence can be effectively utilized at the receiver side. 
         [0074]    Next, a third embodiment will be described in detail with reference to  FIG. 13  and  FIG. 14 .  FIG. 13  shows a configuration of a communication system according to the third embodiment and  FIG. 14  shows an example of a transmission sequence generated by a transmitter of the communication system according to the third embodiment as well. The communication system according to the third embodiment is altered in the configuration of the transmitter  10  of the communication system according to the first embodiment described before. In the following description, components common to  FIG. 1  are denoted with common symbols and numerals and redundant description will be omitted. 
         [0075]    As shown in  FIG. 13 , a transmitter  30  of this embodiment includes a first transmission sequence generator  300 , a second transmission sequence generator  301 , a mixer  302 , a local oscillator  101 , a first frequency converter  102 , a second frequency converter  104 , a combiner  106 , and a transmitting antenna  108 . The local oscillator  101 , the first frequency converter  102 , the second frequency converter  104 , the combiner  106 , and the transmitting antenna  108  respectively are components common to the first embodiment. Further, a receiver  35  of this embodiment further includes a demodulator  323  in addition to the configuration of the receiver  15  according to the first embodiment. 
         [0076]    The first transmission sequence generator  300  corresponds to the transmission sequence generator  100  according to the first embodiment, and generates as a first transmission sequence a transmission sequence to be transmitted as a broadcast signal including the concrete information. The second transmission sequence generator  301  generates a second transmission sequence having a symbol rate faster than that of the first transmission sequence. Since the second transmission sequence is used to segment and randomize the first transmission sequence, the second transmission sequence is not required to have a particular meaning as a data sequence, and a signal made by QPSK-modulating a random sequence can be used, for example. Correlation between the first transmission sequence and another interference wave can be made low by randomizing the first transmission sequence by the second transmission sequence. Here, the symbol rate of the second transmission sequence is larger in a value than the symbol rate of the first transmission sequence, and it is desirable that the symbol rate of the second transmission sequence is a faster rate than a bandwidth of an assumable interference wave, for example. However, even if this rate cannot be realized, by making the rate be about fourfold, better signal detection is possible than in a case without randomizing. In the following description, the second transmission sequence generator  301  generates a QPSK signal having a symbol rate N-fold higher than the first transmission sequence generated by the first transmission sequence generator  300 . 
         [0077]      FIG. 14  is a diagram showing relationship between the first transmission sequence and the second transmission sequence. The second transmission sequence has an N-fold symbol rate compared with the first transmission sequence. The first transmission sequence is a signal (for example, an OOK-modulated signal) to be sent as a broadcast signal, concrete information of the signal, such as a modulation method, an error correction encoding method, an encode rate, or a frame format being modulated, while the second transmission sequence is a signal made by QPSK-modulating a random sequence. 
         [0078]    The mixer  302  multiplies the first transmission sequence generated by the first transmission sequence generator  300  and the second transmission sequence generated by the second transmission sequence generator  301  together. Here, since a bit rate of the first transmission sequence is set to be lower than a bit rate of the second transmission sequence, the mixer  302  outputs a signal made by subjecting the second transmission sequence to modulation of the first transmission sequence. 
         [0079]    Based on a local signal of the local oscillator  101 , the first frequency converter  102  converts an output signal of the mixer  302  to a high frequency signal of a frequency f 1 , and similarly, the second frequency converter  104  converts the second transmission sequence generated by the second transmission sequence generator  301  to a high frequency signal of a frequency f 2 . The combiner  106  combines an output signal of the first frequency converter  102  and an output signal of the second frequency converter  104 , and the transmitting antenna  108  radiates the combined high frequency signal to a space as a radio wave. 
         [0080]    The receiver  35  has a commonality with the receiver  25  in the second embodiment shown in  FIG. 8  in terms of the configuration, only a function of the demodulator  323  being different. A received signal is distributed and inputted to a first mixer  112  and a second mixer  118 . The first mixer  112  multiplies the received signal by a local signal of a frequency Δf generated by a local oscillator  114 , and a complex conjugate processor  116  outputs a complex conjugate signal of an output signal of the first mixer  112 . The second mixer  118  multiplies the received signal and an output signal of the complex conjugate processor  116  together, and supplies obtained signals to an LPF  120 . The LPF  120  passes a baseband signal among the signals obtained in the second mixer  118 . As a result, the baseband signal whose center frequency is DC is inputted to a signal detector  122  and the demodulator  323 . 
         [0081]    The signal detector  122  judges existence of the transmitter  30  and presence/absence of a broadcast signal, based on presence/absence of the baseband signal. The demodulator  323  reproduces the first transmission sequence based on a DC component of the baseband signal. 
         [0082]    Subsequently, an operation of the communication system according to the third embodiment will be described. The first transmission sequence generator  300  generates the first transmission sequence including the concrete information, while the second transmission sequence generator  301  generates the second transmission sequence of the symbol rate N-fold of the first transmission sequence. The second transmission sequence is distributed into two and inputted to the mixer  302  and the second frequency converter  104 . 
         [0083]    The mixer  302  multiplies the first transmission sequence and the second transmission sequence together and inputs an obtained transmission sequence to the first frequency converter  102 . Here, the local oscillator  101  supplies local signals to the first frequency converter  102  and the second frequency converter  104 , and the first frequency converter  102  converts the second transmission sequence modulated by the first transmission sequence to a high frequency signal of a frequency f 1  and inputs an obtained signal to the combiner  106 . Similarly, the second transmission converter  104  converts the second transmission sequence to a high frequency signal of a frequency f 2  and inputs an obtained signal to the combiner  106 . At this time, a frequency difference between each is given Δf. The combiner  106  combines the inputted high frequency signals and inputs the combined signal to the transmitting antenna  108 , and the transmitting antenna  108  radiates an electric wave to a space. As a result, a radio wave having spectra of the frequencies f 1  and f 2  as a broadcast signal is transmitted as the broadcast signal. 
         [0084]    When the receiving antenna  110  receives the high frequency signal from the transmitter  30 , the high frequency signal is distributed into two, one being inputted to the first mixer  112 , the other being inputted to the second mixer  118 , respectively. The first mixer  112  multiplies the high frequency signal from the receiving antenna  110  and the local signal of the frequency Δf of the local oscillator  114  together. The complex conjugate processor  116  outputs the complex conjugate signal of the signal from the first mixer  112 . 
         [0085]    On the other hand, the signal received by the receiving antenna  110  is distributed and inputted also to the second mixer  118 . The second mixer  118  multiplies the signal received by the receiving antenna  110  and the output signal of the complex conjugate processor  116  together. As a result of the multiplication, the component of the broadcast signal of the frequency f 2  is converted to the baseband signal. 
         [0086]    The LPF  120  passes mainly the baseband signal among the signals obtained in the multiplication in the second mixer  118 . The signal filtered by the LPF  120  is sent to the signal detector  122  and the demodulator  323 . The signal detector  122  judges existence of the transmitter  30  and presence/absence of the broadcast signal based on presence/absence of the baseband signal. 
         [0087]    The demodulator  323  demodulates the baseband signal sent from the LPF  120  and reproduces the broadcast signal sequence. The output of the LPF  120  is the baseband signal, whose amplitude is subject to modulation of the first transmission sequence. Thus, the demodulator  323  reproduces an original first transmission sequence based on a polarity and the like of the direct current component of the output of the LPF  120 . 
         [0088]    As stated above, also in the communication system of this embodiment, it is possible to precisely reproduce the first transmission sequence including the concrete information and the like at a receiver side. It should be noted that any sequence can be used for the second transmission sequence as long as phase modulation is applied thereon. In the example described above, it is described as a random sequence is used as the second transmission sequence, but a modulation signal or transmission bit other than the above random sequence can bring a similar effect. 
         [0089]    Next, a fourth embodiment will be described in detail with reference to  FIG. 15  and  FIG. 16 .  FIG. 15  shows a configuration of a transmitter of a communication system according to the fourth embodiment, while  FIG. 16  explains an operation of the communication system according to the fourth embodiment. The communication system of this embodiment is altered from the communication systems according to the first to third embodiments in terms of a configuration of a transmitter. Thus, components common to the transmitter according to the first to third embodiments are denoted with the same symbols and numerals and redundant description will be omitted. 
         [0090]    As shown in  FIG. 15 , a transmitter  40  in the communication system of this embodiment is made by replacing the local oscillator  101  in the configuration of the transmitter  10  according to the first embodiment shown in  FIG. 1  with a frequency controllable local oscillator (VFO)  401 . The VFO  401  supplies local signals for frequency conversion to a first frequency converter  402  and a second frequency converter  404 . The VFO  401  can change its oscillation frequency fa based on an instruction from an unshown controller. 
         [0091]    The first frequency converter  402  and the second frequency converter  404  correspond to the first frequency converter  102  and the second frequency converter  104  in the first embodiment respectively, and are set to be able to control a conversion frequency based on the local signal sent from the VFO  401 . For example, when the local signal of the frequency fa is received from the VFO  401 , the first frequency converter  402  converts a baseband signal to a high frequency signal of a frequency f 1  and the second frequency converter  404  converts the baseband signal to a high frequency signal of a frequency f 2  as well. Here, as stated before, a frequency difference Δf (=f 2 −f 1 ) is known in a receiver side. 
         [0092]    It is configured that, if the frequency of the local signal from the VFO  401  is changed to fb from fa, the conversion frequency of the first frequency converter  402  is controlled to become f 3  from f 1  and the conversion frequency of the second frequency converter  404  is controlled to become f 4  from f 2  but the frequency difference between the frequencies f 3  and f 4  is always Δf ( FIG. 16 ). In other words, the first frequency converter  402  and the second frequency converter  404  in this embodiment are common to the first to the third embodiments in that frequency conversion is performed in a manner that the frequency difference becomes Δf, and are different in that the conversion frequency is controllable. 
         [0093]    An operation of the communication system of this embodiment will be described. In an example shown in  FIG. 15 , the VFO  401  supplies the local signals of the frequency fa to the first frequency converter  402  and the second frequency converter  404 . A transmission sequence generated by a transmission sequence generator  100  is converted to a high frequency signal of the frequency f 1  by the first frequency converter  402  and converted to a high frequency signal of the frequency f 2  by the second frequency converter  404 . When an oscillation frequency of the VFO  401  is controlled to become fb from fa by the instruction from the unshown controller, the first frequency converter  402  and the second frequency converter  404  convert the transmission sequence to high frequency signals of frequencies f 3  and f 4  respectively, with the frequency difference Δf being fixed. The high frequency signals of the frequencies f 3  and f 4  are combined by a combiner  106  to be radiated from an antenna  108  as a radio wave. 
         [0094]    Here, there is considered a case that a receiver having a configuration common to the first to third embodiments receives the radio wave radiated from the transmitter  40 . As described hereinabove, since the broadcast signal is detected using only the frequency difference Δf in the receivers according to the first to third embodiments, even when the frequencies of the broadcast signal transmitted by the transmitter  40  are changed to become f 3  and f 4  from f 1  and f 2 , as long as the frequency different Δf thereof is fixed, existence of the transmitter  40  and the broadcast signal can be detected. 
         [0095]    In other words, as shown in  FIG. 16 , assume that during times T 1  to T 2 , the VFO  401  oscillates the local signal of the frequency fa and the transmitter transmits the broadcast signal of the frequencies f 1  and f 2  (f 2 −f 1 =Δf). When the VFO  401  controls the frequency of the local signal to become fb from fa at the time T 2 , the broadcast signal transmitted by the transmitter is the frequencies f 3  and f 4  respectively, with the frequency difference Δf being maintained. Even if the transmitter side operates as above, the receiver can detect existence of the transmitter and the broadcast signal, and demodulate the broadcast signal, as long as the receiver knows the frequency difference Δf of the broadcast signal paired. 
         [0096]    The communication system of this embodiment is effective in a circumstance in which an interference source exist in particular. For example, if the interference source exists in the frequency f 1 , it is not desirable to transmit the broadcast signal at the same frequency f 1 . On the other hand, if the transmitter side does not know the frequency transmitted by the interference source, it is difficult to evade interference. Then, by the transmitter changing the transmitting frequency (from f 1  to f 3  in  FIG. 16 ) with time, the interference can be evaded after a given time passes, even if part of the signal overlaps an interference signal transmitted by the interference source. 
         [0097]    As stated above, according to a communication system of this embodiment, since a transmitter can control a transmission frequency, communication which is resistant to interference is possible. It should be noted that though it is described as the transmitter  40  is provided with the VFO  401  instead of the reference oscillator  400  in the first embodiment in the above description, the transmitters according to the modification example ( FIG. 7 ) of the first embodiment, the second embodiment ( FIG. 8 ) and the third embodiment ( FIG. 13 ) can be provided with similar VFOs to be able to control transmission frequencies. 
         [0098]    Next, an application example of the communication systems according to the first to fourth embodiments will be described.  FIG. 17  is a schematic diagram showing an example in which a plurality of radio systems coexist in the same communication range. 
         [0099]    If the plurality of radio systems coexist in the same frequency band, it is necessary for each child station to identify a parent station of the radio system to which the child station belongs. For example, a cellular terminal identifies a corresponding base station, or a wireless LAN terminal identifies a corresponding AP (Access Point). 
         [0100]    Here, there is considered a case that radio systems A and B are the communication systems according to the first to fourth embodiments described hereinabove. A frequency difference Δf 1  for the radio system A and a frequency difference Δf 2  for the radio system B are prescribed, and respective parent stations transmit broadcast signals s and t, and broadcast signals u and v at different frequencies from each other. 
         [0101]    The child station belonging to the radio system A starts a receiving operation with the frequency difference being Δf 1 . In this case, since the child station of the radio system A can detect and demodulate the broadcast signals s and t, subsequent data communication can be realized based on reproduced concrete information. Similarly, the child station belonging to the radio system B starts a receiving operation with the frequency difference being Δf 2 , whereby broadcast signals u and v can be detected and demodulated, so that subsequent data communication can be realized. 
         [0102]    As stated above, in the communication systems according to the first to fourth embodiments, making a frequency difference Δf being different value per a communication system enables a receiver side to identify the system to which the receiver belongs. 
         [0103]    Subsequently, another application example will be described.  FIG. 18  is a frequency spectrum atlas showing an example of a case that the communication systems according to the first to fourth embodiments are applied to a concrete system. 
         [0104]    A receiver converts a frequency of a band of 3 to 4 GHz to a baseband signal, samples the signal at a sampling rate of 2 GHz, and takes in the signal as a digital signal. Also, a transmitter transmits OOK-modulated broadcast signals (w, x in  FIG. 18 ) at any frequency among the band of 3 to 4 GHz with the broadcast signals being apart from each other by a frequency Δf. The OOK-modulated broadcast signals each has a bandwidth of 2 MHz, that is, a symbol length of 0.5 μsec, has a burst of 0.8 msec, and is transmitted repeatedly per 0.8 msec. Then, a number of symbols included in one burst is 1600. Further, one symbol becomes  1000  samples by sampling in the receiver. The broadcast signals w and x include idle frequency information (y in  FIG. 18 ) in which communication is performed after synchronization is established, a communication method and the like. 
         [0105]    Under these circumstances, the receiver first starts reception of the frequency band of 3 to 4 GHz. For example, if the receiver is the receiver  15  according to the first embodiment, the receiver  15  converts a frequency of a received signal with a bandwidth of 1 GHz by Δf, and after applying a complex conjugate processing, multiplies the obtained signal and the received signal itself together. Then, the broadcast signals apart by Δf 1  are multiplied by each other so that a baseband signal whose center frequency is a direct current can be obtained. 
         [0106]    If a pass bandwidth of the LPF  120  of the receiver  15  is 2 MHz being a symbol speed, a sample sequence of 1000 samples constituting each symbol of an OOK signal being the baseband signal is in-phase added and outputted from the LPF  120 . As a result, it becomes possible to obtain a gain of 30 dB against a noise. This gain is effective also to an interference wave, if the interference wave can be regarded as a white noise, as a result of multiplication. Since the signal outputted from the LPF  120  is an OOK-modulated signal, the signal detector  122  can reproduce a bit sequence of the broadcast signal by an OOK-demodulation processing. In this case, the idle frequency information included in the broadcast signal is reproduced, and subsequent communication is performed at a frequency indicated by y in  FIG. 18 . 
         [0107]    If the pass bandwidth of the LPF  120  of the receiver  15  is 1.25 kHz being an inverse number of the burst length, all of the OOK signals of 1600 symbols forming the burst and further 1000 samples forming the symbols of the OOK signals are in-phase added. As a result, a gain of about 62 dB against the noise or white interference can be obtained. In this case, since the pass bandwidth of the LPF  120  is narrow, demodulation of the OOK signal is difficult, but detection of the broadcast signal itself is possible. 
         [0108]    As described above, according to each of above embodiments, a broadcast signal can be detected and demodulated even if a transmission frequency of the broadcast signal is unknown. This is effective in a cognitive radio communication system and the like in which a communication frequency is dynamically changed to perform communication. In other words, in the cognitive radio communication system, since a transmitter must use a frequency which is not used by another communication system, a frequency at which a broadcast signal is transmitted can always change. In each embodiment, since the transmitter is detected based on a frequency difference between the broadcast signals transmitted simultaneously, instead of the frequency itself of the broadcast signal, the receiver is able to detect the broadcast signal even if the transmission frequency is arbitrary. 
         [0109]    It should be noted that though in the above embodiments, it is described as the broadcast signal includes the concrete information, but this is not indispensable. In other words, after synchronization is established between the transmitter and the receiver by the broadcast signal, subsequent communication may be continued at the frequency of the broadcast signal. 
         [0110]    Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.