Abstract:
The digital frequency-hopping (FH) transceiver and the method thereof can be applied to FH communication system with a short FH time. The transceiver employs a digital signal processing (DSP) in baseband (BB) circuits to implement FH without need to adjusting the RF carrier frequency. Wherein, the digital FH transmitter performs DSP on a digital BB transmission signal to frequency-hop between BB sub-bands corresponding to sub-bands of a channel, converts it into an analog signal, and then modulates the analog signal to the channel sub-bands for transmission. The digital FH receiver then performs carrier demodulation, analog-to-digital conversion and DSP on a received signal to regain the original digital BB transmission signal.

Description:
BACKGROUND OF THE INVENTION 
   (a). Field of the Invention 
   This invention relates to a communication system, and more particularly, to the frequency hopping transceiver of the communication system. 
   (b). Description of the Prior Arts 
   Frequency-hopping (FH) communication system is a commonly used technique. One of the examples is the multi-band orthogonal frequency division multiplexing (MB-OFDM) system. The MB-OFDM system hops between three adjacent sub-bands in the channel, as shown in  FIG. 1 . In  FIG. 1 , f c1 , f c2 , f c3  are the central frequencies of the three adjacent sub-bands. B stands for a symbol rate of the MB-OFDM system (i.e., the bandwidth of the OFDM signal). T OFDM  stands for a symbol interval. The OFDM signal frequency-hops to another sub-band every symbol interval in accordance with a specific sequence. 
   In prior art, a frequency-hopping controller within a baseband circuit of the transmitter is used to control the frequency of a carrier signal generated by a carrier frequency synthesizer, and the baseband signal can be modulated according to the carrier frequency for transmission. In the baseband circuit of a conventional receiver, a packet detector is used to detect if a packet has been received. If the packet is detected, it will activate the frequency-hopping controller to sequentially output a frequency-hopping control signal to the carrier frequency synthesizer, and the carrier frequency synthesizer will then output the carrier signal. The received radio frequency (RF) signal will be demodulated according to the carrier frequency to produce the original baseband signal. 
   Please refer to  FIG. 2 .  FIG. 2  is a block diagram of the conventional transmitter of the MB-OFDM system. A signal mapper  201  converts an input into a mapped signal. An inverse fast Fourier transform (IFFT) device  202  is used for converting the mapped signal into a time-domain signal. A guard interval (GI) adding circuit  203  is used for adding the time-domain signal with a guard interval and then generating a baseband OFDM signal. The baseband OFDM signal is converted into an analog signal by a parallel-to-serial converter (P/S)  204  and a digital-to-analogue converter (DAC)  205 . The sampling frequency f s  of the DAC  205  is 
   
     
       
         
           
             
               f 
               s 
             
             ≥ 
             
               1 
               
                 T 
                 OFDM 
               
             
           
           = 
           B 
         
       
     
   
   This analog signal will pass through a transmission filter  206 , in which the cutoff frequency (marked as f cutoff ) is B/2. A frequency-hopping controller  209  controls the frequency of the oscillating signal of a frequency synthesizer  208 , and a mixer  207  modulates the analog signal according to the frequency of the oscillating signal to produce a RF signal. As shown in  FIG. 1 , the carrier frequency output by the frequency synthesizer  208  is
 
f c =f c1 , f c2 , or f c3  
 
   In which, f c2 −f c1 =B and f c3 −f c2 =B. 
   Please refer to  FIG. 3 , which shows a block diagram of the conventional receiver of the MB-OFDM system. The frequency of an oscillating signal of a frequency synthesizer  308  is first set at an initial frequency (one of f c1 , f c2  and f c3 ) A mixer  307  is used for demodulating a received RF signal according to the frequency of the oscillating signal into a baseband signal that will then pass through a low pass filter (LPF)  306  with a cutoff frequency of B/2. After the filtered baseband signal is sampled by an analog-to-digital converter (ADC)  305 , the sampled signal is monitored by a packet detector  310  to set appropriate frequency-hopping time points. The frequency synthesizer  308  outputs different carrier frequencies according to the appropriate frequency-hopping time point controlled by a frequency-hopping controller  309 . On the other hand, the sampled signal from the ADC  305  passes through a GI removing circuit  303  and a serial-to-parallel converter (S/P)  304 , and then enters a fast Fourier transform (FFT) device  302  to be converted into a frequency-domain signal. After the compensation of a channel compensation device  311 , a signal-demapping circuit  301  is used to generate the originally transmitted signal. 
   However, due to the very short frequency-hopping time stipulated by the MB-OFDM system, the frequency-hopping mechanism needs a very fast reaction rate. Also, as shown in  FIG. 4 , when the previously described mechanism is used for frequency hopping, transient impairment may happen to damage the performance of the MB-OFDM system. In  FIG. 4 , we can see that T FH  is relatively smaller than T GI  and T FFT  within a symbol interval T OFDM . This means the time provided for frequency hopping is shorter. Therefore, the transient impairment is easy to happen in the transmitting signal within the period of T FH . 
   Moreover, in the receiver as shown in  FIG. 3 , if the packet detector  310  unfortunately makes a mistake, or the frequency-hopping controller  309  makes a wrong decision, it is very possible to lose signal due to improper setting of the frequency-hopping time point. This will cause unrecoverable continuous mistakes and damage the performance of the MB-OFDM system seriously. 
   SUMMARY OF THE INVENTION 
   It is therefore one of objectives of this invention to provide a digital frequency-hopping transceiver and a method thereof to resolve the above-mentioned problem. 
   According to an embodiment of this invention, a transmitting method used in a frequency-hopping communication system is provided. The frequency-hopping communication system hops between N sub-bands of a channel for signal transmission (N&gt;1). The transmitting method comprises: generating a digital frequency-shifted signal according to a digital baseband signal, wherein the digital frequency-shifted signal comprises information of the digital baseband signal and hops between N baseband sub-bands corresponding to the N sub-bands of the channel; converting the digital frequency-shifted signal into an analog frequency-shifted signal; modulating the analog frequency-shifted signal into a frequency-hopping transmission signal; and transmitting the frequency-hopping transmission signal through the N sub-bands of the channel. 
   According to an embodiment of this invention, a receiving method used in a frequency-hopping communication system is provided. The frequency-hopping communication system comprises a transmitter, which converts a baseband transmitting signal into a frequency-hopping signal and transmits the frequency-hopping signal to a receiver through a channel comprising N sub-bands. The receiving method comprises: demodulating the frequency-hopping signal into an analog baseband signal according to an oscillation frequency; converting the analog baseband signal into a digital baseband signal; and generating a baseband receiving signal according to frequency dehopping of the digital baseband signal, wherein the baseband receiving signal comprises information of the baseband transmitting signal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a diagram of frequency-hopping mechanism of the MB-OFDM system. 
       FIG. 2  illustrates a block diagram of the conventional transmitter of the MB-OFDM system. 
       FIG. 3  illustrates a block diagram of the conventional receiver of the MB-OFDM system. 
       FIG. 4  illustrates transient impairment generated during frequency hopping by the conventional technology. 
       FIG. 5  illustrates a block diagram of an embodiment of a frequency-hopping transmitter according to the present invention. 
       FIG. 6  is a block diagram of a preferred embodiment of the frequency-hopping transmitter according to the present invention. 
       FIG. 7  is a block diagram of an embodiment of a frequency-hopping receiver according to the present invention. 
       FIG. 8  is a block diagram of a preferred embodiment of the frequency-hopping receiver according to the present invention. 
       FIG. 9  is a flowchart of an embodiment of the frequency-hopping transmitting method according to the present invention. 
       FIG. 10  is a flowchart of an embodiment of the frequency-hopping receiving method according to the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   For easier narration, the frequency-hopping communication system mentioned in all embodiments of this section hops between N sub-bands of a channel for signal transmission (N&gt;1), and is described as the MB-OFDM system. However, the present invention is not limited thereto. 
     FIG. 5  illustrates a block diagram of an embodiment of a frequency-hopping transmitter  50  according to the present invention. The frequency-hopping transmitter  50  includes a frequency-hopping module  51 , a transmitting converter  52  and a modulator  53 , as shown in  FIG. 5 . The frequency-hopping module  51  generates a digital frequency-shifted signal, which hops between N first baseband sub-bands, according to a digital baseband transmission signal. The digital frequency-shifted signal keeps the information of the digital baseband transmission signal, and the N first baseband sub-bands have a corresponding relationship with the N sub-bands of the channel. The transmitting converter  52  is coupled to the frequency-hopping module  51 , and converts the digital frequency-shifted signal into an analog frequency-shifted signal. The modulator  53  is coupled to the transmitting converter  52 , and modulates the analog frequency-shifted signal according to a carrier frequency into a frequency-hopping transmission signal, which is then transmitted by an antenna. The frequency-hopping transmission signal hops among the N sub-bands of the channel according to a frequency-hopping sequence of the digital frequency-shifted signal. 
     FIG. 6  illustrates a more detailed block diagram of the frequency-hopping transmitter  50  of this invention. As shown in  FIG. 6 , the frequency-hopping transmitter  50  is located in the transmitter of the frequency-hopping communication system. The transmitting converter  52  includes a DAC  521 , which receives the digital frequency-shifted signal from the frequency-hopping module  51  and converts it into an analog signal. In order that the bandwidth of the analog signal can cover the whole variable frequency range, the sampling frequency f s1  of the DAC  521  has to be at least the difference B total  of the highest frequency and the lowest frequency of the N sub-bands of the channel. The transmitting converter  52  also includes a transmission filter  522  that filters the analog signal output by the DAC  521  to generate the analog frequency-shifted signal. In a preferred embodiment, the transmission filter  522  is a low-pass filter, which has a cut-off frequency in between B total /2 and f s1 -B total /2 to prevent from the aliasing effect and signal distortion. 
   The frequency-hopping module  51  includes a frequency up-converter  511 , a digital low-pass filter  512 , a shift signal generator  514 , a multiplier  513  and a frequency-hopping controller  515 . As mentioned earlier about  FIG. 2 , the signal to be transmitted will become a baseband signal (i.e., the digital baseband transmission signal as shown in  FIG. 5 ) after being processed by the circuit blocks  201  to  204 . The frequency up-converter  511  will then up-convert the baseband signal into an up-converted signal according to an up-converting factor M 1  (i.e., insert (M 1 -1) zeros in between any two sampling points). The low-pass filter  512  will filter the up-converted signal and produce a filtered signal. In a preferred embodiment, M 1  is equal to f s1 /B and the cut-off frequency of the low-pass filter  512  is π/M 1 , in which, B is the Symbol Rate of the communication system. One objective for the selection of the up-converting factor M 1  and the cut-off frequency of the low-pass filter  512  is to extend the frequency range that the digital frequency spectrum (i.e., from −π to π) can represent, such that digital signal processing (DSP) can be applied for frequency hopping. Another objective is to keep the bandwidth of the analog frequency-shifted signal, which is produced after digital-to-analog conversion, at the Symbol Rate B. 
   The shift signal generator  514  produces N transmission shift signals corresponding to the N first baseband sub-bands. The frequency-hopping controller  515 , coupled to the shift signal generator  514 , selects one of the N transmission shift signals according to a predetermined sequence, and outputs the selected transmission shift signal. The multiplier  513  receives the filtered signal from the low-pass filter  512  and the selected transmission shift signals from the shift signal generator  514 , and then sequentially shifts the filtered signal to each corresponding first baseband sub-band, thereby producing the digital frequency-shifted signal that hops among the N first baseband sub-bands. The hopping sequence of the digital frequency-shifted signal is set up in accordance with the need of the communication system, and there are no other limitations. In a preferred embodiment, the channel of the communication system includes N adjacent sub-bands. Every sub-band has a bandwidth of B, and therefore, B total =N×B. If the sampling frequency f s1  is set as NB, then the cut-off frequency of the transmission filter  522  is NB/2, M 1 =N, and the N transmission shift signals produced by the shift signal generator  514  can be 
   
     
       
         
           
             c 
             n 
           
           = 
           
             ⅇ 
             
               j 
               ⁢ 
               
                   
               
               ⁢ 
               2 
               ⁢ 
               π 
               ⁢ 
               
                   
               
               ⁢ 
               
                 Ω 
                 c 
               
               ⁢ 
               n 
             
           
         
       
     
     
       
         
           
             In 
             ⁢ 
             
                 
             
             ⁢ 
             which 
           
           , 
           
             
               Ω 
               c 
             
             = 
             
               - 
               
                 
                   ( 
                   
                     N 
                     - 
                     1 
                   
                   ) 
                 
                 
                   2 
                   ⁢ 
                   N 
                 
               
             
           
           , 
           
             - 
             
               
                 ( 
                 
                   N 
                   - 
                   3 
                 
                 ) 
               
               
                 2 
                 ⁢ 
                 N 
               
             
           
           , 
           … 
           ⁢ 
           
               
           
           , 
           
             
               ( 
               
                 N 
                 - 
                 3 
               
               ) 
             
             
               2 
               ⁢ 
               N 
             
           
           , 
           
             
               ( 
               
                 N 
                 - 
                 1 
               
               ) 
             
             
               2 
               ⁢ 
               N 
             
           
         
       
     
   
   In the embodiment of  FIG. 6 , the modulator  53  includes a local oscillator  531  and a mixer  532 . The oscillation frequency of the local oscillator  531  is determined according to the corresponding relationship of the N first baseband sub-bands and the N sub-bands of the channel as mentioned before. In this embodiment, the N sub-bands and the corresponding N first baseband sub-bands are each distanced by a frequency shift amount. The frequency shift amount equals the central frequency f c  of the channel; therefore, the oscillation frequency is set as f c . The mixer  532  modulates the analog frequency-shifted signal output by the transmitting converter  52  into the frequency-hopping transmission signal mentioned above according to the oscillating frequency of the local oscillator  531 . The frequency-hopping transmission signal is then transmitted through the antenna. 
     FIG. 7  illustrates a block diagram of an embodiment of a frequency-hopping receiver  70  according to this invention. Both the frequency-hopping receiver  70  and the frequency-hopping transmitter  50  in  FIG. 5  can be used in the same frequency-hopping communication system, thereby restoring the frequency-hopping transmission signal transmitted through the channel by the frequency-hopping transmitter  50  back to the original digital baseband transmission signal. As shown in FIG.  7 , the frequency-hopping receiver  70  includes a frequency-dehopping module  71 , a receiving converter  72  and a demodulator  73 . The demodulator  73  can demodulate the frequency-hopping transmission signal received from the channel into an analog baseband signal. The receiving converter  72  is coupled to the demodulator  73 , and converts the analog baseband signal into a digital baseband signal. This digital baseband signal retains the information of the frequency-hopping transmission signal and hops among N second baseband sub-bands, in which the N second baseband sub-bands have a corresponding relationship with the N sub-bands of the channel. The frequency-dehopping module  71  can receive the digital baseband signal from the receiving converter  72 , and perform digital signal processing thereon (including performing the frequency dehopping for the digital baseband signal), thereby restoring the original digital baseband transmission signal. 
     FIG. 8  illustrates a block diagram of a preferred embodiment of the frequency-hopping receiver  70  of this invention. As shown in  FIG. 8 , the frequency-hopping receiver  70  is located in the receiver of the MB-OFDM system. The demodulator  73  includes a local oscillator  731  and a mixer  732 . The oscillation frequency of the local oscillator  731  is determined by the corresponding relationship of the N second baseband sub-bands and the N sub-bands of the channel. In the embodiment of  FIG. 8 , the N sub-bands and the corresponding second baseband sub-bands are each distanced by a frequency shift amount. The frequency shift amount equals the central frequency f c  of the channel, and therefore, the oscillation frequency is set as f c . The mixer  732  is coupled to the local oscillator  731  and the receiving converter  72 , and demodulates the received frequency-hopping transmission signal into the analog baseband signal according to the oscillation frequency of the local oscillator  731 . 
   In the embodiment of  FIG. 8 , the receiving converter  72  includes a low-pass filter  722  which filters the analog baseband signal from the mixer  732 . The receiving converter  72  also includes an ADC  721 , which converts the filtered analog baseband signal into the digital baseband signal shown in  FIG. 7 . The sampling frequency f s2  of the ADC  721  has to be at least the difference B total  of the highest frequency and the lowest frequency of the N sub-bands of the channel, so that the converted digital signal bandwidth (i.e., from −π to π) can cover the whole variable frequency range of the communication system. Since the mixer  732  performs signal demodulation according to the central frequency f c , the cutoff frequency of the low-pass filter  722  can be set as B total /2, thereby filtering out the noise that does not belong to the variable frequency range of the communication system. 
   In the embodiment of  FIG. 8 , the frequency-dehopping module  71  includes a shift signal generator  714 , N multipliers  713 , N digital low-pass filters  712 , N frequency down-converters  711 , a detector  716 , a frequency-dehopping controller  715  and a multiplexer  717 . The shift signal generator  714  can generate N reception shift signals corresponding to N second baseband sub-bands. The digital baseband signal output by ADC  721  is applied to N routes, each sent into one of the N multipliers  713  and multiplied by one of the N reception shift signals. Each of the N digital low-pass filters  712  is individually coupled to one of the N multipliers  713 , and filters the output result of the corresponding multiplier  713  to generate a filtered signal. The cutoff frequency of the N digital low-pass filters  712  is π/M 2 , wherein M 2  equals to f s2 /B. Each of the N frequency down-converters  711  is individually coupled to one of the N digital low-pass filters  712 , and down-converts one of the N filtered signals according to a down-converting factor M 2  and then outputs a down-converted signal. In one embodiment, the channel of the MB-OFDM system includes N adjacent sub-bands. The bandwidth of each sub-band is B, and therefore, B total =N×B. If the sampling frequency f s2  of the ADC  721  is set as NB, then M 2 =N, and the N reception shift signals produced by the shift signal generator  714  can be 
   
     
       
         
           
             c 
             
               n 
               , 
               k 
             
           
           = 
           
             ⅇ 
             
               
                 - 
                 j 
               
               ⁢ 
               
                   
               
               ⁢ 
               2 
               ⁢ 
               
                   
               
               ⁢ 
               π 
               ⁢ 
               
                   
               
               ⁢ 
               
                 Ω 
                 
                   c 
                   , 
                   k 
                 
               
               ⁢ 
               n 
             
           
         
       
     
     
       
         
           
             In 
             ⁢ 
             
                 
             
             ⁢ 
             which 
           
           , 
           
             
               Ω 
               
                 c 
                 , 
                 k 
               
             
             = 
             
               
                 - 
                 
                   1 
                   2 
                 
               
               + 
               
                 
                   
                     2 
                     ⁢ 
                     k 
                   
                   + 
                   1 
                 
                 
                   2 
                   ⁢ 
                   N 
                 
               
             
           
           , 
           
             k 
             = 
             0 
           
           , 
           1 
           , 
           … 
           ⁢ 
           
               
           
           , 
           
             N 
             - 
             1. 
           
         
       
     
   
   The digital baseband signal that originally hops among the N second baseband sub-bands will be frequency-dehopped after the digital signal processing (including frequency shifting, filtering, and down-converting) mentioned above, and will return to the original digital baseband transmission signal (i.e., the baseband OFDM signal in this embodiment), which can be collected from the N down-converted signals. The detector  716  is coupled to each frequency down-converter  711  to detect the down-converted signals that form the original digital baseband transmission signal. The frequency-dehopping controller  715  is coupled to the detector  716 , and generates a frequency-dehopping control signal according to the result of detection. The multiplexer  717  is coupled to the N frequency down-converters  711 , and sequentially selects one of the N frequency down-converters  711  according to the frequency-dehopping control signal, thereby outputting the digital baseband transmission signal. In one embodiment, the detector  716  is a packet detector, which will set up an appropriate frequency-hopping time point when a packet is detected, and will then activate the frequency-dehopping controller  715  to output the frequency-dehopping control signal according to a predetermined frequency-hopping sequence of the system. Lastly, after the restored digital baseband transmission signal sequentially passes through the circuit blocks  303 ,  304 ,  302 ,  311  and  301 , the original signal transmitted by the system transmitter can be obtained. This part of signal processing is similar to the like part of  FIG. 3 , and will not be described again here. 
   In another embodiment, the frequency-hopping transmitter  50  in  FIG. 5  and the frequency-hopping receiver  70  in  FIG. 7  can be used separately. For example, the embodiment of the system transmitter shown in  FIG. 6  can be used together with the conventional receiver as shown in  FIG. 3 . Also, the embodiment of the system receiver shown in  FIG. 8  can be used together with the conventional transmitter as shown in  FIG. 2 . 
     FIG. 9  is a flowchart of a preferred embodiment of the frequency-hopping transmitting method according to the present invention. The flow corresponds to the frequency-hopping transmitter  50  in  FIG. 6 , and comprises the following steps:
         Step  901 : Up-converting a digital baseband transmission signal to output an up-converted signal according to an up-converting factor M 1 ;   Step  902 : Filtering the up-converted signal to generate a filtered signal according to a cutoff frequency π/M 1 ;   Step  903 : Generating N transmission shift signals corresponding to N first baseband sub-bands, wherein the N first baseband sub-bands are corresponding to the N sub-bands of the channel;   Step  904 : Multiplying the filtered signal by the N transmission shift signals to shift the filtered signal to the corresponding first baseband sub-bands, thereby producing a digital frequency-shifted signal;   Step  905 : Converting the digital frequency-shifted signal into an analog signal according to a sampling frequency f s1 , wherein the sampling frequency f s1  is not smaller than B total ;   Step  906 : Filtering the analog signal according to a first cutoff frequency to generate an analog frequency-shifted signal, wherein the first cutoff frequency is between B total /2 and f s1 −B total /2; and   Step  907 : Modulating the analog frequency-shifted signal to generate a frequency-hopping transmission signal according to the corresponding relation between the sub-bands, and transmitting the frequency-hopping transmission signal through the channel.       
     FIG. 10  is a flowchart of a preferred embodiment of the frequency-hopping receiving method according to this invention. The flow corresponds to the frequency-hopping receiver  70  in  FIG. 8 , and comprises the following steps:
         Step  1001 : Demodulating a frequency-hopping transmission signal received from the channel to produce an analog baseband signal;   Step  1002 : Filtering the analog baseband signal in accordance with a cutoff frequency B total /2;   Step  1003 : Converting the filtered analog baseband signal into a digital baseband signal in accordance with a sampling frequency f s2 ;   Step  1004 : Generating N reception shift signals that correspond to N second baseband sub-bands;   Step  1005 : Multiplying the digital baseband signal with the N reception shift signals to produce N multiplication results;   Step  1006 : Respectively filtering the N multiplication results to generate N filtered signals in accordance with a cutoff frequency π/M 2 ;   Step  1007 : Down-converting the N filtered signals to generate N down-converted signals according to a down-converting factor M 2 ; and   Step  1008 : Selectively outputting one of the N down-converted signals to generate the original digital baseband transmission signal.       
   While the present invention has been shown and described with reference to the preferred embodiments thereof and in terms of the illustrative drawings, it should not be considered as limited thereby. Various possible modifications and alterations could be conceived of by one skilled in the art to the form and the content of any particular embodiment, without departing from the scope and the spirit of the present invention.