Patent Publication Number: US-7583742-B2

Title: Multi-band DMT receiver

Description:
This application is a continuation of U.S. application Ser. No. 09/686,784, entitled “Multi-Band DMT Receiver,” filed Oct. 10, 2000. 

   FIELD OF THE INVENTION 
   The present invention relates to the demodulation of a multi-band DMT signal in a receiver, and particularly but not exclusively to the demodulation of a multi-band DMT signal in the transceiver of a modem. 
   BACKGROUND TO THE INVENTION 
   Referring to  FIG. 1  there is shown an example of a multi-band signal transmitted in the upstream (or uplink) of a communications system. In this example it is assumed that the multi-band signal comprises two bands. As shown in  FIG. 1 , a first band is defined between the frequencies f 1  and f 2 , and a second band is defined between the frequencies f 3  and f 4 . A downstream frequency band may be provided between the frequencies f 2  and f 3 , and a further downstream frequency band may be provided beyond the frequency f 4 . 
   Techniques for transmitting and receiving such multi-band signals are well known. For example, in modem technology a multi-carrier signal having multiple frequency bands is transmitted. 
   At the receive side, such multi-band signals require a large amount of processing. The processing speed of the receiver is determined by the highest frequency of the multi-band signal. That is, the receiver has to operate at a speed such that the received signals having the highest frequencies can be processed within system constraints. 
   It is an object of the present invention to provide an improved multi-band DMT receiver in which an improved processing of the received signal is achieved. 
   SUMMARY OF THE INVENTION 
   According to a first aspect of the invention there is provided a receiver for receiving a multi-band signal modulated using an inverse discrete Fourier transform, comprising: a plurality of demodulators, each demodulator for demodulating a respective one of the plurality of bands in a multi-band signal wherein each demodulator includes a discrete Fourier transform. Thus, the processing of the multi-band signal is spread amongst more than one demodulator, so that each discrete Fourier transform (DFT) can be optimised for a particular frequency band. 
   The process speed of each demodulator may then be determined by the respective frequency band. That is, if a particular demodulator processes a frequency band having a high frequency, then a corresponding high sampling speed is required in the respective demodulator. A lower frequency band requires a lower sampling speed in the respective demodulator. Thus the process speed of each demodulator is preferably determined by the respective frequency band of the signal processed therein. 
   Each demodulator may further include an equaliser connected to the output of the discrete Fourier transform. Each demodulator may further include a filter for filtering the received signal prior to the discrete Fourier transform. 
   The multi-band signal may be generated by nulling selected tones in the modulator. In addition to. Or alternatively, the multi-band signal may be generated by filtering the output of the modulator. 
   The receiver may comprise part of a transceiver. In such a transceiver, each demodulator may further include an echo canceller for removing an echo associated with the signal in a transmitter of the transceiver from the received signal. The echo canceller may be connected to remove the echo at the inputs of the discrete Fourier transform. Each echo canceller may comprise an adaptive filter. 
   In a second aspect the present invention provides a method of demodulating a multi-band signal modulated using an inverse discrete Fourier transform, comprising the step of: providing a demodulator for each respective band in a multi-band signal, wherein each demodulator performs a discrete Fourier transform. 
   Each demodulator may further comprise an equalisation step. Each demodulator may further filter the received signal prior to the discrete Fourier transform. 
   The multi-band signal may be generated by nulling selected tones in the modulator. In addition to. Or alternatively, the multi-band signal may be generated by filtering the output of the modulator. 
   The demodulating step may be carried out in a transceiver. In a transceiver, each demodulator further performs an echo cancellation step to remove an echo associated with a signal in a transmitter of the transceiver from the received signal. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
     The invention will now be described with regard to an illustrative example with reference to the accompanying drawings in which: 
       FIG. 1  illustrates the frequency spread of a two-band signal in one direction; 
       FIG. 2  illustrates a multi-band transmission system in accordance with a preferred implementation of the present invention; 
       FIGS. 3(   a ) and  3 ( b ) illustrate alternative implementations of the splitter of the receiver of  FIG. 2 ; and 
       FIG. 4  illustrates a DMT receiver in accordance with a preferred embodiment of the present invention. 
   

   DESCRIPTION OF PREFERRED EMBODIMENT 
   In the following illustrative example the present invention will be described by way of reference to a particular implementation in which a modem transceiver transmits a DMT (discrete multi-tone) signal, and the DMT signal is received by a transceiver of another modem. 
   Referring to  FIG. 2  there is illustrated a first modem  214  and a second modem  218 . For the purposes of this example it is assumed that the first modem  214  transmits a multi-band signal to the second modem  218 . Therefore only the transmitter portion of a transceiver of the first modem  214  is discussed in detail and only a receiver portion of the transceiver of the second modem  218  is discussed in detail. 
   In accordance with known techniques, the transmitter of the first modem  214  includes an inverse discrete Fourier transform  204 , a digital to analogue converter  218 , and a hybrid  212 . As the transmitter described herein is a DMT transmitter, there is also provided a cyclic prefix (CP) insertion block  205 . The inverse discrete Fourier transform (IDFT) block  204  receives on a plurality of signal lines  202  data to be encoded for transmission. The thus encoded data is output by the IDFT block  204  in series on line  207 . The operation of the IDFT block  204  is outside the scope of the present invention, and its implementation will be well know to one skilled in the art. The CP insertion block inserts a 32 sample guard band in front of the 512 sample symbols generated by the IDFT. As known in the art, the CP insertion includes repeating the last 32 sample of the 512 sample symbol to thereby generate a 544 sample symbol. 
   Modulated and encoded data output by the CP insertion block  205  on line  206  is converted into an analogue signal on line  210  by the digital to analogue converter  208 , and the hybrid  212  transmits the analogue signal on a channel  216 . 
   The plurality of signals on line  202  are provided by a transmitter which generates a discrete multi-tone (DMT) signal with N/2 tones with a tone spacing of ΔF. Consequently the IDFT generates a transform having N points. Thus, the bands of the multi-band signal are generated by nulling certain tones at the input to the IDFT  204 . In addition, the nulling may be assisted by appropriate filters at the output of the IDFT  204 . 
   After the CP insertion the symbols may be optionally windowed. Windowing shapes the symbols before transmission in order to reduce the leakage in the adjacent band. Windowing multiplies a certain number of samples at the beginning and end of a symbol by a weighting function. Usually the number of windowed samples is smaller than the length of the guard-band. The shaping of two contiguous symbols could overlap. This windowing principle is used, for example, in the VDSL multi-carrier standard. 
   The analogue signal on a channel  216  is received by the transceiver of the modem  218 . The transceiver of the modem  218  includes a hybrid  220  and a splitter  221 . In accordance with the present invention, the receiver of the modem  218  in addition comprises a plurality, in this example two, of receivers  224   a  and  224   b . In this example it is assumed that the signal transmitted by the transmitter of the first modem  214  corresponds to the signal of  FIG. 1  and includes two frequency bands. In general the signal will include a plurality n of frequency bands, and the receiver of the modem  218  will be provided with a plurality n of the receivers  224 . 
   In addition the modem  218  includes transmitter circuitry  223  which provides a signal to be transmitted to the hybrid  220  on line  225 . 
   Each of the demodulators  224   a  and  224   b  is constructed identically, and the various components therein have identical reference numerals except for the designation of a or b. Thus the receiver  224   a  will be described hereinafter, and it will be understood that demodulator  224   b  is constructed in exactly the same manner. 
   Before discussing the receivers  224 , reference is made to  FIGS. 3(   a ) and  3 ( b ) which illustrate in block diagram form respective digital and analogue implementations of the splitter  221 . 
   In a digital implementation, the signal on line  222  is converted into an analogue signal on line  316  by an analogue to digital converter  300 , and then presented to respective low and high pass filters  302   a  and  302   b . The output of the low pass filter  302   a  on line  314   a  is processed by block  304 , which in turn generates a signal to the first receiver  224   a . The block  304  is a downsampler. The principle of this device is to reduce the sampling speed by a certain factor. For example a downsampler by 2 divides the sampling speed by 2 between its input and output. The division is done by taking one sample every two samples. For the input sequence 1 2 3 4 5 6 7 8 9 10 . . . to the downsampler, the output sequence is 1 3 5 7 9 11 . . . . The output of the high pass filter  302   b  on line  314   b  forms the signal on line  223   b  to the second receiver. 
   In an analogue implementation, the signal on line  222  is provided to respective low and high pass filters  310   a  and  310   b . The outputs of these filters on lines  315   a  and  316   b  are converted into digital form by analogue to digital converters  312   a  and  312   b , which generate the signals  223   a  and  223   b  to the receivers  224   a  and  224   b.    
   At the output of the splitter both streams are sampled at their respective Nyquist frequency even if the ADC is running at the Nyquist frequency of the highest band. 
   Each filter stream in the digital implementation of  FIG. 3(   a ), and each stream in the analogue implementation of  FIG. 3(   b ), can be processed with a DFT of Fs,k/ΔF points. This is because each stream need only process the points for its particular band. Each stream is sampled at a frequency Fs,k, where Fs,k is at least two times the maximum frequency of the band. 
   The receiver  224   a  comprises a time equaliser  234   a , a subtractor  238   a , a cyclic prefix removal block  239   a , a discrete Fourier transform (FFT)  244   a , a frequency equaliser  248   a , and an echo canceller  242   a . In this preferred embodiment the echo canceller  242   a  is comprised of an adaptive filter. 
   The time equaliser  234   a  is preferably a finite impulse response (FIR) filter. 
   The digitised version of the received signal for the particular frequency band is provided on line  223   a  by a respective one of either the digital or analogue splitters of  FIGS. 3(   a ) and  3 ( b ). In accordance with conventional techniques, the echo canceller  242   a  preferably comprises an adaptive filter and receives a representation on line  225  of the signal in the transceiver for the modem  218  which is being transmitted by the hybrid  220 . The echo canceller  242   a  then provides an estimate of the echo associated with this transmitted signal on line  262   a . The subtractor  238   a  subtracts the estimate of the echo on line  262   a  from the time equalised received signal on line  236   a , to generate an estimate of the received signal on line  240   a . As is known in the art, the signal on line  240   a  is used to control the echo canceller  242   a  to adjust the estimate of the echo on line  262   a.    
   The cyclic prefix (CP) removal block  239   a  operates in the reverse manner to the cyclic prefix (CP) insertion block  205  to remove the 32 samples of the cyclic prefix, forming a guard band, from the 544 samples of the received symbol. When windowing is applied at the transmitter, the cyclic removal process is identical. 
   The estimate of the received signal on line  241   a , after the cyclic prefix removal block, is then input to the discrete Fourier transform  244   a . The outputs of the discrete Fourier transform on line  246   a  are provided to the frequency equaliser  248   a  for equalisation. The thus equalised signals provided on line  250   a  are provided for further processing in the receiver of the modem  218 . The CP removal block  239   a , the discrete Fourier transform  244   a  and the equaliser  248   a  each receive a clocked signal on line  264   a  for controlling the speed of the operations performed therein. 
   The receiver  224   b  is similarly constructed. 
   Thus in accordance with the invention the demodulation of the different bands of the multi-band signal are processed independently such that each modulator can be optimised to perform for that particular frequency band. In the present example it is shown that the receiver  224   a  demodulates the lower frequency band of  FIG. 1 , and the receiver  224   b  demodulates the higher frequency band of  FIG. 1 . In the example that the highest frequency within the lower frequency band is a frequency of 200 kHz, then the sampling speed of the demodulator  224   a  must be at least 400 kHz. If, for example, the highest frequency within the higher frequency band is 2 MHz, then the sampling speed must be at least 4 MHz. Thus lower frequencies can be processed at a lower sampling speed. 
   This contrasts with prior art arrangements, where even low frequency received signals have to be sampled at a sampling speed dictated by the highest possible frequency of the multi-band signal. Therefore in prior art arrangements the 200 kHz signal is processed at a sampling speed of 4 MHz. This is particularly advantageous for the implementation of the echo canceller  242 . For the low frequency bands the echo canceller need only be processed at the low sampling speed (that is at the Nyquist speed), rather than always having to be processed at the speed determined by the Nyquist speed of the highest frequency signal. 
   The main advantage of the invention is that the processing of the lower band, or bands, can be performed at a lower speed. Primarily this means that the time equaliser  234  and the echo canceller  242  consume less CPU processing. 
   Each of the filters  302  or  310  implement the necessary processing to select the frequency band for the respective receiver. 
   The above example presents the invention with particular reference to receipt of a discrete multi-tone (DMT) signal. The invention may be advantageously applied in environments such as asymmetric digital subscriber line (ADSL) technology, or very-high-data-rate digital subscriber line (VDSL) technology for example.