Abstract:
FFT demodulation of OFDM symbols is achieved using a frame synchronization pulse generated by deriving absolute values of successive complex samples of the OFDM symbol, determining the difference between such values and other values separated therefrom by a period representing the useful part of the OFDM symbol, integrating the difference values over a plurality of symbols and determining the sample position representing the point at which the integrated difference values substantially change.

Description:
FIELD OF THE INVENTION 
   This invention relates to OFDM modulation. It is particularly concerned with the generation of a synchronisation pulse representing an OFDM symbol boundary, for example for use in Fourier Transform demodulation. 
   BACKGROUND OF THE INVENTION 
   OFDM systems are well known. Various techniques have been used for synchronisation of OFDM receivers. Some of these techniques require transmission of a special synchronisation signal. Other techniques rely on a standard OFDM signal, in which a complete symbol comprises a “useful part” and a “guard space”, the guard space sometimes being referred to as a guard interval, cyclic extension or cyclic prefix. 
   The guard space precedes the useful part of the symbol and contains a repeat of the data at the end of the useful part. (This is equivalent to having a guard space after the useful part, containing data which is the same as that at the beginning of the useful part.) 
   Synchronisation techniques which rely upon the duplicated data in the guard space generally operate by performing a cross correlation between complex samples spaced apart by the length of the useful part of the symbol. This generates a timing pulse which is used in Fourier Transformation of the received signal. The timing of the pulse is such that the Fourier Transform window contains only data from a single symbol. 
   If the timing is incorrect, inter-symbol-interference (ISI) occurs. However, the use of the guard space allows a certain amount of variation in the timing of the pulse while still avoiding ISI. The guard space should be longer than the longest expected spread of delays amongst signals received via different paths. The guard space is relatively small compared with the useful part of the signal; typically, the guard space may contain Nu/32, Nu/16, Nu/8 or Nu/4 samples, where Nu is the number of samples in the useful part of the symbol. 
   It would be desirable to provide a simpler and less expensive technique for synchronisation, and preferably one which can produce more accurate results. 
   Aspects of the present invention are set out in the accompanying claims. 
   According to a further aspect, a synchronisation pulse for a Fourier Transform demodulator is generated by taking absolute values of complex samples of an OFDM symbol, determining the difference between absolute values which are separated by the length of the useful part of the signal, and generating the synchronisation pulse in response to determining the time at which there is a substantial increase in the difference value. 
   The invention also extends to apparatus for generating such a synchronisation pulse, and to a method and an apparatus for receiving an OFDM signal using such techniques. 
   Using the method of the invention, the difference between the absolute values of the complex values will be very small when those complex values are equal. Thus, this difference will remain small whenever the samples in a guard space are subtracted from the corresponding values within the useful part of the symbol. When the values cease to be small, this represents the end of the symbol, and this point can be used to allow an FFT demodulator window to be aligned with the OFDM symbol. 
   It is found that, for example because of slight drift in the local oscillator, the complex samples of an OFDM signal can suffer different phase rotations within the period of a single symbol. Thus, although nominally the data in a guard space should correspond to part of the data in the useful part of the symbol, the actual complex values derived from the signal may differ because of this phase rotation. However, by taking the absolute values of the complex samples, such differences are reduced substantially. Furthermore, because the system handles real, rather than complex, values, data storage requirements are reduced. The complexities and inaccuracies associated with multipliers can also be avoided. The arrangement can also produce better accuracy than these systems which rely on integrating the output of a cross-correlator, and better acquisition times than prior art systems relying on phase locked loops (PLL&#39;s). 
   Preferably, the difference values are integrated over a number of symbols. Preferably this is achieved using an infinite impulse response (IIR) filter. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An arrangement embodying the arrangement will now be described by way of example with reference to the accompanying drawings, in which: 
       FIG. 1  is a block diagram of an OFDM receiver; 
       FIG. 2  represents an OFDM signal; 
       FIG. 3  schematically illustrates the processing carried out in a Fourier Transform synchroniser circuit of a receiver according to the present invention; 
       FIG. 4  shows a typical trace of an absolute difference signal arising in the synchronising circuit of  FIG. 3 ; and 
       FIG. 5  is a typical output from a rising edge detector of the synchronising circuit. 
   

   DETAILED DESCRIPTION 
   Referring to  FIG. 1 , an OFDM receiver  2  comprises an antenna  4  which receives a signal and presents it to a down-converter  6  which converts the RF signal to an IF signal. This is then converted into a baseband signal by an IF-to-baseband converter  8 . This produces at its output complex samples of each transmitted OFDM symbol. These complex samples are digitised by an analog-to-digital (A/D) converter  10 , and delivered to a Fast Fourier Transform (FFT) circuit  14 . The FFT circuit  14  converts the samples from the time domain to the frequency domain, and the symbol data at the output is provided to a channel estimator and corrector  16  and a decoder  17 . If desired, the complex samples from the A/D converter  10  and/or a signal from the channel estimator and corrector  16  may also be delivered to a frequency synchronisation circuit  18 , which is used to control the local oscillator frequencies used by the down converter  6  and the IF-to-baseband converter  8 ; however, it is also possible to implement the recever without requiring such feedback. 
   The complex samples are also delivered to a symbol synchronisation circuit  20 , which generates a synchronisation pulse for use by the Fast Fourier Transform (FFT) circuit  14 . The FFT circuit  14  requires the synchronisation pulse so that each transformation operation is aligned with the start of the OFDM symbol. 
   The circuit described so far is known from the prior art. The present invention is directed to a novel and inventive technique for use in the symbol synchronisation circuit  20 . 
   Referring to  FIG. 2 , it is assumed that an OFDM symbol consists of Nu+Ng samples, representing Nu samples in the useful part U of the signal, preceded by Ng samples in the guard space G. The Ng samples in the guard space G contain the same data as the last Ng samples of the useful part U of the symbol (as indicated, in respect of one of the symbols, by hatching). 
     FIG. 3  is a schematic block diagram representing the functions performed by the circuit  20 . 
   The complex samples from the IF-to-baseband converter  8  are converted into absolute values by an absolute value converter  21 . These output values are delivered to a subtractor  22  and to delay circuit  24 , which delays the values by an amount corresponding to the length of the useful part U of the OFDM symbol. Accordingly, the delay circuit  24  delays the absolute values by a period corresponding to Nu samples. The subtractor  22  subtracts the delayed absolute values from the non-delayed values received from the absolute value converter  21 . The resultant difference value is converted into an absolute value by a second absolute value converter  26 . The output of the absolute value converter  26  will therefore be substantially zero during the period that the last Ng samples contained in the useful part U are present at the input of the absolute value converter  21 , because at this time the subtractor will be receiving those samples at its positive input and, at its negative input, samples of equal value derived from the guard space G. At other times, the output of the absolute value converter  26  would adopt values determined by the correlation, if any, between data in successive symbols, but in practice this would be essentially random. 
   The output of the absolute value converter  26  will therefore comprise successive absolute difference values, each associated with a respective sample of the OFDM symbol. These difference values are then passed to an integrating circuit  28 , which serves to integrate each absolute difference value over a number of symbols. 
   This is achieved by multiplying each difference value by a factor G 1  in a multiplier  30 , the output of which is delivered to a positive input of a summing circuit  32 . The output of the summing circuit is sent to a delay circuit formed by a first-in, first-out (FIFO) memory  34  which serves to delay each of its input signals by a period corresponding to an entire OFDM symbol, i.e. Nu+Ng samples. The output of the delay circuit  34  is delivered to a positive input of the summer  32 , and also to a further multiplying circuit  36  which multiplies the output by G 2 . The resultant of this is sent to a negative input of the summer  32 . 
   As a consequence of this arrangement, the output of the summer comprises successive samples S(n,m), where n represents the number of the sample within the OFDM symbol and m represents the symbol, and wherein:
 
 S ( n,m )= G 1( V ( n,m ))+(1 −G 2) S ( n,m −1)
 
and wherein V(n,m) represents the absolute value for the sample position n of symbol m.
 
   A typical output of the integrating circuit  28  is shown in  FIG. 4 . It will be noted that there are spaced periods in which the output is substantially zero, corresponding to the periods in which the last Ng symbols of the useful part U are present at the input of the absolute value converter  21 . These periods are separated by intervals in which there is a substantial amount of random data. The distinction between the two types of intervals is enhanced by integration over a number of symbol periods. 
   The output of the summing circuit  32  is delivered to a rising edge detector  38 , a typical output of which is shown in  FIG. 5 . This output is delivered to a peak detector  40 , which detects the trailing edge of each of the periods corresponding to the presence of the guard interval at the output of the delay circuit  34 . The peak detector  40  outputs a logic high signal precisely once per OFDM frame, which is used as the sync pulse denoting the end of the OFDM symbol FFT window. 
   In the preferred embodiment, the delay circuit  34  comprises a plurality of memory elements or registers arranged in a FIFO array. Instead of taking the output of summer  32  for processing in real time, the contents of the registers in the delay circuit  34  can be processed to determine the register (and hence the sample) corresponding to the end of the OFDM symbol. 
   Whichever arrangement is used, the edge detection and peak detection can be achieved using either hardware or software. The rising edge detector  38  may be implemented by a FIR filter, which may have variable coefficients which are altered following switch on or re-tuning to speed up the locking on to the signal. For example, the FIR filter may be arranged to subtract from the value of the latest sample the values of the preceding n samples. This will give a significant positive value only at the end of the symbol, when the difference between the absolute values ceases to be zero. The value n may be altered not only when acquiring a signal, but also in dependence on, for example, the risk of multi-path interference; when there is significant multi-path interference, there will be noise in the initial part of the period where the difference between the absolute values ought ideally to be zero, so a smaller value for n would be appropriate. 
   The invention has been described in the context of an OFDM receiver, in which the synchronisation pulse is used to define the window of samples on which a Fast Fourier Transformation is performed. However, the invention is also useful in other circumstances in which there is a need for a synchronisation pulse representing the symbol boundaries; for example, such a pulse would be valuable in a repeater where full FFT demodulation is not performed. 
   The functional elements described herein can be implemented either in dedicated hardware or in software.