Abstract:
A receiver circuit processes a received signal to determine information regarding a guard interval therein. The receiver circuit typically forms digital samples from the received signal; generates a correlation function with peaks at positions that are determined by the size of the guard interval and a position of the guard interval within the received signal. The receiver circuit includes one or more comb filters, each corresponding to a possible guard interval size, each receiving a series of locations at which the correlation function exceeds a predetermined threshold, and each producing an output that provides information about the guard interval size and/or position. This advantageously allows the receiver circuit to determine the guard interval size quickly, for example, even in the case of received signals with relatively low signal-noise ratio.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 10/340,854, entitled “Guard interval analysis method and apparatus,” filed on Jan. 10, 2003, now U.S. Pat. No. 7,424,048 which is incorporated by reference, which claims the effective filing date under 35 USC §§120 and 363 to PCT International Application No. PCT/EP01/07655, entitled “Receiver Circuit”, filed Jul. 4, 2001 designating the U.S. and published under PCT Article 21(2) in English as International Publication No. WO 02/09383 A1 entitled “Receiver Circuit,” of which this application is a continuation, which PCT application claims priority to Great Britain Patent Application No. 0017131.4, filed Jul. 12, 2000. U.S. patent application Ser. No. 10/340,854 claims priority under 35 USC 119(a) to Great Britain Patent Application No. 0017131.4, filed Jul. 12, 2000, which is incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a communication receiver, in particular for receiving signals in which a portion of a transmitted signal forms a guard interval. 
     2. Description of Related Art 
     The European DVB-T (Digital Video Broadcasting-Terrestrial) standard ETS 300 744 for digital terrestrial television (DTT) uses Coded Orthogonal Frequency Division Multiplexing (COFDM) of transmitted signals, which are therefore grouped into blocks and frames. 
     It is necessary to transmit the DTT signals over transmission paths which are of uncertain quality. In particular, the area close to the transmission path may include objects such as tall buildings, which cause reflections. That is, a signal may be received at a receiver twice, once on a direct path from the transmitter, and then, after a short delay, as an indirect reflected path. As is well known, this can cause inter-symbol interference (ISI) in the receiver. To reduce this problem, DVB-T COFDM signals include a cyclic prefix guard interval to each active symbol. Specifically, the end portion of the active symbol is repeated before the current active symbol. 
     These cyclic signals must be correctly removed before demodulation, or the demodulation performance can be seriously degraded. Before the cyclic signals can be removed, it is necessary to know the duration of the guard interval. This is not a constant value, because the presence of the guard interval reduces the efficiency with which the transmission channel can be used, and so it is advantageous to vary the size of the guard interval to be the minimum allowed by the conditions. 
     The receiver must therefore include a mechanism for determining the size of the guard interval. The chosen mechanism must be robust enough to deal with noisy signals, and with signals which suffer from multipath and co-channel interference. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a receiver circuit which overcomes some of the disadvantages of the prior art. 
     For example, it may be advantageous for the receiver circuit to be able to determine the guard interval size quickly, even in the case of received signals with relatively low signal-noise ratio. 
     In a first aspect, the invention relates to a receiver circuit for analyzing a received signal in regard to a guard interval therein, comprising: a sampler for forming digital samples of the received signal; a correlator for generating from the digital samples a correlation function with peaks at positions which are determined by the size of the guard interval and a position of the guard interval within the received signal; and one or more comb filters, each corresponding to a possible guard interval size, each receiving a series of sample locations at which the correlation function exceeds a predetermined threshold to the series of sample locations. 
     Further, the invention relates in a second aspect to a method of determining a guard interval size of a received signal, the method comprising: forming digital samples of the received signal; generating from the digital samples a correlation function with peaks at positions that are determined by the size of the guard interval and a position of the guard interval within the received signal; determining a series of sample locations at which the correlation function exceeds a predetermined threshold; applying a plurality of comb filters, each corresponding to a possible guard interval size, to the series of sample locations; and determining the guard interval size from outputs of said comb filters. 
     In a third aspect, the invention relates to a method of processing a received signal having guard intervals. The method includes as step of correlating the received signal with a delayed version of itself and selecting peaks at which a function developed from the self-correlation exceeds a predetermined threshold to identify peaks of interest. The method further includes a step of applying a comb filter corresponding to a guard interval size to a selected peak series, developed from the peaks of interest, in order to discriminate peaks that are expected to provide useful information about the guard interval. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified block schematic diagram of a receiver circuit according to the invention. 
         FIG. 2  is a block schematic diagram of a guard detection block component used in the receiver circuit of  FIG. 1 . 
         FIG. 3  is a flow chart of a method according to one aspect of the invention. 
         FIG. 4  is an explanatory diagram of the value of an averaged correlation function produced by a component of the guard detection block of  FIG. 2 . 
         FIG. 5  is a further explanatory diagram describing aspects of the steps used in the method of  FIG. 3 . 
         FIG. 6 , comprising  FIGS. 6(   a )- 6 ( d ), is a further explanatory diagram showing the results obtained using the method of  FIG. 3  and the data of  FIGS. 4 and 5 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a simplified block diagram of a receiver circuit made in accordance with the present invention. Typically, in the exemplary case of a digital terrestrial television signal receiver, for example receiving signals using the DVB-T standard with Coded Orthogonal Frequency Division Multiplexing, the receiver includes an antenna (not shown) and a tuner (not shown) for receiving the signals. As shown in  FIG. 1 , received signals are supplied to a demodulator  2 , which mixes the received RF input signal down to baseband in-phase (I) and quadrature (Q) components and produces digital samples. The baseband I and Q components are supplied to a mode detection block  4 , which determines whether the received signal was transmitted in 2 k or 8 k mode, and provides a mode detection output. The baseband I and Q components are also supplied to a synchronizer block  6 , which removes frequency and sampling rate offsets, and then to a window position block  8 . Following determination of the window position, the received signals are applied to a guard removal block  10 , FFT block  12 , equalizer  14 , demapper  16 , and FEC block  18 . The resulting MPEG data is output to a demultiplexing device (not shown) for ultimate display. 
     The receiver also includes a guard detection block  20 , which determines the size and position of the guard interval, for supply to the window position block  8  and guard removal block  10 , allowing them to operate accurately. 
     The guard detection block  20  is illustrated schematically in  FIG. 2 , and a method for determining the guard interval size will be described with reference to  FIG. 2 , and to  FIG. 3 , which shows a flow chart illustrating the inventive method. 
     The method of  FIG. 3  includes a first step, step  201 , of receiving the digital samples at respective I and Q inputs  22  ( FIG. 2 ). For each sample, the complex conjugate of the sample is formed, and the sample is applied to a delay block  24  ( FIG. 2 ), in step  203  ( FIG. 3 ). The delay block  24  is a FIFO buffer of A taps, and thus applies a delay equivalent to the duration of A samples, where A is the FFT size (that is, for example, 2048 in 2 k mode or 8192 in 8 k mode), and is the duration of the active part of the symbol, expressed in samples. 
     Then, in step  205  of the method, a correlation function is calculated in a complex multiplication block  26  ( FIG. 2 ), by multiplying each input symbol with the appropriate symbol obtained in step  203  with the delay applied thereto, thereby obtaining a measure of the correlation between them. The results are then supplied to a summing block  28  (in  FIG. 2 ), in which, in step  207  ( FIG. 3 ), a running average is calculated over a number of samples g, which equals the smallest allowed guard interval size, that is, 1/32 of the active symbol or +E,fra, 1/32 of the FFT size. Thus, g=64 samples in 2 k mode and 256 samples in 8 k mode. The output average value is then obtained by applying the result from block  28  to a block  30 , which takes the modulus of the output complex numbers. 
     The calculated correlation function is a measure of the degree to which each sample is correlated with the sample appearing in the input signal after a particular delay. The function is averaged over time to provide a measure of the correlation between a particular sequence of samples and the sequence of samples appearing after that delay. Because it is known that the guard interval contains samples which are repeats of samples from the active symbol appearing later in the signal, a high value of the correlation function may be an indication that it is the samples of the guard interval that are being examined. 
       FIG. 4  is an explanatory graph of the value of this averaged correlation function, obtained from block  30 , showing how it varies over a large number of samples, for one particular set of trial data. 
     It can be seen that the correlation function has a value that varies in an unpredictable way, because of chance correlations between the samples at different times, but that includes a number of peaks. These peaks can again arise because of chance correlations, or may arise because it is the samples of the guard interval that are under examination. 
     In step  209  of the method shown in  FIG. 3 , the value of the correlation function is then compared with a threshold value in a thresholding block  32  of  FIG. 2 , and it is only positions at which the value of the correlation function exceeds the threshold value which are investigated further, in order to determine whether the large value of the correlation function is due to chance or indicates the position of the guard interval. A counter  33  is connected to the thresholding block  32 , and used to create a dead period immediately after the threshold is crossed, during which the output is zero. The length of the dead period may for example be half of one symbol period. 
     The use of the threshold value and dead period provide robustness. The presence of multipath and/or co-channel interference can distort the shape of the peak in the correlation function. However, it will not remove the rising edge of the peak. Thus, the use of the threshold value means that the method relies not on the shape of the peak (which is subject to this distortion), but on the periodicity of the peaks (which is not significantly affected by the interference). Further, the use of the dead period (provided this is not too short) ensures that multiple successive spurious peaks are not recorded. 
     The selection of the threshold value is clearly important. If the value is too high, some of the points of potential interest will be missed, while, if it is too low, too many points will be examined, possibly leading to an inaccurate estimate of the guard interval size. It may be possible to set the threshold in initial settings of the receiver circuit. Alternatively, a further algorithm can adjust the threshold value, by starting with a high value, and reducing it until a stable value of the guard interval size is obtained. A further alternative is to measure the range of the correlation signals and to set the threshold at a fraction of the maximum. For example, values in the range of 0.6 to 0.8 of the maximum have been found to work. It is assumed hereafter that a suitable threshold has been adopted. 
     With a suitable threshold selected,  FIG. 5  shows the points at which the value of the correlation function exceeds the threshold value. Based on the particular correlation function illustrated in  FIG. 4 , the points shown in  FIG. 5  are obtained by using a threshold value of 0.8. 
     This procedure is then repeated until either a predetermined number of samples have been examined, or a predetermined number of points have been obtained. 
     The procedure described above is one method of obtaining a function that has peaks corresponding to the guard interval location. Similar functions could be obtained in other ways, for example by performing a subtraction rather than a complex correlation in step  205  of the inventive method. 
     The spacing of the spikes in the graph of  FIG. 5  indicate which guard interval is present. However, it is difficult to measure this directly for two reasons. Firstly, some spikes are missing, where the peaks did not reach the threshold set in the thresholding block  32  ( FIG. 3 ). Secondly, the exact location of the spikes is noisy, due to channel noise and self-noise, which mean that the positions of peaks in the correlation are subject to noise. Thirdly, some peaks will be due to noise only, particularly where the threshold has been set too low, or where the noise power is high. 
     In order to resolve these problems, the function illustrated in  FIG. 5  is filtered further. 
     Specifically, as shown at step  211  in  FIG. 3 , each of a number of comb filters  34  is applied to the detected series of points at which the threshold is exceeded. This is shown schematically in  FIG. 2 . 
     Each comb filter  34  has a pattern of zeroes and ones, the ones forming the teeth of the comb. The teeth of the comb have a finite width, which deals with the lateral positional noise in the spacing of the spikes. Moreover, the fact that some spikes will be missing from the series, as discussed above, is accommodated by providing a large number of teeth. For ease of illustration,  FIG. 2  shows a comb filter in which the teeth have a width of four samples and a spacing of nine samples. In one practical example, the teeth of the comb filter have a width of 32 samples and a spacing which is equal to the total symbol size, that is, the sum of the active symbol size and the guard interval size. In other words, the spacing of the teeth in the comb filter is equal to the repetition spacing of the guard interval. In 2 k mode, testing for a guard interval size of 1/32 of the active symbol size, the spacing is 2048+64=2112 samples. 
     There is one comb filter  34  for each possible guard interval size. Thus, if there are four possible sizes of the guard interval, there are four comb filters. 
     The outputs from each comb filter  34  are summed in a block  36 , and added into an accumulator forming part of a circular buffer  38 . The pointer then moves on to the next accumulator. At the end of the buffer, it jumps back to the start. Just as there is one comb filter  34  for each possible guard interval size, similarly, there is one buffer connected to each comb filter, and hence one buffer for each possible guard interval size. It should be noted that, because the different possible guard sizes are tested for in parallel, the time taken to complete the process is largely independent of the number of possible sizes. 
     Thus, in one embodiment, the described technique uses large amounts of memory within the receiver. However, it is only necessary to perform the required guard interval size detection once, during an acquisition phase, at a time when there is a large amount of RAM in the receiver which is not being used for other purposes. 
     The length of each buffer is equal to the symbol length expressed in samples, and thus corresponds to the spacing of the teeth in the corresponding comb filter. Therefore, in 2 k mode, testing for a guard interval size of 1/32 of the active symbol size, the buffer consists of 2048+64=2112 accumulators. 
     In other words, a correlation value is calculated between a first function which comprises a series of regularly spaced delta functions (the comb filter), and a second function which comprises a delta function at each of the positions at which the correlation exceeds the threshold. 
     At one position of the comb filter relative to the function shown in  FIG. 5 , the spikes line up with the teeth, and give a high output. When the spikes and the filter teeth do not line up, the output is low, because the spikes align mainly with zeroes in the comb filter. The results must be averaged over many symbols because, if the transmitted signal had a guard size equal to ¼ of the active symbol size, part of the symbol would align with the comb filter teeth even for the comb which is detecting the 1/32 guard size. This partial overlap is removed by the averaging process, because the alignment would occur at different points in the buffer. The averaging process also deals with the problem of missing spikes, or spikes occurring due to noise only. 
     Outputs from the buffers  38  are supplied to MAX blocks  40 , which find the positions, and heights, of the biggest accumulator outputs. 
       FIG. 6  shows the result of this process for the data used in  FIGS. 3 through 5 . Specifically,  FIG. 6(   a ) shows the output of the comb filter corresponding to a guard interval size of ¼ FFT size (512 samples);  FIG. 6(   b ) shows the output of the comb filter corresponding to a guard interval size of ⅛ FFT size (256 samples);  FIG. 6(   c ) shows the output of the comb filter corresponding to a guard interval size of 1/16 FFT size (128 samples); and  FIG. 6(   d ) shows the output of the comb filter corresponding to a guard interval size of 1/32 FFT size (64 samples). 
     Outputs from the MAX blocks  40  are supplied to a further MAX block  42 , which outputs the index of whichever input was the largest. 
     From  FIG. 6 , it can be seen that there is a clear peak in the output of the comb filter which corresponds to a guard interval size of 64 samples ( FIG. 6   d ). As a result, this is determined to be the actual guard interval size. 
     The fact that the signals are analyzed by the four (in this example) comb filters and buffers in parallel means that the start up time, required to find the guard interval size, can be reduced. 
     Moreover, the position of the peak can be used to determine the position of the start of each symbol. Specifically, the position of the peak within the circular buffer indicates the position along the input stream of samples at which the averaging window aligns directly with the cyclic prefix. This allows the position of the start of the active part of the symbol to be calculated, at least approximately, very quickly. This means that the acquisition phase of operation of the receiver can be completed quickly. 
     There is therefore described a method of analyzing the guard interval which allows the size to be detected quickly, even in the presence of relatively large noise signals. The method is robust in noise, and also robust enough to handle signals with severe multipath and/or co-channel interference. Moreover, the method is of low complexity, and can be implemented in a VLSI circuit.