Abstract:
A receiver circuit is for processing a received signal which includes at least a first portion and a second portion which repeats the content of the first portion after a repeat interval. For example, the receiver may be for DVB-T signals using COFDM. In order to ensure that the estimated symbol start position is accurate, the receiver calculates two correlation values, namely an early correlation and a late correlation. The early correlation is measured between samples ahead of an assumed first portion start position and ahead of an assumed second portion start position, and the late correlation is measured between samples behind an assumed first portion end position and behind an assumed second portion end position. When the assumed start and end positions are accurate, the early and late correlations are equal, and so the assumed start and end positions are controlled to equalize the early correlation and the late correlation.

Description:
TECHNICAL FIELD OF THE INVENTION 
     This invention relates to a receiver circuit, in particular for receiving signals in which a portion of a transmitted signal is repeated after a known time interval. 
     BACKGROUND OF THE INVENTION 
     The European DVB-T (Digital Video Broadcasting-Terrestrial) standard for digital terrestrial television (DTT) uses Coded Orthogonal Frequency Division Multiplexing (COFDM) of transmitted signals, which are therefore transmitted as OFDM symbols. 
     Received signals are sampled in a receiver, and accurate reception and demodulation of signals therefore requires accurate knowledge of the positions of the beginning and end of each OFDM symbol. 
     In particular, DVB-T COFDM signals include a cyclic prefix to each active symbol, which is repeated after a known and fixed time interval. These cyclic prefixes must be correctly removed before demodulation, or the demodulation performance can be seriously degraded. 
     The fact that the prefix in the COFDM signals is repeated can be used initially to find the position of the prefix, by calculating a running correlation between received portions which are received separated by the known time interval. A very high correlation will indicate the presence of a repeated portion. However, this does not allow correction for any changes in position caused by subsequent variations in sampling rate. 
     SUMMARY OF THE INVENTION 
     The present invention provides a receiver which overcomes some of the disadvantages of the prior art. 
     This invention relates in a first aspect to a receiver which can maintain the assumed position of the active symbols in the signal accurately, as compared with the actual position in the received signal, thereby advantageously allowing feedback control of the sample position of the receiver. 
     According to a second aspect of the invention, there is provided a method of processing received signals, and controlling the sampling position of a receiver. 
     In particular, according to the invention, there is provided a receiver circuit, comprising: 
     a sampler, for taking digital samples of a received signal, said received signal including at least a first portion and a second portion which repeats the content of the first portion after a repeat interval; 
     a processing device, for processing the digital samples on the basis of an assumed position of the first and second portions in the received signal; 
     at least one correlator for measuring: 
     a first correlation between a first group of samples including at least samples around the beginning of the first portion of the signal, and a second group of samples including at least samples around the beginning of the second portion of the signal; and 
     a second correlation between a third group of samples including at least samples around the end of the first portion of the signal, and a fourth group of samples including at least samples around the end of the second portion of the signal; 
     means for comparing the measured first and second correlations to produce a comparison output; and 
     means for determining the assumed position of the first and second portions on the basis of the comparison output in order to tend to equalize the first and second correlations. 
     Preferably, the first, second, third and fourth groups of samples include samples immediately preceding and immediately following the respective beginning or end point of the first or second portion. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic illustration of a part of a receiver circuit in accordance with the invention. 
         FIG. 2  is an explanatory diagram provided for a better understanding of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  shows a section of a receiver circuit relevant to 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 will include an antenna (not shown), and a tuner (not shown) for receiving the signals and downconverting to an intermediate frequency. 
     The receiver further includes a sampler  10  which receives signals, after conversion to baseband, at an input  12 . For example, the sampler is preferably a voltage controlled crystal oscillator with an analog-digital converter or a digital resampler, for producing baseband digital I and Q samples. In this example, the sampler produces (64/7) Msamples/second for both I and Q samples. The sampler is controllable in the sense that its sampling position can be adjusted. Output signals from the sampler  10  may be supplied to processing device (not shown) that, amongst other things, remove the cyclic components which proceed each active symbol. In order to be able to do this accurately, the sampling position of the sampler  10  must be controlled such that the assumed position of the start of each symbol accurately coincides with the actual position in the received signal. Where the sampler  10  is a resampler, this control of the sampling position is achieved by adjusting its phase. 
     The received COFDM signal includes a portion which is repeated after a known and fixed time interval. Specifically, in this example it includes a portion which is 64 samples long, and which is repeated after an interval (the repeat interval) of 2048 samples (measured from the start of the portion to the start of the repeated portion). 
     It will be appreciated that the order in which signals are downconverted to baseband, converted to I and Q, and sampled, is not relevant to the invention. 
     It should also be noted that, while several parameters quoted herein relate specifically to the current United Kingdom specification for DVB-T, the values of such parameters are not relevant to the invention, which may be applied to any suitable signal format. 
     The sampled signal output from the sampler  10  is supplied to a first delay element  14  and a second delay element  16 , which applies a delay having a duration of two samples. The first delay element effectively advances the signal by a duration of two samples. Of course, this is not possible. In practice, the first delay element actually applies a delay of twice two samples, and there is no second delay element, with the extra two sample delay being compensated later in the processing. The exact duration of the delays is not critical, as it could be any number of samples, conveniently an integer number. A small delay gives improved noise performance, while a large delay increases the range of errors which can be corrected in each measurement and correction cycle. 
     The signal from the first delay element  14  is applied to a first correlation combiner  18 , which includes a third delay element  20 , which applies a delay equal to the repeat interval, that is, 2048 samples. A correlator  22  receives as a first input the signal from the first delay element  14 , and as a second input the delayed output from the third delay element  20 . 
     The correlation between these two inputs is determined on a sample-by-sample basis in the correlator  22 , and output to a further block  24 , which includes an integrator  26 . The integrator  26  accumulates the results of the individual sample-by-sample correlations determined by the correlator  22 , and a sampling switch  28  gates the output and resets the integrator  26  to provide an output correlation value, measured over the whole 64 samples of the repeated portion of the signal, to a first input of a subtractor  30 . A running correlation is used initially to find the position of the repeated portion of the signal, so that the correlations described above are calculated only for the repeated portion of the signal. 
     Because the first delay element  14  effectively advanced the signal, this output is regarded, as an early correlation. 
     Similarly, the output from the second delay element  16  is applied to a second correlation combiner  32 , which includes a fourth delay element  34 , which applies a delay equal to the repeat interval. Thus, with a repeat interval of 2048 samples, the fourth delay element  30  applies a delay of 2048 samples. A second correlator  36  receives as a first input the signal from the second delay element  16 , and as a second input the further delayed output from the fourth delay element  34 . 
     The correlation between these two inputs is determined on a sample-by-sample basis in the correlator  36 , and output to an further block  38 , which includes an integrator  40 . The integrator  40  accumulates the results of the individual sample-by-sample correlations determined by the correlator  36 , and a sampling switch  42  gates the output and resets the integrator to provide an output correlation value, measured over the whole 64 samples of the repeated portion of the signal, to a second input of the subtractor  30 . 
     Because the second delay element  16  delayed the signal, this output is regarded as a late correlation. 
     The correlation result for each OFDM symbol, R, is the magnitude of the complex correlation across N samples of the cyclic repeat: 
             R   =     ❘         ∑     m   =   0       N   -   1       ⁢           ⁢       x   m     ⁢     x     m   +     N   R             ❘             
where * denotes the complex conjugate of a complex value, x k  are the samples of the signal and N R  is the number of samples between a sample of the cyclic prefix and its repeat. Either x m  or x m+N     R    maybe conjugated in this calculation and m=0 is taken to be the first sample of the assumed start of the cyclic prefix for a particular symbol.
 
     The early correlation can be written as: 
               R   E     =     ❘         ∑     m   =   0       N   -   1       ⁢           ⁢       x     m   -   2       ⁢     x     m   -   2   +     N   R       *         ❘             
and the late correlation as:
 
     
       
         
           
             
               R 
               L 
             
             = 
             
               ❘ 
               
                 
                   
                     ∑ 
                     
                       m 
                       = 
                       0 
                     
                     
                       N 
                       - 
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                       x 
                       
                         m 
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                         + 
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                           N 
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                 ❘ 
               
             
           
         
       
     
     The subtractor  30  receives the two correlation values as inputs, and therefore provides an output signal which is a measure of the difference between the correlation values calculated in the correlation combiners  18 ,  32  respectively. The full significance of this will be described in more detail with reference to  FIG. 2  below. 
     More specifically, the difference between the correlation values is taken to be proportional to the time error in the initially assumed sampling position. Thus: 
                 Δ   ⁢           ⁢   t     ∝     ❘         ∑     m   =   0       N   -   1       ⁢           ⁢       X     m   -   2       ⁢     X     m   -   2   +     N   R       *         ❘     -     ❘         ∑     m   =   0       N   -   1       ⁢           ⁢       X     m   +   2       ⁢     X     m   +   2   +     N   R       *         ❘                 
The output signal from the subtractor  30  is supplied to a feedback loop filter  44  which appears in a feedback loop  46 , and the output thereof is applied to the sampler  10  to control the sampling position.
 
     Thus, if the result of the correlation calculations is that the input signal is found to be more closely correlated with the delayed signal or the effectively advanced signal, a correction is applied to the sampling position which will tend to equalize these correlations. 
     A more detailed explanation of the operation of the circuit will now be given with reference to  FIG. 2 . 
       FIG. 2  is a partial schematic illustration (not to scale) of the time history of a digitally sampled received COFDM signal. The signal includes a first portion  50 , and a second portion  52 , which is identical thereto and can therefore be seen as a repeat of the first portion. The signal also includes a third portion  54 , and a fourth portion  56 , which is identical thereto an can therefore be seen as a repeat of the third portion. The first, second, third and fourth portions  50 ,  52 ,  54 ,  56  may each have a duration 58 to 64 samples. 
     The start of the second portion is 2048 samples after the start of the first portion, and the start of the fourth portion is 2048 samples after the start of the third portion. Thus, the repeat period is 2048 samples. Therefore if either the first or third portion of the signal were delayed by 2048 samples, it would be found to be exactly correlated (ignoring distortions, noise, etc.) with the signal actually being received at that time. 
     When demodulating signals, it is important to know exactly when to expect to receive the start of each active symbol. This also allows other data, for example the cyclic prefixes which appear before each active symbol, to be removed. An error can mean that the receiver has a reduced ability to remove “ghost” images from the received signal, or can mean that the receiver is unable to reproduce any picture at all. 
       FIG. 2  shows a delay  60  of 2048 samples as applied by the third delay element  20  to a signal portion  62  that is two frames in advance of the portion  50  that is to be repeated, and which produces a delayed signal portion  64 . Thus, the correlator  22  measures the correlation between the delayed signal portion  64  and the signal portion actually received at the same time. To the extent that signal portion  62  overlaps with signal portion  50 , the delayed signal portion  64  is perfectly correlated (again ignoring distortions, noise, etc.) with the signal portion actually received at the same time. However, to the extent that signal portion  62  does not overlap with signal portion  50 , the delayed signal portion  64  is broadly uncorrelated with the signal portion actually received at the same time. 
       FIG. 2  also shows a delay  66  of 2048 samples as applied by the fourth delay element  34  to a signal portion  68  that is two samples behind the portion  50  that is to be repeated, and which produces a delayed signal portion  70 . Thus, the correlator  36  measures the correlation between the delayed signal portion  70  and the signal portion actually received at the same time. To the extent that signal portion  68  overlaps with signal portion  50 , the delayed signal portion  70  is perfectly correlated (again ignoring distortions, noise, etc.) with the signal portion actually received at the same time. However, to the extent that signal portion  68  does not overlap with signal portion  50 , the delayed signal portion  70  is broadly uncorrelated with the signal portion actually received at the same time. 
     If the assumed sampling position is exactly synchronized with the transmitted signal, then the signal portion  62  would begin exactly two samples before the signal portion  50 . The delayed signal portion  64  would then be correlated with the signal portion actually received at the same time for 62 samples out of 64, and uncorrelated for the remaining 2 samples out of 64. Similarly, the delayed signal portion  70  would then be correlated with the signal portion actually received at the same time for 62 samples out of 64, and uncorrelated for the remaining 2 samples out of 64. 
     Thus, taken over many OFDM symbols, the average values of the measures of correlation, as determined by the two correlation combiners  18 ,  32 , would be exactly equal. 
     If, by contrast, the sampling position were slightly in advance of the received signal, the signal portion  62  would overlay with signal portion  50  for longer than before, and the delayed signal portion  64  would be more highly correlated with the signal portion actually received at the same time. At the same time, the signal portion  68  would overlap with signal portion  50  for a shorter time than before, and the delayed signal portion  70  would be less highly correlated with the signal portion actually received at the same time. 
     Conversely, if the sampling position were slightly retarded relative to the received signal, the signal portion  62  would overlap with signal portion  50  for a shorter time than before, and the delayed signal portion  64  would be less highly correlated with the signal portion actually received at the same time. At the same time, the signal portion  68  would overlap with signal portion  50  for a longer time than before, and the delayed signal portion  70  would be more highly correlated with the signal portion actually received at the same time. 
     Returning to  FIG. 1 , therefore, a zero output from the filter  44  is produced when the symbol start position of the receiver is exactly synchronized with the received signal, and produces no change in the sampling position. However, a non-zero output from the filter  44  is produced when the sampling position of the receiver is not exactly synchronized with the received signal, and is fed back to control the sampler  10  to produce a change in the sampling position. This change acts to bring the sampling position of the receiver into synchronization with the received signal. 
     The offset period of two samples, as described above, will often be greater than the actual offset. That being so, the last 60 samples of the signal portion  62  should be exactly correlated (again ignoring distortions, noise, etc.) with the last 60 samples of the signal portion  64 , with any uncorrelation being confined to the first 4 samples. It is therefore sufficient to calculate the correlation only during these first 4 samples. Similarly, the first 60 samples of the signal portion  68  should be exactly correlated (again ignoring distortions, noise, etc.) with the first 60 samples of the signal portion  70 , with any uncorrelation being confined to the last 4 samples. It is therefore sufficient to calculate the correlation only during these last 4 samples. 
     In other words, we can assume that, on average, the difference between the overlapping portions of the two correlations is zero. Hence, it is possible to use the following approximation, if calculated over a sufficiently large number of symbols. 
     
       
         
           
             
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     This modification therefore advantageously reduces the calculations and storage required. 
     The use of an offset period of two samples means that this is the largest error which can be corrected in each measurement and correction cycle. In the event that the actual offset is greater than two samples, then a correction of two samples is applied in each cycle, until the offset becomes less than two samples. 
     There are therefore disclosed a receiver circuit, and a method of controlling a sampling position therein, which allows exact synchronization to be achieved between the sampling position and the received sample position.