Abstract:
A receiver, for example a receiver of broadcast digital terrestrial television signals modulated using COFDM (Coded Orthogonal Frequency Division Multiplexing), imposes a phase adjustment on a received signal. Phase adjustment may be effected, for example, by sample alignment of the signal, such as for cyclic prefix removal, or by shifting a window setting for a Fast Fourier Transform (FFT) processor. Before channel estimation or decoding is performed on the information stream, the information stream is derotated to compensate for the phase adjustment previously imposed on the received signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the effective filing date under 35 USC §§ 120 and 363 to PCT International Application. No. PCT/GB00/04001, entitled “Television Receiver”, filed Oct. 18, 2000 designating the U.S. and published under PCT Article  21 ( 2 ) in English as International Publication No. WO 02/05550 A1 entitled “Television Receiver,” of which this application is a continuation, which PCT application claims priority to Great Britain Patent Application No. 0017132.2, filed Jul. 12, 2000. This application claims priority under 35 USC 119(a) to Great Britain Patent Application No.: 0017132.2, filed Jul. 12, 2000. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates to a communication receiver.  
           [0004]    2. Description of Related Art  
           [0005]    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 grouped into blocks and frames.  
           [0006]    After reception in the television receiver, the signals are sampled, for example using a resampler, and are mixed down to baseband. The start of each active symbol is found, and then the active symbols are applied to a Fast Fourier Transform (FFT) processor, and subsequently to a channel estimator, to extract the wanted information.  
           [0007]    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 echoes. 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 echo. Further, there may be no direct line of sight from the transmitter to the receiver, in which case the receiver will only receive echoes. The effect of this is that the first signal received may not necessarily have the strongest power. There will therefore be combinations of pre-echoes arriving before the strongest signal and echoes arriving afterwards.  
           [0008]    As is well known, this scenario can cause inter-symbol interference (ISI) in the receiver. To reduce the effects associated with this problem, DVB-T COFDM signals include a cyclic prefix guard interval for each active symbol. Specifically, a portion of the active symbol is repeated before the next active symbol.  
           [0009]    Once the received signal is converted down to baseband, if there is a large echo present, a time domain correlation between samples which are an active window length apart yield large powers in the guard interval of the echo. These correlations can be used to correctly position the window when large echoes are present, although the technique is not as effective for smaller echoes. If the smaller echoes lag the larger ones, then correct positioning of the windowing relative to the first large echo (or relative to the main signal if no large pre-echo is present), will result in a good solution. On the other hand, if the smaller echo is a pre-echo, this may not be the case, as the pre-echo will be introducing ISI.  
           [0010]    One solution to this problem is to pull back the window position, calculated using the correlations in time, which can avoid ISI, but which rotates the signal in the frequency domain. Large rotations in the frequency domain can adversely affect the performance of the channel estimator. Moreover, the guard interval prefix must be removed before the signals are further processed. The initial position of the prefix can be found, and it is also preferable to allow correction for any changes in position caused by subsequent variations in sampling rate. Again such corrections have the effect of rotating the signal in the frequency domain.  
         SUMMARY OF THE INVENTION  
         [0011]    There are many possible reasons for wanting to rotate a received signal, either forwards or backwards, in the frequency domain. However, such rotations can have an adverse effect on channel estimation.  
           [0012]    According to a first aspect of the invention, there is provided a receiver circuit which includes a derotator circuit, that is a circuit which can apply a rotation that is equal and opposite to that previously applied, before a signal is applied to a channel estimator.  
           [0013]    According to a second aspect of the invention, there is provided a method of processing received signals, that includes applying a rotation which is equal and opposite to that previously applied, before the signal is applied to a channel estimator.  
           [0014]    Thus, the rotation that is applied can compensate for that previously applied, thereby improving channel estimation, and ultimately improving signal reception. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 is a simplified block schematic diagram of a receiver circuit in accordance with the invention.  
         [0016]    [0016]FIG. 2 is an illustration of the operation of the derotator shown in the receiver circuit of FIG. 1. 
     
    
     DETAILED DESCRIPTION  
       [0017]    [0017]FIG. 1 shows simplified block diagram of a receiver circuit or system in accordance with the present invention. It will be appreciated that many of the receiver functions can be carried out in a different order from that illustrated in FIG. 1 and as described below, and that FIG. 1 is exemplary only.  
         [0018]    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 signals and downconverting the received signals to an intermediate frequency.  
         [0019]    As shown in FIG. 1, the receiver further includes a further mixer stage  10 , for downconverting to baseband, and a resampler  12 , for obtaining digital samples of in-phase ( 1 ) and quadrature (Q) components of the signal.  
         [0020]    The sampler is controllable in the sense that its sampling position can be adjusted.  
         [0021]    Output signals from the resampler  12  are supplied to a processing device  14  that removes the cyclic components preceding each active symbol. In order to be able to do this accurately, the sampling position of the resampler  12  must be controlled such that the assumed position of the start of each symbol accurately coincides with the actual position in the received signal. This control of the sampling position is achieved by adjusting the phase of the resampler  12  under control of a resampler controller  16 . Such adjustments of the phase, in effect, rotate the signal in the phase plane.  
         [0022]    An algorithm to track the resampler displacement offset should in general not have large corrections in any particular symbol. However, it may be advantageous for it to be able to do so.  
         [0023]    The baseband I- and Q-data signals are supplied to a Fast Fourier Transform (FFT) processor  18 . However, in order to avoid any problems of inter-symbol interference (ISF) which may be caused by pre-echo signals, that is, attenuated versions of the main signal which arrive at the receiver before the main signal does, the FFT window may be pulled back in time. Again, this has the effect of rotating the spectrum of the main signal.  
         [0024]    After processing is performed by the Fast Fourier Transform processor  18 , the data signals are supplied to a derotator block  20 .  
         [0025]    The operation of the derotator  20  is now described with reference to FIG. 2. FIG. 2 shows the values of the I- and Q-samples at one particular illustrative moment in time. Ignoring the effect of the rotation of the signal introduced by the resampler position correction algorithm and the Fast Fourier Transform processor window position pullback, the sample values would be at the position marked PI in FIG. 2.  
         [0026]    However, the resampler position correction algorithm has altered the position of the signal by SP1 samples, and the Fast Fourier Transform processor window position has been pulled back by a further SP2 samples, which have introduced a rotation which means that, thereafter, the sample values are at the position marked P 2  (as shown in FIG. 2).  
         [0027]    Each sample change in the window position produces a phase ramp across the frequency spectrum from 0 on the DC bin to 360°, or 2π radians, on the final bin of the FFT processor. Therefore, if the Fast Fourier Transform size is N (which may, for example, be 2048 samples), and n is the bin offset, rotation by a number of samples SP, where SP=SP1+SP2, produces a rotation of 0 radians, where:  
         θ=2 πn ( SP/N )  
         [0028]    The derotator  20  therefore detects the amount by which the Fast Fourier Transform processor window position has been pulled back, that is, SP2 samples. The derotator  20  also detects the size of the correction applied to the resampler position in each symbol, and hence the cumulative correction, that is, SP1 samples. The derotator  20  then forms the sum SP of SP1 and SP2, and calculates the total applied rotation θ, as described above.  
         [0029]    As is well known, a rotation of a complex value can be achieved by complex multiplication, and, in this case, an equal and opposite rotation is applied to compensate for that previously applied.  
         [0030]    Specifically, the corrected sample position S 2 , having I- and Q-values IS2 and QS2, where:  
           S   2 = IS 2+ jQS 2,  
         [0031]    is obtained from the input sample position S 1 , having I- and Q-values IS1 and QS1, where:  
           S   1 = IS 1 +jQS 1,  
         [0032]    by means of the complex multiplication:  
           IS 2+ jQS 2=( IS 1 +jQS 1) e   −jθ .  
         [0033]    Referring again to FIG. 1, the output data signal output by the derotator  20  is then input to a channel estimator  22  including an equalizer, demultiplexer and deinterleaver  24  and decoder  26 , which recover the originally transmitted bitstream, in a generally conventional way.  
         [0034]    The channel equalizer relies upon the channel being steady for multiple symbols. If a large resampler displacement offset is added, then the large phase ramp introduced will introduce an apparent rapid change in the channel and thus degrade the channel equalizer performance. The performance of the channel estimator can be optimized by removal in the derotator  20  of any previously applied rotation, thus improving the performance of the device. Specifically, the derotator can compensate for the introduced phase ramps, and therefore rapid movements in window position are possible, without degrading performance.  
         [0035]    For example, in a mobile environment, the window position may advantageously be rotated either forwards or backwards. Although the invention has been described above in terms of a forwards rotation being compensated by a backwards derotation, it will be appreciated that the invention is equally applicable to compensating a backwards rotation of the window, by means of a forward rotation.  
         [0036]    The receiver system has been described herein with all of the components on a single device, such as a large scale integrated circuit. However, it will be appreciated that the different functions may be achieved in different devices, and in different ways from those described.