TELEVISION RECEIVER

Abstract: Television receiver, and in particular a receiver for broadcast digital terrestrial television signals, where the transmitted signal is modulated using COFDM (Coded Orthogonal Frequency Division Multiplexing), includes a fast fourier transform processor; a phase adjustment circuit for changing the phase of signals to be applied to the fast fourier transform processor; and a channel estimator. As disclosed, the receiver further comprises a derotator for applying a rotation to the fast fourier transform processor output signal, before application to the channel estimator, to counteract any adjustment applied by the phase adjustment circuit.

Description: TELEVISION RECEIVER TECHNICAL FIELD This invention relates to a television receiver, and in particular to a receiver for broadcast digital terrestrial television signals, where the transmitted signal is modulated using COFDM (Coded Orthogonal Frequency Division Multiplexing) . 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 grouped into blocks and frames . 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. 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. As is well known, this can cause inter-symbol interference (ISI) in the receiver. To minimize this problem, DVB-T COFDM signals include a cyclic prefix guard interval to each active symbol. Specifically, a portion of the active symbol is repeated before the next active symbol. 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 will 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 so good 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 there is no large pre-echo) , 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. One solution to this 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 can reduce 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 There are many possible reasons for wanting to rotate a signal, either forwards or backwards, in the frequency domain. However, such rotations can have an adverse effect on channel estimation. According to a first aspect of the invention, there is provided a television receiver circuit which includes a derotator circuit, that is a circuit which can apply a rotation which is equal and opposite to that previously applied, before a signal is applied to a channel estimator. According to a second aspect of the invention, there is provided a method of processing received signals, which includes applying a rotation which is equal and opposite to that previously applied, before the signal is applied to a channel estimator. Thus, the rotation which is applied can compensate for that applied previously, thereby improving channel estimation, and ultimately improving signal reception. BRIEF DESCRIPTION OF DRAWINGS Figure 1 is a block schematic diagram of a receiver circuit in accordance with the invention. Figure 2 is an illustration of the operation of the circuit of the invention. DETAILED DESCRIPTION OF THE INVENTION Figure 1 shows a television receiver circuit in block schematic form. It will be appreciated that many of the receiver functions can be carried out in a different order from that illustrated in Figure 1 and described below. 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 further mixer stage 10, for downconverting to baseband, and a resampler 12, for obtaining digital samples of in-phase (I) and quadrature (Q) components of the signal. The sampler is controllable in the sense that its sampling position can be adjusted. Output signals from the resampler 10 are supplied to a processing device 14 which removes the cyclic components which precede 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 under the control of a resampler controller 16. Such adjustments of the phase, in effect, rotate the signal in the phase plane. An algorithm to track the resampler diplacement 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. 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 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 . After processing in the Fast Fourier Transform processor 18, the data signals are supplied to a derotator 20. The operation of the derotator 20 will be described with reference to Figure 2. Figure 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. 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 P2. Each sample change in the window position will produce 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, will produce a rotation of θ radians, where: θ = 2π.n.SP/N 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. 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. Specifically, the corrected sample position S2, having I- and Q- values IS2 and QS2, where: S2 = IS2 + j .QS2, is obtained from the input sample position SI, having I- and Q- values ISI and.QSl, where: SI = ISI + j .QS1, by means of the complex multiplication: IS2 + J.QS2 = (ISI + j .QS1) .e "jθ . The output data signal from the derotator 20 is then supplied to a channel estimator 22 including an equaliser, demultiplexer and deinterleaver 24 and decoder 26, which recover the originally transmitted bitstream, in a generally conventional way. The channel equaliser 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 equaliser performance. The performance of the channel estimator can be optimised 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. 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. The circuit 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.

Claims: CLAIMS 1. A receiver circuit, comprising: a fast fourier transform processor; a phase adjustment circuit for changing the phase of signals to be applied to the fast fourier transform processor; and a channel estimator, and further comprising: a derotator for applying a rotation to the fast fourier transform processor output signal, before application to the channel estimator, to counteract any adjustment applied by the phase adjustment circuit. 2. A receiver circuit as claimed in claim 1, wherein the fast fourier transform processor operates with a processor window having a window position, and the phase adjustment circuit pulls back the window position of the fast fourier transform processor. 3. A receiver circuit as claimed in claim 1, comprising a resampler for forming digital samples of a received signal, the resampler having a sample position, wherein the phase adjustment circuit adjusts the sample position of the resampler. 4. A receiver circuit as claimed in claim 1, wherein the receiver is a television receiver for COFDM signals. 5. A method of processing a received signal, including: applying a phase change to the received signal; applying the signal with the phase change to a fast fourier transform processor; and applying the fast fourier transform processor output signal to a channel estimator; and further including: applying a rotation to the fast fourier transform processor output signal before application to the channel estimator, to counteract any applied phase change . 6. A method as claimed in claim 5, wherein the fast fourier transform processor operates with a processor window having a window position, and the step of applying a phase change comprises pulling back the window position of the fast fourier transform processor. 7. A method as claimed in claim 5, comprising forming digital samples of a received signal by means of a resampler, the resampler having a sample position, wherein the step of applying a phase change comprises adjusting the sample position of the resampler.

Inventors: FOXCROFT THOMAS, VICKERS MARTIN, FOXCROFT, THOMAS, VICKERS, MARTIN
Applicants: CONEXANT SYSTEMS INC, FOXCROFT THOMAS, VICKERS MARTIN, CONEXANT SYSTEMS, INC., FOXCROFT, THOMAS, VICKERS, MARTIN
Application year: 2000
Main classification: H04N5
IPC codes: H04L27, H04N5