OFDM DEMODULATOR WITH PHASE CORRECTION OF THE I-Q SIGNALS

Abstract: A method and apparatus for processing a received signal in a radio frequency receiver. In-phase and quadrature signal components are detected and a degree of correlation between the detected in-phase and quadrature signal components is then estimated. A correction factor is applied to at least one of said detected in-phase and quadrature signal components to minimise the degree of correlation between them.

Description: OFDM DEMODULATOR WITH PHASE CORRECTION OF THE I-Q SIGNALS TECHNICAL FIELD OF THE INVENTION This invention relates to a method of signal correction, and to a circuit for carrying out such correction. More specifically, the invention relates to the correction of nominal in-phase and quadrature received signals, in order to avoid errors which arise because those received signals are other than exactly orthogonal. In particular, the invention is described with reference to a digital terrestrial television demodulator. However, the same correction technique could be used for any nominal in-phase and quadrature received signals, provided that, at the transmitter, the information contents on the in-phase and quadrature signals are not correlated, and provided that the signals are pseudo-random. 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. A DVB-T receiver therefore typically includes an analog tuner for receiving the signals and downconverting to an intermediate frequency, an analog-digital converter, a digital demodulator, and a MPEG coder for producing output video signals. It will be realised that, although these different stages have been described, there can be overlap between the stages, and some or all functions can advantageously be combined onto a single chip. SUMMARY OF THE INVENTION If the analog tuner is used to demodulate the signal into separate in-phase (I) and quadrature (Q) signals, then, typically, the generated I and Q signals are not always reliably separated by exactly 90° of phase, as they should be. When those signals are then used as the input to a demodulator, any phase imbalance can cause additional noise, due to cross-talk between the I and Q signals. This invention relates to a technique for correcting for any such imbalances. In particular, the invention relies on an appreciation that, if the phase difference between the received nominal in-phase and quadrature signals is anything other than exactly 90°, there will be a degree of correlation between the received signals. Moreover, the degree of correlation can be used as a measure of the amount of phase imbalance, allowing an appropriate correction to be applied so that the signals used for further processing are truly orthogonal . According to the invention, there are therefore provided a signal processor, and a receiver, which apply a correction to a received signal, based on a phase imbalance estimated from a measured degree of correlation between nominal in-phase and quadrature signals . BRIEF DESCRIPTION OF DRAWINGS Figure 1 is a block schematic diagram of a receiver device in accordance with the invention. Figure 2 is a block schematic diagram of a preprocessing circuit forming part of the receiver device of Figure 1. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Figure 1 is block schematic diagram of a receiver device in accordance with the invention. The invention is described herein with reference to its application in the reception of digital terrestrial television (DTT) signals using the European DVB-T standard based on Coded Orthogonal Frequency Division Multiplexing (COFDM) , although it will be appreciated that its use is independent of the type of signals being received. Specifically, the invention can be applied to digital cable and digital television receivers using any suitable standard, and indeed any receiver for nominal in-phase and quadrature received signals, provided that, at the transmitter, the information contents on the in-phase and quadrature signals are not correlated, and provided that the signals are pseudo-random. Figure 1 shows an antenna 2 , for receiving broadcast signals, which are supplied to an analog tuner 4. The tuner 4 includes a mixer for downconversion of the received signals to a first intermediate frequency, and an analog-digital converter, and supplies input signals to the demodulator 6 in the form of digitised samples of the nominal received in-phase (I) and quadrature (Q) signals. The demodulator 6 then removes the COFDM modulation, and supplies output signals to a coder 8, which converts the digital signals into a form suitable for display, for example based on the MPEG-2 standard. The form of the coder is generally known to the person skilled in the art. In accordance with the invention input signals received by the demodulator 6 are sent first to a pre- processor 10, which is described in detail below, and then to the demodulation circuitry 12. The form of the demodulation circuitry is generally known to the person skilled in the art. Although Figure 1 shows several discrete blocks, it will be appreciated that the different stages may be integrated as far as is desirable, for example onto a single chip, or other arrangements of functions can be used. Moreover, the order of the stages can be selected as desired. For example, I and Q demodulation can take place in the analog domain or in the digital domain. Further, the A/D converter may be separate from the tuner. Figure 2 is a block schematic diagram of the preprocessing stage 10. Again, Figure 2 shows a number of discrete blocks, the functions of some or all of which may in fact be carried out together in, for example, a digital signal processor (DSP) . The pre-processing preferably takes the form of a feedback loop, in which a correction is applied to the input signals, and the corrected signals are then used to analyse the requirement for further correction. The circuit receives digitised samples of the nominal in-phase and quadrature signals In, Qn respectively, received from the tuner 4. A correction for any detected phase imbalance is applied in a correction block 20, to give corrected in-phase and quadrature output signals I * , Q * , respectively. These corrected signals are sampled, and fed to a correlation calculation block 22, which determines the degree of correlation between the corrected signals. Since, ideally, the in-phase and quadrature signals should be separated by exactly 90° of phase, they should be completely orthogonal, that is, there should be no correlation between them. The correlation calculation block 22 can use any one of several known measures of correlation. However, in one embodiment of the invention, it uses the linear correlation coefficient or product-moment correlation coefficient, also known as Pearson's correlation coefficient. For each sample x ± , y A of the I and Q components (with the x values being samples from the I component signal, and y values being samples from the Q component signal) , and with the mean values of x and y being x and y respectively, this coefficient, r, is defined as follows : - r=- ∑i ( x r - y ^E f^ F If the phase difference between the nominal in- phase and quadrature signals In and Qn is exactly 90°, as intended, the value of the correlation coefficient will tend, over a large enough number of signal samples, towards zero. The measured value of the correlation coefficient is then supplied to block 24, which uses the measured correlation coefficient to obtain an estimate of the phase difference between the corrected in-phase and quadrature signals I * and Q * . Any variation of this phase difference from 90° is described as a phase imbalance. For example, block 24 may be a lookup table, in which the values of the coefficient corresponding to particular phase imbalances are stored. Since it is believed that the relationship between phase imbalance and correlation coefficient is substantially linear, block 24 may simply apply a scaling factor to the measured correlation coefficient to obtain an estimate of the phase imbalance. The phase imbalance estimate is then supplied to a loop filter 26, which forms a stable estimate of the mean phase imbalance, which is then supplied to the correction block 20. Where it is desired simply to apply a scaling factor to the measured correlation coefficient to obtain the estimated phase imbalance, this scaling factor may in fact be applied through the loop filter 26 without requiring a separate conversion block 24. The required form of the correction can be illustrated by defining the required uncorrelated input signals applied to the inphase and quadrature components as A and B respectively. If the recovered I axis is then assumed to be unaffected, but the recovered Q axis is not exactly perpendicular thereto, but rather is at (90+ φ) ° thereto, the recovered Q axis is rotated by φ° with respect to the transmit Q axis. The signal on the recovered Q axis is then given by: Q n = _4cosφ + Esinφ which can be solved to obtain the required value of B because we know the value of A from the received I signal, and φ is the calculated phase imbalance. This correction can then be applied to the received nominal quadrature input signal Qn, to obtain the corrected quadrature signal Q * . This correction is of lower accuracy if there is also gain imbalance in the tuner, or elsewhere, that is, if the nominal in-phase and quadrature signals In, Qn have been subject to slightly different gains. Specifically, the relationship between the phase imbalance and the correlation co-efficient may not be directly proportional in the event of a gain imbalance. The relationship may still be substantially linear, but not passing through the origin. Preferably, therefore, the pre-processor 10 also applies a gain correction to the nominal in-phase and quadrature signals In, Qn. More specifically, the variance of In and Qn can be estimated from individual samples thereof. Any difference between the variances can be taken to indicate a gain imbalance, and this can then be used to apply a correction to the measured correlation coefficient before this is used to estimate the phase imbalance. Alternatively, automatic gain control circuits may be used to remove any gain imbalance between the nominal in-phase and quadrature signals before the correlation coefficient is calculated. After correction of the in-phase and quadrature components, the corrected components I * , Q * can then be used in further processing stages of the demodulator. There is thus disclosed a system for removing noise components caused by phase imbalance resulting from the action of the tuner.

Claims: CLAIMS 1. A receiver correction device, comprising: an input for receiving detected in-phase and quadrature signal components; a phase estimator, for estimating a degree of correlation between the detected in-phase and quadrature signal components; and a correction circuit, for applying to at least one of said detected in-phase and quadrature signal components a correction factor, to minimise the degree of correlation between them. 2. A receiver correction device as claimed in claim 1, further comprising a feedback loop, wherein the phase estimator estimates the degree of correlation between the detected in-phase and quadrature signal components after application of the correction factor thereto. 3. A receiver correction device as claimed in claims 1 or 2 , wherein the phase estimator estimates a phase imbalance from the estimated degree of correlation between the detected in-phase and quadrature signal components . 4. A receiver circuit, comprising: a tuner circuit, including an analog-digital converter and a downconverter, for forming in-phase and quadrature signal components; and a receiver correction circuit as claimed in any preceding claim. 5. A receiver circuit as claimed in claim 4, further comprising an OFDM demodulator. 6. A method of processing a received signal in a radio frequency receiver, the method comprising: receiving detected in-phase and quadrature signal components; estimating a degree of correlation between the detected in-phase and quadrature signal components; and applying to at least one of said detected in-phase and quadrature signal components a correction factor, to minimise the degree of correlation between them. 7. A method as claimed in claim 6, wherein the degree of correlation between the detected in-phase and quadrature signal components is estimated after application of the correction factor thereto. 8. A method as claimed in claims 6 or 7, wherein a phase imbalance is estimated from the degree of correlation between the detected in-phase and quadrature signal components 9. A method as claimed in claim 6, further comprising receiving transmitted signals and applying them to an analog-digital converter, and downconverting them to form in-phase and quadrature signal components.

Inventors: PARKER JONATHAN, PARKER, JONATHAN
Applicants: CONEXANT SYSTEMS INC, PARKER JONATHAN, CONEXANT SYSTEMS, INC., PARKER, JONATHAN
Application year: 2000
Main classification: H04L27
IPC codes: H03L7, H04B1, H04J11, H04L27