The present invention relates to the field of radiocommunications and in particular to the processing performed in a receiver upstream of a digital equalizer.
It applies in systems where the available spectrum is subdivided into adjacent frequency bands so as to support different communications.
An example is the so-called GSM (“Global System for Mobile communications”) European cellular radiocommunication system in which the spectrum allocated around 900 or 1800 MHz is subdivided into frequency bands spaced 200 kHz apart, each of these bands forming the subject of a time-division multiplexing according to the TDMA scheme (“Time Division Multiple Access”). The GSM system uses a frequency hopping technique to combat the channel fading and to increase the capacity of the system. GSM frequency hopping consists, for a communication set up on a TDMA channel defined in particular by a given time slot of successive TDMA frames, in changing the communication frequency from one frame to the next from among the carriers, spaced 200 kHz apart, allocated to the system, according to a hopping pattern which is known to the transmitter and to the receiver.
In a radiocommunication receiver, the filtering operations performed upstream of the channel equalizer may correspond to a fixed or adaptive filter.
The specification of a fixed filter depends on the assumptions made about the noise and the interferers. If the filter is optimized to enhance the performance in terms of sensitivity, then the robustness of the receiver to interferers which may be present in the adjacent frequency channels is decreased. Conversely, an excellent filter as regards robustness to interferers in the adjacent channels degrades the performance in terms of sensitivity.
An adaptive filter makes it possible to achieve a better compromise. However, the use of an adaptive filter has hitherto been possible only in the case of stationary channels, making it possible to determine the reception filter in a reliable manner.
In certain cases, this stationarity condition is not fulfilled. For example, in the GSM system, the frequency hopping at the TDMA frame rate of 4.615 ms, carried out between two signal bursts transmitted in two successive frames, constantly modifies the interference conditions and therefore the optimal structure of the reception filter.
It is conventional to decompose the filtering upstream of the channel equalizer into the cascading of a filter matched to the spectrum of the modulation and of a filter in the Nyquist band, referred to as “whitening” filter. This whitening filter must guarantee noise residuals (thermal noise+interference of the other channels) which are as independent as possible at the input of the channel equalizer. It is known that this structure affords an optimal protection scheme (see G. D. Forney Jr.: “Maximum-Likelihood Sequence Estimation of Digital Sequences in the Presence of Intersymbol Interference”, IEEE Trans. Inform. Theory, Vol. IT-18, May 1972, pages 363-378).
The present invention aims to allow adaptive estimation of the whitening filter, even in the presence of a weakly stationary transmission channel, such as that of GSM.