Signal processing method
A demapping scheme, having a low computational cost, suitable for any transmission in which only exhaustive demapping can guarantee good performance. The scheme, proposed in this document, can be used in any transmission based on no differential modulation. For example the proposed scheme can be directly applied to a transmission on flat fading AWGN channels or to a transmission on frequency selective channels after equalization or on each sub-carrier of an OFDM System. The proposed solution can be applied for the demapping of any communication system. The proposed scheme can be used for rotated and un-rotated constellation.
1. A method of processing a signal received through a channel and modulated according to a modulation scheme that involves a constellation of symbols representing groups of bits, comprising the steps of:
filtering and demodulating the signal;
extracting a sample of the demodulated signal;
splitting a constellation plane into sectors delimited by thresholds on a real component or on an imaginary component,
identifying a sector boundary closest to the sample,
selecting a subset of constellation points belonging to two sectors adjacent to the identified sector boundary,
computing distances of the sample from the selected constellation points; and
demapping the sample based on the computed distances without using the points of the constellation not included in said subset.
2. The method of claim 1 , wherein said sectors are delimited by thresholds in the real and the imaginary components providing a largest gain in the channel.
3. The method of claim 1 , wherein the selecting step extracts, from the constellation, for every bit in the sample of the demodulated signal, a first sub-set associated to i-th transmitted bit, of the constellation points having a high probability of having been transmitted assuming the i-th transmitted bit equal to 0, and a second sub-set associated to i-th transmitted bit, of the constellation points having an high probability of having been transmitted assuming the i-th transmitted bit equal to 1.
4. The method of claim 1 , including a step of determining said thresholds according to conditions of the channel.
5. The method of claim 1 , wherein the step of demapping includes computing log-likelihood ratios relative to values of bits in the sample.
6. The method of claim 1 , wherein the step of demapping includes computing a shortest distance between the sample and the constellation points in the subset corresponding to values ‘0’ and ‘1’ of a bit.
7. The method of claim 1 , wherein demapping includes the step of reverting scaling and/or distortion in the channel.