NON-COHERENT SEQUENCE ESTIMATION RECEIVER FOR DIGITAL MODULATIONS

摘要

Sequences of information symbols (ä Ð ü) are used to digitally modulate a carrier according to either M-PSK or M-QAM or CPM schemes. The modulated carrier is transmitted on the radio channel and the original sequence is non-coherently estimated at the receiver using the Viterbi algorithm. The radio channel either considered ideal or not is affected by Gaussian noise. The received signal is filtered by a filter matched to the transmitted pulse. In case the channel is considered as non-ideal and/or the selected modulation is CPM, in other words in presence of intersymbol interference, an additional whitening filtering is carried out on the received signal. The filtered signal is sampled at symbol time and the complex samples (xn) are accumulated into a sequential memory containing N most recent samples. The stored samples enter the Viterbi processor to be estimated according to MLSE. Branch metrics used by the Viterbi processor are obtained starting from the known theoretical analytical expression valid for the non-coherent MLSE receiver. The analytical expression is assumed as a sequence general metric subjected to be obtained through recurrent updating of a sequence partial metric defined up to the n discrete instant. An incremental metric of unlimited memory is obtained subtracting from the sequence partial metric at the n discrete instant the sequence partial metric at an immediately preceding discrete instant. The length of the unlimited memory is truncated to include the only N most recent observable complex samples (xn), obtaining the analytical expression of the branch metric ( lambda (s)_n) for a trellis diagram built up on the basis of all possible subsequences of symbols (ä Ð ü) having length N (fig.2).