Receivers for CPM signals in frequency-selective multipath channels

The author derives receiver structures for continuous-phase-modulation signals in frequency-selective channels. Both optimum (maximum-likelihood sequence estimation, or MLSE) and suboptimum (linear equalizer) receivers are considered. Coherent reception with known channel impulse response is assumed. The performance is evaluated by simulation. In deciding between optimum MLSE and suboptimum equalizer receivers, the important issue is the complexity of implementation. In the optimum MLSE receiver the number of states in the Viterbi algorithm increases exponentially with the total length of CPM- and channel memory. Also, the correction terms for the metric increments must be stored. In the suboptimum equalizer receiver, channel memory does not increase the number of states. However, the matrix form of the equalizer requires a large storage and operation count. In adaptive implementations, one needs to update the various matrices {X_i} and {e_i} from some channel estimate. This may also be computationally expensive. A potentially crucial disadvantage of the equalizer receiver is the fact that its performance may for some channels become worse than the performance of the simple AWGN (adaptive white Gaussian noise) receiver. The optimum MLSE receiver is, in contrast, robust