MLSE and soft-output equalization for trellis-coded continuous phase modulation

Trellis-coded continuous phase modulation (TC-CPM) is attractive for mobile radio channels due to excellent power and bandwidth efficiencies. This paper presents two equalizer structures for TC-CPM on multipath fading ISI channels. An equivalent discrete-time model is developed by combining the tapped-delay-line model of the frequency-selective channel and by oversampling at the receiver. The fractionally-spaced MLSE equalizer performs CPM demodulation, TCM decoding, and channel equalization by exploiting the finite state nature of the ISI-corrupted TC-CPM signal. Both simulation and analytical results show diversity-like improvement when performing joint MLSE decoding and equalization. For the interleaved soft-output equalizer, the soft symbol metric is delivered to the TCM decoder by using a forward and a backward recursion algorithm. Three variants of the soft-output equalizer are examined. We conclude that the backward recursion is essential to partial-response CPM schemes and with moderate complexity, the soft-output equalizer can have substantial advantage over a non-interleaved MLSE equalizer