Multilevel CPM with 4-D trellis coding and linear receivers

Presents two multilevel constant-envelope continuous-phase modulation (CPM) schemes with 4-D trellis coding. The receiver is composed of a simple quadrature demodulator, followed by a symbol-rate sampler, and a Viterbi decoder matched to the code trellis. The first modulation is a quaternary CPM scheme whose phase transitions over a symbol interval are those of π/4-shift QPSK. With respect to results of Sari et al. (1994), the present authors make an unconstrained optimization of the receive filter, and achieve a further gain of 0.7 dB in signal-to-noise ratio (SNR). This modulation outperforms MSK by 3.5 dB, in addition to reducing the signal bandwidth by 20%. Next, they introduce an octonary (8-level) CPM scheme whose phase transitions are those of π/8-shift 8PSK. The same trellis code and receive filter optimization are also applied to this modulation, which is shown almost to achieve the bit error rate (BER) performance of MSK, while saving some 60% of the transmitted signal bandwidth