Coded and uncoded M-PSK and CPM signals on nonlinear bandpass channels: an approach to a fair comparison

The performances of several coded and uncoded M-ary phase-shift keying (M-PSK) schemes and two representative continuous-phase modulation (CPM) schemes are compared on a given nonlinear bandpass channel with adjacent channel interference (ACI). It is shown that, due to their constant envelope, the CPM schemes are not degraded by the nonlinearity and allow saturated operation of the high-power amplifier (HPA). In this case the only degradation is produced by ACI. The PSK schemes are assumed to be filtered with conventional square-root Nyquist filters and are very bandwidth efficient. The results of simulations are presented for 4-PSK, offset 4-PSK, coded and uncoded 8-PSK, minimum-shift keying (MSK), and a four-stage CPM scheme with smoothed frequency impulse (M=4, h=1/2, 3RC). It is shown that coded 8-PSK maintains or improves its gain over 4-PSK in the nonlinear channel, when the operating point of HPA is carefully optimized. MSK suffers mainly from ACI, and for narrow channels it is outperformed even by 4-PSK. The 3RC-CPM scheme shows nearly the same performance as coded 8-PSK