HYBRID TRELLIS-CODED 8/4-PSK MODULATION SYSTEMS

Combined convolntional codes with multilevel phase-shift keying (trellis-coded PSK modulation) can yield both power- and bandwidth-efficient transmission. By using codes of rate 2/3 followed by 8-PSK modulation, a gain of 3-6 dB is obtained over uncoded 4-PSK for an ideal coherent transmission on the white Gaussian channel. This gain is achieved without bandwidth expansion and without change of data rate. In the presence of carrier phase offset, it has been shown that trellis-coded 8-PSK systems are more sensitive than uncoded 4-PSK. The conventional trellis-coded systems have a limited pull-in range of ± 22.5° for the phase error in the recovery loop. If the phase error is outside this interval, the carrier recovery fails, causing a random-walk situation with a long error burst. For systems which operate on channels with phase variations like jitter and fading, this random walk and cycle slipping is a serious problem. A more robust performance can be achieved by using rate 2/3 trelliscoded 8-PSK signals and 4-PSK signals in a time-varying manner. Only the mapper from the output of the binary convolutional code to the signal point number to be transmitted has to be periodically time-varying. The remaining parts of the system are time invariant. In its simplest form, trellis-coded 8-PSK and 4-PSK signals are alternating in time. We have also looked at systems where the mixture of 8-PSK and 4-PSK signals vary, with a short periodic sequence of time-varying mapping rules. The distance spectra and error probability are evaluated with and without phase offset. Simulation results of bit error rate (BER) and performance of the recovery loop (5 curve) are also presented. Several options exists for designing the decision-directed carrier recovery and for shaping the S curve. The cycle slip rate of the classical system at 7.5° rms jitter is about 25%, but (ess than 0.1% in the new schemes. This is achieved with almost no penalty in BER in blocks received without cycle slips. Comparisons to conventional rate 2/3 trellis-coded 8-PSK are given. We conclude that systems which are more robust against jitter can be achieved by means of time-varying hybrid trellis-coded 8/4-PSK systems. © 1990 IEEE