Bandwidth-efficient correlative trellis-coded modulation schemes

A novel digital modulation scheme is proposed that allows to increase the data rate compared to uncoded modulation when the out-of-band power is fixed. If the data rate remains unchanged, this corresponds to a bandwidth reduction that is achieved by shaping the signal spectrum by the introduction of correlation in the transmitter. This correlation has to be resolved in the receiver by a maximum likelihood sequence estimator. Since the sequence of transmitted symbols can be represented by a trellis, this modulation scheme is called correlative trellis coded modulation (C-TCM). The transmitter is first optimized with respect to bandwidth-efficiency, i.e. it is designed to minimize the resulting out-of-band power. Where possible, a secondary optimization with respect to power-efficiency is performed, such that the signal-to-noise ratio required at the receiver to guarantee a certain error rate is minimum. The optimization shows that for an encoder with memory one the well-known modu¬ lation schemes duobinary and dicode are optimum. The mappers that lead to a minimum average or peak power are presented. The method derived for the optimization of the transmitter is applicable for any number of delay cells. In this report we present results for encoders with up to five delay cells, where new modulation schemes with time-limited and Nyquist pulse shaping filters are found. It is shown that the optimum transmitters can also be represented as partial response systems. Considering the power-efficiency of the C-TCM scheme, it is shown that the remark¬ able increase of data rate of up to 300% compared to uncoded quaternary modulation is realized at the cost of about 1.5 dB additional signal power per delay cell. Compar¬ ing C-TCM with uncoded M-ary quadrature amplitude modulation, however, C-TCM allows to increase the data rate by up to 0.5 bit/s/Hz for a given performance or to reduce the signal power by about 1 dB for a given data rate.