IMPROVEMENT OF TURBO TRELLIS-CODED MODULATION SYSTEM

A structure and an iterative decoding algorithm of a turbo trellis-cod ed modulation system, proposed by Robertson and et al.. are improved. For the encoder, removal of the odd-even constraint of an interleaver is discussed and a structure which removes a serial connection of an i nterleaver and a deinterleaver is proposed. The latter makes encoding delay nearly half. A decoding algorithm which is a natural extension o f the standard decoding algorithm to TCM is proposed. In the proposed algorithm, logarithm of an a posteriori probability ratio is divided i nto three component values: an a priori value, a channel value and an extrinsic information. The extrinsic information is transferred to the next decoding stage as an a priori value. The proposed algorithm is e asier to understand than the Robertson's algorithm in which a combinat ion of the channel value and the extrinsic information is transferred to the next stage. Simulation results show the proposed algorithm real izes equivalent or better performance than the Robertson's algorithm. The removal of the odd-even constraint gives a little better performan ce than that with odd-even constraint in some conditions. By this impr ovement, bit error rate of 10(-5) is obtained at E-b/N-0 0.4 dB from t he Shannon limit for 2 bit/symbol transmission with 8-PSK modulation.