Joint source channel decoding and iterative symbol combining with turbo trellis-coded modulation

A new joint source channel decoding (JSCD) scheme for turbo trellis-coded modulation is proposed which exploits symbol-a-priori probabilities to improve decoding performance. To obtain symbol-a-priori probabilities, we propose a new method which uses a time-stretched bit-level trellis of a variable length code (VLC) decoder alongside a symbol level trellis. Iterative symbol combining (ISC) is also integrated with the proposed JSCD scheme so that multiple transmissions of the same data packet can be combined to improve decoding. Essentially JSCD with ISC (JSCD-ISC) exploits two types of a-priori information. The first one being the symbol-a-priori probabilities derived from source statistics, and the second one is obtained from the saved extrinsic information of previously transmitted copies of the data. Several simulations with different VLC sources like Huffman and reversible variable length codes (RVLC), and for different number of retransmissions have been performed. The most striking result is obtained with RVLC, whereby using only two retransmissions, the JSCD-ISC scheme outperforms a conventional scheme using three retransmissions by 0.2 dB.