Source-dependent channel coding of CELP speech over land mobile radio channels

We consider the problem of reliably transmitting CELP-encoded speech over a land mobile radio channel. We first quantify the "residual redundancy" inherent in the line spectral parameters (LSPs) of Federal Standard 1016 CELP. This is done by modeling the quantized LSPs as first- and second-order Markov chains; these models indicate that as many as one-third of the LSP bits are redundant. We then consider methods by which that residual redundancy can be exploited by an appropriately designed channel decoder. Before transmission, the LSPs are encoded with a forward error control (FEC) code; we consider both Reed-Solomon codes and convolutional codes. Soft-decision decoders that exploit the residual correlation in the LSPs are implemented assuming a Rayleigh fading environment. Simulation results using BPSK and DQPSK modulation indicate coding gains of 2 to 5 dB over soft-decision decoders that do not exploit the residual correlation.