Joint Iterative Decoding of Trellis-Based VQ and TCM

A joint video and channel coded system employing an iteratively decoded serial concatenation of a vector quantization (VQ) based video codec and a trellis-coded modulation (TCM) scheme is proposed. The video codec imposes VQ-induced code constraints, which may be completely described by a trellis structure, which is employed as the basis for optimal minimum mean-squared-error VQ-encoding and -decoding. In the latter case, the Bahl-Cocke-Jelinek-Raviv (BCJR) algorithm is employed to facilitate the iterative exchange of soft information between the VQ and TCM decoder. An error-free video reconstruction quality is supported using 16-Ievel quadrature amplitude modulation (16QAM) based TCM for transmission over Rayleigh-fading channels at a signal-to-noise ratio (SNR) per bit of 5.25 dB. This value is within 1.29 dB of the Rayleigh channel´s capacity at our system´s effective bandwidth-efficiency of 2 bits/s/Hz. Owing to its ability to exploit the VQ-induced code constraints during iterative decoding, the joint video and channel coding approach is found to consistently outperform the Shannonian source and channel separation philosophy. This is achieved at the cost of a 1.6 times higher computational complexity. Finally, the convergence of the iterative decoder is investigated with the aid of a novel so-called extrinsic information transfer (EXIT) chart