Iteratively Decoded Irregular Variable Length Coding and Trellis Coded Modulation

In this paper we propose a novel Irregular Variable Length Coding (IrVLC) scheme for near-capacity joint source and channel coding. We employ a number of Variable Length Coding (VLC) component codebooks having different coding rates for encoding appropriately chosen fractions of the input source symbols. These fractions may be designed With the aid of EXtrinsic Information Transfer (EXIT) charts so that the EXIT curve shape of the IrVLC codec may be matched to that of a serially concatenated channel codec as closely as possible. In this way, an open EXIT chart tunnel may be created even at low Eb/N0 values that are close to the channel capacity. We detail the proposed serially concatenated and iteratively decoded IrVLC and Trellis Coded Modulation (TCM) designs that are capable of operating within 0.6 dB of the uncorrelated narrowband Rayleigh fading channel's capacity at an effective bandwidth efficiency of 1.56 bit/s/Hz, assuming ideal Nyquist filtering. By contrast, the equivalent-rate VLC-based bench-marker schemes were found to be capable of operating at 1.1 dB from capacity, which is nearly double the discrepancy of the proposed IrVLC-TCM scheme.