Symbol Message Passing Decoding of LDPC Codes for Orthogonal Modulations

A simple decoder forq-ary low-density parity-checkcodes is studied, termed symbol message passing. The decoderpasses hard decisions from aq-ary alphabet. For orthogonalmodulations over the additive white Gaussian channel for whichthe modulation order and the field orderqare equal, it is shownthat the extrinsic messages can be modelled as observations of aq-ary symmetric channel, allowing to work out density evolutionequations. A stability analysis is provided which emphasizes theinfluence of degree-3variable nodes. Simulation results showperformance gains for increasingqw.r.t. binary low-densityparity-check codes with bit-interleaved coded modulation, andpotential savings in decoding complexity