Design of fixed-point iterative decoders for concatenated codeswith interleavers

摘要

We discuss the effects of quantization on the performance of theniterative decoding algorithm of concatenated codes with interleavers.nQuantization refers here to the log-likelihood ratios coming from thensoft demodulator and to the extrinsic information passed from one stagenof the decoder to the next. We discuss the cases of a single soft-inputnsoft-output (SISO) module, in its additive log-likelihood versionn(L-SISO), performing sequentially all iterations (an implementationnsolution coping with medium-low data rate as compared with the hardwarenclock), and that of a pipelined structure in which a dedicated hardwarenis in charge of each SISO operation (an implementation suitable for highndata rates). We give design rules in both cases, and show that ansuitable rescaling of the extrinsic information yields almost idealnperformance with the same number of bits (five) representing bothnlog-likelihood ratios and extrinsic information at any decoder stage