Design of fixed-point iterative decoders for concatenated codes with interleavers

We discuss the effects of quantization on the performance of the iterative decoding algorithm of concatenated codes with interleavers, Quantization re fers here to the log-likelihood ratios coming from the soft demodulator and to the extrinsic informations passed from one stage of the decoder to the next. We discuss the cases of a single sob-input sea-output (SISO) module, in its additive log-likelihood version (L-SISO), performing sequentially al l iterations (an implementation solution coping with medium-low data rate a s compared with the hardware clock), and that of a pipelined structure in w hich a dedicated hardware is in charge of each SISO operation (an implement ation suitable for high data rates). We give design rules in both cases, an d show that a suitable rescaling of the extrinsic informations yields almos t ideal performance with the same number of bits (five) representing both l og-likelihood ratios and extrinsic information at any decoder stage.