Abstract:

When turbo decoder is implemented by Log-MAP algorithm, it is necessary to calculate the correction term by look-up table (or other ways). The required precision of the state metrics, log-likelihood ratio, etc., are calculated and a quantization scheme is presented. The required precision of the correction term is determined by the quantile interval of channel value for the quantizing error propagation. The quantizing error propagation shows the reason why the excellent results could be obtained with 8 stored values in the table when the quantile interval of channel value is 0.25. Also it shows that the Max-Log-MAP algorithm just like Log-MAP algorithm is optimal one because of the ignorable correction term for the quantizing error propagation under high signal noise ratio. Then a non-uniform look-up, table with 6 stored values is developed to calculate the correction term. It reaches the precision of the look-up table with 16 stored values, which is more accurate than 8 stored values table. At the same time it is easier to be implemented than the 8 stored values table. The simulation verifies these conclusions.

Key words: Turbo decoding, Log-MAP algorithm, look-up table, error propagation