Abstract: Since soft input is necessary for Turbo decoder, thedemodulator located before the decoder should offer the soft output. The commonly used soft output demodulating-algorithms, log-MAP and its approximation max-log-MAP are described. The simulation showed that the performance differenceof the two algorithms for the same decoding error rate is within 0.1 dB , butthe complexity of log-MAP is much higher. The combination of max-log-MAP demodulator and log-MAP Turbo decoder can achieve the almost optimal performance with reduced complexity of hardware implementation.

Key words: high order modulation, log-MAP, max-log-MAP, softoutput