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In this paper, a new approximated MAP algorithm for soft bit decision from QAM symbols is proposed for Gray Coded QAM signals, based on the Max-Log-MAP and a Gray coded QAM signal can be separated into independent two Gray coded PAM signal, M-PAM on I axis with M symbols and N-PAM on Q axis with N symbols. The Max-Log-MAP used distance comparisons between symbols to get the soft bit decision instead of mathematical exponential or logarithm functions. But in accordance with the increase of the number of symbols, the number of comparisons also increase with high complexity. The proposed algorithm is used with the Euclidean distance and constituted with plain arithmetic functions, thus we can know intuitively that the algorithm has low implementing complexity comparing to conventional ones. Key Words : Soft Decision, Log Likelihood, QAM, Gray Mapping, Max-Log-MAP *  : (kamihyun@gwnu.ac.kr) 09 10 04  (1 09 11 30, 2 09 12 11)  09 12 16

Approximated MAP Algorithm for Gray Coded QAM Signals