An Hmm Soft-Output Decoder for Qam Signals with a Clustered Constellation

This paper presents a new HMM soft-output decoder for QAM signals transmitted through narrow-band flat-fading channels, in the case where the signal constellation has points which are clustered into groups. Parallel HMM filters are used for QAM detection, with one filter for each group of constellation points. This greatly reduces the number of computations required. A Kalman Filter (KF) is then applied for channel estimation. The resulting adaptive algorithms appear as coupled KF and HMM filters. As well as having reduced computations, this approach does not require so called ‘hard-decoding-decisions’ to be made, which result s in better robustness to noise.