An All-Digital Signal Demodulation Scheme for High Frequency (HF) QAM Signals

In this paper, an all-digital demodulation scheme is introduced and investigated for HF QAM signals in MIL-STD-188-110B. Symbol timing, adaptive equalization and carrier recovery techniques are involved in it. Symbol timing is carried out by a maximum mean power estimate (MMPE) algorithm which avoids the conventional loop and decreases the computational complexity. A decision-directed (DD) mode carrier recovery loop runs jointly with adaptive equalizer. It eliminates intersymbol interference (ISI) and phase error better than separate structure. The non-linear decision feedback equalizer (DFE) is also adopted. Compared to linear equalizers, it has better performance on compensation for channel distortion. In this scheme, the signal synchronization preamble is equalized and phase compensated firstly in order to open the signal eye pattern, then adaptive equalization and carrier synchronization are carried out for data symbols. Simulation shows this all-digital demodulation scheme is feasible and efficient for HF QAM signals. It provides theoretical foundation for the DSP implementation