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A simple soft-demapping scheme for Rectangular Quadrature Amplitude Modulation (R-QAM) signals is developed for a low-cost 60 GHz OFDM baseband chip design, conforming to the IEEE802.15.3C standard. The scheme is equally applicable to a generic bit-interleaved Orthogonal Frequency Division Multiplexing (OFDM) modulation for both Square QAM (S-QAM) and R-QAM constellations. The channel coding employs the concatenation of convolutional and Reed-Solomon (RS) codes. Simulation shows that this simple soft-demapping scheme effectively achieves the same performance of an optimum LogLikelihood Ratio (LLR) demapper. The exact formula for the difference between the proposed approximate LLR and the optimum LLR is derived.
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