Two-Level Quantized Soft-Output Demodulation of QAM Signals With Gray Labeling: A Geometric Approach

We propose a geometric approach to the design of a look-up table-based two-level quantized soft-output (SO) demodulator for coded schemes with quadrature amplitude modulation (QAM) signals with Gray bit labeling. This allows to derive quantized bit logarithmic likelihood ratios (LLRs) directly from the observables, without requiring the actual computation, followed by quantization, of LLRs at the output of the demodulator. The proposed approach is applied to a bit-interleaved coded modulation (BICM) scheme. The obtained results show a limited performance loss with respect to unquantized SO demodulation.