A Universal Low-Complexity Symbol-to-Bit Soft Demapper

High-order constellations are commonly used for achieving high bandwidth efficiency in most communication systems. However, the complexity of the multiplication operations associated with the standard max-sum approximation of the maximum a posteriori probability in the log-domain (Max-Log-MAP) symbol-to-bit demapper is very high. In this contribution, we conceive a low-complexity universal soft demapper, which reduces the demapper's complexity considerably for the binary-reflected Gray-labeled pulse amplitude modulation (PAM), phase shift keying (PSK), quadrature amplitude modulation (QAM), and amplitude phase-shift keying (APSK) relying on product constellation labeling (product-APSK). Our theoretical analysis demonstrates that the proposed demapper has exactly the same performance as the Max-Log-MAP demapper for the Gray-labeled PAM, PSK, and QAM. Our theoretical analysis and simulation results also demonstrate that for the Gray-labeled product-APSK, the performance degradation of the proposed simplified soft demapper is negligible for both 64-ary and 256-ary constellations compared with the Max-Log-MAP demapper.