Turbo Equalization for Gray-Coded M-ary QAM with Bit-Level Soft Decisions

This paper investigates the soft-in soft-out (SISO) equalization of multilevel QAM symbols in coded inter-symbol interference (ISI) channels. Unlike the conventional approach of performing equalization at the symbol level, the proposed scheme targets the channel bits directly. This solution can be seen to belong to a family of SISO detection schemes which we call Bit-Level Equalization and Soft Detection (BLESD), stemming from the minimization of variational free energy given different postulates about the prior and posterior distributions of the channel bits. Simulation results demonstrate that the bit-level approach outperforms the symbol-level alternative in terms of error rate in the Porat-Friedlander channel.