Universal soft demapper for M-ary PAM with reduced complexity

In this paper, we propose an efficient soft demapper for M-ary pulse amplitude modulation (PAM) schemes. The proposed demapper estimates soft decision bit information (SDBI) for any M-ary PAM symbols using a universal type of hardware structure. The proposed method performs as well as the Max-log method, without any exhaustive search process. Instead, it maps a detected PAM symbol to a specific region, with only one constellation symbol to estimate the distance. In this way, the proposed method requires approximately one distance estimation per bit. We first derive the equations to estimate SDBI for a few Gray-mapped M-ary PAM schemes, and subsequently present compact universal equations applicable to any M-ary PAM scheme. The simulation results confirm that the proposed method produces exactly the same performance as the Max-log method with greatly reduced computational complexity.