On the channel capacity and iterative demapping of generalized 4PAM over AWGN channel

The concept of generalized 4PAM (G4PAM) is introduced in this paper, where each 4PAM symbol includes one high bit and one low bit with non-uniformly spaced constellation. The channel capacity of G4PAM constrained additive white Gaussian noise (AWGN) channel is obtained via numeric calculation, so as these of the high and low bits in terms of independent demapping. Numeric results show that traditional hierarchal G4PAM has worse capacity property, and Gray mapping G4PAM constellation with different hierarchical degree suffers differently due to independent demapping. Further more, since independent demapping results in capacity loss, iterative demapping algorithms for both hard-output and soft-output decoding are presented.