Iterative soft onion peeling for multi-access and broadcast channels

Abstract

The concept of iterative multiuser detection for a multiple access adder AWGN channel is coupled with turbo code used for each user. The iterative decoding exchanges the likelihood information both among the two subcodes of each user and among the users. The information exchange among users is performed through subtraction of the mean values of contributions of other users from each symbol. In addition, the variance of the equivalent noise is reestimated for each symbol and each user at each step. When the iterative procedure is applied to equal-power equal-rate case, it fails to converge at aggregate rates significantly beyond 0.5 bit/symbol, due to excessive S/(N+other users) ratio in the first iteration. We are interested in a multiple access scheme which is symmetric with respect to signaling method used by each user, but with significantly higher aggregate capacity. We overcome this problem by splitting each user into several subusers of equal rate but unequal power. Assignment of different powers to different users creates an onion structure. This structure is used implicitly by the iterative decoding process, as the higher power users converge first, and then the lower power users converge as the residual multi-user noise reduces. This breaks the 0.5 bit/symbol barrier and enables to reach 2 bits/symbol and beyond, at the expense of a linear increase in the number of iterations required. The approach of unequal power assignment can be directly applied to multiple users with power disparities, where each user can be interpreted as an independent user. Application to a degraded broadcast channel is addressed.