The optimization of constellation rotation on the multi-user superposition modulation system

In the modulation process of non-orthogonal multiple access system, the modulation structure can be depicted as a factor graph constituted by user nodes, channel nodes and connections between these two kinds of nodes. For a given factor graph with arbitrary number of user nodes, the constellation angles of the user nodes can be optimized to improve the mutual information, or sum capacity of the system. However, the multiplicity of integral on computing mutual information would rise in proportion to the number of channel nodes and make it unsolvable. In this paper, first, we provide a generalized optimization formula based on mutual information without multiple integral. Second, we proposed a low-complexity searching algorithm to search for the optimal angle set of the user nodes. The simulation results show the optimal angle set can improve the performance of multi-user superposition system.