Downlink Cooperative Transmission by Superposition Modulation: Performance Analysis and Power Allocation Strategy

In this paper, we proposed 2 new downlink cooperative transmission strategies based on superposition modulations: Scheme (a), QPSK superimposed modulation and scheme (b), orthogonal 4-PAM superimposed modulation. In both schemes, Rayleigh fading and path loss effect are introduced into channel model. We analyzed the symbol error rate (SER) performances and derived their theoretical expressions. The analytical SER expression of scheme (b) is achieved. Furthermore, we investigated the optimal power allocation problem through both analytical and numerical approach. Finally, the derived analytical expressions were verified by simulation results. Our proposed superposition scheme (b) can outperform the classical downlink cooperative scheme by 2 dB, and the performance advantage of Scheme (a) achieves by 5 dB.