Superposition Coding and Analysis for Cooperative Multiple Access Relay System

The conventional straightforward relaying scheme named decode-and-forward (DF) protocol is used at relay to decode the data received from source and forward the decoded data to the destination to improve the spatial diversity gain. Although it is a simple scheme, the total resource efficiency is low for multiple sources. Superposition coding has been known as a spectral efficient technique in wireless networks. However, when it is applied to the uplink multiple access relay systems, it suffers from performance degradation caused by the power allocation between the symbols of multiple sources. In this paper, we propose to remedy this problem by using hierarchical modulation at the source node and superposition coding at the relay node to enhance the bit energy. We investigate how hierarchical modulation can be incorporated and optimized with superposition coding. Specially, we discuss the capacity behavior of the proposed scheme and compare with the baseline, namely the capacity behavior of the existing DF scheme and the capacity behavior of the superposition coding. Our results are encouraging in that superposed hierarchical modulation significantly improves the better block error rate and capacity performance in multiple access relay channels.