Superposition Coding Based User Combining Schemes for Non-Orthogonal Scheduling in a Wireless Relay System

We consider a wireless system where multiple users are served in Downlink (DL) by one Base Station (BS) and one Relay Station (RS). Previous research has shown that, combining two users' messages via Superposition Coding (SC) could enhance their achievable rate and fairness, in both two-user and multi-user systems. In this work, we propose two novel SC schemes referred as Relayed/Direct (RD-SC) and Relayed/Relayed SC (RR-SC) schemes, that combine the messages to a pair of Relayed/Direct users or Relayed/Relayed users into three or four SC layers, where the number of SC layers is equal to the number of available links. For each scheme, power allocation under sum-rate maximization is analyzed, enabling to derive the optimal power ratios to each SC layer numerically. Then, we design the non-orthogonal Max Rate-Two User-SC (MR-TU-SC) Scheduler and Proportional Fair-Two User-SC (PF-TU-SC) Scheduler where a pair of selected users are allocated simultaneously based on RD-SC and RR-SC schemes. The simulation results show that the proposed schedulers significantly outperform the conventional orthogonal schedulers where a single user is allocated on a unit resource block, in terms of system throughput, fairness and outage, while approaching the upper bound performance.