Rate-Splitting Multiple Access in Wireless Backhaul HetNets: A Decentralized Spectral Efficient Approach

In this paper, we investigate the application of rate-splitting multiple access (RSMA) in a two-tier wireless backhaul heterogeneous network (HetNet), where a macro base station (MBS) simultaneously transmits wireless access signals to multiple macro-cell users (MCUs) and wireless backhaul signals to small base stations (SBSs) by leveraging RSMA. Furthermore, to explore the potential advantage of common streams in RSMA systems, we develop a 'Hybrid RSMA' scheme in which the MBS only employs RSMA to encode the backhaul messages while each MCU's message is directly encoded without rate-splitting. We formulate an optimization problem to maximize the system's spectral efficiency (SE) by jointly considering transmit precoding and rate allocation at the MBS and SBSs. To solve the formulated non-convex problem, we first propose an iterative centralized algorithm based on successive convex approximation (SCA). Then, we further develop an efficient decentralized algorithm that can be executed in parallel at the MBS and each SBS based on the local channel state information with fewer signaling exchanges. Simulation results show that the application of RSMA can achieve higher SE over conventional non-orthogonal multiple access (NOMA) and space division multiple access (SDMA) under different network loads. Particularly, 'Hybrid RSMA' has a greater performance improvement than 'RSMA' in the underloaded system.