Rate-splitting multiple access for downlink communication systems

Abstract

Space-Division Multiple Access (SDMA) utilizes Multi-User Linear Precoding (MU--LP) to separate users in the spatial domain and relies on fully treating any residual multi-user interference as noise. Non-Orthogonal Multiple Access (NOMA) uses linearly precoded Superposition Coding with Successive Interference Cancellation (SC--SIC) to superpose users in the power domain and relies on user grouping and ordering to enforce some users to fully decode and cancel interference created by other users. In this thesis, we develop a novel multiple access framework, called Rate-Splitting Multiple Access (RSMA). RSMA is a more general and powerful multiple access scheme for downlink communication systems that contains SDMA and NOMA as special cases. RSMA relies on linearly precoded Rate-Splitting (RS) with SIC to partially decode the interference and partially treat the interference as noise. It softly bridges the two extreme interference management strategies, namely, fully treats interference as noise (as in SDMA) and fully decodes interference (as in NOMA).

We first study the spectrum efficiency of RSMA in downlink Multiple Input Single Output Broadcast Channel (MISO BC) by solving the problem of maximizing the Weighted Sum Rate (WSR) with the transmit power constraint and individual QoS rate constraint. A Weighted Minimum Mean Square Error (WMMSE)-based algorithm is proposed to solve the WSR maximization problem. Numerical results show that RSMA provides a smooth transition between SDMA and NOMA. The spectrum efficiency of RSMA outperforms that of SDMA and NOMA in a wide range of network loads (underloaded and overloaded regimes) and user deployments (with a diversity of channel directions, channel strengths and qualities of channel state information at the transmitter). RSMA provides rate and QoS enhancements over NOMA at a lower computational complexity for the transmit scheduler and the receivers (number of SIC layers).

We then investigate the Energy Efficiency (EE) of RSMA in MISO BC. A Successive Convex Approximation (SCA)-based algorithm is proposed to solve the EE maximization problem. Numerical results show that RSMA also bridges and outperforms SDMA and NOMA in the realm of EE. The EE region achieved by RSMA is always equal to or larger than that achieved by SDMA and NOMA in a wide range of network loads and user deployments. Therefore, we conclude that RSMA is not only more spectrally efficient, but also more energy efficient than SDMA and NOMA.

Finally, we study the application of RSMA in cooperative multi-cell systems as well as non-orthogonal unicast and multicast transmission systems. Numerical results show that, in a fully cooperative multi-cell network, RSMA achieves significant WSR improvement over SDMA and NOMA in a wide range of inter-user and inter-cell channel strength disparities. In the non-orthogonal unicast and multicast transmission systems, we demonstrate that the rate region and EE region of RSMA with one layer of RS are always equal to or larger than those of SDMA. Importantly, the performance gain of RSMA comes at no additional cost for the receivers since one layer of SIC is required to separate unicast and multicast streams in the conventional MU--LP based SDMA strategy.