Rate Splitting With Finite Constellations: The Benefits of Interference Exploitation vs Suppression

Rate-Splitting (RS) has been shown recently to be a powerful approach for the design of non-orthogonal transmission, multiple access, and interference management strategies in multi-user multi-antenna systems. RS, through the split of messages into common and private parts, relies on the transmission of common streams decoded by all users, and private streams decoded only by its intended user. This enables RS to bridge the extreme of fully decode interference and fully treat interference as noise. In this paper, we depart from Gaussian signaling and study RS under finite input alphabet for multi-user multi-antenna system and propose a constructive interference (CI) exploitation approach to further enhance the sum-rate achieved by RS. To that end, new analytical expressions for the ergodic sum-rate are derived for two precoding techniques of the private messages, namely, 1) a traditional interference suppression zero-forcing (ZF) precoding approach, 2) a closed-form CI precoding approach. Our analysis is presented for perfect channel state information at the transmitter (CSIT), and is extended to imperfect CSIT knowledge. A novel power allocation strategy, specifically suited for the finite alphabet setup, is derived and shown to lead to superior performance for RS over conventional linear precoding not relying on RS (NoRS). The results in this work validate the significant sum-rate gain of RS with CI over the conventional RS with ZF and NoRS.