Rate-Splitting for Multi-Antenna Non-Orthogonal Unicast and Multicast Transmission: Spectral and Energy Efficiency Analysis
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In a non-orthogonal unicast and multicast transmission system, a multicast stream intended to all the users is superimposed in the power domain on the unicast streams. A conventional approach is based on Multi-User Linear Precoding (MU--LP) where each user relies on Successive Interference Cancellation (SIC) to first remove and decode the multicast stream before decoding its intended unicast stream. In this paper, we investigate the application of power-domain Non-Orthogonal Multiple Access (NOMA) and linearly-precoded Rate-Splitting (RS) in the non-orthogonal unicast and multicast transmission. Two different objectives are studied, namely maximizing the Weighted Sum Rate (WSR) and maximizing the Energy Efficiency (EE). Numerical results show that the proposed RS-assisted transmission strategy is more spectrally and energy efficient than the conventional MU--LP in a wide range of user deployments (with a diversity of channel directions, channel strengths and qualities of channel state information at the transmitter) and network loads (underloaded and overloaded regimes). Moreover, the WSR and EE performance gains of RS come without any increase in the receiver complexity compared with MU--LP. In comparison with the proposed NOMA-assisted transmission strategies, RS achieves better WSR and EE performance in a wide range of user deployments and network loads with much less complex receivers. Hence, we conclude that the proposed RS-assisted strategy is superior for downlink multi-antenna non-orthogonal unicast and multicast transmission.
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