Two-Layered Superposition of Broadcast/Multicast and Unicast Signals in Multiuser OFDMA Systems
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We study optimal delivery strategies of one common and K independent messages from a source to multiple users in wireless environments. In particular, we consider a two-layered superposition of broadcast/multicast and unicast signals in a downlink multiuser OFDMA system that is the main multiple access technology in the 4G and 5G mobile standards. In the literature and industry, the two-layer superposition is often considered as a pragmatic approach to make a compromise between the simple but suboptimal orthogonal multiplexing (OM) and the optimal but complex fully-layered non-orthogonal multiplexing. In this work, we show that only two-layers are necessary to achieve the maximum sum-rate when the common message has higher priority than the K individual unicast messages, and OM cannot be sum-rate optimal in general. We develop an algorithm that finds the optimal power allocation over the two-layers and across the OFDMA radio resources. The considered two-layer architecture is sum-rate optimal, enhances the cell-edge user performance, and retains decoding simplicity at the receivers. Two main use-cases are considered: i) Multicast and unicast multiplexing when K users with uplink capabilities request both common and independent messages, and ii) broadcast and unicast multiplexing when the common message targets receive-only devices and K users with uplink capabilities additionally request independent messages. Finally, we develop a transceiver design for broadcast/multicast and unicast superposition based on LTE-A-Pro physical layer and show with numerical evaluations that the capacity improvements can be translated into significant practical performance gains compared to the orthogonal schemes included in the 3GPP specifications.
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