Non-Orthogonal Unicast and Broadcast Transmission via Joint Beamforming and LDM in Cellular Networks

Research efforts to incorporate multicast and broadcast transmission into the cellular network architecture are gaining momentum, particularly for multimedia streaming applications. Layered division multiplexing (LDM), a form of nonorthogonal multiple access (NOMA), can potentially improve unicast throughput and broadcast coverage with respect to traditional orthogonal frequency division multiplexing (FDM) or time division multiplexing (TDM), by simultaneously using the same frequency and time resources for multiple unicast or broadcast transmissions. In this paper, the performance of LDM-based unicast and broadcast transmission in a cellular network is studied by assuming a single frequency network (SFN) operation for the broadcast layer, while allowing for arbitrarily clustered cooperation for the transmission of unicast data streams. Beamforming and power allocation between unicast and broadcast layers, and hence the so-called injection level in the LDM literature, are optimized with the aim of minimizing the sum-power under constraints on the user-specific unicast rates and on the common broadcast rate. The problem is tackled by means of successive convex approximation (SCA) techniques, as well as through the calculation of performance upper bounds by means of semidefinite relaxation (SDR). Numerical results are provided to compare the orthogonal and non-orthogonal multiplexing of broadcast and unicast traffic.