Optimal joint rate and power allocation for layered multicast with superposition coding in cellular systems

Abstract

We propose a layered multicast transmission scheme with superposition coding for cellular systems, i.e., at a base station a basic multicast stream (BMS) and an enhanced multicast stream (EMS) are superimposed and transmitted, the same BMS is repeatedly transmitted multiple times to ensure most users in the cell receive basic qualities of the service, while in each transmission different EMSs are transmitted to make the users with good channel conditions receive higher qualities of the service. In this paper, the optimal joint rate and power allocation for the layered multicast scheme is studied. Specifically, we first give a proof on the claim that the system delay of a BMS is minimized if the transmission rate of the BMS is set according to a fixed user selection ratio in each transmission. Then subject to fixed transmit power and power allocation, we derive the optimal transmission rate of a BMS that minimizes the system delay of the BMS, and the optimal transmission rate of an EMS that maximizes the average throughput of the EMS. Finally, by balancing the tradeoff between the system delay of a BMS and the average throughput of an EMS, we find the optimal joint rate and power allocation for the layered multicast scheme. Numerical results show that the optimized layered multicast scheme outperforms the conventional schemes in terms of the system delay of a BMS and the average throughput of an EMS.