Device-to-device assisted two-stage cooperative multicast with optimal resource utilization

Device-to-device (D2D) communication has the potential to facilitate wireless multicast by retransmissions among users in proximity to each other. In this paper, the D2D assisted cooperative multicast (CM) is investigated in a new scenario, where a large number multicast recipients are densely distributed within a small area (called a D2D cluster). In this scenario, D2D multicast is innovatively employed in the retransmission stage, and a new relay selection scheme named nearest center protocol (NCP) is proposed. Moreover, the optimal time allocation between the two transmission stages is investigated, with the aim of minimizing the total time consumption of the two-stage CM. The time allocation problem is then formulated as an optimization problem and solved by the sequential quadratic programming (SQP) method. Simulation results show that the D2D assisted two-stage CM can achieve higher cost-efficiency than the conventional one-stage multicast, and the gain becomes larger when the D2D cluster moves farther from BS. Besides, the optimal time allocation between the two transmission stages also varies with the location of the D2D cluster, which means equal time allocation is no longer appropriate to be used in our new scenario.