Minimizing delay in D2D-assisted resource-efficient two-stage multicast in LTE access networks

Device-to-device (D2D) communication has been proposed to improve the throughput of multicast from the base station to user equipment (UE) in LTE-based access networks. Such a multicast is divided into two stages where the first stage is from the base station to one or more relay UEs, while the second stage is from the relay UE(s) to the rest of UEs via D2D communication, where D2D-capable UEs form clusters to maintain their short-range transmission characteristic. Previous works did not consider taking advantage of D2D's capability of reusing uplink resource blocks that have been allocated to ordinary UEs (or CUE, cellular-network UE). Previous works also mainly focused on maximizing the throughput sum of firststage and second-stage transmissions. We note that the multicast delay is a more meaningful performance metric for time-sensitive streaming data multicast and high throughput sum does not directly translate to low delay. In this paper, we study the D2D-assisted two-stage multicast that allows D2D communication to reuse CUEs' uplink resource blocks. We choose the multicast delay as the main performance objective and evaluate two relay UE selection schemes in their effectiveness in minimizing the multicast delay in comparison to one-stage multicast. Our results show that two-stage multicast performs better than one-stage multicast in both minimizing delay and maximizing resource efficiency when the cluster is far away from the base station and the cluster size is small. 1