Low-complexity and energy-efficient processing for dual-hop D2D communications with simultaneous multicast and unicast

In this paper, a novel low-complexity processing scheme for energy-efficient dual-hop device-to-device (D2D) multicast and unicast communication is proposed. The main goal is to provide low-delay transmission while minimizing relay-device processing and resource usage. Since decoding increases processing delay and energy consumption, no decoding is performed at the relaying device, and full channel state information (CSI) is not required at the relay for either reception or transmission. For multicast traffic, a new spatial modulation (SM) scheme is implemented at both hops, where multicast information is conveyed through antenna-index selection. For the unicast stream, a novel full-rate quasi-orthogonal space–time line code (QOSTLC) is proposed for the first hop, while a full-rate quasi-orthogonal space–time block code (QOSTBC) is employed at the second hop. Additional relay-side processing is introduced to increase symbol orthogonality in the QOSTLC structure and improve bit error rate (BER) performance. Simulation results are presented for different system parameters.