Energy and overhead aware adaptive forwarding strategy for multi-hop device-to-device communications

Device-to-device (D2D) communications in cellular networks enable direct transmissions between user equipments (UEs). If the source UE (SUE) and the destination UE (DUE) are far away from each other or the channel between them is too weak for direct transmission, then multi-hop D2D communications, where relay UEs (RUEs) forward the SUE's data packets to the DUE, can be used. In this paper, we propose an energy-efficient adaptive forwarding strategy for multi-hop D2D communications to optimally choose between the best RUE forwarding (BRF) mode and the cooperative RUEs beamforming (CRB) mode, depending on which of them provides the higher instantaneous energy efficiency. We analyse the associated average energy efficiency considering the overhead for obtaining channel state information, forwarding mode selection and cooperative beamforming. Simulation results show that multi-hop D2D communications with the proposed adaptive forwarding strategy exhibit a significantly higher energy efficiency than the BRF, CRB, direct D2D communications and conventional cellular communications.