Energy- and Spectral-Efficient Adaptive Forwarding Strategy for Multi-Hop Device-to-Device Communications Overlaying Cellular Networks

Device-to-device (D2D) communications in cellular networks facilitate direct transmissions between user equipment (UEs). In the case that direct transmission is not favourable due to long distances or poor channel conditions between the source UE (SUE) and the destination UE (DUE), multi-hop D2D communications can be deployed, where relay UEs (RUEs) forward the data received from SUE to the DUE. In this paper, we propose an adaptive forwarding strategy for multi-hop D2D communications to optimally choose between the best relay forwarding (BRF) mode and the cooperative relays beamforming (CRB) mode with the optimal number of RUEs, depending on which of them provides the higher instantaneous energy efficiency (EE). We analyse the associated average EE taking into account the circuit power consumption and the overhead for obtaining channel state information (CSI), forwarding mode selection and cooperative beamforming. In order to reduce the overhead for mode selection, we propose a distributed adaptive forwarding mode selection strategy. Simulation results reveal that multi-hop D2D communications utilizing the proposed adaptive forwarding strategy achieves significantly higher EE than the BRF, CRB with the optimal number of RUEs, direct D2D communications, and conventional cellular communications.