Mode Switching and Power Allocation for Relay-Assisted Cooperative Device-to-Device Communications

The fifth generation (5G) networks have changed the world we live in, and device-to-device (D2D) communications are expected to further enhance 5G by enabling direct communication between two adjacent user equipments (UEs) without bypassing the next generation Node B. However, direct D2D communications are insufficient for establishing high-quality links across long distances. This article, by taking relay-assisted cooperate D2D communications into account, studied the mode switching and power allocation problems to maximize the joint channel capacity while meeting the signal-to-interference-plus-noise ratio requirements of D2D UEs (DUEs) and cellular UEs (CUEs). In particular, the proposed mode switching scheme included two phases. In the first phase, the optimal power allocation problems of two relay-assisted modes (i.e., relay and cooperative modes) were formulated and solved, respectively. In the second phase, we presented an optimal relay selection algorithm to find the relay UE that maximizes the joint channel capacity together with a mode selection algorithm among direct D2D, relay D2D, and cooperative D2D modes. The simulation results demonstrated the advantages of the proposed approach in terms of joint channel capacity, energy efficiency, and success probability compared with other benchmark approaches in three typical D2D deployment scenarios.