Energy efficient power allocation for relay-aided D2D communications in 5G networks

As one of the key technologies for the fifth generation (5G) wireless networks, device-to-device (D2D) communications allow user equipment (UE) in close proximity to communicate with each other directly. Forwarded by a relay, the relay-aided D2D (RA-D2D) communications can not only be applied to communications in much longer distance but also achieve a high quality of service (QoS). In this paper, we first propose a two-layer system model allowing RA-D2D links to underlay traditional cellular uplinks. Then we maximize the energy efficiency of the RA-D2D link while satisfying the minimum data-rate of the cellular link. The optimal transmit power at both D2D transmitter and D2D relay sides is obtained by transforming the nonlinear fractional programming into a nonlinear parameter programming. Simulation results show that our proposed power allocation method is more energy efficient than the existing works, and the proposed RA-D2D scheme outperformed direct D2D scheme when the distance between two D2D users is longer.