Energy Efficiency and Spectrum Efficiency of Multihop Device-to-Device Communications Underlaying Cellular Networks

Device-to-device (D2D) communications are usually considered to be two user equipment units (UEs) communicating directly without going through the central base station (BS). In fact, they can be further broadened to multihop D2D communications in which a UE may help other UEs communicate with each other or assist other UEs to communicate with the BS. In this paper, we investigate a scenario of multihop D2D communications where one UE may help other two UEs to exchange information with a two-time-slot physical-layer network coding scheme. We analyze the average energy efficiency and spectral efficiency of multihop D2D communications under Rayleigh fading channels and get close analytical approximations based on Taylor series expansion. Comparisons with direct D2D communications and traditional cellular communications through BS are provided. The optimal UE transmission powers of these three different modes to maximize the energy efficiency are also derived.