Resource and Energy Efficient Device to Device Communications in Downlink Cellular System

n this paper, we investigate the energy efficiency (EE) optimization for a downlink orthogonal frequency division multiple access (OFDMA) system with overlaying Device-to-Device (D2D) communications. Joint EE optimization is highly complex while a two-stage solution will utilize most of the bandwidth for cellular users and not providing sufficient bandwidth for D2D users. Using resource efficiency (RE) optimization approach that balances the bandwidth usage and EE, we propose a two-stage solution that optimizes RE for base station (BS) and EE for D2D pairs. To achieve higher EE, D2D communications operate in non-orthogonal mode, where each resource block (RB) not being assigned to the cellular users are reused by multiple D2D pairs. By exploiting a range of optimization tools including fractional programming, Dinkelbach approach, Lagrange dual decomposition, difference of convex functions, and concave-convex procedure, the original non-convex problem is transformed and we present an iterative two-stage RE-EE solution. Simulation results demonstrate that the proposed resource allocation scheme provides comparable EE performance to a two-stage EE-EE solution with significant gain on EE for D2D users, and achieves much higher EE compared to a sum rate maximization scheme.