Energy-efficient relay placement and power allocation for two-hop D2D relay networks

With device-to-device (D2D) communications, a user terminal (UT) can be used as a relay node to support multi-hop transmission so that cell-edge or deeply faded users can obtain a better connective experience. In this paper, we investigate energy-efficient transmission for D2D-enabled cooperative networks. We aim to maximize the energy efficiency (EE) of the uplink transmission while guaranteeing the minimum data rate requirement via joint D2D relay node (DRN) placement and power allocation. To solve the problem, we first decompose it into four sub-problems depending on the minimum data rate requirement and the power limits at the UT and DRN and then derive a closed-form solution for each problem. Numerical results demonstrate that the maximum EE can be always achieved with the proposed DRN placement and power allocation.