Joint relay selection and resource optimization in BS-assisted multi-hop D2D networks with RF energy harvesting

Device-to-device (D2D) communication is a key enabling technology for sixth-generation wireless networks due to its potential to improve spectral efficiency, extend network coverage, and reduce communication latency. However, the performance of multi-hop D2D communications is constrained by limited relay energy, dynamic wireless channels, and the complexity of relay selection and resource allocation. To address these challenges, this paper proposes a base station (BS)-assisted multi-hop D2D communication framework with radio frequency (RF) energy harvesting. Relay nodes harvest energy from dedicated BS transmissions and subsequently participate in decode-and-forward multi-hop forwarding. The energy-efficiency maximization problem is formulated as a mixed-integer nonlinear fractional programming (MINFP) problem that jointly optimizes relay selection, transmit power allocation, and transmission time scheduling under energy-causality and transmit-power constraints. To efficiently solve this problem, a Dinkelbach-based alternating optimization framework integrating utility-based relay selection and graph-based multi-hop routing is developed. Simulation results demonstrate that, under the considered simulation settings, the proposed framework improves energy efficiency by approximately 20–40% compared with the considered benchmark schemes.