Joint Power Allocation and Power-Splitting Optimization for Underlay D2D Communications in SWIPT-Enabled DF Relay Networks

This paper investigates underlay device-to-device (D2D) communication in a simultaneous wireless information and power transfer (SWIPT)-enabled two-hop decode-and-forward cellular relay network. The D2D transmitter transmits both information and cancelable energy signals, with the energy signal contributing to energy harvesting at the cellular relay while being canceled before information decoding. The signal powers and relay power splitting ratio are jointly optimized to maximize the D2D rate while preserving the end-to-end cellular rate achieved in the absence of D2D transmission. The resulting nonconvex problem is reformulated as an equivalent tractable problem, and its globally optimal solution is derived in closed form. Analytical results demonstrate that the proposed scheme performs at least as well as the baseline and achieves a positive D2D rate even when the baseline cannot support D2D transmission. We also derive a finite asymptotic upper bound on the D2D rate. Numerical results validate the theoretical analysis and demonstrate substantial performance gains over the baseline scheme.