Energy efficiency maximization in IRS-aided cooperative WPCN with non-linear EH users

This work investigates energy efficiency (EE) maximization in intelligent reflecting surfaces (IRS) aided wireless-powered communication network with hybrid access point (HAP) and non-linear energy harvesting (EH) users. For system designers, maximizing EE is important to ensure a sustaining operation of the energy constrained wireless-powered communication network. The HAP powers the users in energy transmission mode (ETM), and it receives information from the users in cooperative manner through information transmission mode (ITM). The focus is on maximizing EE at the HAP by jointly optimizing IRS phase shifts along with transmit time and power allocation at users. The inherent non-convexity of the problem arises from the non-linear EH model at users and the practical non-linear design of IRS phase shifts. To address this, a decoupling strategy is employed, and optimal ITM IRS phase shifts are obtained using the penalty-based method. Subsequently, an alternating optimization approach is proposed where optimal IRS phase shifts in ETM are iteratively updated, incorporating a semidefinite relaxation technique. By utilizing the obtained optimal phase shifts for both ITM and ETM, the joint optimization problem is iteratively solved to maximize EE while considering time and power allocation, employing Dinkelbach's algorithm. Simulation results showcase a substantial enhancement of the system performance using proposed scheme compared to benchmark methods.