Optimizing TTD-Based Wideband Hybrid Beamforming for Energy-Efficient Wireless Power Transfer

Radio frequency wireless power transfer (WPT) is a promising technology to charge low-power devices in future wireless systems. In this letter, energy-efficient WPT in wideband systems is studied, ensuring that each device meets its minimum energy harvesting (EH) requirement under both perfect and imperfect channel state information (CSI) scenarios. To this end, the digital beamformer, the true time delayers (TTDs), and the phase shifter-based analog beamformer are jointly optimized. Since the energy beamforming flexibility depends on the base station (BS) antenna’s architecture, the BS employs two hybrid beamforming architectures, sub-connected (SC) and fully-connected (FC) TTD, which charge EH devices in the near-field region. Since minimizing the transmit power is non-convex and decision variables are highly coupled, an alternating optimization algorithm based on semi-definite programming and semi-definite relaxation is proposed to solve the original problem. Furthermore, a matrix scale reduction scheme is leveraged to decrease computational complexity. Simulation results show that the SC architecture performs better than the FC architecture in both perfect and imperfect CSI scenarios. Additionally, the SC architecture decreases convergence time relative to the FC architecture by more than ten times.