Secure and Energy-Efficient Beamforming for Simultaneous Information and Energy Transfer

Some next-generation wireless networks will likely involve the energy-efficient transfer of information and energy over the same wireless channel. Moreover, densification of such networks will make the physical layer more vulnerable to cyber attacks by potential multi-antenna eavesdroppers. To address these issues, this paper considers transmit time-switching (TS) mode, in which energy and information signals are transmitted separately in time by the base station (BS). This protocol is not only easy to implement but also delivers the opportunity for multi-purpose beamforming, in which energy beamformers can be used to jam eavesdroppers during wireless power transfer. In the presence of imperfect channel estimation and multiantenna eavesdroppers, the energy and information beamformers and the transmit TS ratio are jointly optimized to maximize the worst-case user secrecy rate subject to energy constrained users' harvested energy thresholds and a BS transmit power budget. New robust path-following algorithms, which involve one simple convex quadratic program at each iteration are proposed for computational solutions of this difficult optimization problem and also the problem of secure energy efficiency maximization. The latter adds further complexity due to additional optimization variables appearing in the denominator of the secrecy rate function. Numerical results confirm that the performance of the proposed computational solutions is robust against channel uncertainties.

I. Introduction

Next-generation communication networks offer the potential to transfer information and energy through the same wireless communication channel, where energy constrained users (UEs) would be able to not only receive information but also harvest energy [1]–[3]. The information transfer generally aims at high signal-to-interference-plus-noise-ratio (SINR) while the energy transfer aims at a high-power ambient signal [4], [5]. Early work in this area considered systems in which information and energy are transferred simultaneously by the same signals. To realize both wireless energy harvesting (EH) and information decoding (ID) in such systems, UE receivers need to split the received signal for EH and ID either by power splitting (PS) or time switching (TS) [6], [7]. Our recent work in [8] shows that such protocols, particularly the PS approach at the receiver, is not only complicated and inefficient for practical implementation, but also not necessary. It is much more efficient to transfer information and energy separately, in which case the UE receiver does not need any sophisticated hardware for this purpose.