Secure Beamforming Design with Bidirectional Secondary Transmissions in Wireless-Powered Cognitive Radio Networks

In wireless-powered cooperative cognitive radio networks, guaranteeing the secure transmission of the primary's signal is a critical problem. In this paper, we focus on the primary's information security during the transmission phase when some potential eavesdroppers exist in the range of secondary systems. In particular, we consider a new spectrum sharing protocol that multiplexes the secondary transmissions with both the primary and cognitive transmissions. Moreover, we aim to jointly optimize energy harvesting duration and beamforming vectors to maximize the secondary transmission rate while satisfying the secrecy quality-of-service (QoS) requirement of the primary system and power power constraint of the secondary system. To solve this optimization problem, we adopt a two-stage procedure to obtain the optimal solutions. Simulation results show that the proposed scheme can greatly achieve the transmission rate of the secondary system while can provide a higher secure rate for the primary system compared to the zero-forcing (ZF) scheme.