Robust Design of Secure IRS-aided MISO Broadcasting for SWIPT and Spectrum Sharing

We consider an intelligent reflecting surface (IRS)-aided secondary multiple-input single-output (MISO) broadcast system for simultaneous wireless information and power transfer (SWIPT) in a spectrum underlay setup. The secondary transmitter (ST) regards the primary receivers (PR) as possible eavesdroppers. We propose an inter-system coordination protocol that enables acquisition at the ST of control information to facilitate interference management. We assume availability of imperfect channel state information (CSI) regarding the relevant direct and IRS-cascaded links at the ST. We aim at jointly optimizing the transmit precoding, artificial noise (AN) covariance, and reflect beamforming matrices, so that the transmit power of the ST is minimized subject to the quality-of-service (QoS) requirements of the information decoding and energy harvesting secondary receivers (IDR/EHR), the security and interference constraints of the PRs, and the unit-modulus constraints of the IRS phase shifts. We obtain convex approximations of the probabilistic constraints by employing Bernstein-type and first-order Taylor inequalities. We derive a robust outage-constrained design by developing an alternating minimization algorithm that makes use of the semi-definite relaxation (SDR) method and the penalty convex-concave procedure (CCP). Our design takes into account the additional interference incurred at the PRs by the IRS-reflected transmissions of the primary transmitter (PT) itself, which serves its users in an IRS-blind manner. Numerical simulation results reveal the performance gains of the proposed scheme over benchmark strategies and highlight the impact of the system parameters on the performance.