Optimal transmit precoding and phase shift design for intelligent reflecting surface-assisted MIMO SWIPT systems with secrecy consideration

Intelligent reflecting surface (IRS) has recently stimulated an upsurge of research interest due to its capability of enhancing the spectral and energy efficiencies for future sixth generation (6G) wireless communication systems. Each reflective element of IRS can be dynamically reconfigured, allowing the wireless propagation direction to be controlled in a favorable way. As such, IRS induces extra degrees of freedom, which is appealing for simultaneous wireless information and power transfer (SWIPT) systems, as it helps to achieve better information decoding and energy harvesting performances, thus motivating the incorporation of IRS into SWIPT systems. In SWIPT systems, energy users rely on information beam steering to harvest power. Consequently, the system becomes vulnerable to eavesdropping, and this poses security threats in the communication system. Therefore, to achieve optimal system performance while guaranteeing the transmission security, the joint design of transmit precoding and phase shift matrices considering the secrecy requirements of the information users is imperative, however, the problem has not been well studied. Hence, the objective of this research is to maximize the system's sum-rate by jointly optimizing the transmit precoding and phase shift matrices for IRS-assisted multi-user multiple-input multiple-output (MIMO) SWIPT communication systems with secrecy consideration. By applying the minimum mean squared error (MMSE) method, the problem is transformed into a more tractable form, which can be handled by the block coordinate descent (BCD) algorithm. Then, the successive convex approximation (SCA) method and Penalty Convex-Concave Procedure (CCP) are implemented to deal with the non-convex subproblems. Simulation results show the effectiveness of the IRS and the performance gain of the proposed algorithm.