Global Optimization for RIS-Assisted Wireless Communications: from Physics and Electromagnetic Perspectives

Reconfigurable intelligent surfaces (RISs) are envisioned to be a disruptive wireless communication technique that is capable of reconfiguring the wireless propagation environment. In this paper, we study a far-field RIS-assisted multiple-input single-output (MISO) communication system operating in free space. To maximize the received power of the receiver from the physics and electromagnetic nature point of view, an optimization, including beamforming of the transmitter, phase shifts of the RIS, orientation and position of the RIS is formulated and solved. After exploiting the property of line-of-sight (LoS), we derive closed-form solutions of beamforming and phase shifts. For the non-trivial RIS position optimization problem in arbitrary three-dimensional space, a dimensional-reducing theory is proved. The simulation results show that the proposed closed-form beamforming and phase shifts are near-optimal solutions. Besides, the RIS significantly enhances the performance of the communication system when it is deployed at the optimal position.