Global Optimization for IRS-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-ofsight (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. Xin Cheng (e-mail: 118104010558@njust.cn), Yan Lin, Weiping Shi, and Jiayu Li are with the School of Electronic and Optical Engineering, Nanjing University of Science and Technology, Nanjing, 210094, China. Cunhua Pan is with the School of Electronic Engineering and Computer Science , Queen Mary University of London, Mile End Road London E1 4NS, U.K. Feng Shu is with the School of Information and Communication Engineering, Hainan University, Haikou 570228, China. and also with the School of Electronic and Optical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China. Yongpeng Wu is with the Shanghai Key Laboratory of Navigation and Location Based Services, Shanghai Jiao Tong University, Minhang 200240, China. Jiangzhou Wang is with the School of Engineering and Digital Arts, University of Kent, Canterbury CT2 7NT, U.K.