**Nguyen Minh Tran, Muhammad Miftahul Amri, Je Hyeon Park, Sa Il Hwang, Dong In Kim and Kae Won Choi \***

Department of Electronic, Electrical and Computer Engineering, Sungkyunkwan University (SKKU), Suwon 16419, Korea; nmtran@skku.edu (N.M.T.); miftahulamri@g.skku.edu (M.M.A.); pjw8251@skku.edu (J.H.P.); fortyone@skku.edu (S.I.H.); dikim@skku.ac.kr (D.I.K.)

**\*** Correspondence: kaewonchoi@skku.edu

Received: 18 October 2019; Accepted: 22 November 2019; Published: 25 November 2019

**Abstract:** We propose and implement a novel 1-bit coding metasurface that is capable of focusing and steering beam for enhancing power transfer efficiency of the electromagnetic (EM) wave-based wireless power transfer systems. The proposed metasurface comprises 16 × 16 unit cells which are designed with a fractal structure and the operating frequency of 5.8 GHz. One PIN diode is incorporated within each unit cell and enables two states with 180◦ phase change of the reflected signal at the unit cell. The two states of the unit cell correspond to the ON and OFF states of the PIN diode or "0" and "1" coding in the metasurface. By appropriately handling the ON/OFF states of the coding metasurface, we can control the reflected EM wave impinged on the metasurface. To verify the working ability of the coding metasurface, a prototype metasurface with a control board has been fabricated and measured. The results showed that the coding metasurface is capable of focusing beam to desired direction. For practical scenarios, we propose an adaptive optimal phase control scheme for focusing the beam to a mobile target. Furthermore, we prove that the proposed adaptive optimal phase control scheme outperforms the random phase control and beam synthesis schemes.

**Keywords:** 1-bit unit cell; coding metasurface; adaptive beam focusing; wireless power transfer
