Huang, Chongwen Hu, Sha Alexandropoulos, George C. Zappone, Alessio Yuen, Chau Zhang, Rui Renzo, Marco Di Debbah, Merouane, Holographic MIMO Surfaces for 6G Wireless Networks: Opportunities, Challenges, and Trends, IEEE Wireless Communications, (27), 118-125, 2020, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC,