On Channel Reciprocity in Reconfigurable Intelligent Surface Assisted Wireless Networks

Channel reciprocity greatly facilitates downlink precoding in time-division duplexing (TDD) multiple-input multiple-output communications without the need for channel state information feedback. Recently, reconfigurable intelligent surfaces (RISs) have emerged as a promising technology to enhance performance of future wireless networks. However, since artificial electromagnetic characteristics RISs do not strictly follow normal laws nature, it raises question: does hold RIS-assisted TDD networks? After briefly reviewing theorem, this article, we show that there is still networks satisfying certain conditions. We also experimentally demonstrate at sub-6 GHz and millimeter-wave frequency bands by using two fabricated RISs. Furthermore, introduce several approaches realizing nonreciprocal channels. Finally, potential opportunities brought reciprocal/nonreciprocal research directions are outlined.