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… Click to show full abstract
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 the performance of future wireless networks. However, since the artificial electromagnetic characteristics of RISs do not strictly follow the normal laws of nature, it raises a question: does the channel reciprocity hold in RIS-assisted TDD wireless networks? After briefly reviewing the reciprocity theorem, in this article, we show that there is still channel reciprocity for RIS-assisted wireless networks satisfying certain conditions. We also experimentally demonstrate the reciprocity at the sub-6 GHz and millimeter-wave frequency bands by using two fabricated RISs. Furthermore, we introduce several RIS-assisted approaches to realizing nonreciprocal channels. Finally, potential opportunities brought by reciprocal/nonreciprocal RISs and future research directions are outlined.
               
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