Channel reciprocity greatly facilitates downlink precoding in time-division duplexing (TDD) multiple-input multiple-output (MIMO) communications without the need for channel state information (CSI) feedback. Recently, reconfigurable intelligent surfaces (RISs) emerge 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 brings up 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 still exists channel reciprocity for RIS-assisted wireless networks satisfying certain conditions. We also experimentally demonstrate the reciprocity at the sub-6 GHz and the 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.

翻译：海峡对等性极大地便利了时间分配(TDD)多投入多产出(MIIMO)通信的下行编码前的编码,而不需要国家信息频道(CSI)反馈。最近,可重新配置的智能表面(RIS)成为提高未来无线网络性能的有希望的技术。然而,由于RIS的人工电磁特性并不严格遵循正常的自然法则,它提出了一个问题:频道对等性是否维持在RIS协助的TDD无线网络中?在本篇文章中简要回顾对等理论之后,我们表明,在RIS协助的无线网络仍然存在着符合某些条件的对等性渠道。我们还通过使用两个编造的RIS实验性展示了5GH和毫米波频带的对等性。此外,我们引入了几种由RIS协助实现非对等性渠道的方法。最后,我们概述了互惠性/非对等性RIS带来的潜在机会和今后的研究方向。