On Channel Reciprocity in Reconfigurable Intelligent Surface Assisted Wireless Network

TL;DR

This paper investigates whether channel reciprocity holds in RIS-assisted TDD wireless networks, demonstrating its validity under certain conditions through experiments at different frequency bands and discussing nonreciprocal approaches.

Contribution

It shows that channel reciprocity can still exist in RIS-assisted networks under specific conditions and provides experimental validation at sub-6 GHz and millimeter-wave bands.

Findings

Reciprocity holds under certain conditions in RIS-assisted TDD networks

Experimental validation at sub-6 GHz and millimeter-wave bands

Discussion of nonreciprocal RIS approaches

Abstract

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.