Which Channel in 6G, Low-rank or Full-rank, more needs RIS from a Perspective of DoF?

TL;DR

This paper investigates how reconfigurable intelligent surfaces (RIS) can enhance degrees of freedom in low-rank 6G channels, especially in scenarios with limited or no direct link, by potentially transforming them into full-rank channels for improved spatial multiplexing.

Contribution

It extends RIS applications to low-rank and no-link scenarios, demonstrating potential for significant DoF enhancement and rate improvements in 6G wireless communications.

Findings

RIS can dramatically increase DoF in low-rank channels.

Distributed RIS placement can boost LoS channels from rank-one to full-rank.

RIS-enabled channels enable multiple parallel data streams.

Abstract

Reconfigurable intelligent surface (RIS), as an efficient tool to improve receive signal-to-noise ratio, extend coverage and create more spatial diversity, is viewed as a most promising technique for the future wireless networks like 6G. As you know, RIS is very suitable for a special wireless scenario with wireless link between BS and users being completely blocked, i.e., no link. In this paper, we extend its applications to a general scenario, i.e., rank-deficient channel, particularly some extremely low-rank ones such as no link, and line-of-sight (LoS, rank-one). Actually, there are several potential important low-rank applications like low-altitude, satellite, UAV, marine, and deep-space communications. In such a situation, it is found that RIS may make a dramatic degrees of freedom (DoF) enhancement over no RIS. By using a distributed RISs placement, the DoF of channel from BS to user in LoS channel may be even boosted from a low-rank like 0/1 to full-rank. This will achieve an extremely rate improvement via spatial parallel multiple-stream transmission from BS to user. In this paper, we present a complete review of making an in-depth discussion on DoF effect of RIS.