RIS Deployment in 6G: Do Low-Rank or Full-Rank Channels Benefit More From DoF Enhancement?

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 Internet of Things (IoT) networks. RIS is very suitable for a special wireless scenario with wireless link between BS and IoT devices 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). Several important IoT application scenarios inherently exhibit low-rank channel characteristics, including 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 exploiting a distributed RIS deployment strategy, the channel from the BS to the IoT device in a LoS scenario can be transformed from a low-rank state (e.g., rank zero or one) to full-rank, thereby substantially enhancing the available DoF. This will achieve an extremely rate improvement via spatial parallel multiple-stream transmission from BS to IoT device. In this paper, we present a complete review of making an in-depth discussion on DoF effect of RIS.