Large Gain Degradation of Reflective Intelligent Surfaces in Realistic Environments

TL;DR

Reflective Intelligent Surfaces (RIS) offer limited coverage benefits in realistic urban environments at 28 GHz due to gain degradation caused by channel angle spread, often performing worse than ambient scatter.

Contribution

The paper derives a simple formula quantifying RIS gain degradation from channel angle spread and evaluates its impact in realistic urban scenarios.

Findings

A 0.3 m x 0.3 m RIS at 28 GHz provides only 5 dB gain over ambient scatter at 200 m.

Angle spread causes approximately 14 dB reduction in RIS gain, below ambient levels.

A 1 m x 1 m RIS offers less than 2 dB advantage after considering gain degradation.

Abstract

Reflective Intelligent Surfaces (RIS) are considered promising in improving coverage in Non-Line of Sight (NLOS) wireless links, especially at mm wave or higher frequency bands. Coverage provided by RIS is here compared to coverage from such ambient propagation mechanisms as scattering from street poles (e.g. lampposts), and corner diffraction. A simple formula for RIS gain degradation due to channel angle spread is derived. It is found an ideal 0.3 m x 0.3 m RIS at 28 GHz promises to deliver only about 5 dB more power at 200 m around an urban street corner than the ambient scatter already there. Consideration of angle spread brings about some 14 dB drop in RIS power, bringing it well below ambient mechanisms. A 1 m x 1 m RIS at 28 GHz, offers under 2 dB advantage over ambient scatter after including the 25 dB gain degradation due to angle spread. This raises questions about usefulness of RIS-assisted coverage extension in realistic environments.