Active Simultaneously Transmitting and Reflecting (STAR)-RISs: Modelling and Analysis

TL;DR

This paper models and analyzes active STAR-RISs with reflection-type amplifiers, deriving outage probabilities, scaling laws, and demonstrating their advantages over passive STAR-RISs in communication systems.

Contribution

It introduces a hardware model for active STAR-RISs, derives analytical expressions for performance metrics, and compares their effectiveness to passive counterparts.

Findings

Active STAR-RISs achieve the same diversity orders as passive ones.

Average received SNRs scale with M and M^2 for active and passive STAR-RISs.

Active STAR-RISs outperform passive ones in outage probability, especially with fewer elements.

Abstract

A hardware model for active simultaneously transmitting and reflecting reconfigurable intelligent surfaces (STAR-RISs) is proposed consisting of reflection-type amplifiers. The amplitude gains of the STAR element are derived for both coupled and independent phase-shift scenarios. Based on the proposed hardware model, an active STAR-RIS-aided two-user downlink communication system is investigated. Closed-form expressions are obtained for the outage probabilities of both the coupled and independent phase-shift scenarios. To obtain further insights, scaling laws and diversity orders are derived for both users. Analytical results confirm that active STAR-RIS achieves the same diversity orders as passive ones while their scaling laws are different. It is proved that average received SNRs scale with M and M^2 for active and passive STAR-RISs, respectively. Numerical results show that active STAR-RISs outperform passive STAR-RISs in terms of outage probability especially when the number of elements is small.