Simultaneously Transmitting and Reflecting (STAR)-RISs: Are they Applicable to Dual-Sided Incidence?

TL;DR

This paper develops models for dual-sided STAR-RISs that can transmit and reflect signals simultaneously, analyzing their applicability and performance in NOMA and OMA uplink systems, with insights into outage probabilities and system optimization.

Contribution

It introduces a hardware and signal model for dual-sided STAR-RISs and analyzes their performance in uplink communication systems, highlighting the impact of transmission and reflection ratio adjustments.

Findings

NOMA outperforms OMA in outage probability.

Symmetrical transmission and reflection coefficients are proven for scalar impedance elements.

Adjusting amplitude ratios reduces outage probability error floors.

Abstract

A hardware model and a signal model are proposed for dual-sided simultaneously transmitting and reflecting reconfigurable intelligent surfaces (STAR-RISs), where the signal simultaneously incident on both sides of the surface. Based on the proposed hardware model, signal models for dual-sided STAR-RISs are developed. For elements with scalar surface impedance, it is proved that their transmission and reflection coefficients on both sides are identical. Based on the obtained symmetrical dual-sided STAR model, a STAR-RIS-aided two-user uplink communication system is investigated for both non-orthogonal multiple access (NOMA) and orthogonal multiple access (OMA) schemes. Analytical results for the outage probabilities for users are derived in the high transmit signal-to-noise ratio (SNR) regime. Numerical results demonstrate the performance gain of NOMA over OMA and reveal that the outage probability error floor can be lowered by adjusting the ratio between the amplitudes of transmission and reflection signals.