Active RISs: Modeling and Optimization

TL;DR

This paper models and optimizes active RIS architectures that amplify signals to overcome double path loss in wireless communications, providing theoretical analysis, practical design insights, and validation through simulations.

Contribution

It introduces new models for active RIS hardware designs, derives performance metrics, and develops optimization frameworks for system configuration.

Findings

Active RISs can significantly mitigate double path loss effects.

The proposed models enable effective optimization of phase shifts and amplifier gains.

Numerical results show active RISs outperform passive ones in energy efficiency.

Abstract

Reconfigurable Intelligent Surfaces (RIS)-empowered communication has emerged as a transformative technology for next generation wireless networks, enabling the programmable shaping of the propagation environment. However, conventional RISs are fundamentally limited by the double path loss effect, which severely attenuates the reflected signals. To overcome this, active RIS architectures, capable of amplifying impinging signals, have been proposed. This chapter investigates the modeling, performance analysis, and optimization of active RISs, focusing on two hardware designs: a dual-RIS structure with a single Power Amplifier (PA), and a reflection amplification structure at the unit cell level using tunnel diodes. For the PA-based design, a comprehensive mathematical model is developed, and closed-form expressions for the received signal-to-noise ratio, bit error probability, and Energy Efficiency (EE) are derived. An optimization framework for configuring the phase shifts and amplifier gain is proposed to maximize system capacity under power constraints. Regarding the second design, the integration of a tunnel diode into the unit cell is carefully studied by analyzing its I-V characteristic, enabling the derivation of the negative resistance range and the power consumption model. Furthermore, the intrinsic phase-amplitude coupling of the reflection coefficient is characterized through compact linear algebra formulations, enabling practical optimization of active RISs. Extensive numerical simulations validate the theoretical analyses, demonstrating that active RISs can effectively overcome the double path loss limitation and achieve favorable EE trade-offs compared to passive RISs. Finally, the trade-off between the available power budget and the number of active elements is examined, revealing that a higher number of active elements does not always lead to optimal performance.