Active Reconfigurable Intelligent Surface-Aided Wireless Communications

TLDR

Reconfigurable Intelligent Surfaces can reshape the wireless environment, but passive RIS requires many reflecting elements, leading to large surface size and high power consumption. This work introduces an active RIS that uses active loads to amplify incident signals, aiming to overcome passive RIS limitations. The authors study an active RIS in a SIMO system, balancing signal amplification against RIS‑related noise by jointly optimizing the reflecting coefficient matrix and receive beamforming via an alternating optimization algorithm that employs closed‑form MMSE beamforming and sequential convex approximation for the coefficients. With a fixed RIS power budget, the active RIS enhances link strength and, according to simulations, outperforms conventional passive RIS‑aided systems.

Abstract

Reconfigurable Intelligent Surface (RIS) is a promising solution to reconfigure the wireless environment in a controllable way. To compensate for the double-fading attenuation in the RIS-aided link, a large number of passive reflecting elements (REs) are conventionally deployed at the RIS, resulting in large surface size and considerable circuit power consumption. In this paper, we propose a new type of RIS, called active RIS, where each RE is assisted by active loads (negative resistance), that reflect and amplify the incident signal instead of only reflecting it with the adjustable phase shift as in the case of a passive RIS. Therefore, for a given power budget at the RIS, a strengthened RIS-aided link can be achieved by increasing the number of active REs as well as amplifying the incident signal. We consider the use of an active RIS to a single input multiple output (SIMO) system. However, it would unintentionally amplify the RIS-correlated noise, and thus the proposed system has to balance the conflict between the received signal power maximization and the RIS-correlated noise minimization at the receiver. To achieve this goal, it has to optimize the reflecting coefficient matrix at the RIS and the receive beamforming at the receiver. An alternating optimization algorithm is proposed to solve the problem. Specifically, the receive beamforming is obtained with a closed-form solution based on linear minimum-mean-square-error (MMSE) criterion, while the reflecting coefficient matrix is obtained by solving a series of sequential convex approximation (SCA) problems. Simulation results show that the proposed active RIS-aided system could achieve better performance over the conventional passive RIS-aided system with the same power budget.