RIS-assisted future wireless communication systems

Abstract

Reconfigurable intelligent surfaces (RISs) are considered as a promising technique for enhancing the network performance and coverage in future wireless communication systems, in which a RIS will absorb, reflect and relay the signal from the transmitters to the receivers. The reflected signals can be combined coherently to improve the received signal or destructively to suppress interference. In addition, the reliability and nearly zero-delay are also notable advantages of RISs in underpinning reliable and low-cost future wireless communications.

The majority of the RIS studies are based on idealistic and over simplified system models, while the physical electromagnetic limitations are not taken into consideration. Motivated by this observation, in this thesis, we investigate RIS-aided systems from a physical-based point of view. First, we introduce aperture efficiency into the system, which is an important parameter when it comes to antenna arrays. We derive it as a parameter which only depends on the distance of the source and the destination, the directivity of the source antenna, and the size of the surface. Moreover, we propose a pathloss model based on the aperture efficiency. Finally, we are able to calculate the effective size of the surface depending on different frequencies, which is important for future RIS design.

We also investigate the outage probability performance of RIS-aided Massive MIMO (M-MIMO) downlink systems, where maximum-ratio transmitting (MRT) precoding scheme is deployed at the base station (BS) and serves multiple single antenna users (UEs) through a RIS. In particular, the model we consider is a generalized and three-dimensional (3D) line-of-sight (LoS) channel model, where we also consider the loss on the RIS, i.e. aperture efficiency. A novel closed-form expression for the outage probability and an approximation (using Markov's inequality) are derived. Finally, we provide an asymptotic analysis of the system.


Conventional RISs play an important role in extending the connectivity and improving the data rate of future wireless communication systems. However, passive RISs have their limitations and, thus, a hybrid-relay RIS (HR-RIS) scheme is proposed to reap the benefits of relaying systems with high power consumption but higher throughput and passive RIS systems with cascaded fading effects but low complexity. In particular, we investigate the performance of HR-RIS in a M-MIMO system with zero-forcing (ZF) processing at the BS, when the channel state information (CSI) is unavailable. We first model the uplink/downlink channels, and derive the linear minimum mean square error (LMMSE) estimate of the effective channels. We then derive a closed-form expression for the signal to interference and noise ratio (SINR) and SE, and propose a beamforming training scheme to acquire the channels estimates at each user. Then, we formulate a max-min fairness problem which maximizes the worst SE among the users. Finally, we provide some useful engineering insights with our asymptotic analysis and numerical results.

Summarizing, this thesis proposes novel performance evaluation methods based on the aperture efficiency, as well as more sophisticated system models to improve the design of conventional RIS in future wireless systems. Moreover, the thesis develops a reliable channel estimation for both uplink and downlink scenarios, and a max-min fairness solution for HR-RIS-aided M-MIMO systems.

Date of Award	Jul 2023
Original language	English
Awarding Institution	Queen's University Belfast
Supervisor	Michalis Matthaiou (Supervisor) & Hien-Quoc Ngo (Supervisor)