Intelligent Reflecting Surface-Aided Wireless Communications: A Tutorial

Intelligent reflecting surface (IRS) is an enabling technology to engineer the radio signal propagation in wireless networks. By smartly tuning the signal reflection via a large number of low-cost passive reflecting elements, IRS is capable of dynamically altering wireless channels to enhance the communication performance. It is thus expected that the new IRS-aided hybrid wireless network comprising both active and passive components will be highly promising to achieve a sustainable capacity growth cost-effectively in the future. Despite its great potential, IRS faces new challenges to be efficiently integrated into wireless networks, such as reflection optimization, channel estimation, and deployment from communication design perspectives. In this paper, we provide a tutorial overview of IRS-aided wireless communications to address the above issues, and elaborate its reflection and channel models, hardware architecture and practical constraints, as well as various appealing applications in wireless networks. Moreover, we highlight important directions worthy of further investigation in future work.

I. Introduction

Although the fifth-generation (5G) wireless network is still under deployment worldwide, both academia and industry have been enthusiastically looking into future beyond 5G (B5G) such as the sixth-generation (6G) wireless network that targets at meeting more stringent requirements than 5G, such as ultra high data rate and energy efficiency, global coverage and connectivity, as well as extremely high reliability and low latency [1]. These requirements, however, may not be fully achieved with the existing technology trends for accommodating 5G services (e.g., enhanced mobile broadband (eMBB), ultra-reliable and low latency communication (URLLC), and massive machine-type communication (mMTC)), which mainly include [2]–[5] •

deploying increasingly more active nodes such as base stations (BSs), access points (APs), relays, and distributed antennas/remote radio heads (RRHs) to shorten the communication distance for achieving enhanced network coverage and capacity, which, however, incurs higher energy consumption and deployment/backhaul/maintenance cost, as well as the more severe and complicated network interference issues;

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packing substantially more antennas at the BSs/APs/relays to harness the enormous massive multiple-input-multiple-output (M-MIMO) gains, which requires increased hardware and energy cost as well as signal processing complexity;

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migrating to higher frequency bands such as millimeter wave (mmWave) and even terahertz (THz) frequencies to utilize their large and available bandwidth, which inevitably results in deploying even more active nodes and mounting them even more antennas (i.e., super MIMO) so as to compensate for their higher propagation loss over distance.