Abstract
Reconfigurable intelligent surfaces (RISs) or intelligent reflecting surfaces (IRSs) are regarded as one of the most promising and revolutionizing techniques for enhancing the spectrum and/ or energy efficiency of wireless systems. These devices are capable of reconfiguring the wireless propagation environment by carefully tuning the phase shifts of a large number of low-cost passive reflecting elements. In this article, we aim to answer four fundmental questions: 1) Why do we need RISs? 2) What is an RIS? 3) What are RIS's applications? 4) What are the relevant challenges and future research directions? In response, eight promising research directions are pointed out.
Highlights
Fifth-generation (5G) wireless networks are being rolled out worldwide, the key physical layer technology therein is massive multiple-input multiple-output (MIMO) operating in the sub-6 GHz bands, while millimeter wave communication, originally envisioned as one of three pivotal technologies in 5G networks, has not been widely adopted
Multicast transmission based on content reuse has attracted wide research attention, since it is capable of mitigating the tele-traffic, it will play a pivotal role in future wireless networks
Conventional angle/location estimation algorithms may not be applicable for reconfigurable intelligent surfaces (RISs)-aided networks, when the direct channel spanning from the base stations (BSs) to the user is blocked
Summary
Fifth-generation (5G) wireless networks are being rolled out worldwide, the key physical layer technology therein is massive multiple-input multiple-output (MIMO) operating in the sub-6 GHz bands, while millimeter wave (mmWave) communication, originally envisioned as one of three pivotal technologies in 5G networks, has not been widely adopted. Some innovative applications, such as immersive virtual reality, high-fidelity holographic projections, digital twins, connected robotics and autonomous systems, industrial internet of things, intelligent transportation system and braincomputer interfaces, are expected to be supported by 6G-andbeyond communications [1]. These applications entail high quality-of-service (QoS) requirements such as extremely high data rates, ultra-high reliability, and ultra-low latency, which cannot be readily supported by the existing systems. The array gain of massive MIMO techniques at the base stations (BSs) mitigates the path loss at high frequencies, but fails to solve the blockage problem. RISs can be installed on large flat surfaces (e.g., walls or ceilings indoors, buildings or signage outdoors) in order to reflect radio-frequency (RF) energy around obstacles and create a virtual line-of-sight (LoS) propagation path between a mmWave source and the destination
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