Intelligent Reflecting Surface Aided Wireless Communications A Tutorial
Abstract : 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.
? We assume that only a line-of-sight (LOS) channel exists among the BS, the RISs, and the UE due to propagation characteristics of tera-hertz (THz) spectrum bands that may be used in 6G wireless communication systems.
? There exist several studies related to the RIS in LOS environments for mmWave and THz communications.
? Obviously, the non-RIS case is not effected by the number of RISs because it is based on the environment in which RISs do not exist.
? Many studies have been performed to enhance PLS for future wireless networks, such as massive MIMO, non-orthogonal multiple access (NOMA), Internet of Things (IoT) networks, and mmWave and THz communications .
? This problem has not been addressed before in channel estimation for conventional wireless systems without IRS and thus is new and also non-trivial to solve in general.
? To resolve this problem, proposed an efficient AO-based algorithm by extending that in the narrow-band MIMO case and leveraging the convex relaxation technique.
? Despite the above benefits, the active and passive beamforming designs in the multi-user case are closely coupled and usually lead to more complicated optimization problems as compared to that in the single-user case.
? Besides, such constraints also complicate the IRS reflection design and render the corresponding optimization problems more difficult to solve as compared to their continuous counterparts.
• In particular, many RISaided techniques have been proposed to improve both data rate and energy efficiency for 6G wireless communication systems.
• In the proposed technique, each RIS first pseudo-randomly generates multiple reflection matrices and utilizes them for both pilot signal duration (PSD) in uplink and data transmission duration (DTD) in downlink.
• Reconfigurable intelligent surfaces (RISs) or intelligent reflecting surfaces (IRSs) have been proposed to achieve high spectral and energy efficiency for future 6G wireless communication systems
• It was shown that the proposed RCR-based scheduling technique can achieve better performance than the network without any RIS and than a random scheduling technique.
? In particular, the path loss of IRSreflected channel captures its average power and is thus essential to the link budget analysis and performance evaluation of IRS-aided communications.
? This also offers a flexible way to trade-off between the hardware cost and reflection performance in practice, as amplitude control is generally of lower cost to implement as compared to phase control.
? IRS reflection modelling and practical hardware design are critical to its reflection optimization and achievable performance gains in IRS-aided wireless systems.
? In practice, increasing the number of reflecting elements given the same IRS size is generally helpful in achieving more fine-grained passive beamforming and thus enhanced performance.
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