Intelligent reflecting surface (IRS) is a revolutionary and transformative technology for achieving spectrum and energy efficient wireless communication cost-effectively in the future. Specifically, an IRS consists of a large number of low-cost passive elements each being able to reflect the incident signal independently with an adjustable phase shift so as to collaboratively achieve three-dimensional (3D) passive beamforming without the need of any transmit radio-frequency (RF) chains. In this paper, we study an IRS-aided single-cell wireless system where one IRS is deployed to assist in the communications between a multi-antenna access point (AP) and multiple single-antenna users. We formulate and solve new problems to minimize the total transmit power at the AP by jointly optimizing the transmit beamforming by active antenna array at the AP and reflect beamforming by passive phase shifters at the IRS, subject to users' individual signal-to-interference-plus-noise ratio (SINR) constraints. Moreover, we analyze the asymptotic performance of IRS's passive beamforming with infinitely large number of reflecting elements and compare it to that of the traditional active beamforming/relaying. Simulation results demonstrate the significant performance gain achieved by the proposed scheme with IRS over a benchmark massive MIMO system without using IRS. We also draw useful insights into optimally deploying IRS in future wireless systems. Index TermsIntelligent reflecting surface, joint active and passive beamforming, phase shift optimization.

Intelligent reflecting surface (IRS) is a new and revolutionizing technology that is able to significantly improve the performance of wireless communication networks, by smartly reconfiguring the wireless propagation environment with the use of massive low-cost passive reflecting elements integrated on a planar surface. Specifically, different elements of an IRS can independently reflect the incident signal by controlling its amplitude and/or phase and thereby collaboratively achieve fine-grained three-dimensional (3D) passive beamforming for directional signal enhancement or nulling. In this article, we provide an overview of the IRS technology, including its main applications in wireless communication, competitive advantages over existing technologies, hardware architecture as well as the corresponding new signal model. We focus on the key challenges in designing and implementing the new IRS-aided hybrid (with both active and passive components) wireless network, as compared to the traditional network comprising active components only. Furthermore, numerical results are provided to show the great performance enhancement with the use of IRS in typical wireless networks.

The future of mobile communications looks exciting with the potential new use cases and challenging requirements of future 6th generation (6G) and beyond wireless networks. Since the beginning of the modern era of wireless communications, the propagation medium has been perceived as a randomly behaving entity between the transmitter and the receiver, which degrades the quality of the received signal due to the uncontrollable interactions of the transmitted radio waves with the surrounding objects. The recent advent of reconfigurable intelligent surfaces in wireless communications enables, on the other hand, network operators to control the scattering, reflection, and refraction characteristics of the radio waves, by overcoming the negative effects of natural wireless propagation. Recent results have revealed that reconfigurable intelligent surfaces can effectively control the wavefront, e.g., the phase, amplitude, frequency, and even polarization, of the impinging signals without the need of complex decoding, encoding, and radio frequency processing operations. Motivated by the potential of this emerging technology, the present article is aimed to provide the readers with a detailed overview and historical perspective on state-of-the-art solutions, and to elaborate on the fundamental differences with other technologies, the most important open research issues to tackle, and the reasons why the use of reconfigurable intelligent surfaces necessitates to rethink the communication-theoretic models currently employed in wireless networks. This article also explores theoretical performance limits of reconfigurable intelligent surface-assisted communication systems using mathematical techniques and elaborates on the potential use cases of intelligent surfaces in 6G and beyond wireless networks.INDEX TERMS 6G, large intelligent surfaces, meta-surfaces, reconfigurable intelligent surfaces, smart reflect-arrays, software-defined surfaces, wireless communications, wireless networks. I. INTRODUCTIONAccording to the February 2019 report of Cisco [2], by the year of 2022, the number of networked devices and connections will reach up to 28.5 billions, and 12.3 billions of them will consist of mobile-ready devices and connections.The associate editor coordinating the review of this manuscript and approving it for publication was Feng Li.

Transparent, flexible and high efficient power sources are important components of organic electronic and optoelectronic devices. In this work, based on the principle of the previously demonstrated triboelectric generator, we demonstrate a new high-output, flexible and transparent nanogenerator by using transparent polymer materials. We have fabricated three types of regular and uniform polymer patterned arrays (line, cube, and pyramid) to improve the efficiency of the nanogenerator. The power generation of the pyramid-featured device far surpassed that exhibited by the unstructured films and gave an output voltage of up to 18 V at a current density of ∼0.13 μA/cm 2 . Furthermore, the as-prepared nanogenerator can be applied as a self-powered pressure sensor for sensing a water droplet (8 mg, ∼3.6 Pa in contact pressure) and a falling feather (20 mg, ∼0.4 Pa in contact pressure) with a low-end detection limit of ∼13 mPa. KEYWORDS: Nanogenerator, transparent, polymer, pressure sensor T he integration of flexible and transparent characteristics is an important component in the new organic electronic and optoelectronic devices 1−3 and has been achieved for various applications, including transistors, 4,5 lithium-ion batteries, 6 supercapacitors, 7,8 pressure sensors, and artificial skins. 9−12 Indeed, building flexible transparent energy conversion and storage units plays a key role in realizing fully flexible and transparent devices. In 2006, our group demonstrated the first piezoelectric ZnO nanogenerator that successfully converted mechanical energy into electric energy. 13 Since then, various nanogenerators (NGs) based on piezoelectric effect have been demonstrated. 14−17 As an important part in this field, some studies on fully integrated flexible and transparent NGs have been reported. 18−21 Almost all of them are based on piezoelectric ZnO nanowires and the entire device requires sophisticated design and a high degree of integration.The general physical process for energy conversion has three important steps: charge generation, charge separation, and charge flow. These steps were accomplished in piezoelectric NGs by employing the piezoelectric potential created under strain. Recently, we have developed a flexible triboelectric generator (TEG) using all-polymer based materials. 22 By stacking two thin polymer films made of Kapton and polyester (PET), a charge generation, separation, and induction process can be achieved through a mechanical deformation of the polymer films as a result of the triboelectric effect. This is a simple, low-cost, readily scalable fabrication process of generator that can convert random mechanical energy in our living environment into electric energy using the well-known triboelectric effect. Furthermore, through rational design, this new mode of power generation can be developed to build a high-output, flexible, and transparent NG.To make the device transparent and improve the power generation density, three approaches were employed in this research: (i) replacing Kapton ...

Lithium metal batteries (LMBs) are among the most promising candidates of high‐energy‐density devices for advanced energy storage. However, the growth of dendrites greatly hinders the practical applications of LMBs in portable electronics and electric vehicles. Constructing stable and efficient solid electrolyte interphase (SEI) is among the most effective strategies to inhibit the dendrite growth and thus to achieve a superior cycling performance. In this review, the mechanisms of SEI formation and models of SEI structure are briefly summarized. The analysis methods to probe the surface chemistry, surface morphology, electrochemical property, dynamic characteristics of SEI layer are emphasized. The critical factors affecting the SEI formation, such as electrolyte component, temperature, current density, are comprehensively debated. The efficient methods to modify SEI layer with the introduction of new electrolyte system and additives, ex‐situ‐formed protective layer, as well as electrode design, are summarized. Although these works afford new insights into SEI research, robust and precise routes for SEI modification with well‐designed structure, as well as understanding of the connection between structure and electrochemical performance, is still inadequate. A multidisciplinary approach is highly required to enable the formation of robust SEI for highly efficient energy storage systems.