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Analytical Modeling of the Path-Loss for Reconfigurable Intelligent Surfaces – Anomalous Mirror or Scatterer ?
... Thus, to begin with, we need to obtain the reflected electric field induced by individual RIS element. The Huygens-Fresnel principle, which is suitable for analyzing scalar wave propagation problems, can be applied to derive the electric field as the vector wave can be separated into scalar waves in different directions [39]. However, the authors in [39] only consider the 2-D plane. ...
... The Huygens-Fresnel principle, which is suitable for analyzing scalar wave propagation problems, can be applied to derive the electric field as the vector wave can be separated into scalar waves in different directions [39]. However, the authors in [39] only consider the 2-D plane. Thus, in this section, we leverage the Helmholtz-Kirchhoff integral theorem, which gives a more rigorous mathematical expression based on wave equation for Huygens-Fresnel principle, to derive the power flow of RIS. ...
Reconfigurable intelligent surface (RIS), composed of nearly passive elements, is regarded as one of the potential paradigms to support multi-gigabit data in real-time. However, in traditional CSI (channel state information) driven frame, the training overhead of channel estimation greatly increases as the number of RIS elements increases to intelligently manipulate the reflected signals. To conveniently use the reflected signal without complex CSI feedback, in this paper we propose a position-aided phase configuration scheme based on the property of Fresnel zone. In particular, we design the impedance based discrete RIS elements with joint absorption mode and reflection mode considering the fabrication complexities, which integrated the property of the Fresnel zone to resist the impact of position error. Then, with joint absorption and 1-bit reflection mode elements, we develop the two-step position-aided ON/OFF states judgement (TPOSJ) scheme and the frame structure to control the ON/OFF state of RIS, followed by analyzing the impacts of mobility and position error on our proposed scheme. Also, we derive the Helmholtz-Kirchhoff integral theorem based power flow. Simulations show that the proposed scheme can manipulate the ON/OFF state intelligently without complex CSI, thus verifying the practical application of our proposed scheme.
... In the sum-distance law, the energy decreases with the square of the distance, in contrast to the fourth power of the distance for the product-distance law. The works in [8], [12]- [14] discussed in detail the near-field and far-field formulations with mirrors and scatterers. In [8], it was introduced that the energy is inversely proportional to the square and the fourth power of the distance for the near-field and far-field cases, respectively. ...
... In [8], it was introduced that the energy is inversely proportional to the square and the fourth power of the distance for the near-field and far-field cases, respectively. The work in [12] used the scalar diffraction theory to prove that the sum-distance and product-distance laws hold in the near and far fields, respectively. In the paper [13], along with verification experiments, it is concluded that the sum-distance and product-distance laws hold for nearfield broadcasting and far-field beamforming, respectively. ...
An accurate and fast prediction technique is necessary for the radiation pattern of the reconfigurable intelligent surface (RIS) to quantitatively and efficiently evaluate the performance of RIS. Based on the uniform theory of diffraction (UTD), this paper proposes a UTD-type solution of the physical optics approximation (PO) for RIS modeled by a continuous planar surface in a two-dimensional environment. The authors validate the proposal under different scenarios in an indoor environment (0.1-20 m) at the terahertz bands (100-300 GHz), by comparing them with those computed using the Fresnel approximation, the Fraunhofer approximation, PO, and the full-wave approach based on the method of moment (MoM). The simulated results show that compared to MoM, the proposal and PO achieve good accuracy with a smaller error of less than 1 dB, while the Fresnel and Fraunhofer approximations present imperfect accuracy with an error of larger than 1 dB in the near-field region. Moreover, the proposal outperforms the PO in terms of faster calculation time by approximately 70%-76%, and the computational time of the proposal is improved by approximately 46,190-125,460 times compared to MoM. Furthermore, the computational complexities of the proposal, the Fresnel approximation and the Fraunhofer approximation are O(N0), compared to that of PO and MoM by O(N) and O(N2), respectively, where N and O(∙) are the number of sampling points and the notation of order, respectively. Therefore, the proposal can achieve a good balance between accuracy and computational cost.
... changed by the change of the propagation environment and user distribution [9], the intelligent reflecting surface (IRS), which can control reflection characteristics such as reflection direction and phase, has been attracting attention [4,6,[10][11][12][13][14][15][16][17][18][19]. ...
... Subsequently, the reflection power and direction of the actual IRS are evaluated. However, since the RCS is defined in the far-field region of the IRS, the measurement system of the RCS pattern is deployed to satisfy far-field conditions, such as the distance between the IRS and each transmitting and receiving antenna [17]. Assuming an IRS with several tens of wavelengths, a distance of more than several tens of thousands of wavelengths is required for each transmitter and receiver. ...
Intelligent reflecting surfaces (IRSs) have been attracting attention as a solution to coverage hole problems in millimeter-wave communication areas and are considered one of the key technologies of next-generation mobile communication systems. To utilize the IRS for mobile networks, the reflection power of the IRS should be evaluated in advance to estimate the coverage enhancement. Because the IRS becomes large, up to several tens or hundreds of wavelengths, a reflection power evaluation method for a large IRS is required. Although the reflection amplitude and phase of the reflecting element are available for the physical optics (PO)-based evaluation method, they depend on the location of the reflecting element because of the mutual coupling between adjacent reflecting elements, and should be measured at each location of the reflecting element. Since the number of reflecting elements in large IRS becomes several tens of thousands, measurement becomes difficult. In addition, although the radar cross section (RCS) is available for reflection power evaluation in the far field, a massive measurement system of several hundreds of meters is required to satisfy the far-field condition of the IRS. For these reasons, the evaluation of large IRS is challenging. To solve this problem, we propose a practical evaluation method for large IRS by synthesizing the RCS patterns of small IRSs (sub-IRSs). Since the influence of the mutual coupling between sub-IRSs depends on the size of the sub-IRSs, we formulated the mutual coupling on the IRS and evaluated the proposed method by changing the size of the sub-IRSs. The measurement results in an anechoic chamber verified that the proposed method can estimate the RCS pattern of a large IRS with a reflection power difference of less than 1dB and the correlation between the estimated RCS pattern and the actual RCS pattern exceeds more than 0.87.
... This separation ensures no inter-element interference [25]. The IRS is in the Fraunhofer farfield of the AP and UE and, therefore, the IRS channels can be modelled using a simple geometric model with the IRS being treated as a single reflector with no mutual coupling [26] - [27]. Channels G and H_r are modelled as; ...
In the recent past, there has been a growing need for ultra-low latency and high-data-rate communication. In Non-Line-of-Sight (NLoS) communication, the channel capacity and accuracy of transmission are significantly affected by interferences, lowering the Quality of Service (QoS). An intelligent Reflecting Surface (IRS) has risen as a potential solution to challenges associated with NLOS communication including low data rate, multipath fading, and high BER. However, to leverage the performance gains of the IRS, effective and highly accurate channel estimation is crucial as it facilitates optimal phase shift optimization. This work investigated the performance of four main channel estimation algorithms in an IRS-aided system; LS, DD, DFT, and MMSE in terms of their BERs and effects on the convergence behavior of the Stochastic Convex Approximation (SCA) algorithm following the Armijo rule. The objectives of this work were to determine how different channel estimation schemes influence the BER and test the different rates of convergence. Results indicate that in cases without statistical knowledge of the channel, the DD method provides the best performance. The main advantage of the DD method is that it effectively tracks the possibly varying channels and provides an effective update technique that is not dependent on pilot symbols. This work shows that the communication needs, complexity, and accuracy should be carefully considered when selecting the channel estimation method for IRS-aided communication systems. The outcomes of this research have a critical role in shaping future wireless communication systems by aiding in the adoption of the most optimal channel estimation schemes that fit with specific user needs and resource constraints.
... R ECONFIGURABLE intelligent surfaces (RISs) have been proposed as a promising technology for 6G networks, as they provide satisfactory rate performance with lower energy consumption than traditional antenna arrays [1]- [4]. This can unlock unprecedented levels of energy efficiency (EE), a key performance indicator of 6G networks [5]. ...
This work studies the problem of secrecy energy efficiency maximization in multi-user wireless networks aided by reconfigurable intelligent surfaces, in which an eavesdropper overhears the uplink communication. A provably convergent optimization algorithm is proposed which optimizes the user’stransmit power, metasurface reflection coefficients, and base station receive filters. The complexity of the proposed method is analyzed and numerical results are provided to show the performance of the proposed optimization method.
... R ECONFIGURABLE intelligent surfaces (RISs) have been proposed as a promising technology for 6G networks, as they provide satisfactory rate performance with lower energy consumption than traditional antenna arrays [1]- [4]. This can unlock unprecedented levels of energy efficiency (EE), a key performance indicator of 6G networks [5]. ...
This work studies the problem of secrecy energy efficiency maximization in multi-user wireless networks aided by reconfigurable intelligent surfaces, in which an eavesdropper overhears the uplink communication. A provably convergent optimization algorithm is proposed which optimizes the user's transmit power, metasurface reflection coefficients, and base station receive filters. The complexity of the proposed method is analyzed and numerical results are provided to show the performance of the proposed optimization method.
... RISs have remarkable helpfulness which is enhancing the communication coverage even in NLoS case [32]. It can work as mirror or lens, transmitter, receiver or as an anomalous reflector or steerable reflector [33]- [35] RISs can works in different frequency ranges, it can work from about some gigahertz (GHz) to mm wave bands where RISs can be exploited to obtain significant benefits in terms of coverage. In greater ranges, i.e., 0.1-1 terahertz (THz), high performance gains can be obtained but the propagation may suffer from severe path loss, high sensitive to obstacles blockages, atmospheric absorption and also there is a hardware challenges and limitations [36] [37]. ...
Wireless localization technologies have undergone a paradigm shift with the advent of the emerging trend which is the Reconfigurable Intelligent Surfaces (RISs). This paper presents a comprehensive survey of the state-of-the-art RIS-aided localization techniques, exploring the transformative impact of intelligent surfaces on location-based services. The survey comprehensively reviews existing wireless localization methods, ranging from the localization techniques in the wireless communications generations. It addition the challenges posed by conventional localization methods are discussed, including accuracy and coverage issues. In response to these challenges, the incorporation of RIS is explored as a promising paradigm to enhance the precision and reliability of wireless localization. The core of the paper focuses on the integration of RIS into localization systems, highlighting how these RISs can be strategically deployed to enhance accuracy, mitigate multipath effects, integration in different propagation environment and solve non feasible localization problems. The survey encompasses a wide range of applications, including outdoor positioning, indoor positioning, and Internet of Things (IoT) devices, showcasing the versatility of RIS-aided localization across various scenarios. Also, critical design aspects are examined, including propagation regions, hardware requirements, deployment strategies, necessary measurements, multiplexing, and signalling systems.
... (17)]. This behavior is consistent (and provides a link) with path-loss models (see for example [49], [50]), and shows the generality of the impedance model for arbitrary RIS structures. ...
We devise an end-to-end communication channel model that describes the performance of RIS-assisted MIMO wireless links. The model borrows the impedance (interaction) matrix formalism from the Method of Moments and provides a physics-based communication model. In configurations where the transmit and receive antenna arrays are distant from the RIS beyond a wavelength, a reduced model provides accurate results for arbitrary RIS unit cell geometry. Importantly, the simplified model configures as a cascaded channel transfer matrix whose mathematical structure is compliant with widely accepted, but less accurate, system level RIS models. A numerical validation of the communication model is presented for the design of binary RIS structures with scatterers of canonical geometry. Attained results are consistent with path-loss models adopted in communication system design, and have been validated with full-wave simulations. Our results shows that the applicability of communication models based on mutual impedance matrices is not restricted to canonical minimum scattering RIS unit cells.
... Comprising numerous low-cost passive reflective elements on a planar metasurface, RIS can manipulate the phase and amplitude of signals by controlling multiple reflective elements through an intelligent controller [16]. Since RIS is programmable, deploying RIS in a wireless environment allows inducing different reflection amplitudes and phases in response to incoming signals [17]. Through the optimization of phase shifts in RIS, the signal energy at the receiver is increased, leading to improved achievable rates. ...
Utilizing unmanned aerial vehicle (UAV) technology in communication holds promise for meeting the increasing data rate demands in future wireless systems due to its flexibility. Meanwhile, reconfigurable intelligent surface (RIS) has garnered increased attention for their potential to enhance wireless communication performance through intelligent control of the transmission environment. In this paper, we first combine the UAV and the RISs to construct an Internet of Things (IoT) uplink transmission system, where the UAV serves as an aerial relay to collect data from IoT terminal (IT) and forward it to base stations (BS), while RISs assist communication to reduce congestion. Then, a parallel factor (PARAFAC) tensor model is formulated at the BS. At last, the iterative alternating least squares (ALS) algorithm and the closed-form singular value decomposition (SVD) algorithm are derived to fit the constructed tensor model for joint channel estimation and symbol detection. Compared with the competitive algorithms, the two proposed algorithms offer lower computational complexity and superior channel estimation performance. Furthermore, the proposed algorithms exhibit good symbol detection capabilities even at higher transmission rates. The numerical results demonstrate the effectiveness of the proposed algorithms.
... Specifically in radio localization, the deployment of Reconfigurable Intelligent Surfaces (RISs) creates additional reflected signal paths, providing new degrees of freedom to the localization model [7], [8]. The RIS primarily performs two operations: it aggregates and directs the energy from its elements [9], and it concentrates incoming electromagnetic waves towards the drone's location [10]. ...
The concept of Reconfigurable Intelligent Surfaces (RIS) has emerged as a promising method for communications and the localization of aeronautical vehicles. In this paper, we explore the impact of hardware impairments on three-dimensional (3D) drone localization within a single-input single-output (SISO) system assisted by RIS. Our methodology begins by modeling the channel from the base station (BS), equipped with a single-antenna transmitter, to each RIS at known positions. This model accounts for hardware impairments at the BS, particularly beam downtilt, which influences the accuracy of drone location estimation. Moreover, we model the channel from the RIS to the drone, employing exhaustive beam sweeping in both azimuth and elevation angles to estimate the Angles of Departure (AODs) from the RIS to the drone. We adopt a unique phase noise (PN) model for each element within the RIS and assess the impact of these impairments on angle and location estimation accuracy through extensive simulations. Additionally, we examine the effects of RIS configuration and the Inter-Site Distance (ISD) between two RIS units on localization performance. An Unscented Kalman Filter (UKF) algorithm is integrated for tracking of the drone trajectory. Our simulation results demonstrate that the RIS-assisted 3D drone localization approach achieves significant accuracy despite various impairments. The findings of this paper underscore the potential of RIS-enabled 3D drone localization to maintain high accuracy under hardware impairments, paving the way for future research in RIS-enabled drone localization systems.
... To obtain tractable E-field results, the work in [6] analyzed the E-field for three working modes categorized by the phaseshifting capacity, where the different scaling laws were observed as a function of distance. With E-field expressions, the received power is generally evaluated in most existing works [7]- [12]. ...
Reconfigurable intelligent surface (RIS) used as infrastructure in wireless networks has been a trend, thanks to its low cost and high flexibility. Working in many ways including reflective mirrors and phase-shifted surfaces, RIS is able to enhance the coverage in communications and provide more degrees of freedom for sensing. However, the key issue lies in how to place RIS in accordance with the regulations for electromagnetic field (EMF) exposure, which requires refined evaluations. In this paper, we first investigate the regulations in terms of E-field. Then, relevant deployment characteristics are evaluated jointly: the minimum distance from the base station (BS) to the RIS, and the minimum height of the RIS are given for a given BS power limit and as a function of the number of RIS elements. The ray-tracing simulations verify the correctness of our analysis. Besides, different frequency ranges (FRs) and radiation patterns of RIS elements are investigated. The results show that the EMF exposure risk is negligible when RIS works in the reflective-only (RO) mode. However, when it works in the beamforming (BO) mode, its placement should be well specified based on our analytical framework to comply with the regulations of E-field limit in general public scenarios. Finally, we provide an E-field measurement methodology and low-cost solutions in terms of general wireless networks and 5G standalone networks, which pave the way for real-world evaluation in future work.
... In this situation, the transmitter can focus all its power toward the RIS, and similarly, the RIS can focus the signal toward the receiver. While a carefully placed anomalous mirror can perform this task, RISs have extra degrees of freedom in configuring the phase of the signals to optimize various system performance metrics [39], [41], [42] such as sum rate [43], energy efficiency [12], [44], or user fairness [45], [46]. ...
The emergence of various technologies demanding both high data rates and precise sensing performance, such as autonomous vehicles and internet of things devices, has propelled an increasing popularity of integrated sensing and communication (ISAC) in recent years. ISAC offers an efficient framework for communication and sensing where both functionalities are carried out in a shared spectrum, utilizing the same hardware, beamformer and waveform design. At the same time, intelligent metasurfaces have been identified as an architectural enabler for the upcoming sixth-generation (6G) of wireless communication due to their ability to control the propagation environment in an energy-efficient manner. Due to the potential of metasurfaces to enhance both communication and sensing performance, numerous papers have explored the performance gains of using metasurfaces to improve ISAC. This survey reviews the existing literature on metasurface-assisted ISAC, detailing the associated challenges and opportunities. To provide a comprehensive overview, we commence by offering relevant background information on standalone metasurface-assisted communication and metasurface-assisted sensing systems, followed by a discussion on the fundamentals of ISAC. The core part of the paper then summarizes the state-of-the-art studies on metasurface-assisted ISAC with metasurfaces employed as separate entities placed between the transmitter and receiver, also known as reconfigurable intelligent surfaces, with an emphasis on its two levels of integration: radio-communications co-existence and dual-function radar-communications. We also review the current works in the area of holographic ISAC where metasurfaces are used to form part of ISAC transmitter. Within each category, the challenges, opportunities and future research directions are also highlighted.
... 2127-2134 Journal homepage: http://iieta.org/journals/mmep Originally developed to improve propagation coverage even in NLoS cases, the RIS can function as a steerable-reflector, anomalous-reflector, lens, mirror transmitter, or receiver [11][12][13]. It can operate from some GHz to millimeter-wave bands [14]. ...
... In [41], the authors employed antenna theory to study the path loss of RISs in the far-field and proved that a RIS is capable of acting as an anomalous mirror under far-field propagation. The authors in [62] proved that the performance of each configuration can be different and needs to be judged based on the complexity and the amount of environmental information needed for a proper operation. They used an analytical approach based on the diffraction theory and the Huygens-Fresnel principle to model the path loss in both the near and far fields of RISs. ...
Terahertz (THz) band will play an important role in enabling sixth generation (6G) envisioned applications. Compared with lower frequency signals, THz waves are severely attenuated by the atmosphere temperature, pressure, and humidity. Thus, designing a THz communication system must take into account how to circumvent or diminish those issues to achieve a sufficient quality of service. Different solutions are being analyzed: intelligent communication environments, ubiquitous artificial intelligence, extensive network automation, and dynamic spectrum access, among others. This survey focuses on the benefits of integrating intelligent surfaces (ISs) and THz communication systems by providing an overview of IS in wireless communications with the scanning of the recent developments, a description of the architecture, and an explanation of the operation. The survey also covers THz channel models, differentiating them based on deterministic and statistical channel modeling. The IS-aided THz channels are elucidated at the end of the survey. Finally, discussions and research directions are given to help enrich the IS field of research and guide the reader through open issues.
... RIS-assisted communications provide network planning engineers with new system design freedom, becoming one of the potential key technologies for sixth generation (6G) wireless mobile networks. At present, both academia and industry are striving to explore and evaluate the performance of RIS-assisted wireless communication networks by means of path loss models [3] and experimental measurements [4] of RIS-assisted links. ...
Reconfigurable intelligent surface (RIS) is emerging as a promising technology to achieve coverage enhancement. This paper develops a tractable analytical framework based on stochastic geometry for performance analysis of RIS-assisted millimeter wave networks. Based on the framework, a two-step cell association criterion is proposed, and the analytical expressions of the user association probability and the coverage probability in general scenarios are derived. In addition, the closed-form expressions of the two performance metrics in special cases are also provided. The simulation results verify the accuracy of the theoretically derived analytical expressions, and reveal the superiority of deploying RISs in millimeter wave networks and the effectiveness of the proposed cell association scheme to improve coverage. Furthermore, the effects of the RIS parameters and the BS density on coverage performance are also investigated.
... Due to its significant advantages, numerous research efforts have been dedicated to exploring various aspects of RISaided communications in recent literature, such as channel modeling [25], [26], channel estimation [2], [3], modulation and encoding [27], outage probability analysis [28] and performance evaluation w.r.t. SE maximization [23], [29], SNR maximization [6]- [8], [30], sum-rate maximization [17], [20], [31], [32], achievable rate maximization [12]- [14], [33], power minimization [15], [16], [34], [35], and EE maximization [4], [36]- [38]. ...
Re-configurable intelligent surface (RIS)-aided communication has been envisaged as a frontier scheme to enable ultra-high spectral efficiency (SE) and energy efficiency (EE) for next-generation communication. This paper investigates an unconventional framework of RIS-aided full-duplex (FD) multiuser multiple-input multiple-output (MIMO) communication and analyzes its resource efficiency (RE), a preferable performance metric for realizing trade-off between SE and EE maximization. In particular, we focus on the RE maximization problem via a joint optimization of transmit covariance, optimal receive covari-ance, and phase-shift matrices for each RIS subject to the given constraint on the power budget. To solve the formulated non-convex problem, we propose two optimization approaches: a) policy gradient-based deep-reinforcement learning (DRL) algorithm based on a Markov decision process formulation for a stochastic-time varying channel and b) alternate optimization (AO) algorithm based on general approximations and majorization-minimization (MM) for static channel conditions. Simulation results validate the out-performance of the considered RIS-aided FD-MIMO system compared to the counterpart system with half-duplex (HD) mode and without RIS case. The proposed DRL algorithm achieves comparable RE performance with reduced computational complexity and running time compared to the traditional AO-based algorithm. Index Terms-Deep reinforcement learning (DRL), alternate optimization (AO), spectral and energy efficiency trade-off, re-configurable intelligent surface (RIS), spectral efficiency (SE), full-duplex (FD), multiuser multiple-input multiple-output (MIMO).
... Therefore, the users or receivers can be located in either the far-field or the near-field region of an RIS. The authors of [14], [15] derive closed-form expressions for computing the intensity of the electric field (E-field) in two asymptotic regimes that are representative of the farfield and near-field regions of an RIS. They also characterize the scaling laws of the EM field scattered by an RIS as a function of its size, the transmission distances, and the wave transformations. ...
The ability of reconfigurable intelligent surfaces (RIS) to produce complex radiation patterns in the far-field is determined by various factors, such as the unit cell’s design, spatial arrangement, tuning mechanism, the communication and control circuitry’s complexity, and the illuminating source’s type (point/planewave). Research on RIS has been mainly focused on two areas: first, the optimization and design of unit cells to achieve desired electromagnetic responses within a specific frequency band, and second, exploring the applications of RIS in various settings, including system-level performance analysis. The former does not assume any specific full radiation pattern on the surface level, while the latter does not consider any particular unit cell design. Both approaches largely ignore the complexity and power requirements of the RIS control circuitry. As we progress toward the fabrication and use of RIS in real-world settings, it is becoming increasingly necessary to consider the interplay between the unit cell design, the required surface-level radiation patterns, the control circuit’s complexity, and the power requirements concurrently. In this paper, we propose a benchmarking framework comprising a set of simple and complex radiation patterns. Using full-wave simulations, we compare the relative performance of various RISs made from unit cell designs that use PIN diodes as control elements in producing the full radiation patterns in the far-field of the RIS under point/planewave source assumptions. We also analyze the control circuit complexity and power requirements and explore the tradeoffs of various designs.
... Importantly, this is consistent (and provides a link) with path-loss models (see for example [46,47]), and provides an assessment of the impedance model for arbitrary RIS structures. The small error of the channel capacity obtained using the proposed expression stems from the effect of the mutual coupling among transmitting antennas, receiving antennas and the RIS that is neglected in its formulation. ...
We devise an end-to-end communication channel model that describes the performance of RIS-assisted MIMO wireless links. The model borrows the impedance (interaction) matrix formalism from the Method of Moments and provides a physics-based communication model. In configurations where the transmit and receive antenna arrays are distant from the RIS beyond a wavelength, a reduced model provides accurate results for arbitrary RIS unit cell geometry. Importantly, the simplified model configures as a cascaded channel transfer matrix whose mathematical structure is compliant with widely accepted, but less accurate, system level RIS models. A numerical validation of the communication model is presented for the design of binary RIS structures with scatterers of canonical geometry. Attained results are consistent with path-loss models: For obstructed line-of-sight between transmitter and receiver, the channel capacity of the (optimised) RIS-assisted link scales as , with R RIS-receiver distance at fixed transmitter position. Our results shows that the applicability of communication models based on mutual impedance matrices is not restricted to canonical minimum scattering RIS unit cells.
... Based on general scalar diffraction theory and the Huygens-Fresnel principle, [106] proposed a calculating method for the received power of RIS in closed form. [107] proposed an end-to-end and EM-compliant electromagnetic compatibility model considering the mutual coupling between units of RIS. ...
Existing literature reviews predominantly focus on the theoretical aspects of reconfigurable intelligent surfaces (RISs), such as algorithms and models, while neglecting a thorough examination of the associated hardware components. To bridge this gap, this research paper presents a comprehensive overview of the hardware structure of RISs. The paper provides a classification of RIS cell designs and prototype systems, offering insights into the diverse configurations and functionalities. Moreover, the study explores potential future directions for RIS development. Notably, a novel RIS prototype design is introduced, which integrates seamlessly with a communication system for performance evaluation through signal gain and image formation experiments. The results demonstrate the significant potential of RISs in enhancing communication quality within signal blind zones and facilitating effective radio wave imaging.
... In addition, the path loss (PL) scaling law for RIS-assisted channel, which is also inseparable from phase shift designs, acts as one crucial role in RIS-related research. In existing works, such as [13], [14], [27]- [32], the authors formulated that the PL scaling law in the far field is proportional to (d 1 d 2 ) 2 , where d 1 and d 2 are the distance from transmitter (Tx) and receiver (Rx) to the center of RIS, respectively. However, the existing studies were mainly analyzed under ideal continuous phase shifts of RIS. ...
Reconfigurable intelligent surface (RIS) has aroused a surge of interest in recent years. In this paper, we investigate the joint phase alignment and phase quantization on discrete phase shift designs for RIS-assisted single-input single-output (SISO) system. Firstly, the phenomena of phase distribution in far field and near field are respectively unveiled, paving the way for discretization of phase shift for RIS. Then, aiming at aligning phases, the phase distribution law and its underlying degree-of-freedom (DoF) are characterized, serving as the guideline of phase quantization strategies. Subsequently, two phase quantization methods, dynamic threshold phase quantization (DTPQ) and equal interval phase quantization (EIPQ), are proposed to strengthen the beamforming effect of RIS. DTPQ is capable of calculating the optimal discrete phase shifts with linear complexity in the number of unit cells on RIS, whilst EIPQ is a simplified method with a constant complexity yielding sub-optimal solution. Simulation results demonstrate that both methods achieve substantial improvements on power gain, stability, and robustness over traditional quantization methods. The path loss (PL) scaling law under discrete phase shift of RIS is unveiled for the first time, with the phase shifts designed by DTPQ due to its optimality. Additionally, the field trials conducted at 2.6 GHz and 35 GHz validate the favourable performance of the proposed methods in practical communication environment.
The design of Reconfigurable Intelligent Surfaces (RISs) is typically based on treating the RIS as an infinitely large surface that steers incident plane waves toward the desired direction. In practical implementations, however, the RIS has finite size and the incident wave is a beam of finite
k
-content, rather than a plane wave of δ-like
k
-content. To understand the implications of the finite extent of both the RIS and the incident beam, here we treat the RIS as a spatial filter, the transfer function of which is determined by both the prescribed RIS operation and the shape of the RIS boundary. Following this approach, we study how the RIS transforms the incident
k
-content and we demonstrate how, by engineering the RIS shape, size, and response, it is possible to shape beams with nontrivial
k
-content to suppress unwanted interference, while concentrating the reflected power to desired directions. We also demonstrate how our framework, when applied in the context of near-field communications, provides the necessary insights into how the wavefront of the beam is tailored to enable focusing, propagation with invariant profile, and bending, beyond conventional beamforming.
The design of Reconfigurable Intelligent Surfaces (RISs) is typically based on treating the RIS as an infinitely large surface that steers incident plane waves toward the desired direction. In practical implementations, however, the RIS has finite size and the incident wave is a beam of finite k-content, rather than a plane wave of -like k-content. To understand the implications of the finite extent of both the RIS and the incident beam, here we treat the RIS as a spatial filter, the transfer function of which is determined by both the prescribed RIS operation and the shape of the RIS boundary. Following this approach, we study how the RIS transforms the incident k-content and we demonstrate how, by engineering the RIS shape, size, and response, it is possible to shape beams with nontrivial k-content to suppress unwanted interference, while concentrating the reflected power to desired directions. We also demonstrate how our framework, when applied in the context of near-field communications, provides the necessary insights into how the wavefront of the beam is tailored to enable focusing, propagation with invariant profile, and bending, beyond conventional beamforming.
This work proposes a provably convergent and low complexity optimization algorithm for the maximization of the secrecy energy efficiency in the uplink of a wireless network aided by a Reconfigurable Intelligent Surface (RIS), in the presence of an eavesdropper. The mobil users' transmit powers and the RIS reflection coefficients are optimized. Numerical results show the performance of the proposed methods and compare the use of active and nearly-passive RISs from an energy-efficient perspective.
Realizing the wireless environmental sensing is another desired function of reconfigurable intelligent surface (RIS), in addition to enhancing the performance of wireless communication systems. In this paper, we design a holographic RIS-aided computational imaging system, which consists of a transmitter, a holographic RIS, a rectangular target and a receiver. Here, the target is composed of a series of discrete segments, each of which possesses a constant scattering density. The sensing task of the proposed system is to estimate the scattering densities of the target, which corresponds to the term
computational imaging
. The term
holographic
means that the RIS is modeled as a physically continuous surface with a physically continuous phase shift pattern, which can be approximately considered as as having massive (possibly infinite) number of elements within a finite space. Both the RIS and the target are subject to the electromagnetic boundary conditions, whose scattered fields are computed by the equivalent current method and the physical equivalent. Based on the computed scattered fields of the target, we derive the pathloss of the proposed system. In order to perform the imaging, we alter the phase shift pattern of the RIS such that the main energy of its scattered fields is focused towards different segments of the target successively, which then produces multiple measurements of the scattering densities and simultaneously ensures a low pathloss. After all measurements are completed, the scattering densities of the target can be estimated with the observed measurement vector and the reconstructed sensing channel, i.e., the computational imaging is accomplished. Simulation results show that the proposed imaging strategy performs well if the system parameters are designed properly.
This paper presents the development of a novel communication model for reconfigurable intelligent surface (RIS)-assisted wireless systems, utilizing a 2D formulation of electro-magnetic (EM) fields based on Green's theorem. The proposed 2D model is both simple and accurate, adhering to the laws of electromagnetism for the generation, propagation, and scattering of EM fields. Specifically, we establish a direct relationship between the scattered field and the current and impedance of the scattering antenna, resulting in a tractable expression for the total field scattered by the RIS. Moreover, our model successfully explains and analyzes the various diffraction orders present in the scattered field. Through this analysis, we derive an analytical model that predicts and elucidates the different diffraction order components, including the central and replica components. The presented communication model offers valuable insights and predictions for RIS-assisted wireless systems, paving the way for improved system design and performance optimization.
Reconfigurable intelligent surface (RIS), composed of nearly passive elements, is regarded as one of the potential paradigms to support multi-gigabit data in real-time. However, in traditional CSI (channel state information) driven frame, the training overhead of channel estimation greatly increases as the number of RIS elements increases to intelligently manipulate the reflected signals. To conveniently use the reflected signal without complex CSI feedback, in this paper we propose a position-aided phase configuration scheme based on the property of Fresnel zone. In particular, we design the impedance based discrete RIS elements with joint absorption mode and reflection mode considering the fabrication complexities, which integrated the property of the Fresnel zone to resist the impact of position error. Then, with joint absorption and 1-bit reflection mode elements, we develop the two-step position-aided ON/OFF states judgement (TPOSJ) scheme and the frame structure to control the ON/OFF state of RIS, followed by analyzing the impacts of mobility and position error on our proposed scheme. Also, we derive the Helmholtz-Kirchhoff integral theorem based power flow. Simulations show that the proposed scheme can manipulate the ON/OFF state intelligently without complex CSI, thus verifying the practical application of our proposed scheme.
For wireless communication, the impact of blockage and propagation losses poses significant challenges to robust channel establishment. Reconfigurable Intelligent Surfaces (RIS) have been proposed as a promising technology for mitigating the coverage limitations inherent in many microwave and millimeter-wave wireless applications. However, wireless links facilitated by RIS are also susceptible to detrimental effects from blockages. In this paper, we present a spatial phase shift distribution (SPSD) method that alleviates adverse effects caused by blockages near RIS, exemplified for mmWave wireless scenarios. The proposed method refines the SPSD by accounting for distortions caused by blockages. This is achieved by modifying the conventional beamforming formula, which typically considers only the beam steering phase distribution between the incident wave and the desired direction. Through numerical simulations and by fabricating an RIS prototype for experimental validation, we demonstrate that the proposed method restores an average signal strength gain of approximately 3.5 dB in various scenarios, compared to traditional beamforming techniques in the presence of blockages. The proposed method can be used to optimize the transmit power of communication systems using RIS or to enhance the signal-to-noise ratio.
This paper investigates reconfigurable intelligent surface (RIS) aided multi-cell non-orthogonal multiple access (NOMA) networks with stochastic geometry methods. Under Rayleigh and Nakagami-m fading channels, we provide two types of approximate channel models to depict RIS channels, i.e., the N-fold convolution model and the curve fitting model. The analysis reveals that the N-fold convolution model is accurate and tractable when ignoring inter-cell interference, while the curve fitting model can evaluate the impact of inter-cell interference with a small error. The N-fold convolution model provides accurate diversity orders compared to other existing approaches such as the central limit model. Based on these channel models, we derive the closed-form analytical and asymptotic expressions of coverage probabilities and ergodic rates for two paired NOMA users. The analytical results demonstrate that: i) When we ignore inter-cell interference, the diversity order of the typical user is equal to the number of Rayleigh fading channels; and ii) For Nakagami-m fading channels with coefficient
m
, the diversity order is equal to
m
times of the channel number. Numerical results show that: i) RISs are capable of enhancing the coverage performance and ergodic rates of the proposed network; and ii) RISs provide extra flexibility for NOMA decoding orders.
In this paper, we study the performance limit of the cascaded line-of-sight (LoS) multiple-input-multiple-output (MIMO) system consisting of a transmitter (Tx) and a receiver (Rx) both equipped with uniform linear arrays (ULAs), and an intelligent reflecting surface (IRS) that enables communications between the Tx and the Rx through the cascaded LoS Tx-IRS-Rx link. We investigate the potential gain of the cascaded LoS MIMO system achieved by the optimization of the array orientations, especially when the Tx and the Rx are in the near-field of the IRS. Under a recently established near-field channel model, we formulate the problem of maximizing the input-output mutual information (MI) of the cascaded LoS MIMO system over active and passive beamforming as well as Tx and Rx array orientations. We give analytical solutions to the problem under asymptotic conditions, such as in the high/low signal-to-noise ratio (SNR) regime or with sufficiently large Tx-IRS and IRS-Rx distances. For non-asymptotic cases, we propose an alternating optimization method to solve the problem. We show that compared with only optimizing active and passive beamforming, the cascaded LoS MIMO system can harvest a significant MI gain from the additional optimization over the array orientations.
Instead of traditional acoustic communication, optical wireless communication (OWC) remains as a prominent solution in terms of ensuring high data rates for underwater sensor network (USN) including autonomous underwater vehicles (AUVs), remotely operated vehicles (ROVs), submarines, and other underwater platforms. This study is devoted to the performance analysis of underwater optical wireless communication (UOWC) systems using higher-order mode optical beam source and performance improvement with the application of intelligent reflecting surface (IRS) by using realistic parameters. To make the study as comprehensive as possible, the effects of beam misalignment, attenuation (absorption- and scattering-induced), and underwater turbulence are included. The analytical expressions are obtained for probability density function (PDF), cumulative distribution function (CDF) and outage probability (OP) of UOWC channel. The benefit of IRS application is observed to attain at significant levels for UOWC.
In this paper, we build up a new intelligent reflecting surface (IRS) aided multiple-input multiple-output (MIMO) channel model, named the cascaded LoS MIMO channel, that is applicable to both near-field and far-field scenarios. The proposed channel model consists of a transmitter (Tx) and a receiver (Rx) both equipped with uniform linear arrays (ULAs), and an IRS is used to enable communications between the transmitter and the receiver through the line-of-sight (LoS) links seen by the IRS. When modeling the reflection of electromagnetic waves at the IRS, we take into account the curvature of the wavefront on different reflecting elements. Based on the established channel model, we show that an IRS-assisted MIMO channel is able to support spatial multiplexing solely by the cascaded LoS links. We generalize the notion of
Rayleigh distance
originally coined for the single-hop MIMO channel to
full multiplexing region (FMR)
for the cascaded LoS MIMO channel, where the FMR is the union of all the Tx-IRS and IRS-Rx distance pairs that enable full multiplexing communication. We derive an inner bound of the FMR under a special passive beamforming (PB) strategy named reflective focusing, and provide the corresponding orientation settings of the antenna arrays that achieve full multiplexing.
This chapter starts with the state-of-the-art technology of the intelligent reflecting surface (IRS) which is emerged as a promising new paradigm to achieve smart and reconfigurable wireless propagation environment for beyond fifth-generation (B5G)/sixth-generation (6G) wireless communication systems. The motivation as well as the research background of the IRS is first presented. Then, the potential applications of the IRS in wireless information transmission and wireless energy transmission are highlighted and discussed. The unique property of IRS as well as its difference compared to the other technologies are studied. Next, we introduce the fundamental signal processing technique for IRS to show how it works. In addition, several types of IRS hardware architectures and hardware constraints are introduced and compared. Through the discussion, it is revealed that the IRS can provide significant high passive beamforming gain by adjusting its phase shifts, which motivates the main theme of this monograph.
Reconfigurable intelligent surface (RIS), with its capability of passively beamforming, has aroused a surge of interest in recent years. In this paper, we investigate the joint phase alignment and phase quantization on RIS discrete phase shift designs for RIS-assisted single-input single-output (SISO) system. Firstly, the phenomena of phase distribution in far and near fields of RIS are differentiated. Then, for the purpose of phase alignment, the phase distribution law and its underlying degree-of-freedom (DoF) are analyzed, serving as the guideline of phase quantization strategies. Subsequently, two phase quantization methods, i.e., dynamic threshold phase quantization (DTPQ) and equal interval phase quantization (EIPQ), are proposed to optimize RIS phase shift discretization so as to strengthen the beamforming. DTPQ evidences its capability of achieving the optimal discrete phase alignment with a linear complexity, whilst EIPQ is a simplified method yielding sub-optimal solution yet with a constant complexity. Simulation results demonstrate that both of the proposed methods provide substantial improvements on power gain, stability, and robustness over traditional quantization method. In addition, as a product of RIS beamforming, the path loss (PL) scaling law under discrete phase shifts for RIS channel is unveiled for the first time. The field trials conducted at 2.6 GHz and 35 GHz validate the favourable performance of the proposed methods in practical communication environment. This work may provide reference for the beamforming design of RIS-assisted links, especially in the near field.
Reconfigurable intelligent surfaces (RISs) are programmable metasurface structures that can control the propagation of electromagnetic waves by changing the electrical and magnetic properties of the surface. They can be used to intelligently reconfigure the wireless environment to improve the capacity and coverage of wireless networks. In recent years, numerous theoretical innovations and prototype tests have demonstrated that the RIS has the advantages of low cost, low power consumption, and easy deployment, and creates many potential opportunities and broad application prospects in 5G and future 6G networks. In this paper, starting from the technological development of RISs, we discussed the technical issues of RISs. The standardization of RISs, types of RISs according to operation modes, channel modeling, considerations for hardware implementation, differences from existing communication modules and the need for active RIS implementation, noise and power characteristics to ensure the efficiency of RISs, and performance parameters of RISs and field test results of RISs in indoor and outdoor environments were reviewed. By resolving the current technical issues of RISs, it is expected that RISs will be successfully used for B5G/6G communication through commercialization.
Intelligent reflecting surfaces (IRSs) have been proposed in recent years as a promising technology to enhance signal quality at high frequencies and save energy. In this paper, a Poisson bipolar network model with line segments is used to analyze the energy efficiency (EE) of an IRS-assisted, large-scale network. Specifically, we investigate the performance impact of the IRS configuration, in particular, the number of IRS elements and the phase-shifting resolution of each element. Using customized energy consumption and channel estimation models, we obtain the theoretical trade-off between signal quality and energy consumption as a function of these IRS configurations. The optimal number of elements and phase-shifting resolution of the IRS are also derived. Our results show that IRS technology has great potential for improving the EE of dense networks if their static energy consumption is small enough. Simulation results verify the accuracy of the obtained theoretical results.
A Reconfigurable Intelligent Surface (RIS) redirects and possibly modifies the properties of incident waves, with the aim to restore non-line-of-sight communication links. Composed of elementary scatterers, the RIS has been so far treated as a collection of point scatterers with properties similar to antennas in an equivalent massive MIMO communication link. Despite the discrete nature of the RIS, current design approaches often treat the RIS as a continuous radiating surface, which is subsequently discretized. Here, we investigate the connection between the two approaches in an attempt to bridge the two different perspectives. We analytically find the factor that renders the two approaches equivalent and we demonstrate our findings with examples of RIS elements modeled as antennas with commonly used radiation patterns and properties consistent with antenna theory. We analyze the equivalence between the two theoretical approaches with respect to design aspects of the RIS elements, such as gain, directivity and coupling between elements, with the aim to provide insight into the observed discrepancies, the understanding of which is crucial for assessing the RIS efficiency.
Metasurfaces enable efficient manipulation of electromagnetic radiation. In practice, control over reflection direction is possibly the most useful. Extensive research has been done in the field of anomalous reflectors over the past years, resulting in numerous introduced geometries and several distinct design approaches. Without a comprehensive comparison between design methods, it is difficult to properly select the most appropriate method and the most suitable metasurface geometry. Here, we consider four main approaches that can be used to design anomalous reflectors for large deviations from specular reflection within the same basic structure topology for microwave and millimeter-wave applications. These approaches include the phase-gradient design, which is well-suited for small deviation angles due to its simplicity in design and realization. The second and third approaches involve optimization-based methods at the level of input and grid impedances, respectively. The fourth method is a non-local approach that optimizes supercells of the structure. We analyze and discuss a wide range of performance aspects, such as the power efficiency and losses, angular response, and the scattering pattern of finite-size structures. We believe that our study is particularly interesting for researchers working on metasurfaces, communication technologies, and reconfigurable intelligent surfaces.
The need for unrestricted, high-quality, and high-speed communications in planned sixth generation (6G) wireless systems drives the development and research towards the sub-terahertz (sub-THz) bands which so far have been relatively unused for wireless communications applications. Additionally, the sub-THz bands have gained an increasing interest as a potential spectral region at which to go even beyond the well-known Shannon limits. This review paper provides a technological overview on some of the key hardware aspects of sub-THz wireless communications (at 100–300 GHz), namely antennas, reconfigurable intelligent surfaces (RISs), and reconfigurable antenna systems based on state-of-the-art technologies reported in recent literature. Different technologies of antennas and RISs are compared to understand their possibilities and limitations, and to identify the most promising technological approaches to transform 6G from a vision into a commercially viable solution. The paper also presents the authors’ interpretations of possible hardware and design trends that can shape the future research directions.
Reconfigurable intelligent surfaces (RISs) comprised of tunable unit cells have recently drawn significant attention due to their superior capability in manipulating electromagnetic waves. In particular, RIS-assisted wireless communications have the great potential to achieve significant performance improvement and coverage enhancement in a cost-effective and energy-efficient manner, by properly programming the reflection coefficients of the unit cells of RISs. In this paper, free-space path loss models for RIS-assisted wireless communications are developed for different scenarios by studying the physics and electromagnetic nature of RISs. The proposed models, which are first validated through extensive simulation results, reveal the relationships between the free-space path loss of RIS-assisted wireless communications and the distances from the transmitter/receiver to the RIS, the size of the RIS, the near-field/far-field effects of the RIS, and the radiation patterns of antennas and unit cells. In addition, three fabricated RISs (metasurfaces) are utilized to further corroborate the theoretical findings through experimental measurements conducted in a microwave anechoic chamber. The measurement results match well with the modeling results, thus validating the proposed free-space path loss models for RIS, which may pave the way for further theoretical studies and practical applications in this field.
Future wireless networks are expected to evolve toward an intelligent and software reconfigurable paradigm enabling ubiquitous communications between humans and mobile devices. They will also be capable of sensing, controlling, and optimizing the wireless environment to fulfill the visions of low-power, high-throughput, massively-connected, and low-latency communications. A key conceptual enabler that is recently gaining increasing popularity is the HMIMOS that refers to a low-cost transformative wireless planar structure comprised of sub-wavelength metallic or dielectric scattering particles, which is capable of shaping electromagnetic waves according to desired objectives. In this article, we provide an overview of HMIMOS communications including the available hardware architectures for reconfiguring such surfaces, and highlight the opportunities and key challenges in designing HMIMOS-enabled wireless communications.
Reconfigurable intelligent surfaces (RISs) have the potential of realizing the emerging concept of smart radio environments by leveraging the unique properties of metamaterials and large arrays of inexpensive antennas. In this article, we discuss the potential applications of RISs in wireless networks that operate at high-frequency bands, e.g., millimeter wave (30-100 GHz) and sub-millimeter wave (greater than 100 GHz) frequencies. When used in wireless networks, RISs may operate in a manner similar to relays. The present paper, therefore, elaborates on the key differences and similarities between RISs that are configured to operate as anomalous reflectors and relays. In particular, we illustrate numerical results that highlight the spectral efficiency gains of RISs when their size is sufficiently large as compared with the wavelength of the radio waves. In addition, we discuss key open issues that need to be addressed for unlocking the potential benefits of RISs for application to wireless communications and networks.
The future 5G networks are expected to use millimeter wave (mmWave) frequency bands to take advantage of the large unused spectrum. However, due to the high path loss at mmWave frequencies, coverage of mmWave signals can get severely reduced, especially for non-line-of-sight (NLOS) scenarios as mmWave signals are severely attenuated when going through obstructions. In this work, we study the use of passive metallic reflectors of different shapes/sizes to improve 28 GHz mmWave signal coverage for both indoor and outdoor NLOS scenarios. We quantify the gains that can be achieved in the link quality with metallic reflectors using measurements, analytical expressions, and ray tracing simulations. In particular, we provide an analytical model for the end-to-end received power in an NLOS scenario using reflectors of different shapes and sizes. For a given size of the flat metallic sheet reflector approaching to the size of the incident beam, we show that the reflected received power for the NLOS link is the same as line-of-sight (LOS) free space received power of the same link distance. Extensive results are provided to study the impact of environmental features and reflector characteristics on NLOS link quality.
Intelligent reflecting surfaces can improve the communication between a source and a destination. The surface contains metamaterial that is configured to “reflect” the incident wave from the source towards the destination. Two incompatible pathloss models have been used in prior work. In this letter, we derive the far-field pathloss using physical optics techniques and explain why the surface consists of many elements that individually act as diffuse scatterers but can jointly beamform the signal in a desired direction with a certain beamwidth. We disprove one of the previously conjectured pathloss models.
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.
This work deals with the use of emerging deep learning techniques in future wireless communication networks. It will be shown that data-driven approaches should not replace, but rather complement traditional design techniques based on mathematical models. Extensive motivation is given for why deep learning based on artificial neural networks will be an indispensable tool for the design and operation of future wireless communication networks, and our vision of how artificial neural networks should be integrated into the architecture of future wireless communication networks is presented. A thorough description of deep learning methodologies is provided, starting with the general machine learning paradigm, followed by a more in-depth discussion about deep learning and artificial neural networks, covering the most widely-used artificial neural network architectures and their training methods. Deep learning will also be connected to other major learning frameworks such as reinforcement learning and transfer learning. A thorough survey of the literature on deep learning for wireless communication networks is provided, followed by a detailed description of several novel case-studies wherein the use of deep learning proves extremely useful for network design. For each case-study, it will be shown how the use of (even approximate) mathematical models can significantly reduce the amount of live data that needs to be acquired/measured to implement data-driven approaches. Finally, concluding remarks describe those that in our opinion are the major directions for future research in this field.
Future wireless networks are expected to constitute a distributed intelligent wireless communications, sensing, and computing platform, which will have the challenging requirement of interconnecting the physical and digital worlds in a seamless and sustainable manner. Currently, two main factors prevent wireless network operators from building such networks: (1) the lack of control of the wireless environment, whose impact on the radio waves cannot be customized, and (2) the current operation of wireless radios, which consume a lot of power because new signals are generated whenever data has to be transmitted. In this paper, we challenge the usual “more data needs more power and emission of radio waves” status quo, and motivate that future wireless networks necessitate a smart radio environment: a transformative wireless concept, where the environmental objects are coated with artificial thin films of electromagnetic and reconfigurable material (that are referred to as reconfigurable intelligent meta-surfaces), which are capable of sensing the environment and of applying customized transformations to the radio waves. Smart radio environments have the potential to provide future wireless networks with uninterrupted wireless connectivity, and with the capability of transmitting data without generating new signals but recycling existing radio waves. We will discuss, in particular, two major types of reconfigurable intelligent meta-surfaces applied to wireless networks. The first type of meta-surfaces will be embedded into, e.g., walls, and will be directly controlled by the wireless network operators via a software controller in order to shape the radio waves for, e.g., improving the network coverage. The second type of meta-surfaces will be embedded into objects, e.g., smart t-shirts with sensors for health monitoring, and will backscatter the radio waves generated by cellular base stations in order to report their sensed data to mobile phones. These functionalities will enable wireless network operators to offer new services without the emission of additional radio waves, but by recycling those already existing for other purposes. This paper overviews the current research efforts on smart radio environments, the enabling technologies to realize them in practice, the need of new communication-theoretic models for their analysis and design, and the long-term and open research issues to be solved towards their massive deployment. In a nutshell, this paper is focused on discussing how the availability of reconfigurable intelligent meta-surfaces will allow wireless network operators to redesign common and well-known network communication paradigms.
Electromagnetic metasurfaces can be characterized as intelligent if they are able to perform multiple tunable functions, with the desired response being controlled by a computer influencing the individual electromagnetic properties of each metasurface inclusion. In this paper, we present an example of an intelligent metasurface that operates in the reflection mode in the microwave frequency range. We numerically show that, without changing the main body of the metasurface, we can achieve tunable perfect absorption and tunable anomalous reflection. The tunability features can be implemented using mixed-signal integrated circuits (ICs), which can independently vary both the resistance and reactance, offering complete local control over the complex surface impedance. The ICs are embedded in the unit cells by connecting two metal patches over a thin grounded substrate and the reflection property of the intelligent metasurface can be readily controlled by a computer. Our intelligent metasurface can have a significant influence on future space-time modulated metasurfaces and a multitude of applications, such as beam steering, energy harvesting, and communications.
Programmable wireless environments use unique customizable software processes rather than traditional rigid channel models.
Electromagnetic waves undergo multiple uncontrollable alterations as they propagate within a wireless environment. Free space path loss, signal absorption, as well as reflections, refractions, and diffractions caused by physical objects within the environment highly affect the performance of wireless communications. Currently, such effects are intractable to account for and are treated as probabilistic factors. This article proposes a radically different approach, enabling deterministic, programmable control over the behavior of wireless environments. The key enabler is the so-called HyperSurface tile, a novel class of planar meta-materials that can interact with impinging electromagnetic waves in a controlled manner. The HyperSurface tiles can effectively re-engineer electromagnetic waves, including steering toward any desired direction, full absorption, polarization manipulation, and more. Multiple tiles are employed to coat objects such as walls, furniture, and overall, any objects in indoor and outdoor environments. An external software service calculates and deploys the optimal interaction types per tile to best fit the needs of communicating devices. Evaluation via simulations highlights the potential of the new concept.
Conventional optical components rely on gradual phase shifts accumulated during light propagation to shape light beams. New
degrees of freedom are attained by introducing abrupt phase changes over the scale of the wavelength. A two-dimensional array
of optical resonators with spatially varying phase response and subwavelength separation can imprint such phase discontinuities
on propagating light as it traverses the interface between two media. Anomalous reflection and refraction phenomena are observed
in this regime in optically thin arrays of metallic antennas on silicon with a linear phase variation along the interface,
which are in excellent agreement with generalized laws derived from Fermat’s principle. Phase discontinuities provide great
flexibility in the design of light beams, as illustrated by the generation of optical vortices through use of planar designer
metallic interfaces.
Reconfigurable intelligent surfaces (RISs) are an emerging transmission technology for application to wireless communications. RISs can be realized in different ways, which include (i) large arrays of inexpensive antennas that are usually spaced half of the wavelength apart; and (ii) metamaterial-based planar or conformal large surfaces whose scattering elements have sizes and inter-distances much smaller than the wavelength. Compared with other transmission technologies, e.g., phased arrays, multi-antenna transmitters, and relays, RISs require the largest number of scattering elements, but each of them needs to be backed by the fewest and least costly components. Also, no power amplifiers are usually needed. For these reasons, RISs constitute a promising software-defined architecture that can be realized at reduced cost, size, weight, and power (C-SWaP design), and are regarded as an enabling technology for realizing the emerging concept of smart radio environments (SREs). In this paper, we (i) introduce the emerging research field of RIS-empowered SREs; (ii) overview the most suitable applications of RISs in wireless networks; (iii) present an electromagnetic-based communication-theoretic framework for analyzing and optimizing metamaterial-based RISs; (iv) provide a comprehensive overview of the current state of research; and (v) discuss the most important research issues to tackle. Owing to the interdisciplinary essence of RIS-empowered SREs, finally, we put forth the need of reconciling and reuniting C. E. Shannon’s mathematical theory of communication with G. Green’s and J. C. Maxwell’s mathematical theories of electromagnetism for appropriately modeling, analyzing, optimizing, and deploying future wireless networks empowered by RISs.
In this work we address the distance dependence of reconfigurable intelligent surfaces (RIS). As differentiating factor to other works in the literature, we focus on the array near-field, what allows us to comprehend and expose the promising potential of RIS. The latter mostly implies an interplay between the physical size of the RIS and the size of the Fresnel zones at the RIS location, highlighting the major role of the phase. To be specific, the point-like (or zero-dimensional) conventional scattering characterization results in the well-known dependence with the fourth power of the distance. On the contrary, the characterization of its near-field region exposes a reflective behavior following a dependence with the second and third power of distance, respectively, for a two-dimensional (planar) and one-dimensional (linear) RIS. Furthermore, a smart RIS implementing an optimized phase control can result in a power exponent of four that, paradoxically, outperforms free-space propagation when operated in its near-field vicinity. All these features have a major impact on the practical applicability of the RIS concept. As one contribution of this work, the article concludes by presenting a complete signal characterization for a wireless link in the presence of RIS on all such regions of operation.
Contents: Green's Functions in Mathematical Physics: Time-Independent Green's Functions. Time-Dependent Green's Functions.- Green's Functions in One-Body Quantum Problems: Physical Significance of G. Application to the Free-Particle Case. Green's Functions and Perturbation Theory. Green's Functions for Tight-Binding Hamiltonians. Single Impurity Scattering. Two or More Impurities Disordered Systems.- Green's Functions in Many-Body Systems: Definitions. Properties and Use of the Green's Functions. Calculational Methods for g. Applications.- Appendix A: Analytic Behavior of G(z) Near a Band Edge.- Appendix B: The Renormalized Perturbation Expansion (RPE).- Appendix C: Second Quantization.- References.- Subject Index.
Non-uniform metasurfaces (electrically thin composite layers) can be used for shaping refracted and reflected electromagnetic waves. However, known design approaches based on the generalized refraction and reflection laws do not allow realization of perfectly performing devices: there are always some parasitic reflections into undesired directions. In this paper we introduce and discuss a general approach to the synthesis of metasurfaces for full control of transmitted and reflected fields and show that perfect performance can be realized. The method is based on the use of an equivalent impedance matrix model which connects the tangential field components at the two sides on the metasurface. With this approach we are able to understand what physical properties of the metasurface are needed in order to perfectly realize the desired response. Furthermore, we determine the required polarizabilities of the metasurface unit cells and discuss suitable cell structures. It appears that only spatially dispersive metasurfaces allow realization of perfect refraction and reflection of incident plane waves into arbitrary directions. In particular, ideal refraction is possible only if the metasurface is bianisotropic, and ideal reflection without polarization transformation requires spatial dispersion with a specific, strongly non-local response to the fields.
Reconfigurable intelligent surfaces vs. relaying: Differences, similarities, and performance comparison
- Di Renzo
Beyond max-SNR: Joint encoding for reconfigurable intelligent surfaces
- R Karasik