Conference Paper

Analytical Modeling of the Path-Loss for Reconfigurable Intelligent Surfaces – Anomalous Mirror or Scatterer ?

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... Moreover, it should be noted that, in this work, the path loss of the IRS signal was modelled proportionally to -1/(d + v) 2 -the sum of the distance, d, between the IRS and the receiver and the distance, v, between the transmitter and the IRS. However, this model has been recently shown to only be reasonable for short ranges [8,9]. ...
... In this paper, a Poisson bipolar system model is investigated with the presence of blockages and IRSs. While embracing the sum of the distances, d and v, for short ranges, in this paper, following the model in [8,9], the path loss of the IRS signal for long ranges is proportional to -1/(d·v) 2 -the product of such distances. The IRS behaves like an anomalous mirror or a scatterer in these different ranges. ...
... For the signals transmitted through a reflection via the IRS, a piece-wise path loss model is considered, accounting for the short-and the long-distance regimes [9]. Formally, the path loss is given as ...
... IRSs have several applications, and, in particular, they are useful for enhancing the coverage in severe non-line-of-sight (NLOS) channel conditions by acting as intelligent mirrors [15]. More in general, IRSs are viewed as a technology enabler for realizing the so-called smart radio environment, i.e., a wireless system in which the environment (e.g., the channel) becomes a variable that can be optimized in addition to the parameters of the communication devices [16]. ...
... where E (V ( ) , B ( ) , G ( )) is the × MSE matrix defined in (15). ...
... From the statistical properties of the information vectors s , we can elaborate (15) as ...
Preprint
In this paper, we consider a multi-user multiple-input multiple-output (MIMO) system aided by multiple intelligent reflecting surfaces (IRSs) that are deployed to increase the coverage and, possibly, the rank of the channel. We propose an optimization algorithm to configure the IRSs, which is aimed at maximizing the network sum-rate by exploiting only the statistical characterization of the environment, such as the distribution of the locations of the users and the distribution of the multipath channels. As a consequence, the proposed approach does not require the estimation of the instantaneous channel state information (CSI) for system optimization, thus significantly relaxing (or even avoiding) the need of frequently reconfiguring the IRSs, which constitutes one of the most critical issues in IRS-assisted systems. Numerical results confirm the validity of the proposed approach. It is shown, in particular, that IRS-assisted wireless systems that are optimized based on statistical CSI still provide large performance gains as compared to the baseline scenarios in which no IRSs are deployed.
... It is important to note, however, that the author in [38] specifically focuses on characterizing the available spatial degrees of freedom of two RISs communicating with each other, instead of RISs that are utilized for reflection or transmission. In [1], we propose a path-loss model that is applicable only to onedimensional RISs that are deployed in a two-dimensional space. Also, the approach in [1] does not account for the vectorial nature of the electromagnetic waves. ...
... In [1], we propose a path-loss model that is applicable only to onedimensional RISs that are deployed in a two-dimensional space. Also, the approach in [1] does not account for the vectorial nature of the electromagnetic waves. ...
... With these preliminary definitions at hand, we can now discuss how to appropriately model the propagation of radio waves in the presence of a metasurface. As recently proved in [1] and [46], the EM field at any point of a volume and in the presence of a metasurface can be obtained by invoking the theory of diffraction and the Huygens-Fresnel principle. In general terms, the theory of diffraction provides one with the mathematical tools for modeling the bending of the radio waves when they encounter an object or a discontinuity, which in our case is a metasurface characterized by its specific effective parameters. ...
Thesis
Full-text available
The emergence of smart radio environment (SRE) as a new paradigm that challenges the status quo in wireless communication has motivated the use of metasurface-based reconfigurable intelligent surface (RIS) to improve the performance limits in wireless communication systems. The main focus of this thesis is the modeling of reconfigurable intelligent surfaces (RIS)-aided communication systems using electromagnetic based methods.Chapter 1 introduces the concept of smart radio environment. We also give the definition of RIS and how RIS can be used in context of SRE. To give some historical perspectives, we also discuss several important milestone papers throughout the development of research activities that lead to the current state of the art.Chapter 2 introduces theoretical concepts that are necessary to understand the results in the subsequent chapters. This chapter is divided into two parts. The first part discusses the metasurfaces modeling where we move from a physics-based microscopic description of a metasurface and introduces a macroscopic representation for it, which is shown to be suitable for application in wireless communications. The second part introduces several analytical approaches that allow us to compute the EM field at any point of a given volume that contains the metasurface.Chapter 3 provides a performance comparison between RISs operating as anomalous reflectors and a decode-and-forward relaying scheme that is representative of competing candidate technologies to realize SREs. The comparison is qualitative and covers multitude metrics. Furthermore, a quantitative comparison in terms of achievable data rates is presented. In particular, the numerical results show that sufficiently large RISs can outperform relay-aided systems in terms of data rate, while reducing the implementation complexity.Chapter 4 proposes an electromagnetic-based analytical characterization of the free-space path-loss of a wireless link in the presence of a RIS that is modeled as a two-dimensional homogenized metasurface made of sub-wavelength scattering elements and that operate either in reflection or transmission mode. The analytical method of physical optics is employed. Closed-form expressions are also obtained in two asymptotic regimes that are representative of far-field and near-field deployments. Based on the proposed approach, the impact of several design parameters and operating regimes is unveiled.Finally, Chapter 5 summarizes the main findings of the thesis and discusses possible future directions that are worth investigating to unlock the full potential of RIS and bring it into practice.
... conference paper [10] and [9], the available contributions are applicable to RISs made of large arrays of inexpensive antennas that are usually spaced half of the wavelength apart, and, therefore, are not homogenizable. In [3], the authors perform a measurement campaign in an anechoic chamber and show that the power reflected from an RIS follows a scaling law that depends on many parameters, including the size of the RIS, the mutual distances between the transmitter/receiver and the RIS (i.e., near-field vs. far-field), and whether the RIS is used for beamforming or broadcasting. ...
... However, the author focuses on charactering the available spatial degrees of freedom of two RISs communicating with each other, rather than on RISs that are utilized for reflection or transmission. In [10], we propose a path-loss model that is applicable only to one-dimensional RISs that are deployed in a two-dimensional space. Also, the approach in [10] does not account for the vectorial nature of the electromagnetic waves. ...
... In [10], we propose a path-loss model that is applicable only to one-dimensional RISs that are deployed in a two-dimensional space. Also, the approach in [10] does not account for the vectorial nature of the electromagnetic waves. ...
Article
In this paper, we introduce a physics-consistent analytical characterization of the free-space path-loss of a wireless link in the presence of a reconfigurable intelligent surface. The proposed approach is based on the vector generalization of Green’s theorem. The obtained path-loss model can be applied to two-dimensional homogenized metasurfaces, which are made of sub-wavelength scattering elements and that operate either in reflection or transmission mode. The path-loss is formulated in terms of a computable integral that depends on the transmission distances, the polarization of the radio waves, the size of the surface, and the desired surface transformation. Closed-form expressions are obtained in two asymptotic regimes that are representative of far-field and near-field deployments. Based on the proposed approach, the impact of several design parameters and operating regimes is unveiled.
... By adaptively adjusting the attenuation and/or phase shift of massive passive reflecting elements, the IRS is able to "program" the wireless propagation channel with low-cost hardware, which shows great potential for improving spectrum efficiency and energy efficiency, especially for millimeter-wave (mmWave) communication system due to its vulnerability to high path-loss and link blockage [1], [2]. Massive research works on the IRS have been conducted recently, including channel modeling [3], [4], beamforming design [5], [6], and hardware experiments [7], [8]. ...
... Such channel characteristic makes the channel-failure coupling issue even worse and introduces 4 more channel uncertainties to the diagnosis. Current literature fails to address the above issues, which motivates us to develop tailored diagnostic techniques that incorporate the hardware and channel characteristics of the IRS system. ...
Preprint
Full-text available
Intelligent reflecting surface (IRS) is a promising technology for enhancing wireless communication systems, which adaptively configures massive passive reflecting elements to control wireless channel in a desirable way. Due to hardware characteristics and deploying environments, the IRS may be subject to reflecting element blockages and failures, and hence developing diagnostic techniques is of great significance to system monitoring and maintenance. In this paper, we develop diagnostic techniques for IRS systems to locate faulty reflecting elements and retrieve failure parameters. Three cases of the channel state information (CSI) availability are considered. In the first case where full CSI is available, a compressed sensing based diagnostic technique is proposed, which significantly reduces the required number of measurements. In the second case where only partial CSI is available, we jointly exploit the sparsity of the millimeter-wave channel and the failure, and adopt compressed sparse and low-rank matrix recovery algorithm to decouple channel and failure. In the third case where no CSI is available, a novel atomic norm is introduced as the sparsity-inducing norm of the cascaded channel, and the diagnosis problem is formulated as a joint sparse recovery problem. Finally, proposed diagnostic techniques are validated through numerical simulations.
... In [25], integral and approximated closed-form expressions of the path loss of a one-dimensional RIS in the far-field and near-field regions are given. The results are obtained by leveraging the general scalar theory of diffraction. ...
... If the transmitter and the receiver are in the near-field region of the RIS, on the other hand, the unit cells that are closer to edge of the RIS provides a smaller contribution if the transmit and receive antennas are steered towards the center of the RIS. This result is consistent with the fact that, as the size of the RIS increases to infinity, the received power from the entire RIS is still finite [25], [26]. ...
Preprint
Full-text available
Reconfigurable intelligent surfaces (RISs) provide an interface between the electromagnetic world of the wireless propagation environment and the digital world of information science. Simple yet sufficiently accurate path loss models for RISs are an important basis for theoretical analysis and optimization of RIS-assisted wireless communication systems. In this paper, we refine our previously proposed free-space path loss model for RISs to make it simpler, more applicable, and easier to use. In the proposed path loss model, the impact of the radiation patterns of the antennas and unit cells of the RIS is formulated in terms of an angle-dependent loss factor. The refined model gives more accurate estimates of the path loss of RISs comprised of unit cells with a deep sub-wavelength size. The free-space path loss model of the sub-channel provided by a single unit cell is also explicitly provided. In addition, two fabricated RISs, which are designed to operate in the millimeter-wave (mmWave) band, are utilized to carry out a measurement campaign in order to characterize and validate the proposed path loss model for RIS-assisted wireless communications. The measurement results corroborate the proposed analytical model. The proposed refined path loss model for RISs reveals that the reflecting capability of a single unit cell is proportional to its physical aperture and to an angle-dependent factor. In particular, the far-field beamforming gain provided by an RIS is mainly determined by the total area of the surface and by the angles of incidence and reflection.
... where a = β l δ 2 The LIS is assumed to act like an anomalous mirror, and the path loss model from [1], [16] is applied. The distance between S and LIS is d 1 = 50 m, between LIS and D is d 2 = 5 m, and between S and D is d 3 = 60 m. ...
... = ln(1 +γ(aK 2 + bK + c)) (16) for K, and then rounding the solution to the nearest integer. Proof: Computation of the first derivative ofR(K) yields: ...
Article
Large intelligent surfaces (LIS) present a promising new technology for enhancing the performance of wireless communication systems. Realizing the gains of LIS requires accurate channel knowledge, and in practice the channel estimation overhead can be large due to the passive nature of LIS. Here, we study the achievable rate of a LIS-assisted single-input single-output communication system, accounting for the pilot overhead of a least-squares channel estimator. We demonstrate that there exists an optimal K*, which maximizes achievable rate by balancing the power gains offered by LIS and the channel estimation overhead. We present analytical approximations for K*, based on maximizing an analytical upper bound on average achievable rate that we derive, and study the dependencies of K* on statistical channel and system parameters.
... From there, the idea of smart environments using electronically reconfigurable surfaces, coined Reconfigurable Intelligent Surfaces (RIS) emerged as a credible major evolution for 6G and has attracted an enormous and growing interest in the wireless communications community. Associated to RIS there are numerous research topics ranging from energy efficient wireless communication [21] to channel modeling [22], [23], RIS based signal modulation and encoding [24], MIMO channel estimation and beamforming [25]- [27], telecommunication performance evaluation [28], mathematical model and optimization method for wavefront shaping with RIS [29]- [31] and even stochastic analysis approach [32]. Yet of the numerous works proposed, a very large part is concerned with theoretical and mathematical approaches, and very few deal with experimental demonstrations of RIS. ...
Article
Full-text available
Originally introduced in the early 2010’s, the idea of smart environments through reconfigurable intelligent surfaces (RIS) controlling the reflections of the electromagnetic waves has attracted much attention in recent years in preparation for the future 6G. Since reconfigurable intelligent surfaces are not based on increasing the number of sources, they could indeed pave the way to greener and potentially limitless wireless communications. In this paper, we design, model and demonstrate experimentally a millimeter wave reconfigurable intelligent surface based on an electronically tunable metasurface with binary phase modulation. We first study numerically the unit cell of the metasurface, based on a PIN diode, and obtain a good phase shift and return loss for both polarizations, over a wide frequency range around 28.5 GHz. We then fabricate and characterize the unit cell and verify its properties, before fabricating the whole 10 cm $\times 10$ cm reconfigurable intelligent surface. We propose an analytical description of the use that can be done of the binary phase RIS, both in the near field (reflectarray configuration) and in the far field (access point extender). We finally verify experimentally that the designed RIS works as expected, performing laboratory experiments of millimeter wave beamforming both in the near field and far field configuration. Our experimental results demonstrate the high efficiency of our binary phase RIS to control millimeter waves in any kind of scenario and this at the sole cost of the energy dissipated by the PIN diodes used in our design.
... We also assume that the intensity of the scattered electromagnetic field decays with the inverse of the distance, and that the IRS are uniformly illuminated by the BS; our model is intended to hold in the far-field regime [32], [24] and when the angular aperture of the IRS, as observed from the BS, is small when compared to the beamwidth of the BS signal. ...
Preprint
Full-text available
We consider a smart radio environment where meta-surfaces are employed to improve the performance of wireless networks working at sub-THz frequencies. To this end, we propose a comprehensive mathematical channel model, taking into account both the ability of the meta-surfaces to redirect the impinging signal towards a desired direction, and the signal reflection due to large objects. We show how the design of both the meta-surface and the transmitter precoder influences the network throughput. Furthermore, we compare several algorithms to optimize the effect of the meta-surfaces in a realistic scenario. As a result, a simpler algorithm that associates network users and meta-surfaces provides a performance comparable to more complex numerical optimization methods. Simulation results suggest how many users are supported in the designed system.
... In [88], a simple path loss model was proposed for RIS communications based on the general scalar theory of diffraction and the Huygens-Fresnel principle. The RIS was modeled as a sheet of electromagnetic material of negligible thickness. ...
Preprint
Full-text available
Reconfigurable intelligent surfaces (RISs) are two dimensional (2D) metasurfaces which can intelligently manipulate electromagnetic waves by low-cost near passive reflecting elements. RIS is viewed as a potential key technology for the sixth generation (6G) wireless communication systems mainly due to its advantages in tuning wireless signals, thus smartly controlling propagation environments. In this paper, we aim at addressing channel characterization and modeling issues of RIS-assisted wireless communication systems. At first, the concept, principle, and potential applications of RIS are given. An overview of RIS based channel measurements and experiments is presented by classifying frequency bands, scenarios, system configurations, RIS constructions, experiment purposes, and channel observations. Then, RIS based channel characteristics are studied, including reflection and transmission, Doppler effect and multipath fading mitigation, channel reciprocity, channel hardening, rank improvement, far field and near field, etc. RIS based channel modeling works are investigated, including largescale path loss models and small-scale multipath fading models. At last, future research directions related to RIS-assisted channels are also discussed.
... Through the research of channel modeling [12,13], channel estimation [14], and performance evaluation of the communication system based on the RIS [15][16][17], the potential benefits of the communication environments based on RIS are discussed. In the resource allocation design of RIS-aided communication, large amounts of work focus on optimizing the spectral efficiency (SE) of the system [18]. ...
Article
Full-text available
As reconfigurable intelligent surfaces (RISs) have been gradually brought to reality, a large amount of research has been conducted to investigate the immense benefits of RISs. That is because RISs enable us to artificially direct the radio wave propagating through the environment at a relatively low cost. This paper investigates the trade-off between spectral efficiency (SE) and energy efficiency (EE) in the RIS-aided multi-user multiple-input single-output downlink. We develop an optimization framework for designing the transmitting precoding at the base station and the phase shift values at the RIS to balance the EE-SE trade-off. The proposed iterative optimization framework for the design includes quadratic transform, alternating optimization, and weighted minimization mean-square error conversion. Simulation results illustrate our optimization framework algorithm exhibits effectiveness and a fast convergence rate.
... For instance, the authors in [15] designed and manufactured a 80 cmˆ30 cm experimental IRS system comprising 1,100 phase shift elements, while an 1 mˆ1 m large IRS prototype composed of 10,000 phase shift elements was demonstrated in [16]. On the other hand, even in free space propagation environments, the equivalent path loss of the BS-IRS-receiver link is in general much larger than that of the unobstructed direct link due to the double-path loss effect [17]. Hence, to fully realize the potential of IRSs, it is necessary to deploy a large number of phase shift elements such that the severe end-to-end path loss of the cascaded IRS channel can be compensated [18], [19]. ...
Preprint
Full-text available
In this paper, we study the optimal resource allocation algorithm design for large intelligent reflecting surface (IRS)-assisted simultaneous wireless information and power transfer (SWIPT) systems. To facilitate efficient system design for large IRSs, instead of jointly optimizing all the IRS elements, we partition the IRS into several tiles and employ a scalable optimization framework comprising an offline design stage and an online optimization stage. In the offline stage, the IRS elements of each tile are jointly designed to support a set of different phase shift configurations, referred to as transmission modes, while the best transmission mode is selected from the set for each tile in the online stage. Given a transmission mode set, we aim to minimize the total base station (BS) transmit power by jointly optimizing the beamforming and the transmission mode selection policy taking into account the quality-of-service requirements of information decoding and non-linear energy harvesting receivers, respectively. Although the resource allocation algorithm design is formulated as a non-convex combinatorial optimization problem, we solve it optimally by applying the branch-and-bound (BnB) approach which entails a high computational complexity. To strike a balance between optimality and computational complexity, we also develop an efficient suboptimal algorithm capitalizing on the penalty method and successive convex approximation. Our simulation results show that the proposed designs enable considerable power savings compared to several baseline schemes. Moreover, our results reveal that by properly adjusting the numbers of tiles and transmission modes, the proposed scalable optimization framework indeed facilitates online design for large IRSs.
... From there, the idea of smart environments using electronically reconfigurable surfaces, coined Reconfigurable Intelligent Surfaces (RIS) emerged as a credible major evolution for 6G and has attracted an enormous and growing interest in the wireless communications community. Associated to RIS there are numerous research topics ranging from energy efficient wireless communication [21] to channel modeling [22], [23], RIS based signal modulation and encoding [24], MIMO channel estimation and beamforming [25]- [27], telecommunication performance evaluation [28], mathematical model and optimization method for wavefront shaping with RIS [29]- [31] and even stochastic analysis approach [32]. Yet of the numerous works proposed, a very large part is concerned with theoretical and mathematical approaches, and very few deal with experimental demonstrations of RIS. ...
Preprint
Full-text available
Originally introduced in the early 2010's, the idea of smart environments through reconfigurable intelligent surfaces (RIS) controlling the reflections of the electromagnetic waves has attracted much attention in recent years in preparation for the future 6G. Since reconfigurable intelligent surfaces are not based on increasing the number of sources, they could indeed pave the way to greener and potentially limitless wireless communications. In this paper, we design, model and demonstrate experimentally a millimeter wave reconfigurable intelligent surface based on an electronically tunable metasurface with binary phase modulation. We first study numerically the unit cell of the metasurface, based on a PIN diode, and obtain a good phase shift and return loss for both polarizations, over a wide frequency range around 28.5 GHz. We then fabricate and characterize the unit cell and verify its properties, before fabricating the whole 10 cm x10 cm reconfigurable intelligent surface. We propose an analytical description of the use that can be done of the binary phase RIS, both in the near field (reflectarray configuration) and in the far field (access point extender). We finally verify experimentally that the designed RIS works as expected, performing laboratory experiments of millimeter wave beamforming both in the near field and far field configuration. Our experimental results demonstrate the high efficiency of our binary phase RIS to control millimeter waves in any kind of scenario and this at the sole cost of the energy dissipated by the PIN diodes used in our design.
... The reason being, through rapid tuning of the associated metasurfaces (MSFs), they transform the physical environment from being an adversary to being an ally in the communication process. Concretely, they enable the operators to engineer the channel propagation characteristics [5][6][7][8][9]. Such functionalities will be critical towards meeting the requirements being laid out for 6G networks [3,10,11]. ...
Article
Full-text available
As the current standardization for the 5G networks nears completion, work towards understanding the potential technologies for the 6G wireless networks is already underway. One of these potential technologies for the 6G networks is reconfigurable intelligent surfaces. They offer unprecedented degrees of freedom towards engineering the wireless channel, i.e., the ability to modify the characteristics of the channel whenever and however required. Nevertheless, such properties demand that the response of the associated metasurface is well understood under all possible operational conditions. While an understanding of the radiation pattern characteristics can be obtained through either analytical models or full-wave simulations, they suffer from inaccuracy and extremely high computational complexity, respectively. Hence, in this paper, we propose a neural network-based approach that enables a fast and accurate characterization of the metasurface response. We analyze multiple scenarios and demonstrate the capabilities and utility of the proposed methodology. Concretely, we show that this method can learn and predict the parameters governing the reflected wave radiation pattern with an accuracy of a full-wave simulation (98.8–99.8%) and the time and computational complexity of an analytical model. The aforementioned result and methodology will be of specific importance for the design, fault tolerance, and maintenance of the thousands of reconfigurable intelligent surfaces that will be deployed in the 6G network environment.
... IRS characterization. IRSs are made of meta-atoms (modeled as elementary spherical scatterer) whose scattered electromagnetic field holds in the far-field regime [15], [16]. The n-th IRS, n = 1, . . . ...
Preprint
Intelligent reflecting surfaces (IRSs) have several prominent advantages, including improving the level of wireless communications security and privacy. In this work, we focus on this aspect and envision a strategy to counteract the presence of passive eavesdroppers overhearing transmissions from a base station towards legitimate users. Unlike most of the existing works addressing passive eavesdropping, the strategy we consider has low complexity and is suitable for scenarios where nodes are equipped with a limited number of antennas. Through our performance evaluation, we highlight the trade-off between the legitimate users' data rate and secrecy rate, and how the system parameters affect such a trade-off.
... Above this we browsed many aspects of RIS applications, I also made notes on RIS' own physical electromagnetic properties. This paper writes about analytical modeling of the path-loss for Reconfigurable Intelligent Surfaces-anomalous mirror or scatterer [32]. A fundamental component for analyzing and optimizing RIS-empowered wireless networks is the development of simple but sufficiently accurate models for the power scattered by an RIS [33]. ...
Preprint
Full-text available
Reconfigurable Intelligent Surface (RIS) has becoming a useful tool in future wireless communication systems for close-distance communication network. This paper we use Reconfigurable Intelligent Surface (RIS) for downlink multi-user communication designed to improve energy collection performance while satisfying wireless information and Power Transfer (WIPT). The designed system consists of an IRS-assisted system consists of a multi-antenna assisted base station (BS) and two opposite multi-antenna assisted information receiver cooperated (RIS) as energy receiver (ERs) that meets energy collection requirements. Based on the electromagnetic property of Reconfigurable Intelligent Surface (RIS), like two mirrors that are opposite each other, setting two Reconfigurable Intelligent Surface (RIS) attached to the city buildings to reflect the sending signals. The transmitting precoding of the Multi-antenna Auxiliary Base Station (BS) and the angular phase transfer matrix of the multi-antenna Auxiliary Information Receiver (IRs) need to be optimized together to maximize the energy harvesting of IoT devices for energy efficiency (EE) of the IRs system and to provide users with the efficiency of the received signal. In order to solve the joint optimization problem effectively, we turn the non-convex maximize problem into the equivalent formal error method based on the mean square, and finally use the iterative algorithm for optimization. As for algorithm, we respectively use MSE method, semidefinite relaxation techniques to simplify transmitting beamforming matrix and the matrix phase shift. Through the observation of simulation data, it can be concluded that the performance optimization method of SDR based on RIS is effective.
... They are constant over several coherence-time intervals [48], since the distance between devices and LIS is much larger than the distance between the LIS' elements. In this far-field regime [4], the intelligent surface is better modeled as a scatterer and the scaling law that governs the intensity of its electric field is a function of the distances' product, as proved in [49] and shown later. ...
Article
Full-text available
Despite many studies already published on large intelligent surfaces (LIS), there are still some gaps in mathematical models in the face of possible scenarios. In this work, we evaluate the performance of a single-input single-output (SISO) system in which an LIS acts as a controllable scatterer. We consider that the direct link between the transmitting and receiving devices is non-existent due to a blockage. Quantization phase errors at the LIS are considered since a high precision configuration of the reflection phases is not always feasible. We derive exact closed-form expressions for the spectral efficiencies, outage probabilities, and average symbol error rate (SER) of different modulations schemes. We assume a more comprehensive scenario in which b bits are dedicated to the phase adjustment of the LIS’ elements. Based on Monte Carlo simulations, we prove the excellent accuracy of our approach and investigate the behavior of the power scaling law and the power required to reach a specific capacity, depending on the number of reflecting elements. We show that an LIS with approximately fifty elements and four dedicated bits for phase quantization outperforms the conventional system without LIS.
... The limitation associated with RIS-assisted wireless communications operating in the near-field is the lack of reliable electromagnetic models for the RISs. Majority of the existing research works are based on far-field models [13,14]. Therefore, it may lead to relatively simplified algorithm designs and performance predictions [15]. ...
Article
Reconfigurable intelligent surfaces (RISs) have recently attracted attention in the implementation of smart radio environment. In this paper, RISs are realized by the near-field focused antennas (NFF). A near-field channel gain model of RIS-assisted wireless communications is developed for an NFF reflectarray antenna based on the physics and electromagnetic nature of the RISs. The developed model entails the computation of the reflectarray aperture efficiency. Also, it takes into account reflectarray reconfigurablility to cope with varying environment, physical factors like the physical dimensions of the RISs, and the radiation patterns of the unit cells. Moreover, it is characterised by a reduction in the complexity. This model is further used in computing the positioning performance bounds and estimating the RIS optimal beamformer weights. For a validation purpose, the model is simulated by using Matlab software, and the results are compared to the simulation results of a near-field model discussed in literature. The comparison shows a very good agreement. Finally, the reflectarray antenna is thinned to achieve a performance comparable to a fully populated reflectarray antenna case using the full wave 3D electromagnetic solver CST Microwave Studio (CST MWS).
... IRS characterization. IRSs are made of meta-atoms (modeled as elementary spherical scatterer) whose scattered electromagnetic field holds in the far-field regime [15], [16]. The n-th IRS, n = 1, . . . ...
Conference Paper
Full-text available
Intelligent reflecting surfaces (IRSs) have several prominent advantages, including improving the level of wireless communications security and privacy. In this work, we focus on this aspect and envision a strategy to counteract the presence of passive eavesdroppers overhearing transmissions from a base station towards legitimate users. Unlike most of the existing works addressing passive eavesdropping, the strategy we consider has low complexity and is suitable for scenarios where nodes are equipped with a limited number of antennas. Through our performance evaluation, we highlight the trade-off between the legitimate users' data rate and secrecy rate, and how the system parameters affect such a trade-off.
... Follow up works were then published over the years, for example [9-11], but the topic remained of little interest to the wireless community, until a series of papers were published in late 2018 and 2019 [11][12][13][14][15]. From there, the idea of smart environments using electronically reconfigurable surfaces, coined Reconfigurable Intelligent Surfaces (RIS) emerged as a credible major technological advancement for 6G and has attracted an enormous and growing interest in the wireless communications community. Associated to RIS there are numerous research topics ranging from energy efficient wireless communication [16] to channel modeling [17,18], RIS based signal modulation and encoding [19], MIMO channel estimation and beamforming [20][21][22], telecommunication performance evaluation [23], mathematical model and optimization method for wavefront shaping with RIS [24-26] and even stochastic analysis approaches [27]. Of the numerous works proposed, a very large proportion are concerned with theoretical and mathematical approaches, and very few deal with experimental demonstrations of RIS [28]. ...
Preprint
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We exploit multi-path fading propagation to improve both the signal-to-interference-plus-noise-ratio and the stability of wireless communications within electromagnetic environments that support rich multipath propagation. Quasi-passive propagation control with multiple binary reconfigurable intelligent surfaces is adopted to control the stationary waves supported by a metallic cavity hosting a software-defined radio link. Results are demonstrated in terms of the error vector magnitude minimization of a quadrature phase-shift modulation scheme under no-line-of-sight conditions. It is found that the magnitude of fluctuation of received symbols is reduced to a stable constellation by increasing the number of individual surfaces, or elements, thus demonstrating channel hardening. By using a second software-defined radio device as a jammer, we demonstrate the ability of the RIS to mitigate the co-channel interference by channel hardening. Results are of particular interest in smart radio environments for mobile network architectures beyond 5G.
... The authors in [147] also showed that in far-field scenarios, the path loss of the transmitter-RIS-receiver link is larger than that of the unobstructed direct transmitter-receiver link. Therefore, for the two links to have the same path loss, the RIS should be equipped with a massive number of phase shift elements [25]. ...
Article
Full-text available
Reconfigurable intelligent surface (RIS)-aided communication is considered as an exciting research topic in academic and industrial communities since it provides an emerging affordable solution to achieve high quality and secure next-generation wireless systems. Especially, the deployment of RIS in multi-user wireless networks promises to reduce system hardware costs, signal processing complexity, as well as energy consumption due to small size, lightweight and ability to actively shape the wireless propagation environment. Further, by realizing a cost-effective radio environment, RIS-aided communication can be implemented to be an appealing technology for future integration with other emerging wireless applications and communication systems. Despite the positive appeal, RISs face new challenges that hinder integrating efficiently into wireless networks, such as network secrecy performance and system sum-rates, as well as achieving efficient deployment design in highly dynamic and time-varying wireless environments. To this end, we overview recent state-of-the-art techniques to address the above issues faced in the integration of RISs with various emerging multi-user communication techniques, such as Unmanned Aerial Vehicles (UAVs), Non-Orthogonal Multiple Access (NOMA), Millimeter Wave (mmWave) and Terahertz (THz) communications, Physical Layer Security (PLS), massive antennas, and Simultaneous Wireless Information and Power Transfer (SWIPT). Finally, we highlight promising future research directions of RIS-aided communication in Cell-Free Massive Multiple-Input-Multiple-Output (MIMO) systems, Rate-Splitting Multiple Access (RSMA), Light Fidelity (LiFi), and Cognitive Radio (CR) systems.
... The RIS elements can independently shift the signal phase and absorbing the signal energy. The reflected signals benefit to wireless transmission due to less energy required [18][19][20][21][22][23][24][25]. In [21], the authors demonstrated an interesting RIS architecture which includes any number of passive reflecting elements, a simple controller for their adjustable configuration, and a single radio frequency (RF) chain for baseband measurements. ...
Article
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With the given scope for new use cases and the demanding needs of future 6th generation (6G) wireless networks, the development of wireless communications looks exciting. The propagation medium has been viewed as a randomly behaving entity between the transmitter and the receiver since traditional wireless technology, degrading the quality of the received signal due to the unpredictable interactions of the broadcast radio waves with the surrounding objects. On the other hand, network operators could now manipulate electromagnetic radiation to remove the negative impacts of natural wireless propagation due to the recent arrival of reconfigurable intelligent surfaces (RIS) in wireless communications. According to recent findings, the RIS mechanism benefits nonorthogonal multiple access (NOMA), which can effectively deliver effective transmissions. For simple design, of RIS-NOMA system, fixed power allocation scheme for NOMA is required. The main system performance metric, i.e., outage probability, needs to be considered to look at the efficiency and capability of transmission mode relying on RIS and NOMA schemes, motivated by the potential of these developing technologies. As major performance metrics, we derive analytical representations of outage probability, and throughput and an accurate approximation is obtained for the outage probability. Numerical results are conducted to validate the exactness of the theoretical analysis. It is found that increasing the higher number of reflecting elements in the RIS can significantly boost the outage probability performance, and the scenario with only the RIS link is also beneficial. In addition, it is desirable to deploy the RIS-NOMA since it is indicated that better performance compared with the traditional multiple access technique.
... So far, several techniques have been theoretically proposed for the design of the desired RIS properties [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] and relevant experiments have been performed in order to verify the predicted RIS performance [12][13][14][15][21][22][23][24][25][26][27]. To achieve the desired wave manipulation, the design involves the determination of the appropriate surface properties, such as surface impedance (or effective electric and magnetic surface conductivities). ...
Preprint
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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 seemingly opposite 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. The equivalence between the two theoretical approaches is analyzed with respect to design aspects of the RIS elements, such as gain and directivity, with the aim to provide insight into the observed discrepancies, the understanding of which is crucial for assessing the RIS efficiency.
... Thus, the beamforming gain of RIS is distance-dependent, in addition to the angles of the incident and scattered waves. As a result of this effect, the conventional collimated beamforming, which only depends on the directions and is optimal in the far-field, suffers from notable performance degradation in the near-field [6], [7]. More specifically, the effective beamforming gain has a limit which is independent of the RIS size or the distance between the RIS and the receiver. ...
Conference Paper
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Beamformer design for large-dimension reconfig-urable intelligent surface (RIS) is investigated. The challenge arises from the expanded near-field of RIS where its beam-forming gain is, in general, distance-dependent due to near-field diffraction. To gain more insights into this effect, we first analyse the performance of conventional collimated beamforming using Fresnel integrals, and compare it with that of focused beam-forming. Then, a two-step beamforming scheme is proposed. The principle of the proposed scheme is to divide the RIS into multiple subarrays followed by steering the sub-beams of the individual subarrays to focus. Providing properly chosen subarray size, the proposed scheme retains the low complexity of collimated beamforming and achieves a near-optimal performance, as shown both analytically and numerically.
... Considering an RIS modeled as a sheet of EM material of negligible thickness, and leveraging the general scalar theory of diffraction as well as the Huygens-Fresnel principle in [109], closed-form expressions for the power reflected from an RIS are presented as functions of the size of the RIS, the distance between the transmitter/receiver and the RIS, and the phase shift matrix configured by the RIS. With the aid of the stationary phase method, the authors identify sufficient conditions under which an RIS acts as an anomalous mirror, indicating that the received power decays as a function of the reciprocal of the sum of the distances between the transmitter/receiver and the RIS. ...
Preprint
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The demanding objectives for the future sixth generation (6G) of wireless communication networks have spurred recent research efforts on novel materials and radio-frequency front-end architectures for wireless connectivity, as well as revolutionary communication and computing paradigms. Among the pioneering candidate technologies for 6G belong the reconfigurable intelligent surfaces (RISs), which are artificial planar structures with integrated electronic circuits that can be programmed to manipulate the incoming electromagnetic field in a wide variety of functionalities. Incorporating RISs in wireless networks has been recently advocated as a revolutionary means to transform any wireless signal propagation environment to a dynamically programmable one, intended for various networking objectives, such as coverage extension and capacity boosting, spatiotemporal focusing with benefits in energy efficiency and secrecy, and low electromagnetic field exposure. Motivated by the recent increasing interests in the field of RISs and the consequent pioneering concept of the RIS-enabled smart wireless environments, in this paper, we overview and taxonomize the latest advances in RIS hardware architectures as well as the most recent developments in the modeling of RIS unit elements and RIS-empowered wireless signal propagation. We also present a thorough overview of the channel estimation approaches for RIS-empowered communications systems, which constitute a prerequisite step for the optimized incorporation of RISs in future wireless networks. Finally, we discuss the relevance of the RIS technology in the latest wireless communication standards, and highlight the current and future standardization activities for the RIS technology and the consequent RIS-empowered wireless networking approaches.
... In this thesis, it is considered to be a constant; (ii) λ l ,G is the l -th non-zero eigenvalue of the matrix W G = (1/N ) G H G and λ l ,H is the l -th non-zero eigenvalue of the matrix W H = (1/N ) HH H ; (iii) v l ,G and u l ,G are the l th eigenvectors of W G and W G H , respectively, that correspond to λ k,G ; (iv) and v k,H and u k,H are the kth eigenvectors of W H and W H H , respectively, that correspond to λ k,H . As mentioned, the SNRs in (3.1) and (3.2) are applicable in the far-field regime, as defined in [118], [119]. Thus, N can be large but it needs to be finite [7, Sec. ...
Thesis
Recently, the emergence of reconfigurable intelligent surface (RIS) has attracted heated attention from both industry and academia. A RIS is a planar surface that consists of a large number of low-cost passive reflecting elements. By carefully adjusting the phase shifts of the reflecting elements, an RIS can reshape the wireless environment for better communication. In this thesis, we focus on two subjects: (i) To study the modeling and optimization of RIS-aided communication systems. (ii) To study RIS-aided spatial modulation, especially the detection using deep learning techniques. Chapter 1 introduces the concept of smart radio environments and RIS. In 5G and future communications, RIS is a key technique to achieve seamless connectivity and less energy consumption at the same time. Chapter 2 introduces RIS-aided communication systems. The reflection principle, channel estimation problem and system design problem are introduced in detail. State-of-the-art research on the problems of channel estimation and system design are overviewed. Chapter 3 investigates the distribution of the signal-to-noise ratio (SNR) as a random variable in an RIS-aided multiple-input multiple-output (MIMO) system. Rayleigh fading and line-of-sight propagation are considered separately. The theoretical derivation and numerical simulation prove that the SNR is equivalent in distribution to the product of three (Rayleigh fading) or two (line-of-sight propagation) independent random variables. Chapter 4 studies the behavior of interference in an RIS-aided MIMO system, where each base station serves a user equipment (UE) through an RIS. The interference at a UE is caused by its non-serving RIS. It is proven that the interference-to-noise ratio is equivalent in distribution to the product of a Chi-squared random variable and a random variable which can be approximated with a Gamma distribution. Chapter 5 focuses on RIS-aided spatial modulation. First, we introduce deep learning aided detection for MIMO systems. Then, by generalizing RIS-aided spatial modulation systems as a special case of traditional spatial modulation systems, we investigate deep learning based detection for RIS-aided spatial modulation systems. Numerical results validate the proposed data-based and model-based deep learning detection schemes for RIS-aided spatial modulation systems. Finally, Chapter 6 concludes the thesis and discusses possible future research directions.
... Analytical modelling applies not only to network planning in the first stage in scenarios using RISs but also to resource management and network control where the adaptive approach is used. The complexity of RIS systems raises the possibility that existing traditional analytical models may become dysfunctional for them [220]. Considering situations mentioned above, uncertainties arise from RIS system configurations and channel dynamics complicate the system design [221]. ...
Preprint
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Deep learning (DL) has proven its unprecedented success in diverse fields such as computer vision, natural language processing, and speech recognition by its strong representation ability and ease of computation. As we move forward to a thoroughly intelligent society with 6G wireless networks, new applications and use-cases have been emerging with stringent requirements for next-generation wireless communications. Therefore, recent studies have focused on the potential of DL approaches in satisfying these rigorous needs and overcoming the deficiencies of existing model-based techniques. The main objective of this article is to unveil the state-of-the-art advancements in the field of DL-based physical layer (PHY) methods to pave the way for fascinating applications of 6G. In particular, we have focused our attention on four promising PHY concepts foreseen to dominate next-generation communications, namely massive multiple-input multiple-output (MIMO) systems, sophisticated multi-carrier (MC) waveform designs, reconfigurable intelligent surface (RIS)-empowered communications, and PHY security. We examine up-to-date developments in DL-based techniques, provide comparisons with state-of-the-art methods, and introduce a comprehensive guide for future directions. We also present an overview of the underlying concepts of DL, along with the theoretical background of well-known DL techniques. Furthermore, this article provides programming examples for a number of DL techniques and the implementation of a DL-based MIMO by sharing user-friendly code snippets, which might be useful for interested readers.
... For instance, the authors in [15] designed and manufactured a 80 cmˆ30 cm experimental IRS system comprising 1,100 phase shift elements, while an 1 mˆ1 m large IRS prototype composed of 10,000 phase shift elements was demonstrated in [16]. On the other hand, even in free space propagation environments, the equivalent path loss of the BS-IRS-receiver link is in general much larger than that of the unobstructed direct link due to the double-path loss effect [17]. Hence, to fully realize the potential of IRSs, it is necessary to deploy a large number of phase shift elements such that the severe end-to-end path loss of the cascaded IRS channel can be compensated [18], [19]. ...
Article
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In this paper, we study the optimal resource allocation algorithm design for large intelligent reflecting surface (IRS)-assisted simultaneous wireless information and power transfer (SWIPT) systems. To facilitate efficient system design for large IRSs, instead of jointly optimizing all the IRS elements, we partition the IRS into several tiles and employ a scalable optimization framework comprising an offline design stage and an online optimization stage. In the offline stage, the IRS elements of each tile are jointly designed to support a set of different phase shift configurations, referred to as transmission modes, while the best transmission mode is selected from the set for each tile in the online stage. Given a transmission mode set, we aim to minimize the total base station (BS) transmit power by jointly optimizing the beamforming and the transmission mode selection policy taking into account the quality-of-service requirements of information decoding and non-linear energy harvesting receivers, respectively. Although the resource allocation algorithm design is formulated as a non-convex combinatorial optimization problem, we solve it optimally by applying the branch-and-bound (BnB) approach which entails a high computational complexity. To strike a balance between optimality and computational complexity, we also develop an efficient suboptimal algorithm capitalizing on the penalty method and successive convex approximation. Our simulation results show that the proposed designs enable considerable power savings compared to several baseline schemes. Moreover, our results reveal that by properly adjusting the numbers of tiles and transmission modes, the proposed scalable optimization framework indeed facilitates online design for large IRSs. Besides, our results confirm that the advocated physics-based model and scalable optimization framework enable a flexible trade-off between performance and complexity, which is vital for realizing the performance gains promised by large IRS-assisted communication systems in practice.
... Owing to the enormous potential benefits promised by the RIS-empowered environments, various research activities have been recently devoted in the literature, covering different aspects, e.g., channel modeling [15], [16], channel estimation [17], [18], modulation and encoding [19], [20], and performance evaluation in RIS-aided wireless networks [21]- [23]. In this work, we focus on the design of resource allocation in RIS-aided wireless transmissions. ...
Preprint
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The emergence of reconfigurable intelligent surfaces (RISs) enables us to establish programmable radio wave propagation that caters for wireless communications, via employing low-cost passive reflecting units. This work studies the non-trivial tradeoff between energy efficiency (EE) and spectral efficiency (SE) in multiuser multiple-input multiple-output (MIMO) uplink communications aided by a RIS equipped with discrete phase shifters. For reducing the required signaling overhead and energy consumption, our transmission strategy design is based on the partial channel state information (CSI), including the statistical CSI between the RIS and user terminals (UTs) and the instantaneous CSI between the RIS and the base station. To investigate the EE-SE tradeoff, we develop a framework for the joint optimization of UTs' transmit precoding and RIS reflective beamforming to maximize a performance metric called resource efficiency (RE). For the design of UT's precoding, it is simplified into the design of UTs' transmit powers with the aid of the closed-form solutions of UTs' optimal transmit directions. To avoid the high complexity in computing the nested integrals involved in the expectations, we derive an asymptotic deterministic objective expression. For the design of the RIS phases, an iterative mean-square error minimization approach is proposed via capitalizing on the homotopy, accelerated projected gradient, and majorization-minimization methods. Numerical results illustrate the effectiveness and rapid convergence rate of our proposed optimization framework.
... where β 1 and β 2 are the path losses of the LOS channel and the RIS-scattering channel. For the RIS-scattering path, different path loss models have been proposed [32]- [36] and we adopt the model in [36] which takes into account the incident and reflection angles of the electromagnetic wave on a RIS, ...
Article
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Reconfigurable intelligent surface (RIS) is thought to be a potential key technique for future wireless communications due to its ability for manipulating the electromagnetic environment smartly. This paper focuses on the rank and capacity analysis when a RIS is introduced into a multiple-input multiple-output (MIMO) system. By establishing a system model for this communication system, various simulations are conducted for identifying the characteristics of the channel. The simulations of different distance between the access point (AP) and user equipment (UE) show that the condition number of the channel is worsen when the distance increases and the role of the RIS in rank improvement is weaken. The spatial distributions of the reciprocal condition number of the channel which corresponds to the RIS locations are obtained and rank-deficient zones are found in different AP, UE and RIS configurations, which depicts the spatial characteristics of the RIS-assisted MIMO channel. The simulations also indicate that the condition number of the channel not only varies with the RIS location, but is also affected by the antenna array size and orientation of AP and UE. In addition, when the AP has a larger amount of antennas than UE, it is advantageous to place the RIS near the UE rather than the AP to achieve better channel condition. Modulation and coding schemes are applied in the simulations for comparison and capacity improvement is witnessed. Beneficial suggestions for RIS deployment in the MIMO system are concluded according to the simulation results.
... Furthermore, θ i specifies the adjustable phase induced by the i-th RE of the RIS, respectively [2]. The total received power expression given in (1) is applicable in the far-field regime, as defined in [53], [54]. Accordingly, N can be large but it needs to be finite [20, Sec. ...
Article
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In this paper, we develop an analytical framework for the statistical analysis of the battery recharging time (BRT) in reconfigurable intelligent surfaces (RISs)-aided wireless power transfer (WPT) systems. Specifically, we derive novel closed-form expressions for the probability density function (PDF), cumulative distribution function, and moments of the BRT of the radio frequency energy harvesting wireless nodes. Moreover, a closed-form expression of the PDF of the BRT is obtained for the special case when the RIS consists of a large number of elements. Capitalizing on the derived expressions, we offer a comprehensive treatment for the statistical characterization of the BRT and study the impact of the system and battery parameters on its performance. Our results reveal that the proposed statistical models are analytically tractable, accurate, and efficient in assessing the sustainability of RIS-assisted WPT networks and in providing key design insights for large-scale future wireless applications. For example, we demonstrate that a 4-fold reduction in the mean time of the BRT can be achieved by doubling the number of RIS elements. Monte Carlo simulation results corroborate the accuracy of the proposed theoretical framework.
Article
The idea of utilizing reconfigurable intelligent surfaces (RISs) that employ an array of severally-controllable meta-material elements to scatter incoming data in a desirable way, in wireless communication systems has created new opportunities for transmission in various scenarios. For instance, RISs are envisioned as potential candidates to achieve smart radio environments in fifth generation (5G) network technology, which is usually described as incapable of controlling the environment. Nevertheless, existing studies analyzing RIS-empowered wireless networks generally discuss the cases with ideal hardware despite the fact that both physical transceiver and RISs suffer from unignorable hardware imperfections, which may exceedingly reduce the system performance in practical communication scenarios. This paper investigates the performance of an RIS-assisted spatial modulation (SM) scheme in the presence of in-phase/quadrature-phase imbalance (IQI) at the base station (BS) constituting a substantial impairment in direct-conversion transceivers. The concept of the RIS-assisted SM scheme is discussed by comparing three different implementation scenarios depending on the awareness of both the channel phases at the RIS and the severity of the IQI at the destination (D). The average bit error rate (ABER) is derived analytically by use of maximum likelihood (ML) detection method to tackle the distorting impact of the IQI taking the effects of path loss into account. Extensive computer simulation results, which verify theoretical findings, show that the performance of the traditional SM schemes with IQI can remarkably be improved by utilizing RISs, while having the knowledge of the adjusted phases at the receiver (Rx) provides substantial gain in terms of energy.
Article
This paper considers a two-user downlink transmission in intelligent reflecting surface (IRS) aided network over fading channels. Particularly, non-orthogonal multiple access (NOMA) and two orthogonal multiple access (OMA) schemes, namely, time division multiple access (TDMA) and frequency division multiple access (FDMA), are studied. The objective is to maximize the system average sum rate for the delay-tolerant transmission. We propose two adjustment schemes, namely, dynamic phase adjustment and one-time phase adjustment. The power budget, minimum average data rate, and discrete unit modulus reflection coefficient are considered as constrains. To solve the problem, two phase shifters adjustment algorithms with low complexity are proposed to obtain near optimal solutions. With given phase shifters and satisfaction of time-sharing condition, the optimal resource allocations are obtained using the Lagrangian dual decomposition. The numerical results reveal that: i) the average sum rate of proposed NOMA network aided by IRS outperforms the conventional NOMA network over fading channels; ii) with continuous IRS adjustment in the fading block, the proposed TDMA scheme performs better than the FDMA scheme; iii) increasing the minimum average user rate requirement has less impact on the proposed IRS-NOMA system than on the IRS-OMA system.
Article
Intelligent reflecting surfaces (IRSs) have the potential to transform wireless communication channels into smart reconfigurable propagation environments. To realize this new paradigm, the passive IRSs have to be large, especially for communication in far-field scenarios, so that they can compensate for the large end-to-end path-loss, which is caused by the multiplication of the individual path-losses of the transmitter-to-IRS and IRS-to-receiver channels. However, optimizing a large number of sub-wavelength IRS elements imposes a significant challenge for online transmission. To address this issue, in this article, we develop a physics-based model and a scalable optimization framework for large IRSs. The basic idea is to partition the IRS unit cells into several subsets, referred to as tiles, model the impact of each tile on the wireless channel, and then optimize each tile in two stages, namely an offline design stage and an online optimization stage. For physics-based modeling, we borrow concepts from the radar literature, model each tile as an anomalous reflector, and derive its impact on the wireless channel for a given phase shift by solving the corresponding integral equations for the electric and magnetic vector fields. In the offline design stage, the IRS unit cells of each tile are jointly designed for the support of different transmission modes, where each transmission mode effectively corresponds to a given configuration of the phase shifts that the unit cells of the tile apply to an impinging electromagnetic wave. In the online optimization stage, the best transmission mode of each tile is selected such that a desired quality-of-service (QoS) criterion is maximized. We consider an exemplary downlink system and study the minimization of the base station (BS) transmit power subject to QoS constraints for the users. Since the resulting mixed-integer programming problem for joint optimization of the BS beamforming vectors and the tile transmission modes is non-convex, we derive two efficient suboptimal solutions, which are based on alternating optimization and a greedy approach, respectively. We show that the proposed modeling and optimization framework can be used to efficiently optimize large IRSs comprising thousands of unit cells.
Article
The emergence of reconfigurable intelligent surfaces (RISs) enables us to establish programmable radio wave propagation that caters for wireless communications, via employing low-cost passive reflecting units. This work studies the non-trivial tradeoff between energy efficiency (EE) and spectral efficiency (SE) in multiuser multiple-input multiple-output (MIMO) uplink communications aided by a RIS equipped with discrete phase shifters. For reducing the required signaling overhead and energy consumption, our transmission strategy design is based on the partial channel state information (CSI), including the statistical CSI between the RIS and user terminals (UTs) and the instantaneous CSI between the RIS and the base station. To investigate the EE-SE tradeoff, we develop a framework for the joint optimization of UTs' transmit precoding and RIS reflective beamforming to maximize a performance metric called resource efficiency (RE). For the design of UT's precoding, it is simplified into the design of UTs' transmit powers with the aid of the closed-form solutions of UTs' optimal transmit directions. To avoid the high complexity in computing the nested integrals involved in the expectations, we derive an asymptotic deterministic objective expression. For the design of the RIS phases, an iterative mean-square error minimization approach is proposed via capitalizing on the homotopy, accelerated projected gradient, and majorization-minimization methods. Numerical results illustrate the effectiveness and rapid convergence rate of our proposed optimization framework.
Preprint
A reconfigurable intelligent surface (RIS) is capable of manipulating electromagnetic waves with its flexibly configurable unit cells, thus is an appealing technology to resist fast fading caused by multi-path in wireless communications. In this paper, a two-path propagation model for RIS-assisted wireless communications is proposed by considering both the direct path from the transmitter to the receiver and the assisted path provided by the RIS. The proposed propagation model unveils that the phase shifts of RISs can be optimized by appropriate configuration for multi-path fading mitigation. In particular, four types of RISs with different configuration capabilities are introduced and their performances on improving received signal power in virtue of the assisted path to resist fast fading are compared through extensive simulation results. In addition, an RIS operating at 35 GHz is used for experimental measurement. The experimental results verify that an RIS has the ability to combat fast fading and thus improves the receiving performance, which may lay a foundation for further researches.
Preprint
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Recently, there has been a flurry of research on the use of Reconfigurable Intelligent Surfaces (RIS) in wireless networks to create dynamic radio environments. In this paper, we investigate the use of an RIS panel to improve bi-directional communications. Assuming that the RIS will be located on the facade of a building, we propose to connect it to a solar panel that harvests energy to be used to power the RIS panel's smart controller and reflecting elements. Therefore, we present a novel framework to optimally decide the transmit power of each user and the number of elements that will be used to reflect the signal of any two communicating pair in the system (user-user or base station-user). An optimization problem is formulated to jointly minimize a scalarized function of the energy of the communicating pair and the RIS panel and to find the optimal number of reflecting elements used by each user. Although the formulated problem is a mixed-integer non-linear problem, the optimal solution is found by linearizing the non-linear constraints. Besides, a more efficient close to the optimal solution is found using Bender decomposition. Simulation results show that the proposed model is capable of delivering the minimum rate of each user even if line-of-sight communication is not achievable.
Technical Report
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This research report is the first systematic technical report on RIS technology in the industry, and is released by the IMT-2030 propulsion group. It first briefly reviews the evolution history and current situation of RIS technology, and then introduces the foundational principle of RIS technology. The typical application scenarios and key enabling technologies of RIS are the focus of this report. As for potential application scenarios, the report gives 7 traditional communication scenarios enhanced by RIS and 10 new application scenarios enabled by RIS. This report further divides the potential key enabling technologies into three categories: hardware structure and regulation mechanism, baseband algorithm and system architecture and deployment. Among them, the hardware structure and regulation mechanism includes two key technologies, the baseband algorithm includes nine key technologies, and the system architecture and deployment include three key technologies. Then, the report analyzes the maturity and challenges of RIS technology. Finally, the report gives a prospect for the development of RIS technology.
Chapter
The diverse and stringent requirements of 6G networks, detailed in the present book, give rise to fully flexible, end-to-end, software-defined network paradigm, where every part of the network can be configured and controlled via software. However, the wireless environment per se – i.e. the wireless medium or channel – is generally assumed uncontrollable, thus, imposing inherent limitations to 6G networks, that arise by the very nature of wireless operation. To this end, the emergence of the newly introduced concept of reconfigurable intelligent surfaces (RISs) challenges the fundamental status quo that the wireless environment is fixed by nature, paving the way for the realization of end-to-end fully reconfigurable 6G networks. RISs are network devices that can control and manipulate the radio waves traversing through the wireless channel. Since RISs reside within the wireless channel and not at the communication endpoints, they can control the wireless channels from within. In this context, the present chapter introduces the concept of RISs within the 6G framework, elaborating on their advantages and limitations. RISs are then compared with other transmission technologies, e.g. phased arrays, multi-antenna transmitters, and relays, while demonstrating the trade-offs governing their operation and applicability. Subsequently, the different types of RIS implementations are presented from a theoretical standpoint, along the available prototypes in the literature; thus, demonstrating how the RIS technology is already a reality, ushering wireless networks into the 6G era.
Article
In this paper, we focus on large intelligent reflecting surfaces (IRSs) and propose a new codebook construction method to obtain a set of predesigned phase-shift configurations for the IRS unit cells. Since the overhead for channel estimation and the complexity of online optimization for IRS-assisted communications scale with the size of the phase-shift codebook, the design of small codebooks is of high importance. We show that there exists a fundamental tradeoff between power efficiency and the size of the codebook. We first analyze this tradeoff for baseline designs that employ a linear phase-shift across the IRS. Subsequently, we show that an efficient design for small codebooks mandates higher-order phase-shift variations across the IRS. Consequently, we propose a quadratic phase-shift design, derive its coefficients as a function of the codebook size, and analyze its performance. Our simulation results show that the proposed design yields a higher power efficiency for small codebooks than the linear baseline designs.
Article
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.
Article
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We demonstrate anomalous reflectors at 140 GHz for the application of 6G communication coverage expansion. The reflectors are designed on cyclo-olefin polymer (COP) substrates on the basis of measured complex permittivity and conductivity at millimeter frequencies obtained with the balancedtype circular disk resonator method. From full-wave simulations, we confirm that nearly perfect anomalous reflection characteristics are realized with suppressed parasitic reflections in undesired directions. The simulated efficiencies excluding the material losses of the reflectors designed with the reflection angle θR = 30°, 45°, 60°, and 75° are 99.5%, 98.9%, 99.7%, and 99.8%, respectively. In addition, we fabricate the designed reflectors on the COP substrates and experimentally evaluate their anomalous reflection performances. The measured overall efficiencies of the reflectors with θR = 450, 600, and 750 are 88.0%, 83.6%, and 81.6%, respectively. This is the first demonstration of highly efficient anomalous reflectors in the D-band.
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Next-generation wireless networks are expected to be highly heterogeneous, multilayered, with embedded intelligence at both the core and edge of the network. In such a context, system-level performance evaluation will be very important to formulate relevant insights into tradeoffs that govern such a complex system. Over the past decade, SG has emerged as a powerful analytical tool to evaluate the system-level performance of wireless networks and capture their tendency toward heterogeneity. However, with the imminent onset of this crucial new decade, where global commercialization of fifth generation (5G) is expected to emerge and essential research questions related to beyond 5G (B5G) are intended to be identified, we are wondering about the role that a powerful tool, such as SG, should play. In this article, we first aim to track and summarize the novel SG models and techniques developed during the last decade in the evaluation of wireless networks. Next, we will outline how SG has been used to capture the properties of emerging radio access networks (RANs) for 5G/B5G and quantify the benefits of key enabling technologies. Finally, we will discuss new horizons that will breathe new life into the use of SG in the foreseeable future, for instance, using SG to evaluate performance metrics in the visionary paradigm of molecular communications. Also, we will review how SG is envisioned to cooperate with machine learning that is seen as a crucial component in the race toward ubiquitous wireless intelligence. Another important insight is Grothendieck's toposes, which is considered as a powerful mathematical concept that can help to solve long-standing problems formulated in SG.
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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.
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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.
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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.
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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.
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Reconfigurable intelligent surfaces vs. relaying: Differences, similarities, and performance comparison
  • Di Renzo
Beyond max-SNR: Joint encoding for reconfigurable intelligent surfaces
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