Unlocking the Power of Reconfigurable Intelligent Surfaces: From Wireless Communication to Energy Efficiency and Beyond

Abstract

Reconfigurable Intelligent Surfaces (RISs) are a class of metamaterials that have gained significant attention in recent years due to their potential to revolutionize wireless communication, sensing, and imaging technologies. RISs consist of a planar array of closely spaced, subwavelengthsized elements that can manipulate electromagnetic waves in a controllable manner. By reconfiguring the geometry, material properties, or phases of the individual elements on the RIS, the surface can be customized to meet specific application requirements. RISs can improve wireless communication by creating virtual channels, reducing interference, and improving overall quality. They can also enhance the efficiency of energy harvesting systems and improve sensing and imaging technologies by manipulating the propagation and scattering of electromagnetic waves. Additionally, RISs could be used to increase privacy and security by selectively blocking or allowing specific frequencies of electromagnetic waves. In this editorial, we provide a brief history of the development of RISs and discuss the design and fabrication of RIS structures. We also discuss RIS technology’s potential applications and benefits, including improved wireless communication, enhanced energy efficiency, advanced sensing and imaging, and increased privacy and security. Finally, we highlight some current research challenges and future directions for RIS technology. Overall, RISs hold great promise for advancing a wide range of technologies and applications, and we expect to see many exciting developments in this area.

1. Introduction

The demand for faster and more reliable wireless communication systems is constantly increasing, and traditional wireless technologies face challenges in meeting these demands [1]. Reconfigurable Intelligent Surfaces (RISs) have emerged as a potential solution to these challenges [2]. RISs are an innovative technology that uses electromagnetic surfaces to manipulate and control electromagnetic waves, offering a new way to improve wireless communication [3]. With the ability to create multiple virtual channels and selectively block or allow specific frequencies of electromagnetic waves, RISs can significantly enhance the performance of wireless communication systems, increase energy efficiency, and even enhance imaging and sensing technologies [4]. As a result, there has been significant research interest in RISs, and they are being investigated as a promising technology for the next generation of wireless communication systems.

RISs are a class of metamaterials that consist of a planar array of closely spaced, subwavelength-sized elements that can manipulate electromagnetic waves in a controllable manner [2]. These surfaces are designed to modify the propagation and radiation of electromagnetic waves, enabling them to selectively reflect, refract, amplify, attenuate, or even redirect electromagnetic waves in a desired direction [5]. RISs are “intelligent” because they can be programmed to adapt to changing environmental conditions, which makes them highly versatile and applicable to a wide range of wireless communication, sensing, and imaging applications [6]. By reconfiguring the geometry, material properties, or phase of individual elements on the RIS, the surface can be customized to meet specific application requirements [6].

RISs have the potential to revolutionize wireless communication, energy harvesting, sensing and imaging, privacy, and security technologies and applications [7]. The potential applications and benefits of RISs are improved wireless communications, enhanced energy efficiency, advanced sensing and imaging, and increased privacy and security [6].

Regarding wireless communication, RISs can create virtual channels between a transmitter and receiver, effectively increasing the capacity of wireless networks [5]. By selectively reflecting or refracting electromagnetic waves, RISs can also reduce interference, increase coverage, and improve the overall quality of wireless communications [5].

Concerning energy efficiency, RISs can be employed to improve the efficiency of energy harvesting technologies by manipulating the electromagnetic waves that are used to extract energy from the environment [8,9]. By focusing and amplifying these waves, RISs can improve energy conversion efficiency and reduce the size and cost of energy harvesting systems [9].

In terms of sensing and imaging, RISs can be used to enhance the performance of such technologies by manipulating the propagation and scattering of electromagnetic waves. By focusing, redirecting, or amplifying these waves, RISs can improve the resolution, sensitivity, and range of sensing and imaging systems [10].

Regarding privacy and security, RISs can be used to create “smart” environments capable of selectively blocking or allowing specific frequencies of electromagnetic waves [11]. This could enhance privacy and security by preventing unauthorized access to wireless networks, or creating secure areas protected from electromagnetic interference [12].

RISs are a relatively new area of research that has gained significant attention in the last few years. Using electromagnetic surfaces to manipulate and control electromagnetic waves has been around for decades, but reconfigurable surfaces are relatively new [6]. Here is a brief history of the development of RISs:

• In the late 1990s and early 2000s, researchers began investigating the concept of metamaterials, artificially engineered materials that can manipulate electromagnetic waves in unique ways [13]. Metamaterials are composed of structures smaller than the wavelength of the electromagnetic waves they manipulate, allowing them to bend, refract, and even absorb these waves in unusual ways [14]. The earliest form of RIS was the electromagnetic bandgap (EBG) structure, designed to reflect specific frequencies of electromagnetic waves and mainly used to reduce electromagnetic interference in electronic devices [15].
• Around 2012, researchers proposed the concept of using “smart walls” composed of metamaterials to improve wireless communication systems [16,17,18]. These smart walls could be programmed to selectively block or allow certain frequencies of electromagnetic waves, effectively creating a wireless network with minimal interference [19].
• In 2019, researchers proposed the concept of using reconfigurable surfaces to improve wireless communication in indoor environments [20]. They showed that by using RISs, it was possible to create multiple virtual channels between a transmitter and receiver, effectively increasing the capacity of wireless networks [21].
• Since then, the research on RISs has expanded rapidly, with many researchers investigating different aspects of RIS design, fabrication, and application [22]. Today, RISs are being investigated as a potential technology for improving wireless communication, increasing the efficiency of energy harvesting, and even enhancing imaging and sensing technologies [23,24].

This editorial and brief overview is divided as follows. In Section 2, the working principles of RISs are discussed. Next, in Section 3, the modeling and simulation of RISs are presented. In Section 4, the implementation aspects of RISs are debated. Then, in Section 5, an overview of potential RIS applications is presented. In Section 6, an outline of current RIS challenges and future research directions are listed. Finally, in Section 7, this overview is concluded with a summary of the key findings.

2. Principles of RISs

RISs are artificial surfaces that can be electronically controlled to manipulate and control electromagnetic waves. These surfaces consist of an array of sub-wavelength elements that can be controlled individually, so as to change the wavefront of the incident electromagnetic waves. The sub-wavelength elements can be realized by various means, such as patch antennas, metallic patterns, and metamaterials [25].

The structure of an RIS typically consists of a substrate, a ground plane, and an array of sub-wavelength elements. The substrate can be made of different materials, such as glass, silicon, or plastic, providing mechanical support to the RIS [26]. The ground plane is typically a conductive material such as copper or aluminum, providing a reference point for electromagnetic waves. The sub-wavelength elements are placed on top of the substrate and the ground plane, forming an array.

The size and spacing of the sub-wavelength elements determine the frequency range of the electromagnetic waves that the RIS can manipulate. The distance between the sub-wavelength elements should be smaller than the wavelength of the incident electromagnetic waves, in order to enable sub-wavelength phase control. The elements are connected to electronic circuits that can adjust the phase shift of each element, enabling the RIS to control the direction, amplitude, and phase of the incident electromagnetic waves [11].

The structure of the RIS can be flexible or rigid, depending on the application. Flexible RIS structures can be used for conformal and wearable applications, while rigid RIS structures can be used for indoor and outdoor applications [27]. The operation of the RIS is based on the principle of wave interference, which is a fundamental phenomenon in electromagnetics. When two or more electromagnetic waves interact, they can either reinforce or cancel each other out, depending on their relative phase and amplitude [28].

In the case of RISs, the electromagnetic waves are incident on an array of sub-wavelength elements, each of which can be electronically controlled to adjust its phase shift. By adjusting the phase shift of each element, the RIS can control the wavefront of the incident electromagnetic waves and create a specific interference pattern. The interference pattern can be designed to either reinforce or cancel out specific frequencies or directions of the incident waves [11].

The RIS operates based on the principle of constructive and destructive interference. When the phase shift of each element is adjusted to reinforce the incident waves, the waves constructively interfere with each other, leading to a stronger and more focused signal. When the phase shift of each element is adjusted to cancel out specific frequencies or directions of the incident waves, the waves destructively interfere with each other, leading to reduced interference and improved signal quality [29].

The physics behind RIS operation is further supported by Snell’s law, which states that the angle of incidence and the angle of reflection of an electromagnetic wave are related to the refractive index of the material it passes through. By adjusting the phase shift of each element, the RIS can control the refractive index of the material it passes through, effectively manipulating the angle of incidence and the reflection of the incident waves [30].

Designing an RIS requires careful consideration of various factors, including the type of application, the frequency range of operation, the required signal quality, the number of elements in the array, and the power consumption. Next, we discuss some of the key design considerations for RIS.

The frequency range of operation is a critical design consideration for RIS. The size and geometry of the RIS elements should be optimized for the frequency range of interest, so as to achieve the desired phase shift range and response time [30].

The spacing between the RIS’s elements is another vital consideration. The spacing should be chosen to achieve the desired interference pattern and minimize the interference between adjacent elements. The element spacing should be smaller than the wavelength of the incident waves, in order to ensure sub-wavelength diffraction and provide the desired phase shift range [31].

The size of the RIS’s elements should be optimized for the frequency range of operation and the desired phase shift range. The size should be small enough to ensure sub-wavelength diffraction, but not too small to cause excessive losses and reduced efficiency [11].

The power consumption of the RIS should be minimized to reduce the overall system cost and improve efficiency. The RIS should be designed to operate with low power consumption by using low-loss materials, efficient electronics, and optimized control algorithms [32].

The number of elements in the RIS array should be optimized for the desired interference pattern and the frequency range of operation. The more elements in the array, the finer the control over the interference pattern, and the higher the complexity and cost [33].

The RIS’s control algorithms should be optimized so as to achieve the desired interference pattern and minimize power consumption. The algorithms should take into account the location of the transmitter and receiver, the desired signal quality, and the interference from other sources [34].

The fabrication process for the RIS should be optimized to achieve the desired element size, spacing, and shape. The fabrication process should be scalable and cost-effective, with high repeatability and low variability [35].

3. RIS Modeling and Simulation

Simulations are essential to designing RISs and predicting their behavior in different scenarios. Next, we provide a detailed overview of simulation tools and techniques used in RIS design.

Electromagnetic simulation software, such as CST Microwave Studio (Dassault Systems, Velizy-Villacoublay, France), Ansys’s HFSS (Ansys, Canonsburg, PA, USA) and Altair’s FEKO (Altair Engineering Inc., Troy, MI, USA, is used to simulate the behavior of electromagnetic waves in RIS structures. These software tools use numerical methods to solve Maxwell’s equations and predict the electromagnetic field distribution, transmission, reflection, and absorption in RIS structures [36].

Numerical optimization techniques, such as genetic algorithms and particle swarm optimization, can optimize the RIS’s design parameters, such as element size, spacing, and shape, for specific performance metrics, such as phase shift range, radiation pattern, and efficiency. These optimization techniques can speed up the design process and improve the overall performance of the RIS [37].

Ray tracing is a simulation technique that models the propagation of electromagnetic waves using ray paths. It is commonly used to simulate the behavior of electromagnetic waves in complex indoor environments where multiple reflections and diffractions occur. Ray tracing software, such as COMSOL (Burlington, MA, USA) and MATLAB (Natick, MA, USA), can predict the signal strength and quality in different scenarios and optimize the RIS design accordingly [38].

Circuit simulation software, such as SPICE (UC Berkeley, Berkeley, CA, USA) and ADS (PathWave Design, Santa Rosa, CA, USA), can model the electronic circuits and control algorithms used in RISs. These software tools can simulate the behavior of electronic components, such as amplifiers, filters, and switches, and optimize the circuit design for the desired performance metrics, such as power consumption and speed [39].

Machine learning techniques, such as deep learning and reinforcement learning, can be used to predict the behavior of RISs in different scenarios and optimize the control algorithms. These techniques can be learned from data and experience to improve the accuracy and efficiency of RIS control [40].

Each simulation approach has its advantages and disadvantages. Electromagnetic simulation software provides accurate and detailed simulation results, but can be computationally intensive, while numerical optimization techniques provide fast optimization results, but may not provide detailed simulation results. Ray tracing is suitable for simulating complex indoor environments, while circuit simulation is suitable for simulating RIS control algorithms. Machine learning provides fast and accurate simulation results, but requires a lot of training data. Therefore, the optimal simulation approach depends on the specific application, the desired performance metrics, and the available resources [40,41].

Experimental validation is a crucial step in the development of RIS technology. It involves verifying the performance of RIS devices under real-world conditions and comparing the results with theoretical predictions obtained from simulations. The following are some of the experimental validation techniques commonly used for RISs.

Antenna measurements are used to verify the radiation characteristics of RIS devices. The RIS device is placed in an anechoic chamber, and the radiation pattern is measured using a calibrated antenna. The measured radiation pattern is compared with the simulation results to validate the RIS’s design [42].

Channel sounding involves measuring the wireless channel response between a transmitter and receiver in the presence of RIS devices. The RIS device is placed in the wireless channel, and the channel response is measured using a channel sounder. The measured channel response is compared to the simulation results in order to to validate the RIS’s design [43].

Beamforming measurements are used to verify the beamforming capabilities of RIS devices. The RIS device is placed in the wireless channel, and the beamforming pattern is measured using a calibrated antenna array. The measured beamforming pattern is compared with the simulation results to validate the RIS’s design [34].

Energy harvesting measurements are used to verify the energy harvesting capabilities of RIS devices. The RIS device is placed in a simulated energy-harvesting environment, and the amount of harvested energy is measured using a power meter. The measured energy harvesting results are compared with the simulation results to validate the RIS’s design [44].

Imaging and sensing measurements verify RIS devices’ imaging and sensing capabilities. The RIS device is placed in a simulated imaging or sensing environment, and the image or sensing results are measured using an imaging or sensing system. The measured results are compared with the simulation results to validate the RIS’s design [23,45].

4. RIS Implementation

RISs are typically fabricated using traditional microfabrication techniques and novel material synthesis methods. Next, we describe some of the most commonly used fabrication techniques for RIS.

Electron beam lithography (EBL) is a high-resolution patterning technique used to create the complex geometries required for RIS devices. A beam of electrons is focused onto a substrate coated with a resist material, and the pattern is defined by selectively exposing the resist material to the electrons. The exposed resist is then etched away, leaving behind the desired pattern [46].

Photolithography is a widely used patterning technique in microfabrication. It involves using light to transfer a pattern from a photomask onto a photosensitive substrate. The pattern is then etched into the substrate using wet or dry etching techniques [47].

Chemical vapor deposition (CVD) is a technique used to deposit thin films of materials onto a substrate. The process involves introducing precursor gases into a reaction chamber, which then reacts to form a solid film on the substrate. CVD is commonly used to deposit metal and dielectric films onto RIS devices [48].

Atomic layer deposition (ALD) is a thin film deposition technique that precisely controls film thickness and composition. The process involves sequentially exposing a substrate to alternating pulses of precursor gases. Each pulse chemically adsorbs onto the substrate, allowing precise control of the deposited film thickness [49].

Inkjet printing is a printing technique that deposits small ink droplets onto a substrate. In the case of RIS devices, inkjet printing is used to deposit conductive and dielectric inks onto the substrate to create the required RIS structure [50]. Three-dimensional printing is an additive manufacturing technique that can fabricate RIS devices with complex geometries. This technique creates a 3D model of the RIS’s structure, and the printer deposits the material layer by layer in order to create the final structure [51].

Overall, the choice of RIS fabrication technique depends on the specific requirements of the RIS device. Each technique has its advantages and disadvantages regarding cost, complexity, scalability, and performance. In general, the choice of fabrication technique depends on the specific application requirements and constraints [51]. Printed circuit board (PCB) fabrication is a popular and mature technique that is relatively low-cost and can produce high-performance RISs with good reproducibility [51]. However, it is limited in terms of design flexibility and scalability. Metamaterial-based techniques, such as photo-lithography and nano-imprint lithography, offer high design flexibility and scalability, but are generally more complex and expensive. Three-dimensional printing is a relatively new technique gaining interest in RIS fabrication due to its high design flexibility and rapid prototyping capabilities. However, the material properties and performance of 3D-printed RISs may not be as good as those produced by other techniques [51]. Overall, the choice of fabrication technique depends on several factors, including the required performance, design flexibility, scalability, and cost.

Experimental implementations of RISs have been conducted in various applications, ranging from wireless communication systems to sensing and imaging. One common approach for experimental RIS implementations is to use a prototype system that consists of a transmitter, an RIS, and a receiver. The RIS is usually designed and fabricated based on a specific application requirement, and its performance is evaluated experimentally by measuring the signal quality and strength at the receiver [23].

RISs have a significantly improved signal quality and coverage area in wireless communication systems. For example, an experimental RIS system was used to create a virtual environment with multiple signal paths that can improve the performance of a wireless network in indoor environments [52]. Another experiment showed that an RIS between a transmitter and a receiver could act as a relay and increase the signal strength and coverage area [53].

In sensing and imaging applications, RISs have been used to enhance the resolution and accuracy of the imaging system. For example, an RIS can be designed and fabricated as a lens or a mirror that can focus or reflect electromagnetic waves. In one experiment, an RIS enhanced the resolution of a synthetic aperture radar (SAR) imaging system by improving the signal-to-noise ratio and reducing the imaging artifacts [54].

5. RIS Applications

RISs have a wide range of potential applications due to their ability to manipulate and control electromagnetic waves. One of the most promising areas for RIS application is wireless communication. RISs can be used to improve wireless communication systems by increasing signal quality, coverage area, and data rate. By creating a virtual environment with multiple signal paths, RISs can improve the performance of a wireless network in indoor environments where signal strength and quality can be compromised [2,55].

Another potential application of RISs is in sensing and imaging. An RIS can be designed and fabricated to act as a lens or a mirror that can focus or reflect electromagnetic waves, enhancing the resolution and accuracy of imaging and sensing systems. RISs can also be used to improve the signal-to-noise ratio and reduce imaging artifacts in synthetic aperture radar (SAR), computerized tomography (CT), and magnetic resonance imaging (MRI) systems. In those applications, RISs act as lenses or mirrors that can focus or reflect electromagnetic waves, leading to higher resolution and accuracy [56].

RISs can also be used to improve the efficiency of energy harvesting systems by optimizing the absorption and reflection of electromagnetic waves. By absorbing specific frequencies of electromagnetic waves, such as solar radiation or radio waves, RISs can convert them into usable energy, making energy harvesting systems more efficient [57].

Another potential application of RISs is in security and defense. RISs can be designed to selectively block or reflect specific frequencies of electromagnetic waves, such as radar waves or thermal radiation. By doing so, RISs can create invisible barriers or cloaking devices, making objects or areas invisible or undetectable and improving security and defense systems [58,59].

Finally, RISs can be used to improve the performance, energy efficiency, and coverage area of Internet of Things (IoT) networks. By creating virtual environments with multiple signal paths, RISs can increase IoT devices’ reliability and data rate and reduce energy consumption, improving their performance and coverage area [60,61].

RISs have the potential to revolutionize many industries and fields due to their ability to control and manipulate electromagnetic waves. In addition to the applications discussed earlier, several other potential RIS applications are worth exploring. One such application is in 5G wireless networks, as well as those beyond 5G [3]. RISs can be strategically placed to enhance signal coverage, reduce interference, and increase the network’s capacity, leading to improved performance and faster data transfer rates [62]. RISs can also be used in the automotive and transportation industry to improve safety and efficiency [63]. By selectively blocking or reflecting specific frequencies of electromagnetic waves, such as radar waves, RISs can improve the accuracy of collision avoidance systems and reduce accidents [64].

Agriculture and environmental monitoring systems can also benefit from RIS technology [65]. By optimizing the absorption and reflection of specific frequencies of electromagnetic waves, such as sunlight and radio waves, RISs can increase solar panels’ efficiency and improve environmental sensors’ accuracy [66].

With continuous technological advancement and the increasing demand for more efficient and reliable systems, there is great potential for future RIS applications [67]. One potential future application of RISs is in the field of space exploration. RISs can enhance spacecrafts’ communication and navigation systems by improving signal quality and reducing interference [68]. RISs can also shield spacecraft from radiation and other harmful electromagnetic waves, improving space missions’ safety and efficiency [69].

Another potential future application of RISs is in the field of robotics. RISs can be used to improve the sensing and control systems of robots by enhancing the resolution and accuracy of imaging and sensing technologies [60]. RISs can also be used to create virtual environments with multiple signal paths that can improve the performance of robots in complex environments [70,71].

RISs can also be used in manufacturing to improve the efficiency and quality of production processes. RISs can be designed and placed strategically to optimize the absorption and reflection of specific frequencies of electromagnetic waves, which can improve the performance of industrial equipment and reduce energy consumption [69].

Moreover, RISs can also be used in entertainment and media to enhance consumers’ viewing and listening experience. RISs can be designed to act as a screen or a speaker that can manipulate and control electromagnetic waves, improving the quality and immersion of audiovisual content [72]. Finally, RISs can also be used in education and research to improve the performance of scientific instruments and equipment. RISs can be designed and fabricated to act as a lens or a mirror that can focus or reflect electromagnetic waves, improving the resolution and accuracy of scientific imaging and sensing technologies [73].

6. Challenges and Future Directions

Several challenges need to be addressed for the widespread adoption of RISs. The current challenges of RISs are design complexity, fabrication complexity, integration with existing systems, power consumption, robustness and reliability, and regulatory and safety issues, which are detailed next.

The design of RIS structures can be complex and time-consuming, requiring advanced simulation tools and techniques. The optimization of RIS structures for specific applications is also challenging, and there is a need for more efficient and automated design methods [74]. The fabrication of RIS structures can also be challenging, requiring advanced techniques such as nanofabrication or 3D printing. The scalability and cost-effectiveness of RIS fabrication methods are also essential considerations [75].

RISs need to be integrated with existing wireless communication or sensing systems, which can be challenging due to compatibility issues or the need for additional hardware. RISs also need to be compatible with different wireless communication standards and protocols, which can add to the complexity of system integration [76]. The operation of RIS structures requires power, and the power consumption can be significant, especially for large-scale RIS applications. There is a need for more efficient power management methods and low-power RIS designs [32].

RIS structures can be susceptible to environmental factors such as temperature, humidity, and mechanical stress, affecting their performance and reliability. There is a need for more robust and reliable RIS designs that can withstand different environmental conditions [70]. RISs may need to comply with regulatory standards related to electromagnetic interference and safety. There is a need for more research and development on the safety and regulatory aspects of RIS applications [61].

There are several potential solutions to the current challenges facing RIS technology. One solution is to improve the design and optimization of RIS structures. This can be achieved by using advanced simulation tools and optimization algorithms that can accurately model the behavior of electromagnetic waves and optimize the RIS structure for specific applications. Machine learning and artificial intelligence techniques can also be used to develop intelligent RIS systems that can adapt to changing environmental conditions and optimize their performance [77].

Another potential solution is to improve the fabrication techniques for RIS structures. This can be achieved by developing new materials and manufacturing processes that can produce complex and high-performance RIS structures at a low cost. 3D printing and nanofabrication techniques can also produce customized RIS structures with high precision and accuracy [78].

In addition, the collaboration between researchers in different fields can help to address the current challenges facing RIS technology. Collaboration between electromagnetics, material science, and manufacturing experts can lead to developing new RIS structures and fabrication techniques optimized for specific applications [79]. Collaboration between researchers and industry can also help bridge the gap between research and commercialization, leading to the development of practical RIS systems for various applications [80].

Finally, government support and funding can be critical in developing and commercializing RIS technology. Government support can provide researchers and companies with the resources needed to develop and test new RIS systems and support for developing standards and regulations that can help ensure RIS technology’s safety and reliability. Government funding can also help accelerate the commercialization of RIS technology by providing financial support for research and development and the production and deployment of RIS systems in real-world applications [81].

Future RIS research can be categorized into two main directions: improving the performance and capabilities of RISs and exploring new applications and domains for RISs. In terms of improving RISs’ performance and capabilities, research efforts can focus on addressing RIS technology’s current challenges. One approach is to develop new materials and fabrication techniques that enable high-performance and low-cost RISs. For example, using advanced nanomaterials such as graphene and carbon nanotubes can improve RIS properties, such as higher efficiency, larger bandwidth, and better controllability [82]. Additionally, developing new simulation and optimization tools can help optimize the design and operation of RISs for specific applications [74].

Another way to improve RISs is to investigate new concepts and architectures beyond the current RIS designs. For example, multi-functional RISs that can perform multiple functions such as sensing, imaging, and communication can significantly enhance the versatility and value of RISs [83]. In addition, incorporating artificial intelligence and machine learning techniques can enable RISs to adapt to changing environments and optimize their performance in real time [56].

On the other hand, exploring new applications and domains for RISs can lead to exciting and innovative uses of this technology. For example, RISs can be used in the emerging field of terahertz communications to enable high-speed and high-capacity wireless communication at terahertz frequencies [84,85]. RISs can also be used in space applications such as satellite communication and sensing, where the high radiation environment and harsh operating conditions require robust and reliable technologies [86,87].

Furthermore, RISs can be used in emerging smart cities and infrastructure areas. They can enhance the performance and efficiency of various systems such as transportation, energy, and communication [88]. RISs can also be used in the field of medicine and healthcare, where they can improve the accuracy and resolution of medical imaging and sensing systems [89,90].

7. Conclusions

RISs have recently gained significant attention in wireless communication and beyond, due to their ability to manipulate and control electromagnetic waves. This review covered a broad range of topics related to RIS, including their operation principles, simulation techniques, fabrication methods, experimental implementations, and potential applications.

Regarding operation principles, it was discussed how RISs can reflect, absorb, and re-transmit electromagnetic waves to achieve desired signal propagation characteristics. Simulation tools and techniques were also presented, including analytical, numerical, and experimental methods. It was noted that the choice of simulation technique depends on the problem’s complexity and the desired accuracy level.

Several fabrication techniques were described, including planar and non-planar approaches. Planar techniques are less expensive and easier to fabricate, while non-planar techniques allow for more complex and flexible designs.

The review also presented a range of potential applications for RISs, including wireless communication, sensing and imaging, energy harvesting, security and defense, IoT, automotive and transportation, medical and healthcare, agriculture and environment, and smart homes and buildings. It was noted that RISs have the potential to improve the performance and efficiency of these systems significantly.

However, several challenges currently exist in developing and implementing RIS technology, including cost, scalability, and integration with existing systems. To address these challenges, potential solutions were discussed, such as developing low-cost fabrication techniques, integrating RISs with existing wireless infrastructure, and using machine learning algorithms to optimize RISs’ performance.

Overall, this review highlights the potential of RIS technology in a wide range of applications and the need for continued research to overcome current challenges and fully realize their potential.

RISs have the potential to revolutionize a wide range of technologies due to their ability to manipulate and control electromagnetic waves. Some potential applications of RISs include improving wireless communication systems, enhancing imaging and sensing systems, improving the efficiency of energy harvesting systems, improving security and defense systems, and enhancing the performance of IoT networks. Additionally, RISs can improve the performance of 5G wireless networks (as well as those beyond 5G), improve the safety and efficiency of automotive and transportation systems, enhance medical imaging and sensing technologies, improve the efficiency and sustainability of agriculture and environmental monitoring systems, and enhance the performance and energy efficiency of smart home and building systems. However, there are also several challenges to overcome, such as efficient and cost-effective fabrication techniques, optimization of RIS designs for specific applications, and integration with existing technologies. Potential solutions include developing new fabrication techniques, using artificial intelligence to optimize RIS designs, and collaborating with experts in other fields to exploit the potential of RISs fully. Overall, the impact of RISs on future technologies could be significant, enabling a wide range of previously impossible or impractical applications.

RISs have emerged as a promising technology for various communication, sensing, energy, security, and IoT applications. However, several challenges need to be addressed to fully realize the potential of RISs, such as fabrication complexity, control complexity, and scalability.

To overcome these challenges, future RIS research should focus on developing more efficient and cost-effective fabrication techniques, advanced control algorithms for real-time operation, and scalable solutions for large-scale deployment. Moreover, interdisciplinary collaborations among researchers from different fields, such as materials science, electrical engineering, computer science, and physics, are essential to accelerate RIS research and development.

Furthermore, future RIS research should also consider the impact of RISs on society, including ethical and regulatory implications, as RISs may have significant implications for privacy, security, and social equity. Therefore, it is crucial to involve stakeholders from diverse backgrounds in RIS research, including industry, academia, government, and civil society, to ensure the responsible and sustainable development of RISs.

Finally, the potential impact of RISs on future technologies is immense, and the ongoing research and development in this field hold great promise for advancing various fields and improving our daily lives. With continued efforts and collaborations, RISs can potentially revolutionize how we communicate, sense, and interact with the world around us.