A Reconfigurable Intelligent Surface Using a 2-Bit Programmable Metasurface for Communications

Recently, a reconfigurable intelligent surface (RIS) that provides alternative wireless propagation paths (by reflecting an electromagnetic wave from a base station in desired directions) has attracted attention as a key facilitating technology for Beyond 5G and 6G wireless communications. This paper presents an improved path-loss model suitable for RIS-aided wireless communications. Unlike previously reported RIS path-loss models, the proposed model considers more practical electromagnetic phenomena (e.g., the incident and reflected gain patterns of the RIS unit cell, the effective received power at the RIS, the reflection phase error of each unit cell, and the specular reflection loss), by applying rigorous analysis. Furthermore, the accuracy of the proposed path-loss model is validated by extensive experimental measurements using our developed RIS prototype. The prototype consists of 576 phase-reconfigurable unit cells operating at 29 GHz, each of which features two PIN diodes. Each PIN diode is independently controlled to produce the desired phase distributions on the RIS. It is shown that the calculated received power in the proposed path-loss model matches the measurement results within a 1 dB margin across various practical test environments.

Reconfigurable intelligent surface (RIS) is promising for future 6G wireless communications. However, the increased number of RIS elements results in the high overhead for channel acquisition and the non-negligible power consumption. Therefore, how to improve the system capacity with limited RIS elements is essential. Unlike the classical regular RIS whose elements are arranged on a regular grid, in this paper, we propose an irregular RIS structure to improve the system capacity. The key idea is to irregularly configure a given number of RIS elements on an enlarged surface, which provides extra spatial degrees of freedom compared with the regular RIS. In this way, the received signal power can be enhanced, and thus the system capacity can be improved. Then, we formulate a joint topology and precoding optimization problem to maximize the capacity for irregular RIS-aided communication systems. Accordingly, a joint optimization algorithm with low complexity is proposed to alternately optimize the RIS topology and the precoding design. Particularly, a tabu search-based method is used to design the irregular RIS topology, and a neighbor extraction-based cross-entropy method is introduced to optimize the precoding design. Simulation results demonstrate that, subject to the constraint of limited RIS elements, the proposed irregular RIS can significantly enhance the system capacity.

Reconfigurable intelligent surface (RIS) and inte-grated sensing and communication (ISAC), as two key potential technologies for future sixth generation (6G) wireless commu-nication networks, are jointly studied in this paper. A novel RIS-assisted ISAC system is proposed for the non-line-of-sight (NLOS) communication scenario, where the base station (BS) can simultaneously communicate with the target and estimate its distance and velocity. The RIS can not only provide the line-of-sight (LOS) path for radar sensing, but also improve the quality of communications by concentrating signal power on the target. The maximum likelihood estimation scheme is designed to estimate the distance and velocity. Furthermore, the communication performance of the proposed scheme is evaluated in term of data rate. Simulation results indicate that the proposed joint system can estimate the distance and velocity of the target well, and concurrently achieve required data rate.

Satellite-aerial-terrestrial network (SATN) is considered as a promising architecture for sixth-generation (6G) wireless communication networks to achieve seamless coverage, flexible wireless access, and high data rate. Moreover, non-orthogonal multiple access (NOMA), and reconfigurable intelligent surface (RIS) can significantly increase spectrum and energy efficiency. Recently, the integration of these two technologies and SATN has attracted a lot of attention both in academia and industry. This survey provides a comprehensive overview of RIS-empowered SATN with NOMA. In particular, the rudimentary knowledge of SATN, NOMA scheme, and RIS technology is presented. Then, the motivations for investigating the NOMA-RIS-assisted SATN are discussed. In addition, we introduce the three usage modes of RIS, two scenarios of NOMA-RIS, and the path loss model of NOMA-RIS-assisted SATN. Next, the system performance is analyzed for a case study. Besides, a comprehensive overview of resource allocation in NOMA-RIS-assisted SATN is provided, where theoretical and artificial intelligence-based methods are compared and analyzed. Moreover, physical layer security and covert communication are selected as two representative security techniques to be discussed in NOMA-RIS-aided SATN. Furthermore, the combination of other emerging technologies with NOMA-RIS-assisted SATN is investigated. Finally, this survey provides a detailed discussion of the main challenges and open issues that need to be deeply investigated from a practical point of view, including channel modeling, channel estimation, deployment strategies, and backhaul control.

Reconfigurable Intelligent Surfaces (RIS) are emerging as a key enabler of 6G wireless networks by dynamically controlling and optimizing RF environments. RIS consists of programmable meta- surfaces that enhance signal propagation through beamforming, interference reduction, and energy efficiency. This paper explores the technical foundations of RIS, its role in overcoming limitations of traditional RF systems, and its advantages in 6G use cases. Deployment strategies, including hybrid and collaborative RIS systems, are analyzed to ensure optimal performance. The paper concludes with insights into the future trajectory of RIS technology and its integration into next-generation wireless networks. Keywords – Reconfigurable Intelligent Surfaces, 6G, RF Signal Enhancement, Beamforming, Wireless Networks.

Energy efficiency (EE) is one of the most important metrics for the beyond fifth generation (B5G) and the future sixth generation (6G) wireless networks. Reconfigurable intelligent surface (RIS) has been widely focused on EE enhancement for wireless networks because it is power-saving, programmable, and easy to be deployed. However, RIS is generally passive and thus difficult to obtain corresponding full channel state information (CSI), which severely impacts the EE enhancement of RIS-assisted wireless communications. To solve this problem, in this paper we propose the new single-active-antenna combined RIS transmitter structure, which can replace traditional multiple antennas to reduce hardware cost and power consumption. Based on the single-active-antenna combined RIS structure, we develop the Dumb RIS-Assisted Random Beamforming (Darb)-based Joint RIS-Elements and Transmit-power optimizAtion (Jeta) scheme, where dumb RIS randomly changes its phase shift according to isotropic distribution only depending on the CSI feedback from users to RIS-assisted transmitter. Then, we jointly design the number of RIS elements and optimize the transmit power to maximize the EE of RIS-assisted wireless communications. Simulation results show that compared with the traditional multi-antenna system, our developed Darb-based-Jeta scheme can significantly increase the EE without the full CSI.

Reconfigurable Intelligent Surface (RIS) is one of the promising techniques for 6G wireless communications, and recently has also been shown to be able to improve secure communications. However, there is a “double fading” effect in the reflection link between base station and user, thus passive RIS only achieves a negligible secrecy gain in typical communications scenarios. In this letter, we propose an active RIS-aided multi-antenna physical layer secrecy transmission scheme, where the active RIS can amplify the signal actively. Our aim is to minimize the transmit power subject to the constraint of secrecy rate. To solve the non-convex optimization problem, a penalty-based alternating minimization (AltMin) algorithm is proposed to optimize both the beamformer at the transmitter and the reflection matrix at RIS. Simulation results show that active RIS can resist the impact of “double fading” effect effectively, and is more energy efficient than passive RIS.

In this paper, a novel framework for guaranteeing ultra-reliable millimeter wave (mmW) communications using multiple artificial intelligence (AI)-enabled reconfigurable intelligent surfaces (RISs) is proposed. The use of multiple AI-powered RISs allows changing the propagation direction of the signals transmitted from a mmW access point (AP) thereby improving coverage particularly for non-line-of-sight (NLoS) areas. However, due to the possibility of highly stochastic blockage over mmW links, designing an intelligent controller to jointly optimize the mmW AP beam and RIS phase shifts is a daunting task. In this regard, first, a parametric risk-sensitive episodic return is proposed to maximize the expected bitrate and mitigate the risk of mmW link blockage. Then, a closed-form approximation of the policy gradient of the risk-sensitive episodic return is analytically derived. Next, the problem of joint beamforming for mmW AP and phase shift control for mmW RISs is modeled as an identical payoff stochastic game within a cooperative multi-agent environment, in which the agents are the mmW AP and the RISs. Two centralized and distributed controllers are proposed to control the policies of the mmW AP and RISs. To <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">directly</i> find a near optimal solution, the parametric functional-form policies for the controllers are modeled using deep recurrent neural networks (RNNs). The deep RNN-based controllers are then trained based on the derived closed-form gradient of the risk-sensitive episodic return. It is proved that the gradient update algorithm converges to the same locally optimal parameters as the deep RNN-based centralized and distributed controllers. Simulation results show that the error between the policies of the optimal and the RNN-based controllers is less than 1.5%. Moreover, the variance of the achievable rates resulting from the deep RNN-based controllers is 60% less than the variance of the risk-averse baseline.

Many emerging technologies, such as ultra-massive multiple-input multiple-output (UM-MIMO), terahertz (THz) communications are under active discussion as promising technologies to support the extremely high access rate and superior network capacity in the future sixth-generation (6G) mobile communication systems. However, such technologies are still facing many challenges for practical implementation. In particular, UM-MIMO and THz communication require extremely large number of radio frequency (RF) chains, and hence suffering from prohibitive hardware cost and complexity. In this article, we introduce a new paradigm to address the above issues, namely wireless communication enabled by programmable metasurfaces, by exploiting the powerful capability of metasurfaces in manipulating electromagnetic waves. We will first introduce the basic concept of programmable metasurfaces, followed by the promising paradigm shift in future wireless communication systems enabled by programmable metasurfaces. In particular, we propose two prospective paradigms of applying programmable metasurfaces in wireless transceivers: namely RF chain-free transmitter and space-down-conversion receiver, which both have great potential to simplify the architecture and reduce the hardware cost of future wireless transceivers. Furthermore, we present the design architectures, preliminary experimental results and main advantages of these new paradigms and discuss their potential opportunities and challenges toward ultra-massive 6G communications with low hardware complexity, low cost, and high energy efficiency.

Amplitude‐phase programmable metasurfaces (APPMs) have attracted significant attention in recent years for their real‐time control of electromagnetic (EM) waves. However, existing APPMs often face challenges such as low energy efficiency, limited amplitude‐tuning range, and low phase‐modulation resolution. Additionally, achieving high‐precision amplitude and phase modulations simultaneously requires complex control circuits, thereby increasing the system cost and complexity. To address these challenges, an amplifying APPM (AAPPM) is proposed that achieves independent wide‐range amplitude tuning and high‐precision phase control through a novel and simplified architecture. Specifically, the designed AAPPM is embedded with gain‐tunable amplifiers, achieving over 5 dB EM‐energy amplification and ≈30 dB dynamic amplitude‐tuning range. Meanwhile, the AAPPM provides the 1‐bit phase modulations by controlling the states of integrated PIN diodes. With the introduction of time‐coding techniques, it can further achieve wide‐range and high‐precision phase controls of harmonics. To demonstrate its capability, a 6×6 AAPPM prototype is designed, simulated, and measured. The results indicate that AAPPM can achieve EM energy amplification, wide‐range dynamic amplitude control, and high‐precision phase modulation. By implementing various time‐coding strategies, the AAPPM facilitates flexible harmonic beam manipulations. With these distinctive features, the AAPPM shows great potential in wireless communication and radar systems.

In recent years, wireless use has exploded. 5G is being implemented globally. The development and implementation of 6G have started, with a target date of the 2030s. Current 3G to 5G technologies have opened the path for high data speeds, low latency, and great dependability in wireless communications. There will be continuous change in the key performance indicators (KPIs) to change, recent trends show that wireless communication system architecture concepts need to go beyond the standard. This paper describes 6G wireless communications and networks vision and needs. Finally, several 6G launch issues are discussed. The design enables migration beyond 5G/6G networks. This study also discusses 6G technology and applications. 6G will include Artificial Intelligence enabled services, reconfigurable intelligent surfaces, tera hertz communication and antenna structure with key applications with the challenges faced at tera hertz communication.

Programmable metasurfaces show great application potential in many fields, so studying their radar cross-section reduction (RCSR) characteristics is very important. Metasurfaces based on polarization conversion and artificial magnetic conductors are widely used for RCSR, but they cannot scan beams. Programmable metasurfaces can dynamically steer beams in-band and reduce RCS by changing the phase of electromagnetic waves. But to meet practical needs, they must be stealthy both in-band and out-of-band. This paper presents a low-profile self-stealth programmable metasurface. Self-stealth describes the capability of the metasurface to achieve in-band and out-of-band RCSR through its intrinsic design, without requiring additional structures such as radomes or absorbing materials. The design principle is to achieve out-of-band RCS reduction without increasing the profile height or adding extra radio frequency components. The meta-atom has a positive-intrinsic-negative (PIN) diode to control the in-band ( f 0 ) x -polarization phase. Switching the PIN diode states gives 1-bit phase modulation. The in-band y -polarization uses a bridge design. Changing the size can get 1-bit phase, and the x - and y -polarizations can be adjusted separately. Also, the top layer has 4 cross-shaped dumbbells and 4 branch structures for out-of-band high ( fh 1 ) and low ( fl 1 ) frequencies. The middle layer has four 45° rotated cross-shaped dumbbells and 4 branch structures for out-of-band high ( fh 2 ) and low ( fl 2 ) frequencies. So, the meta-atom can modulate phase across 5 frequency bands ( fl 2 &lt; fl 1 &lt; f 0 &lt; fh 2 &lt; fh 1 ). The profile height is 0.065λ of the in-band center frequency, which is more conducive to conformal design. A 12 × 12 prototype was made. Simulation and measurement show that the metasurface reduces RCS in 5 bands and its in-band radiation can be used in communication transmission mode, which is good for programmable metasurface technology development.

Thanks to the recent advances in metamaterials, reconfigurable intelligent surface (RIS) has emerged as a promising technology for future 6G wireless communications. Benefiting from its high array gain, low cost, and low power consumption, RISs are expected to greatly enlarge signal coverage, improve system capacity, and increase energy efficiency. In this article, we systematically overview the emerging RIS technology with the focus on its key basics, nine fundamental issues, and one critical problem. Specifically, we first explain the RIS basics, including its working principles, hardware structures, and potential benefits for communications. Based on these basics, nine fundamental issues of RISs, such as ``What's the differences between RISs and massive MIMO?'' and ``Is RIS really intelligent?'', are explicitly addressed to elaborate its technical features, distinguish it from existing technologies, and clarify some misunderstandings in the literature. Then, one critical problem of RISs is revealed that, due to the ``multiplicative fading'' effect, existing passive RISs can hardly achieve visible performance gains in many communication scenarios with strong direct links. To address this critical problem, a potential solution called active RISs is introduced, and its effectiveness is demonstrated by numerical simulations.

In this letter, we present a reconfigurable intelligent surface (RIS)-assisted space shift keying (SSK) modulation and reflection phase modulation (RPM) scheme, where RIS embeds its own information in the reflection phase shift of the reflected radio frequency (RF) signal. More specifically, the RIS simultaneously performs two tasks: i) reflecting the impinging RF signal with a discrete phase shift and ii) embedding the information bits in the phase shift. We perform joint detection for the RPM and SSK symbols using a maximum likelihood detector, and a unified analytical framework is presented for theoretical analysis of the average bit error rate (ABER) and ergodic capacity of the proposed scheme. All numerical results are thoroughly verified through Monte Carlo simulation. Numerical results reveal that the proposed scheme outperforms the conventional SSK scheme.

In this paper, we discuss the new challenges and solutions of managing spectrum in 6G wireless communications when unmanned aerial vehicle (UAV) base stations are equipped with and reconfigurable intelligent surface (RIS). UAV-aided communications can help meet the dynamic traffic demand of mobile crowds. However, the extreme quality of service (QoS) requirement in 6G mobile network poses several challenges of low energy-efficient and insufficient link capacity for UAV communications. However, UAV base station cause the coverage overlapping issues in 3D cellular network and face the energy-limited issue. RIS can adjust the phase shift, amplitude, polarization, and frequency of the incident electromagnetic wave to improve spectral efficiency and reduce interference. The other important spectrum management issue in 6G wireless is to design the efficient and effective spectrum sharing mechanism in a co-existing environment with enhanced mobile broadband (eMBB), ultra-reliable low-latency communications (URLLCs) and massive machine-type communications (mMTCs) traffic flows. The state of the art data-driven deployment strategies and learning-based spectrum management techniques for 6G wireless are surveyed in this paper.