Scholars are expected to continue enhancing the depth and breadth of theoretical research on reconfigurable intelligent surface (RIS) to provide a higher theoretical limit for RIS engineering applications. Notably, significant advancements have been achieved through both academic research breakthroughs and the promotion of engineering applications and industrialization. We provide an overview of RIS engineering applications, focusing primarily on their typical features, classifications, and deployment scenarios. Furthermore, we systematically and comprehensively analyze the challenges faced by RIS and propose potential solutions including addressing the beamforming issues through cascade channel decoupling, tackling the effects and resolutions of regulatory constraints on RIS, exploring the network-controlled mode for RIS system architecture, examining integrated channel regulation and information modulation, and investigating the use of the true time delay (TTD) mechanism for RIS. In addition, two key technical points, RIS-assisted non-orthogonal multiple access (NOMA) and RIS-based transmitter, are reviewed from the perspective of completeness. Finally, we discuss future trends and challenges in this field.
Simultaneously transmitting and reflecting reconfigurable intelligent surfaces (STAR-RISs) have been attracting significant attention in both academia and industry for their advantages of achieving 360◦ coverage and enhanced degrees-of-freedom. This article first identifies the fundamentals of STAR-RIS, by discussing the hardware models, channel models, and signal models. Then, three representative categorizing approaches for STAR-RISs are introduced from the phase-shift, directional, and energy consumption perspectives. Furthermore, the beamforming design of STAR-RISs is investigated for both independent and coupled phase-shift cases. As a recent advance, a general optimization framework, which has high compatibility and provable optimality regardless of the application scenarios, is proposed. As a further advance, several promising applications are discussed to demonstrate the potential benefits of applying STAR-RISs in sixth-generation wireless communication. Lastly, a few future directions and research opportunities are highlighted.
Complex beams play important roles in wireless communications, radar, and satellites, and have attracted great interest in recent years. In light of this background, we present a fast and efficient approach to realize complex beams by using semidefinite relaxation (SDR) optimization and amplitude-phase digital coding metasurfaces. As the application examples of this approach, complex beam patterns with cosecant, flat-top, and double shapes are designed and verified using full-wave simulations and experimental measurements. The results show excellent main lobes and low-level side lobes and demonstrate the effectiveness of the approach. Compared with previous works, this approach can solve the complex beam-forming problem more rapidly and effectively. Therefore, the approach will be of great significance in the design of beam-forming systems in wireless applications.
Reconfigurable intelligent surface (RIS) is widely accepted as a potential technology to assist in communication between base stations (BSs) and users in edge areas. We study the energy efficiency of a RIS-assisted multi-cell communication system with a realistic RIS power consumption model. With the goal of maximizing the energy efficiency of the system, we optimize the transmit beamforming vectors at the BS and the RIS phase shift matrix by a proposed alternative optimization algorithm. First, the transmit beamforming vector is optimized by solving the transformed weighted minimum mean square error (WMMSE) problem. Subsequently, to solve the inconvenience incurred by the discrete relationship between the RIS reflecting unit power consumption and its discrete phase shift, we use a continuous function to approximate their relationship. With this approximation, we can use the majorization minimization (MM) technique to optimize the continuous RIS phase shifts, and then quantize the obtained phase shifts to discrete ones. Simulation results demonstrate that the energy efficiency of the system is effectively optimized by the proposed algorithm.
The use of a reconfigurable intelligent surface (RIS) in the enhancement of the rate performance is considered to involve the limitation of the RIS being a passive reflector. To address this issue, we propose a RIS-aided amplify-and-forward (AF) relay network in this paper. By jointly optimizing the beamforming matrix at AF relay and the phase-shift matrices at RIS, two schemes are put forward to address a maximizing signal-to-noise ratio (SNR) problem. First, aiming at achieving a high rate, a high-performance alternating optimization (AO) method based on Charnes–Cooper transformation and semidefinite programming (CCT-SDP) is proposed, where the optimization problem is decomposed into three subproblems solved using CCT-SDP, and rank-one solutions can be recovered using Gaussian randomization. However, the optimization variables in the CCT-SDP method are matrices, leading to extremely high complexity. To reduce the complexity, a low-complexity AO scheme based on Dinkelbachs transformation and successive convex approximation (DT-SCA) is proposed, where the variables are represented in vector form, and the three decoupling subproblems are solved using DT-SCA. Simulation results verify that compared to three benchmarks (i.e., a RIS-assisted AF relay network with random phase, an AF relay network without RIS, and a RIS-aided network without AF relay), the proposed CCT-SDP and DT-SCA schemes can harvest better rate performance. Furthermore, it is revealed that the rate of the low-complexity DT-SCA method is close to that of the CCT-SDP method.
An array’s degree of freedom (DoF) determines the number of jamming incidents that can be managed and the antijamming performance. Conventional arrays can improve the DoF only by increasing the number of antennas. On the other hand, when the received signal is digitized, high-power jamming will reduce the number of bits used to represent the desired signal, further increasing the difficulty of back-end antijamming based on digital signal processing. In this paper, we propose a joint radio frequency (RF) front-end and digital back-end antijamming scheme based on a metasurface antenna array. The metasurface antennas can rapidly switch patterns when receiving signals, so that a single channel can be equivalent to multiple channels and increase the DoF. We use independent component analysis to estimate the channel and then optimize the array parameters under the minimum signal-tojamming ratio constraint of each antenna. The proposed scheme works well under high-power jamming conditions by suppressing jamming at the RF front end and using a low-precision analog-to-digital converter. Simulation results show that the proposed scheme reduces the bit error rate of the received signals by one order of magnitude compared with the conventional array.
This research investigates the digital-to-analog converter (DAC) free architecture for the digital reconfigurable intelligent surface (RIS) system, where the transmission lines are implemented for reflection coefficient (RC) control to reduce power consumption. In the proposed architecture, the radio frequency (RF) switch based phase shifter is considered. By using a single-pole four-throw (SP4T) switch to simultaneously control the RCs of a group of elements, a 2-bit phase shifter is realized for passive beam steering. A novel modulation scheme is developed to explore the cost effectiveness, which approaches the performance of traditional quadrature amplitude modulation (QAM). Specifically, to overcome the limitation of the phase shift bits, joint frequency-shift and phase-rotation operations are applied to the constellation points. The simulation and experimental results demonstrate that the proposed architecture is capable of providing an ideal transmission performance. Moreover, 64- and 256-QAM modulation schemes could be implemented by expanding the elements and phase bits.
Reconfigurable intelligent surfaces (RISs) have the capability to change the wireless environment smartly. Considering the attenuation of subchannels and crowding users involved in the wideband system, we introduce RISs into the multi-user multi-input single-output (MU-MISO) system with orthogonal frequency division multiplexing (OFDM) for performance enhancement. Maximizing the minimum rate of dense users in an MU-MISO-OFDM system assisted by RIS with an approximate practical model is formulated as the joint optimization problem involving subcarrier allocation, transmit precoding (TPC) matrices at the base station, and RIS passive beamforming. A coalition-game subcarrier allocation (CSA) algorithm is proposed to solve space–frequency resource allocation on subcarriers, which reforms the interference topology among dense users. Fractional programming and convex optimization method are used to optimize the TPC matrices and the RIS passive beamforming, which improves the spectral efficiency synthetically across all subchannels in the wideband system. Simulation results indicate that the CSA algorithm provides a significant gain for dense users. Besides, the proposed joint optimization method shows the considerable advantage of the RISs in the MU-MISO-OFDM system.
The wavefront control of spin or orbital angular momentum (OAM) is widely applied in the optical and radio fields. However, most passive metasurfaces provide limited manipulations, such as the spin-locked wavefront, a static OAM combination, or an uncontrollable OAM energy distribution. We propose a reflection-type multi-feed metasurface to independently generate multi-mode OAM beams with dynamically switchable OAM combinations and spin states, while simultaneously, the energy distribution of carrying OAM modes is controllable. Specifically, four elements are proposed to overcome the spin-locked phase limitation by combining propagation and geometric phases. The robustness of these elements is analyzed. By involving the amplitude term and multi-feed technology in the design process, the proposed metasurface can generate OAM beams with a controllable energy distribution over modes and switchable mode combinations. OAM-based radio communication with four independent channels is experimentally demonstrated at 14 GHz by employing a pair of the proposed metasurfaces. The powers of different channels are adjustable by the provided amplitude term, and the maximum crosstalk is −9 dB, proving the effectiveness and practicability of the proposed method.
Due to the openness of the wireless propagation environment, wireless networks are highly susceptible to malicious jamming, which significantly impacts their legitimate communication performance. This study investigates a reconfigurable intelligent surface (RIS) assisted anti-jamming communication system. Specifically, the objective is to enhance the system’s anti-jamming performance by optimizing the transmitting power of the base station and the passive beamforming of the RIS. Taking into account the dynamic and unpredictable nature of a smart jammer, the problem of joint optimization of transmitting power and RIS reflection coefficients is modeled as a Markov decision process (MDP). To tackle the complex and coupled decision problem, we propose a learning framework based on the double deep Q-network (DDQN) to improve the system achievable rate and energy efficiency. Unlike most power-domain jamming mitigation methods that require information on the jamming power, the proposed DDQN algorithm is better able to adapt to dynamic and unknown environments without relying on the prior information about jamming power. Finally, simulation results demonstrate that the proposed algorithm outperforms multi-armed bandit (MAB) and deep Q-network (DQN) schemes in terms of the anti-jamming performance and energy efficiency.
Physical layer key generation (PKG) technology leverages reciprocal channel randomness to generate shared secret keys. However, multipath fading at the receiver may degrade the correlation between legitimate uplink and downlink channels, resulting in a low key generation rate (KGR). In this paper, we propose a PKG scheme based on the pattern-reconfigurable antenna (PRA) to boost the secret key capacity. First, we propose a reconfigurable intelligent surface (RIS) based PRA architecture with the capability of flexible and reconfigurable antenna patterns. Then, we present the PRA-based PKG protocol to improve the KGR via mitigation of the effects of multipath fading. Specifically, a novel algorithm for estimation of the multipath channel parameters is proposed based on atomic norm minimization. Thereafter, a novel optimization method for the matching reception of multipath signals is formulated based on the improved binary particle swarm optimization (BPSO) algorithm. Finally, simulation results show that the proposed scheme can resist multipath fading and achieve a high KGR compared to existing schemes. Moreover, our findings indicate that the increased degree of freedom of the antenna patterns can significantly increase the secret key capacity.
As a candidate technique to achieve sixth-generation wireless communication (6G), reconfigurable intelligent surface (RIS) has become popular in both academia and industry. For better exploration of the advantages of RIS, we compare the performances of RIS and network-controlled repeater (NCR) in 3GPP release-18. We first theoretically analyze the received signal power and signal-to-noise ratio performances for both RIS and NCR. Then, we simulate the reference signal received power and signal-to-interference-and-noise ratio performances at the system level for both RIS and NCR in the frequency range 1 and frequency range 2 bands. Finally, several insights on engineering applications are given based on the comparison between RIS and NCR.