The reconfigurable intelligent surface (RIS) has attracted significant research interests recently due to its abilities of dynamic channel reconstruction, flexible deployment and reduced power ...consumption. However, a malicious RIS can introduce serious signal degradation and even interception risk. This paper investigates destructive beamforming design from the perspective of a malicious RIS, where the RIS is active and able to amplify the reflected signals from the base station (BS) to an internet-of-things device (IoTD). We consider two scenarios where the BS is known and unknown to the identity of malicious RIS, and the objective is to minimize the received signal-to-noise-ratio (SNR) at the IoTD with the constraints of total power budget and RIS signal amplification. To solve the above nonconvex optimization problem, we firstly propose a low-complexity scheme by integrating several classical beamforming methods with the Taylor expansion approach to solve the original problem for the case of known malicious RIS at BS. While for the unknown malicious RIS case, we propose an alternating optimization scheme by using the successive convex approximation method to obtain the beamforming vector and reflection coefficient matrix iteratively. Finally, numerical results verify that, through the proposed destructive beamforming design, the RIS only brings pain without gain for the signal reception.
Reconfigurable intelligent surfaces (RISs), especially the simultaneous transmitting and reflecting RISs (STAR-RISs), can be integrated with multiple-input mutiple output (MIMO) and non-orthogonal ...multiple access (NOMA) to improve coverage and save power usage. In this letter, we investigate a RIS-MIMO-NOMA system in a certain scenario, with focus on the system's power consumption. To be specific, instead of the traditional clustering-based inter-user NOMA, inter-group NOMA is proposed to reduce total transmit power of RIS-MIMO-NOMA system. In order to solve the power minimization problem, the beamforming and phase shifts optimization are jointly designed. To address the beamforming optimization problem, the MMSE filters are exploited and iterative method is adopted, and in order to optimize phases, sequential rotation (SR) scheme is proposed. The numerical simulation results reveal the connection between the system's parameters and overall transmit power, and show that for RIS-MIMO-NOMA system, the novel inter-group NOMA scheme is better than traditional clustering-based inter-user NOMA scheme in the total power consumption.
Reconfigurable intelligent surface (RIS)-assisted communication have recently attracted the attention of the wireless communication community as a potential candidate for the next 6-th generation ...(6G) of wireless networks. Various studies have been carried out on the RIS technology, which is capable of enabling the control of the signal propagation environment by network operators. However, when an RIS is used in its inherently passive structure, it appears to be only a supportive technology for communications, while suffering from a multiplicative path loss. Therefore, researchers have lately begun to focus on RIS hardware designs with minimal active elements to further boost the benefits of this technology. In this paper, we present a simple RIS hardware architecture including a single and variable gain amplifier for reflection amplification to confront the multiplicative path loss. The end-to-end signal model for communication systems assisted with the proposed amplifying RIS design is presented, together with an analysis focusing on the capacity maximization and theoretical bit error probability performance, which is corroborated by computer simulations. In addition, the major advantages of the proposed amplifying RIS design compared to its passive counterpart are discussed. It is shown that the proposed RIS-based wireless system significantly eliminates the double fading problem appearing in conventional passive RIS-assisted systems and improves the communication energy efficiency.
Reconfigurable intelligent surfaces (RIS) constitute a promising technology for future wireless communications in terms of improving the spectral-efficiency and energy-efficiency. In this context, a ...novel RIS structure, which we refer to as coordinated RIS architecture, is formulated, where different RIS elements can be connected by configurable impedances to eliminate the channel fading. In the proposed RIS architecture, both the RIS element connection pattern and the configurable impedances can be optimized, based on the channel state information (CSI). The proposed architecture exhibits higher optimization flexibility than the state-of-the-art single-connected RIS architecture and group-connected RIS architecture, where only the configurable impedances can be optimized. Specifically, when considering base stations (BS) having a single antenna, the maximal ratio combining (MRC) criterion may be harnessed for designing the RIS element connection pattern, while in the case of multiple BS antennas, the alternating optimization algorithm may be employed for iteratively optimizing the BS's active beamforming vector and the RIS's passive beamforming matrix. Our numerical results show that the proposed coordinated RIS architecture achieves higher power gain than the group-connected RIS architecture having the same number of configurable impedances. Furthermore, the power gain in our proposed RIS architecture tends to that of the fully-connected architecture upon increasing the number of RIS elements, while requiring significantly fewer configurable impedances.
Simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) is a promising implementation of RIS-assisted systems that enables full-space coverage. However, STAR-RIS as ...well as conventional RIS suffer from the double-fading effect. Thus, in this paper, we propose the marriage of active RIS and STAR-RIS, denoted as ASTARS for massive multiple-input multiple-output (mMIMO) systems, and we focus on the energy splitting (ES) and mode switching (MS) protocols. Compared to prior literature, we consider the impact of correlated fading, and we rely our analysis on the two timescale protocol, being dependent on statistical channel state information (CSI). On this ground, we propose a channel estimation method for ASTARS with reduced overhead that accounts for its architecture. Next, we derive a closed-form expression for the achievable sum-rate for both types of users in the transmission and reflection regions in a unified approach with significant practical advantages such as reduced complexity and overhead, which result in a lower number of required iterations for convergence compared to an alternating optimization (AO) approach. Notably, we maximize simultaneously the amplitudes, the phase shifts, and the active amplifying coefficients of the ASTARS by applying the projected gradient ascent method (PGAM). Remarkably, the proposed optimization can be executed at every several coherence intervals that reduces the processing burden considerably. Simulations corroborate the analytical results, provide insight into the effects of fundamental variables on the sum achievable SE, and present the superiority of ASTARS compared to passive STAR-RIS for a practical number of surface elements.
Reconfigurable intelligent surfaces (RISs) have attracted the attention of academia and industry circles because of their ability to control the electromagnetic characteristics of channel ...environments. However, it has been found that the introduction of an RIS may bring new and more serious network coexistence problems. It may even further deteriorate the network performance if these new network coexistence problems cannot be effectively solved. In this paper, an RIS network coexistence model is proposed and discussed in detail, and these problems are deeply analysed. Two novel RIS design mechanisms, including a novel multilayer RIS structure with an out-of-band filter and an RIS blocking mechanism, are further explored. Finally, numerical results and a discussion are given.
In this study, we develop an active reconfigurable intelligent surface (RIS)-assisted multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) prototype compliant with ...the 5G New Radio standard at 3.5 GHz. The experimental results clearly indicate that active RIS plays a vital role in enhancing MIMO performance, surpassing passive RIS. Furthermore, when considering factors such as complexity, energy consumption, and performance, the comparative evaluation between passive RIS and active RIS reinforces the critical role of active RIS in MIMO systems. These findings underscore the practical significance of active RIS in improving MIMO gain in 5G scenarios.
Recently, reconfigurable intelligent surfaces (RIS) have attracted a lot of attention due to their capability of extending cell coverage by reflecting signals toward the receiver. In this letter, we ...analyze the coverage of a downlink RIS-assisted network with one base station (BS) and one user equipment (UE). Since the RIS orientation and the horizontal distance between the RIS and the BS have a significant influence on the cell coverage, we formulate an RIS placement optimization problem to maximize the cell coverage by optimizing the RIS orientation and horizontal distance. To solve the formulated problem, a coverage maximization algorithm (CMA) is proposed, where a closed-form optimal RIS orientation is obtained. Numerical results verify our analysis.
In this paper, we propose and study a multi-functional reconfigurable intelligent surface (MF-RIS) architecture. In contrast to conventional single-functional RIS (SF-RIS) that only reflects signals, ...the proposed MF-RIS simultaneously supports multiple functions with one surface, including reflection, refraction, amplification, and energy harvesting of wireless signals. As such, the proposed MF-RIS is capable of significantly enhancing RIS signal coverage by amplifying the signal reflected/refracted by the RIS with the energy harvested. We present the signal model of the proposed MF-RIS, and formulate an optimization problem to maximize the sum-rate of multiple users in an MF-RIS-aided non-orthogonal multiple access network. We jointly optimize the transmit beamforming, power allocations as well as the operating modes and parameters for different elements of the MF-RIS and its deployment location, via an efficient iterative algorithm. Simulation results are provided which show significant performance gains of the MF-RIS over SF-RISs with only some of its functions available. Moreover, we demonstrate that there exists a fundamental trade-off between sum-rate maximization and harvested energy maximization. In contrast to SF-RISs which can be deployed near either the transmitter or receiver, the proposed MF-RIS should be deployed closer to the transmitter for maximizing its communication throughput with more energy harvested.
Reconfigurable intelligent surfaces (RISs) are recognized as a fundamental enabler for improving energy efficiency in 6G and future networks. However, the power consumption and the reconfiguration ...delay still need improvement for what is required at GHz frequencies, thus delaying their commercial adaptation. On that regard, this study proposes the incorporation of Integrated Circuits (ICs) with MOS varactor loadings as part of the RIS framework, to improve power consumption and speed, while having precise tuning of the reflection phase for individual unit-cells. The presented circuit design features an asynchronous digital circuit responsible for transmitting binary streams to digital-to-analogue converters, which in turn, bias MOS varactors that are directly connected to each unit-cell within the RIS. The use of asynchronous digital control circuits facilitates the development of ultra-low power, high-speed ICs, thereby enhancing the dynamic scalability of the RIS system. Simulated results of the asynchronous circuit are presented on a mature, cost-effective, CMOS 0.18 μm process technology, showing static power consumption of 40,63 μW, dynamic energy consumption of 474.43 pJ and reconfiguration delay of 23.38 ns. The simulations are accompanied by a scalability analysis and a discussion of potential capabilities, offering valuable insights for the future of ICs on RIS systems. The proposed approach and circuit provide flexibility and performance to RIS systems not achievable with conventional control systems due to their benefits of using clockless networking communication.
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