Polarization-Enabled MIMO Bidirectional Device-to-Device Communications via RIS

On performance of multi-RIS assisted multi-user nonorthogonal multiple access system over Nakagami-[formula omitted] fading channels

In this paper, we have studied a model of a relay radio network system using Reconfigurable Intelligent Surface (RIS). Specifically, we used a relay network that uses RIS when there is an extra direct link from the Source (S) to the Destination (D). Next, an approximate closed-form expressions of the Outage Probability (OP) and Ergodic Capacity (EC) are considered. Based on the simulation results of OP and EC, the results show that our proposed system is more optimal than the system using supported RIS without direct link and the system without using RIS. In addition, changing the number of the RIS reflecting elements and the RIS’s location near (S) or (D) has a significant impact on the performance of the system. The analytical expression match the simulation results through the Monte Carlo simulation method. Furthermore, the simulation results of energy efficiency (EE) also show that when the target spectral efficiency (SE), Rth, is high (more than 5.45 b/s/Hz), the system using supported RIS with direct link will help reduce the transmit power and optimize the most energy compared to the other two systems.

Energy efficiency (EE) has become a key performance indicator in designing future wireless networks. In this regard, we propose a novel two reconfigurable intelligent surfaces (RISs) assisted coordinated multipoint (CoMP) network, where two RISs cooperate to reduce energy consumption by constructing an intelligent radio environment. Aiming to maximize this system’s energy efficiency (EE), the joint optimization problem on the cooperative beamforming among base stations (BSs) and passive beamforming among RISs, subject to each user’s data rate requirement, is formulated. An alternative optimization algorithm is proposed by decoupling it into a cooperative beamforming subproblem among BSs and a cooperative reflective coefficients design subproblem among RISs. Dinkelbach’s transform and complex fractional programming algorithms are also invoked to transform these two subproblems into convex optimization problems and obtain the optimal solution for each sub-problem. Numerical results validate the proposed algorithm’s feasibility, fast convergence, and flexibility. It also shows that the proposed scheme with cooperative beamforming design for both BSs and RISs outperforms that without RIS and that with randomly initialized RISs.

In this paper, we explore the communication performance of vehicle-to-vehicle (V2V) networks assisted by a reconfigurable intelligent surface (RIS) and a simultaneous transmitting and reflecting intelligent omni-surface (STAR-IOS) under non-orthogonal multiple access (NOMA) and orthogonal multiple access (OMA) schemes. In particular, we consider that the RIS is close to the transmitter vehicle while the STAR-IOS is near the receiver vehicles. In addition, it is assumed that the STAR-IOS exploits the energy-splitting (ES) protocol for communication and the fading channels between the RIS and STAR-IOS follow composite Fisher-Snedecor <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mathcal {F}$</tex-math></inline-formula> distribution. Under such assumptions, we first use the central limit theorem (CLT) to derive the PDF and the CDF of equivalent channels at receiver vehicles, and then, we derive the closed-form expression of outage probability (OP) under NOMA/OMA scenarios. Additionally, by exploiting Jensen's inequality, we propose an upper bound of the ergodic capacity (EC) and subsequently develop an analytical expression of the energy efficiency (EE) for both NOMA and OMA cases. Further, our analytical results, which are double-checked with the Monte-Carlo simulation, reveal that applying RIS/STAR-RIS in V2V communications significantly enhances the communication performance of intelligent transportation systems (ITS). Besides, the results indicate that considering the NOMA scheme provides better performance in terms of the OP, EC, and EE as compared with the OMA case for the considered V2V communication system.

In this article, the physical layer security (PLS) is investigated for reconfigurable intelligent surface (RIS)-aided wireless communication systems, where one RIS is deployed to assist the communications between a pair of transmitter (Alice) and receiver (Bob), under a passive eavesdropper (Eve) attack. For the Eve, different from the bounded channel state information uncertainty model, the distribution of the Eve's location is introduced into the wiretap link. In the proposed system, considering that the Eve can overhear signals transmitted from Alice or reflected by the RIS, two scenarios are studied for RIS-aided secure communication systems: one is that the Eve distributes close to Alice without the RIS orientation, and the other is that the Eve locates close to Bob and in the presence of the RIS. After investigating the probability distribution functions of the Eve's location and the wiretap link, the novel cumulative density functions (CDFs) of the received signal-to-noise ratios (SNRs) at the Eves are, respectively, derived for the two considered scenarios, taking into account the effects of RIS reflection coefficients, pathloss, and Eve's location distribution. The closed-form expressions for the probability of the nonzero secrecy capacity and the ergodic secrecy capacity are obtained, providing insights into the impact of the Eve's location uncertainty and the RIS design on the secrecy performance. Moreover, based on the derived CDFs for received SNRs at Eves, the secrecy outage probabilities are, respectively, analyzed. Specifically, under the constraint of the secrecy outage probability, the closed forms of the minimum required SNRs at Bob and the number of RIS elements are also obtained. Simulation and analytical results corroborate the derived expressions and reveal the tradeoff between the system's energy efficiency and the number of RIS elements.

RIS-assisted D2D communication in the presence of interference: Outage performance analysis and DNN-based prediction

In this paper, we exploit the benefits of full-duplex (FD) transmission and reconfigurable intelligent surfaces (RISs) by considering a bidirectional full-duplex wireless communication system with the support of RISs. Specifically, cooperative communications are applied at two FD terminals in a RISs-assisted bidirectional full-duplex (RIS-FD) system. In this context, we successfully derive the mathematical expressions of the outage probability (OP) and the ergodic capacity (EC) of the RIS-FD system with a direct link between two terminals and multiple reflecting paths created by RISs over the Rayleigh fading channel. Numerical results show that the performance of the considered RIS-FD system depends significantly on the total number of reflecting elements in the RISs. Particularly, when the total number of reflecting elements is small, the OP and EC saturate at a high signal-to-noise ratio (SNR). However, when the total number of reflecting elements is large enough, OP decreases greatly while EC enhances significantly. In addition, the effect of residual self-interference (SI) caused by FD transmission on OP and EC is remarkable, even the residual SI level is very small. On the other hand, the influence of other system parameters such as the number of RISs (N) and the number of reflecting elements (L) are also investigated to fully consider the performance of the RIS-FD system. An important observation is that when current techniques cannot completely eliminate SI in FD transmission, the use of multiple RISs with larger L can indirectly reduce the impact of SI and significantly improve the performance of the proposed RIS-FD system. Finally, the obtained expressions are validated via Monte-Carlo simulations.

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.

The reconfigurable intelligent surface (RIS)-assisted wireless communication system is studied in this paper, where two full-duplex (FD) users simultaneously communicate with each other. The reception at each user is facilitated through a dedicated RIS. Owing to the geometry and placements of meta-atoms, the analysis incorporates spatial-correlation among the channels related to RIS. Using moment-matching approach, a new analytical framework is presented for spatially-correlated channels considering the optimal phase-shifts from RIS meta-atoms. The effects of residual self-interference (RSI), self-reflections and interference from the RIS of transmitting user are considered in the analysis. The expressions are derived for the outage probability and ergodic capacity for infinite block-length transmission, and for finite block-length transmission, the expressions are obtained for block-error rate (BLER), ergodic capacity and system goodput. To gain further insights, the expression for the asymptotic outage probability is also derived. The finding of this paper confirms the ability of RIS-assisted communication to counter the detrimental effects of self-interference in FD scenario, even in presence of spatial-correlation. The derived expressions are validated using the Monte-Carlo simulations.

A fresh line of research for future wireless networks is called reconfigurable intelligent surface (RIS)-aided communication. We developed one-bit and two-bit discrete phase shifter facilitated RIS for access point (AP) scenario in this study. In the AP scenario, RIS is considered as a source and it can transmit the information. When RIS is utilized as an AP, it can generate the phase angles needed for constellation mapping as well as compensate for channel distortion. The proximity of the radio frequency (RF) signal source to RIS minimizes the impact of fading between RIS and AP. The exact phase compensation of each RIS component is very difficult practically. Consequently, we can enhance the system’s overall performance using a discrete phase shifter. We are evaluating the effectiveness of one- and two-bit discrete phase shifters over intelligent RIS-AP and blind RIS-AP methods.KeywordsAccess point (AP)Blind communicationDiscrete phase shiftersIntelligent communicationReconfigurable intelligent surfaces (RIS)

A novel framework is proposed for the deployment and passive beamforming design of a reconfigurable intelligent surface (RIS) with the aid of non-orthogonal multiple access (NOMA) technology. The problem of joint deployment, phase shift design, as well as power allocation in the multiple-input-single-output (MISO) NOMA network is formulated for maximizing the energy efficiency with considering users particular data requirements. To tackle this pertinent problem, machine learning approaches are adopted in two steps. Firstly, a novel long short-term memory (LSTM) based echo state network (ESN) algorithm is proposed to predict users' tele-traffic demand by leveraging a real dataset. Secondly, a decaying double deep Q-network (D <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> QN) based position-acquisition and phase-control algorithm is proposed to solve the joint problem of deployment and design of the RIS. In the proposed algorithm, the base station, which controls the RIS by a controller, acts as an agent. The agent periodically observes the state of the RIS-enhanced system for attaining the optimal deployment and design policies of the RIS by learning from its mistakes and the feedback of users. Additionally, it is proved that the proposed D <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> QN based deployment and design algorithm is capable of converging within mild conditions. Simulation results are provided for illustrating that the proposed LSTM-based ESN algorithm is capable of striking a tradeoff between the prediction accuracy and computational complexity. Finally, it is demonstrated that the proposed D <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> QN based algorithm outperforms the benchmarks, while the NOMA-enhanced RIS system is capable of achieving higher energy efficiency than orthogonal multiple access (OMA) enabled RIS system.

Active reconfigurable intelligent surface (RIS) emerges as an effective technique to resist the double-fading attenuation of passive RIS. By embedding with power harvesting function, it further evolves to zero-power active RIS, which can effectively enhance the flexibility of RIS deployment without external power demand. Nevertheless, existing works neglected the inherent difficulty of channel estimation (CE) for RIS-assisted systems, and the discrete phase shift constraint in practical deployment. In this paper we design a new element-wise RIS architecture and propose a distributed location-aided transmission scheme with low complexity to enhance the reflected gain for channel state information (CSI)-limited RIS-assisted near-field communications. Specifically, the new element-wise RIS provides dynamic element selection capability with low hardware resources. Based on Fresnel diffraction theory, we construct the mapping from locations in space-domain to phase distributions of waves in phase-domain and reveal the priority of elements for harvesting and reflecting. Then, the distributed beamforming design with the phase of determine-then-align is proposed, where the estimation overhead reduction stems from exempted requirements of RIS-associated CE at base station (BS). The asymptotic analysis indicates that the proposed scheme can achieve the optimal gain with a fixed proportion of reflective elements when RIS is large, followed by simulations to verify its superiority to other protocols.

Reconfigurable intelligent surface (RIS) is regarded as a promising technology to assist indoor millimeter wave (mm-Wave) communication due to its ability to manipulate electromagnetic waves directionally. Due to the complexity of indoor environment, different RIS deployment strategies significantly affect the system performance. However, the existing work investigating RIS indoor deployment ignored the influence of human mobility to indoor signal coverage, therefore a practical indoor deployment strategy remains unknown. In this letter, based on the consideration of human blockage to mm-Wave signal and probability distribution of human location, we propose an indoor RIS deployment strategy with human mobility model to minimize the outage probability, which is more applicable to the realistic indoor environment. An algorithm is presented to minimize the outage probability at a subspace scale of the room geometry and calculate the corresponding RIS location for single RIS deployment and multiple RISs cooperative deployment. Numerical results are provided to display how our deployment strategy works, which indicate that deploying RIS correctly can decrease the indoor communication outage probability significantly.

Reconfigurable intelligent surface (RIS)-aided wireless communications systems are one the promising wireless communication system where the wave can be guided by the RIS. It is envisioned that beyond-5G/6G communication will have a low-cost, high spectral efficiency, high energy efficiency, and smart wireless environment. In this paper, initially, different measurement techniques of the RIS have been discussed, which are available in the literature. Then, a new type of RIS has been proposed. Finally, a different parameter measurement technique for our proposed RIS has been presented. A low-cost FR4 substrate with a height of 1.6 mm was considered to design the RIS in the sub-6 GHz frequency band. Another important thing is that our proposed IRS is a single-layer substrate backed by a copper plate. The area of each unit cell was 42 mm × 42 mm. The RIS was designed to operate at the central frequency of the 3.5 GHz frequency band. The novelty of the proposed RIS is that it is a polarization-independent structure. Thus, polarization-related losses can be overcome using this structure. A 10×10-unit cell array was designed to check the radiation performance. The magnitude of the reflection coefficients was measured in our laboratory for the proposed configuration.

As a promising imaging technique, computational imaging is able to reconstruct the target by transmitting temporal-spatial incoherent signals. With the reconstructed image, a deterministic model of the scene can be established. However, in previous computational imaging systems, the transceiver is an antenna array and each antenna element needs to be modulated respectively, which requires many radio-frequency (RF) chains and complicated signal processing techniques. In addition, the traditional method uses colocated transmitting and receiving antenna which is not conducive to the flexibility of the system. Reconfigurable intelligent surface (RIS) is a revolutionary technology for achieving spectrum and energy efficient in future wireless communication systems. In this paper, we propose distributed computational imaging with reconfigurable intelligent surface. The transmitting antenna, receiving antenna and RIS are distributed in different spatial positions. By deploying reconfigurable intelligent surface, the phase and amplitude of the detecting signals are stochastically modulated by reconfigurable passive elements of RIS, which achieves high spectrum and energy efficiency with low hardware cost. Furthermore, we analyze four models of the detecting signal in detail and subdivide region of interest (ROI) into a collection of grids to formulate the reconstruction process as a sparse recovery problem. Simulation results demonstrate that the proposed distributed computational imaging with RIS can achieve outstanding imaging performance and reconstruct the target effectively.