In this paper, we consider a multi-reconfigurable intelligent surface(RIS)-assisted multi-cell communication system in which we jointly optimize the base station (BS)-RIS-user association and RIS ...deployment locations to maximize the ergodic capacity. In order to derive deterministic approximate expressions for the ergodic capacity, active and passive beamforming are first optimized to maximize the signal power of the relevant path. Then, we propose a two-level nested algorithm to address this mixed-integer nonlinear programming (MINLP) problem. Finally, the effectiveness of the proposed algorithm is validated through numerical simulations.
Simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) offers a promising solution for achieving full space coverage. In this letter, we focus on a STAR-RIS assisted ...downlink communication system, where we jointly optimize the locations, passive transmitting and reflecting beamforming (BF) of the STAR-RISs, and the active BF at the base station (BS), to maximize the sum rate of users in non-line-of-sight (NLoS) areas. We propose a joint deployment and beamforming design algorithm to address the formulated non-convex optimization problem. Simulation results validate the superiority of our proposed algorithm.
As a revolutionary paradigm for controlling wireless channels, reconfigurable intelligent surfaces (RISs) have emerged as a candidate technology for future 6G networks. However, due to the ..."multiplicative fading" effect, the existing passive RISs only achieve limited capacity gains in many scenarios with strong direct links. In this paper, the concept of active RISs is proposed to overcome this fundamental limitation. Unlike passive RISs that reflect signals without amplification, active RISs can amplify the reflected signals via amplifiers integrated into their elements. To characterize the signal amplification and incorporate the noise introduced by the active components, we develop and verify the signal model of active RISs through the experimental measurements based on a fabricated active RIS element. Based on the verified signal model, we further analyze the asymptotic performance of active RISs to reveal the substantial capacity gain they provide for wireless communications. Finally, we formulate the sum-rate maximization problem for an active RIS aided multi-user multiple-input single-output (MU-MISO) system and a joint transmit beamforming and reflect precoding scheme is proposed to solve this problem. Simulation results show that, in a typical wireless system, passive RISs can realize only a limited sum-rate gain of 22%, while active RISs can achieve a significant sum-rate gain of 130%, thus overcoming the "multiplicative fading" effect.
The integration of reconfigurable intelligent surfaces (RISs) and grant-free non-orthogonal multiple access (GF-NOMA) has emerged as a promising solution for enhancing spectral efficiency and massive ...connectivity in future wireless networks. This paper proposes a GF-NOMA communication network enabled by simultaneously transmitting and reflecting RISs (STAR-RIS). In the proposed GF-NOMA, all user equipments (UEs) have instantaneous access to resource blocks (RBs) without the need for grant acquisition and power control as in the traditional grant-based NOMA schemes. Specifically, we have considered two regimes of interest: 1) the max-min fair (MMF) regime and 2) the max-sum throughput (MST) regime. To achieve the required power disparity, a two-level power control mechanism is proposed; initially, the UEs are clustered according to their channel gains. Additionally, we introduce a multi-level GF-NOMA (MGF-NOMA) scheme that adjusts the transmit power levels for each UE in the cluster. The second level of power disparity is achieved through the assignment of STAR-RISs to the clusters and optimal partitioning of STAR-RIS to support each of the cluster members. Specifically, we have also derived the closed-form equations for the optimal partitioning of STAR-RIS within the clusters for both regimes of interest. Simulation results demonstrate that the proposed STAR-RIS-aided MGF-NOMA yields a gain of 60% and 20% in the MST regime with active and passive RIS realization, respectively. Furthermore, the active and passive RIS-based MGF-NOMA achieve nearly the equivalent fairness that can be obtained through optimal power control in the MMF regime. The finding emphasizes the potential of integrating STAR-RIS with GF-NOMA as a robust and promising solution for future wireless communication systems.
This letter theoretically compares the active reconfigurable intelligent surface (RIS)-aided system with the passive RIS-aided system. For a fair comparison, we consider that these two systems have ...the same overall power budget that can be used at both the base station (BS) and the RIS. For active RIS, we first derive the optimal power splitting between the BS's transmit signal power and RIS's output signal power. We also analyze the impact of various system parameters on the optimal power splitting ratio. Then, we theoretically and numerically compare the performance between the active RIS and the passive RIS, which demonstrates that the active RIS would be superior if the power budget is not very small and the number of RIS elements is not very large.
In this paper, we study the statistical characterization and modeling of distributed multi-reconfigurable intelligent surface (RIS)-aided wireless systems. Specifically, we consider a practical ...system model where the RISs with different geometric sizes are distributively deployed, and wireless channels associated to different RISs are assumed to be independent but not identically distributed (i.n.i.d.). We propose two purpose-oriented multi-RIS-aided schemes, namely, the exhaustive RIS-aided (ERA) and opportunistic RIS-aided (ORA) schemes. A mathematical framework, which relies on the method of moments, is proposed to statistically characterize the end-to-end (e2e) channels of these schemes. It is shown that either a Gamma distribution or a Log-Normal distribution can be used to approximate the distribution of the magnitude of the e2e channel coefficients in both schemes. With these findings, we evaluate the performance of the two schemes in terms of outage probability (OP) and ergodic capacity (EC), where tight approximate closed-form expressions for the OP and EC are derived. Representative results show that the ERA scheme outperforms the ORA scheme in terms of OP and EC. In addition, under i.n.i.d. fading channels, the reflecting element settings and location settings of RISs have a significant impact on the system performance of both the ERA or ORA schemes.
Reconfigurable intelligent surfaces (RISs) have been often proposed as a means for restoring nonline-of-sight links in wireless communications. Because the path loss changes with the relative ...distance of the RIS from the transmitter and the receiver and their relative orientations, the quality of the communication is expected to depend strongly on the RIS placement in an RIS-aided link. In this work, the optimal placement of the RIS is studied with respect to both position and orientation and it is shown that even for an ideal RIS, i.e., lossless, polarization preserving, and with a uniform response with respect to the incident angle, the quality of the communication is greatly affected by the RIS orientation. To provide insight into the optimal choice, the analysis is based on an analytical model that treats the RIS as a continuous surface of finite size that steers the incident beam toward a prescribed direction. By imposing a desired minimum threshold on the received power, it is studied how the efficiency of the RIS depends on both its position and orientation. The decisions that have to be made concerning the placement of the RIS are illustrated with examples of D-band indoor scenarios.
Reconfigurable intelligent surface (RIS) has emerged as a promising technology to enhance the spectral efficiency of wireless communication systems. However, if there are many obstacles between the ...RIS and users, a single RIS may not provide sufficient performance. For this reason, a double RIS-aided communication system is proposed in this paper. However, this system also has a problem: the signal is attenuated three times due to the three channels created by the double RIS. To overcome these attenuations, an active RIS is proposed in this paper. An active RIS is almost the same as a conventional RIS, except for the included amplifier. Comprehensively, the proposed system overcomes various obstacles and attenuations. In this paper, an active RIS is applied to the second RIS. To reduce the power consumption of active elements, a partially active RIS is applied. To optimize the RIS elements, the sum of the covariance matrix is found by using channels related to each RIS, and the right singular vector is exploited using singular value decomposition for the sum of the covariance matrix. Then, the singular value of the sum of the covariance value is checked to determine which element is the active element. Simulation results show that the proposed system has better sum rate performance compared to a single RIS system. Although it has a lower sum rate performance compared to a double RIS with fully active elements, the proposed system will be more attractive in the future because it has much better energy efficiency.
In this paper, we consider the uplink channel estimation problem for multi-user communication systems that utilize a hybrid reconfigurable intelligent surface (HRIS). In particular, we are interested ...in the HRIS that has the capability of reflecting and sensing the impinging signals. Our objective is to jointly design the pilot sequences transmitted by user equipments (UEs) and the amount of phase shifts applied by the HRIS for accurate channel estimation. We formulate a multi-objective optimization problem in terms of the mean squared errors of UEs-to-RIS and RIS-to-base station (BS) channels, and approximate it by assuming that the BS has a perfect estimate of the UEs-to-RIS channel. As a result, a sufficient condition is derived for a feasible solution to be Pareto-optimal to the approximated problem, which simultaneously achieves the Cramér-Rao bounds. We propose two joint designs that satisfy the sufficient condition when the pilot length is at least as long as the product of the numbers of HRIS elements and UEs. The proposed design solutions are easy to implement and lead to computationally efficient channel estimations. The average performances of the proposed designs are analyzed under Rayleigh fading. Numerical results demonstrate that the proposed joint designs outperform non-joint designs that use randomized phase shifts and conventional channel estimation methods for passive RISs.
Reconfigurable Intelligent Surface (RIS) draws great attentions in academic and industry due to its passive and low power consumption nature, and has currently been used in physical layer security to ...enhance the secure transmission. However, due to the existence of "double fading" effect on the reflecting channel link between transmitter and user, RIS helps achieve limited secrecy performance gain compared with the case without RIS. In this correspondence, we propose a novel active RIS design to enhance the secure wireless transmission, where the reflecting elements in RIS not only adjust the phase shift but also amplify the amplitude of signals. To solve the non-convex secrecy rate optimization based on this design, an efficient alternating optimization algorithm is proposed to jointly optimize the beamformer at transmitter and reflecting coefficient matrix at RIS. Simulation results show that with the aid of active RIS design, the impact of "double fading" effect can be effectively relieved, resulting in a significantly higher secrecy performance gain compared with existing solutions with passive RIS and without RIS design.
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