1,721,045 research outputs found
Optimizing Reconfigurable Intelligent Surfaces in Multi-User Environments: A Multiport Network Theory Approach Leveraging Statistical CSI
No full textReconfigurable Intelligent Surfaces (RIS) are one of the emerging technologies aimed at meeting the expectations of next-generations wireless systems. In this field, the use of multi-port network models for the characterization and optimization of RIS has emerged in recent years. These models take into account aspects traditionally not considered in communication theory, such as mutual coupling of RIS elements and the presence of structural scattering. In this work, we refer to this model and focus on the problem of maximizing the average achievable rate in a multi-user uplink scenario by leveraging statistical Channel State Information (CSI). This approach significantly reduces the computational burden and communication overhead in CSI estimation compared to schemes requiring instantaneous CSI estimation. These benefits are achieved with performance that, in many cases, is reasonably close to that of the perfect CSI scenario. This is one of the outcomes achievable with the proposed optimization scheme. Moreover, it is shown how in multi-user scenarios, namely in the presence of interference, the use of inadequate models to characterize RIS can lead to very poor performances. For example, models that do not consider structural scattering may fail to account for interference caused by RIS
Message passing resource allocation for the uplink of multicarrier systems
No full textWe propose a novel distributed resource allocation scheme for the up-link of a cellular multi-carrier system based on the message passing (MP) algorithm. In the proposed approach each transmitter iteratively sends and receives information messages to/from the base station with the goal of achieving an optimal resource allocation strategy. The exchanged messages are the solution of small distributed allocation problems. To reduce the computational load, the MP problems at the terminals follow a dynamic programming formulation. The advantage of the proposed scheme is that it distributes the computational effort among all the transmitters in the cell and it does not require the presence of a central controller that takes all the decisions. Numerical results show that the proposed approach is an excellent solution to the resource allocation problem for cellular multi-carrier systems
Optimal radio resources and transmission formats assignment in OFDMA systems
No full textThe problem of assigning radio resources and transmission formats to users in the downlink of an OFDMA network is addressed. In particular, a single cell environment with a realistic interference model and a margin adaptive approach is considered, i.e., the aim is of minimizing the total transmission power while maintaining a certain given rate for each user. The computational complexity issues of the problem are discussed, and an approximation analysis is presented. Heuristic approaches, based on rounding techniques and graph models, are also proposed. Computational experiences show that, in a comparison with a commercial state-of-the-art optimization solver and with alternatives from the literature, the proposed algorithms are effective in terms of solution quality and CPU times
Design of Reconfigurable Intelligent Surfaces by Using S-Parameter Multiport Network Theory - Optimization and Full-Wave Validation
No full textMultiport network theory has been proved to be a suitable abstraction model for analyzing and optimizing reconfigurable intelligent surfaces (RISs) in an electromagnetically consistent manner, especially for studying the impact of the electromagnetic mutual coupling among radiating elements that are spaced less than half of the wavelength apart and for considering the interrelation between the amplitude and phase of the reflection coefficients. Both representations in terms of Z-parameter (impedance) and S-parameter (scattering) matrices are widely utilized. In this paper, we embrace multiport network theory for analyzing and optimizing the reradiation properties of RIS-aided channels, and provide four new contributions. (i) First, we offer a thorough comparison between the Z-parameter and S-parameter representations. This comparison allows us to unveil that typical scattering models utilized for RIS-aided channels ignore the structural scattering from an RIS, which is well documented in antenna theory. We show that the structural scattering results in an unwanted specular reflection. (ii) Then, we develop an iterative algorithm for optimizing, in the presence of electromagnetic mutual coupling, the tunable loads of an RIS based on the S-parameters representation. We prove that small perturbations of the step size of the algorithm result in larger variations of the S-parameter matrix compared with the Z-parameter matrix, resulting in a faster convergence rate. (iii) Subsequently, we generalize the proposed algorithm to suppress the specular reflection due to the structural scattering, while maximizing the received power towards the direction of interest, and analyze the effectiveness and tradeoffs of the proposed approach. (iv) Finally, we validate the theoretical findings and algorithms with numerical simulations and a commercial full-wave electromagnetic simulator based on the method of moments
Two-Timescale Joint Precoding Design and RIS Optimization for User Tracking in Near-Field MIMO Systems
Full text linkIn this paper, we propose a novel framework that aims to jointly design the reflection coefficients of multiple reconfigurable intelligent surfaces and the precoding strategy of a single base station (BS) to optimize the self-Tracking of the position and the velocity of a single multi-Antenna user equipment (UE) that moves either in the far-or near-field region. Differently from the literature, and to keep the overall complexity affordable, we assume that RIS optimization is performed less frequently than localization and precoding adaptation. The proposed procedure leads to minimize the inverse of the received power in the UE position uncertainty area between two subsequent optimization steps. The optimal RIS and precoder strategy is compared with the classic beam focusing strategy and with a scheme that maximizes the communication rate. It is shown that if the RISs are optimized for communications, their configuration is suboptimal when used for tracking purposes. Numerical results show that in typical indoor environments with only one BS and a few RISs operating on millimeter waves, high location accuracy in the range of less than half a meter can be achieved
A Compressive Sampling Data Gathering Approach for Wireless Sensor Networks Using a Sparse Acquisition Matrix With Abnormal Values
No full textApplication of Compressive Sampling (CS) to Wireless Sensor Networks (WSNs), is a very promising field. In particular, CS allows to exactly reconstruct a sparse signal using only a few measurements. Hence, it promises to represent a viable solution for reducing data exchange in WSNs, thus prolonging the network lifetime. On the other hand, natural signals are only approximately sparse and, hence, CS entails a reconstruction error, which limits its applicability in many situations. To cope with this impairment, we first consider a CS scheme based on a sparse acquisition matrix, so that only M over N (M N) randomly chosen nodes in the network send a packet towards the sink. Then, we propose to use a distributed estimation scheme to locally detect whether the data must be forced to transmit or not, thus highly improving the reconstruction quality. © 2012 IEEE
Distributed Digital and Hybrid Beamforming Schemes With MMSE-SIC Receivers for the MIMO Interference Channel
Full text linkThis paper addresses the problem of weighted sum-rate maximization and mean squared error (MSE) minimization for the multiple-input multiple-output (MIMO) interference channel. Specifically, we consider a weighted minimum MSE architecture where each receiver employs successive interference cancellation (SIC) to separate the various received data streams and derive a hybrid beamforming scheme, where the transmitters operate with a number of radio frequency chains smaller than the number of antennas, particularly suited for millimeter-wave channels and 5G applications. To derive our proposed schemes, we first study the relationship between sum-rate maximization and weighted MSE minimization when using SIC receivers, assuming fully digital beamforming. Next, we consider the important - and, as it turns out, highly non-trivial - case where the transmitters employ hybrid digital/analog beamforming, developing a distributed joint hybrid precoding and SIC-based combining algorithm. Moreover, for practical implementation, we propose a signaling scheme that utilizes a common broadcast channel and facilitates the acquisition of channel state information, assuming minimal assistance from a central node such as a cellular base station. Numerical results show that both the proposed weighted MMSE-SIC schemes exhibit great advantages with respect to their linear counterparts in terms of complexity, feedback information, and performance
A new framework for Physical Layer Security in HetNets based on Radio Resource Allocation and Reinforcement Learning
Full text linkDensification of networks through heterogeneous cells deployment is considered a key technology to satisfy the huge traffic growth in future wireless systems. In addition to achieving the required communication capacity and efficiency, another significant challenge arises from the broadcast nature of wireless channels: vulnerability to wiretapping. Physical-layer security is envisaged as an additional level of security to provide confidentiality of radio communications. Typical characteristics of the wireless channel (noise, interference) can be exploited to keep a message confidential from potential eavesdroppers. In particular, heterogeneous networks (HetNet) have inherent security features: while the legitimate user can benefit of the HetNet architecture, the eavesdropper is strongly affected by the inter-cell interference. This paper presents an overview of HetNets intrinsic security benefits, mainly focusing on users association and resource allocation policies. In particular, allocation of radio resources is a poorly investigated topic when related to information security. However, in systems with a large radio resource reuse like HetNets, co-channel interference can be suitably exploited to resist to the eavesdropper. This paper presents a new framework for radio resources allocation using reinforcement learning (Q-learning) to increase the security level in HetNets. A coordinated scheduling among different cells using the same radio resources is proposed based on the exploitation of the spatial information. The goal is to optimize the security at physical layer. The reinforcement learning approach represents a feasible and efficient solution to the proposed problem
Network coding schemes for D2D communications based relaying for cellular coverage extension
No full textAlthoughnetwork-assisted device-to-device (D2D) communications are known to improve the spectral and energy
efficiency of proximal communications, the performance of cooperative D2D schemes in licenced spectrum is less understood
when employed to extend the coverage of cellular networks. In this paper, we study the performance of D2D-based
range extension in terms of sum rate and power efficiency when a relaying user equipment (UE) helps to improve the coverage
for cell edge UEs. In our design, the relaying UE may have own traffic to transmit and receive to/from the cellular
base station (BS) and can operate either in amplify-and-forward (AF) or decode-and-forward (DF) modes and can make
use of either digital or analogue physical (PHY) layer network coding. In this rather general setting, we propose mode
selection, resource allocation and power control schemes and study their performance by means of system simulations.
We find that the performance of the DF scheme with network coding is superior both to the traditional cellular and the
AF-based relaying schemes, including AF with two-slot or three-slot PHY layer network codin
- …
