查看更多>>摘要：In this paper, we consider real-time beamforming design for dynamic wireless environments with varying channels and different numbers of access points (APs) and users in cell-free systems. Specifically, a sum spectral efficiency (SE) maximization optimization problem is formulated for the beamforming design in dynamic wireless environments of cell-free systems. To efficiently solve it, a high-generalization network (HGNet) is proposed to adapt to the changing numbers of APs and users. Then, a high-generalization beamforming module is also designed in HGNet to extract the valuable features for the varying channels, and we theoretically prove that such a high-generalization beamforming module is able to reduce the upper bound of the generalization error. Subsequently, by online adaptively updating about 3% of the parameters of HGNet, an online adaptive updating (OAU) algorithm is proposed to enable the online adaptive real-time beamforming design for improving the sum SE. Numerical results demonstrate that the proposed HGNet with OAU algorithm achieves a higher sum SE with a lower computational cost on the order of milliseconds.
查看更多>>摘要：Cooperative jamming is one of the promising technologies for securing wireless communications. The cooperative jammer transmits jamming to deteriorate the eavesdropping channel, while the authorized receiver implements cooperative jamming cancellation (CJC) using the pre-stored prior information, thus achieving confidential communication at the physical layer. However, the signal transmission will introduce time delay and frequency offset, as well as zero-intermediate-frequency structures that will cause in-phase and quadrature (IQ) imbalances and local oscillator (LO) leakage, which lead to degradation of the CJC performance. In this paper, we specifically analyze the combined effect of time-frequency mismatch, IQ imbalance and LO leakage on the CJC performance by deducing the closed-form expression for the jamming cancellation ratio. A CJC architecture considering time-frequency mismatch, IQ imbalance and LO leakage is proposed, and the corresponding recursive algorithm for CJC is presented. Simulation results show that the proposed algorithm can effectively improve the CJC performance with the presence of time-frequency mismatch, IQ imbalance and LO leakage, and thus improves the security of the communication system.
Aki KarttunenRoman KlusMikko ValkamaJukka Talvitie...
8952-8968页
查看更多>>摘要：Positioning is a vital capability in different radio systems for extracting situational awareness, with path loss (PL)-based positioning playing a crucial role due to its widespread use in wireless standards. In this work, we propose an idle-mode PL-based positioning approach without line-of-sight (LOS) detection that is suitable for millimeter-wave (mmWave) urban networks with directive beams in the base stations (BSs). With the beam gain significantly affecting the observed PLs, we divide the data and models relative to the beam direction rather than to LOS and non-line-of-sight (NLOS) BSs. Different approaches for obtaining the PL model parameter estimates are proposed, including model fitting taking into account the influence of the noise limit and direct optimization based on positioning accuracy using the training data set. In addition to the PLs, azimuth- and elevation-of-departure (AoD and EoD) are estimated, modeled, and used in the positioning calculations, building on maximum likelihood (ML) estimation. Comprehensive numerical results and performance assessments are provided, harnessing ray-tracing (RT) data in a 28GHz urban microcellular environment. The demonstrated median positioning error is under 20m reflecting an improvement of 50%-70% compared to the classical CellID method. Additionally, the results show that the proposed method outperforms machine learning based reference solutions. Finally, the methods and the resulting positioning performance are shown to be robust against variations in the underlying technical parameters, such as the BS transmit beam-width, as well as errors or imperfections in the assumed BS locations and BS orientation information.
查看更多>>摘要：Reconfigurable intelligent surface (RIS) has shown its potential in terahertz (THz) communications, due to its capability to expand coverage and compensate for the severe attenuation of THz signals. This paper investigates a large-scale RIS-assisted THz communications system in the near-field. Nevertheless, the beam squint effect of RIS, caused by the frequency-independent phase shifting circuit, results in severe array gain loss across the wide bandwidth. While true time delays (TTDs) that generate frequency-dependent phase shifts can mitigate beam squint, they often suffer from high power consumption. To address the drawback, we introduce a set of fixed true time delays (FTTDs) with low power consumption and low insertion loss for the RIS. These FTTDs, shared by the elements of RIS, can generate frequency-dependent phase shifts, thereby addressing the beam squint effect. To overcome the limitation of FTTDs being unable to change delays, we propose a dynamic architecture that consists of a switch network and two-layer phase shifters, allowing the elements of RIS to select the FTTDs. Subsequently, we analyze the theoretical array gain of the proposed FTTD-equipped RIS and determine the minimum number of FTTDs required for effective mitigation. Then, we formulate a problem of maximizing the achievable rate and propose a two-stage algorithm. Specifically, in the first stage, we obtain the optimal wideband beamforming design for both the BS and the RIS. In the second stage, we approximate this optimal design with the beamforming design from our proposed architecture. Finally, simulation results demonstrate that our proposed RIS design, with a small number of FTTDs, can achieve a near-optimal achievable rate and higher energy efficiency.
查看更多>>摘要：A novel integrated wireless powered sensing and communication (IWPSAC) framework is proposed. Specifically, a multi-antenna transmitter utilizes a radar signal for sensing targets while enabling multiple Internet of Things (IoT) devices to harvest energy from the signal, each of which employs the collected energy to upload information to an access point (AP). Our setup further considers a reconfigurable intelligent surface (RIS) to integrate sensing, wireless energy transfer (WET) and wireless information transfer (WIT) by optimizing the phase shifts. We formulate an optimization problem to maximize the weighted sum of the communication throughput and the beampattern gain by jointly designing the energy beamforming, transmission time scheduling and RIS phase shifts. The presence of multiple coupled variables in the formulated problem renders the optimization problem non-jointly convex. To address its non-convexity, we first derive a closed-form expression for the optimal RIS phase shifts in the WIT phase. Then, an alternating optimization (AO) algorithm is proposed to solve the trade-off problem iteratively. Concretely, this involves alternating the design of the energy beamforming and the RIS phase shifts for sensing/WET by leveraging the semidefinite programming (SDP) relaxation method. To overcome the high complexity introduced by the SDP, we introduce a low complexity AO algorithm that derives the optimal solutions for energy beamforming, transmission time scheduling, and sensing/WET phase shift using successive convex approximation (SCA), Lagrangian duality methods, Karush-Kuhn-Tucker (KKT) conditions, and the element-wise block coordinate descent (EBCD) approach. Simulation results demonstrate the performance of the proposed algorithms and underscore the superior benefits of the RIS compared to baseline schemes.
查看更多>>摘要：This paper proposes a fast and robust graph-based wavenumber-domain approach for channel estimation in holographic MIMO (HMIMO) systems. Unlike conventional angular-domain methods—prone to mutual coupling, power leakage, and sampling redundancy—our framework resolves HMIMO’s high-dimensional challenges by introducing a wavenumber-domain basis via orthogonal Fourier harmonics (FHs), eliminating dependencies on antenna density. By reformulating channel estimation as its sparse recovery counterpart, we model clustered sparsity using an elliptic Markov random field (EMRF), upon which a graph-cut swap expansion (GCSE) algorithm is developed, leveraging graph-theoretic optimizations for fast convergence and low complexity. Simulations demonstrate that our method achieves robust performance against mutual coupling, varying SNRs, and antenna density with drastically less computing time.
查看更多>>摘要：In this paper, we consider a low earth orbit satellite communication system with a legitimate user and an eavesdropper and investigate its secrecy performance. Considering the beam coverage of the satellites, we propose a system where a friendly jammer satellite transmits artificial noise signals only to eavesdroppers. We obtain the cumulative distribution functions (CDFs) of the signal-to-interference-plus-noise ratios taking into account the Nakagami-m fading and the positions of ground users and satellites. The CDFs are used to derive the analytical forms of the probability of non-zero secrecy capacity, secrecy capacity outage probability, and average secrecy capacity. In addition, for tractability and insight, asymptotic forms are also investigated at a high-power regime. Finally, we present the numerical results to validate the obtained expressions and show the diversity order of the secrecy capacity.
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IEEE