arXiv subjects

Zihang Ding

Publications and source records attributed to Zihang Ding.

4 recordsLinked to original sources

arXiv2026

Movable Antenna-Aided Secure LEO Satellite Networks: Joint Antenna Position and Beamforming Optimization

The broadcast characteristics of sixth-generation (6G) low-earth orbit (LEO) satellite communications raise serious security issues. Movable antenna (MA) technology offers a promising physical layer security (PLS) solution by flexibly reconfiguring antenna positions to exploit additional spatial degrees of freedom. However, in highly dense LEO satellite constellations, the legitimate satellite and potential eavesdropping satellites may exhibit small angular separations, which poses significant challenges for the design of secure transmission schemes. To address this challenge, this paper proposes an MA-assisted secure transmission scheme for time-varying LEO satellite communications, where a ground station equipped with an MA array communicates with a serving satellite, while the other visible satellites are regarded as potential eavesdroppers. We maximize the average secrecy rate by jointly optimizing the transmit beamforming and MA positions. An alternating optimization (AO) framework is developed, where semidefinite relaxation is adopted for the beamforming optimization subproblem, while high-accuracy successive convex approximation (SCA) and low-complexity differential evolution (DE) algorithms are proposed for the MA position optimization subproblem. Numerical results demonstrate that the proposed MA-assisted LEO secure transmission scheme consistently achieves superior performance compared to the conventional fixed-position antenna scheme.

cs.IT↗

arXiv2025

Cell-free versus Conventional Massive MIMO : An Analysis of Channel Capacity based on Channel Measurement in the FR3 Band

Cell-free massive MIMO (CF-mMIMO) has emerged as a promising technology for next generation wireless systems, combining the benefits of distributed antenna systems (DAS) and traditional MIMO technology. In this work, we present the first extensive channel measurements for CF-mMIMO in the mid-band (FR3, 6-24 GHz), using a virtual widely distributed antenna array comprising 512 elements in the urban Macrocell (UMa) environment. Based on the measurement data, this paper compares the channel capacity of CF-mMIMO and Conventional mMIMO under both line-of-sight (LOS) and non-line-of-sight (NLOS) conditions across a range of signal-to-noise ratios (SNRs). We then analyze how channel capacity varies with Rx positions from the perspectives of the full array and of individual subarrays. Finally, we conclude that the 64-element array configuration yields the greatest advantage in channel capacity for CF-mMIMO in the measurement environment considered, with gains of 14.02\% under LOS and 24.61\% under NLOS conditions. This in-depth analysis of channel capacity in the FR3 band provides critical insights for optimizing CF-mMIMO systems in next generation wireless networks.

eess.SP↗

arXiv2025

Optimizing Sensor Placement for Flow Reconstruction in Urban Drainage Networks: A Digital Twin-Based Sparse Sensing Approach

Urban flooding triggered by intense rainfall is becoming increasingly frequent and widespread. While flood prediction and monitoring in high spatio-temporal resolution are desired, practical constraints in time, budget, and technology hinder its full implementation. How to monitor urban drainage networks and predict flow conditions under constrained resources is a major challenge. To address this, we introduced a data-driven sparse sensing (DSS) approach, demonstrated via a digital-twin of the Woodland catchment in Duluth, Minnesota. Specifically, we coupled EPA-SWMM with singular value decomposition and QR factorization-based sensor selection to optimize monitoring locations for system-level flow reconstruction. An ensemble of SWMM simulations, driven by diverse scenarios, provided the necessary hydraulic data to extract the reduced basis and identify informative sensor locations. Cross-event validation showed that three strategically placed sensors among 77 candidate nodes achieved a mean system-level Nash-Sutcliffe efficiency (NSE) of 0.949 across observed storm events. The QR-selected sensor sets were benchmarked against reference sensor configurations obtained from exhaustive searches and Monte Carlo random-placements. This comparison further showed that flow reconstruction based on QR-selected sensors closely tracked the exhaustive optimum while substantially outperforming random placements. We further evaluated the framework's robustness by introducing multiplicative Gaussian noise and simulating individual sensor failures. While the model is relatively resilient to noise, the impact of sensor dropouts depends heavily on the number of sensors allocated and their specific locations.

cs.AI↗

arXiv2025

Far-Field vs. Near-Field Propagation Channels: Key Differences and Impact on 6G XL-MIMO Performance Evaluation

Extremely large-scale multiple-input multiple-output (XL-MIMO) is regarded as a promising technology for next-generation communication systems. However, this will expand the near-field (NF) range, rendering more users more likely to be located in the NF region. In this paper, we aim to answer two questions: What are the new characteristics of the NF channel? Is it necessary to develop new transciver techniques to maintain system performance within the NF region? To this end, we first review current NF channel models and analyze the differences between the existing 3GPP TR 38.901 channel model and the NF channel model, including the spherical wavefront and spatially non-stationarity. Then, we provide examples on how these differences affect the XL-MIMO system performance in terms of beamforming gain and achievable rate. Simulation results demonstrate that, when using far-field (FF) technique under the NF channel, the maximum normalized beam gain loss is less than 3 dB for most users in the NF region defined by Rayleigh distance. Moreover, the achievable rate loss of beam training is less than 3% compared to that realized by NF technique. Finally, we demonstrate the necessity of employing NF transceiver techniques based on simulation results.

eess.SP↗