arXiv subjects

Jiawang Li

Publications and source records attributed to Jiawang Li.

14 recordsLinked to original sources

arXiv2026

Near-Field Sampling for Line Sources

Near-field sampling seeks to represent electromagnetic fields between transmitting and receiving regions using a minimal number of measurement points while preserving the dominant spatial modes. This paper develops a geometry-aware sampling framework based on spatial degrees of freedom (DoF). A view-length formulation is used to derive closed-form expressions for the propagating-mode DoF density for simple line-source geometries, providing both the total DoF and its local distribution. One-DoF sampling points are obtained from equal increments of the cumulative DoF density, yielding an adaptive nonuniform sampling strategy up to the knee of the singular-value spectrum. To improve the representation of the remaining modes beyond the knee, a reactive-mode density is introduced to guide the placement of additional edge samples. An operator-based sampling error functional is formulated and shown to be lower-bounded by the neglected singular values of the continuous channel operator. Numerical results demonstrate that the proposed sampling strategy closely approaches the optimal performance obtained from singular-value decomposition and significantly outperforms sampling based solely on the propagating-mode DoF density.

eess.SP↗

arXiv2026

Degrees of Freedom and Beamforming for Large Intelligent Surfaces

Spatial degrees of freedom (DoF), sampling, and beamforming are fundamental to multi-user large intelligent surfaces (LISs), where electromagnetic fields must be shaped, resolved, and focused at multiple near-field locations. This work estimates the number of DoF using closed-form expressions derived from the mutual shadow area for representative LIS configurations. The resulting DoF predictions are validated through numerical singular-value spectra, whose spectral knee points closely match the theoretical estimates. For line-source configurations, an analytic sampling scheme is developed by partitioning the source or observation line into unit-DoF intervals, enabling the selection of spatial samples. Beamforming results using maximum-ratio transmission and zero-forcing demonstrate that approximately the number of DoF independent beams can be formed. Attempting to exceed this limit results in increased interference and degraded performance. For surface-based LIS configurations, sampling points are instead determined numerically using the discrete empirical interpolation method. The corresponding beamforming results further confirm that the target region can support approximately as many independent beams as predicted by the DoF analysis. Finally, a polarization-aware study reveals that the electric-field components contribute unequally to the DoF and that the total-field DoF is twice that of a single polarization component.

eess.SP↗

arXiv2026

Properties of Near Field Focusing for Three-Dimensional Large Intelligent Surface

This work investigates near-field focusing using a three-dimensional (3D) large intelligent surface (LIS) across frequencies and polarizations. Specifically, the LIS elements are distributed in 3D space within a long corridor, rather than being confined to a single planar aperture, and the focal point is located at a prescribed position in the radiating near field. By formulating optimization problems under both local and global power constraints, we obtain the corresponding optima. For continuous apertures, the optimal current magnitude distribution matches time-reversal (TR) solution under the global constraint and conjugate-phase (CP) solution when the local constraint dominates. When both constraints are active, the solution assigns larger excitation magnitudes to elements closer to the illumination field. This behavior remains invariant with respect to frequency and polarization for a fixed-size LIS. These findings are consistent to the more practical case of using discretized apertures in the form of Hertzian dipole arrays, studied using both analytical results and full-wave simulation. In addition, with the CP method, specific polarizations lead to identical transverse and longitudinal resolution, in contrast, under the TR method, these quantities can differ across polarizations.

eess.SP↗

arXiv2025

Quasi-Closed-Form Driven Near-Field Flat-Top Beamfocusing with Concentric Circular Vertical Polarized Dipole Array For Large Intelligent Surface Applications

This letter presents a near-field flat-top beam synthesis method based on a semi-closed-form approach. First, the feasibility of achieving a flat-top beam in the near field is examined using a closed-form analysis. A circular concentric ring array structure is adopted, and it is observed that circular rings with different radii exhibit distinct gain characteristics along the focal region on the z-axis. Specifically, smaller radii lead to a monotonic increase in electric field strength near the focus, whereas larger radii result in a monotonic decrease. Based on this behavior, parameters such as the number of rings and the initial radius are determined through field superposition. Subsequently, an optimization algorithm is employed to fine-tune the excitation amplitudes of the individual rings in order to suppress sidelobes. The effectiveness of the proposed method is validated through full-wave electromagnetic simulations.

physics.optics↗

arXiv2025

Low-Cost Wideband Tilted Beam Antenna for Millimeter-wave Vehicle Applications

To facilitate vehicle coverage for millimeter-wave applications, this communication presents a low-cost, wideband tilted-beam antenna. A novel design is proposed in which a slot antenna is both directly excited and electromagnetically coupled to a monopole array. This slot-monopole configuration is inherently robust against substrate losses, enabling low-cost fabrication while maintaining high realized gain and compact size. Furthermore, the slot-fed structure effectively excites multiple resonant modes within the monopole array, resulting in a significantly enhanced bandwidth. Experimental results demonstrate that the antenna achieves a -10-dB impedance bandwidth of over 76.5% (20-44.78 GHz) and a peak realized gain of 6.1 dBi.

eess.SP↗

arXiv2025

Single-layer Circular SIW Filtenna With Beam Scanning Capability for 5G Millimeter Wave Communication Applications

In this communication, two novel low-cost single-layer filtering antennas (filtennas) are proposed for millimeter wave (mmWave) applications. The proposed filtennas consists of a compact circular substrate integrated waveguide (SIW) cavity, a metal post close to the center of the cavity for power feeding, a metal post in the center for modes controlling, and a slot for radiating power. In the passband, the fundamental TM010 mode and the TM110 mode in the circular SIW cavity are excited by the feeding post. In addition, thanks to the high-pass characteristics of the cavity, it exhibits more than 20 dB suppression in the lower frequency band. There are three radiation nulls in Filtenna 1 and one radiation null in Filtenna 2 in the upper band which increase the suppression level as high as 18 dB. As a proof of concept, the proposed filtennas are fabricated and measured. It is shown that the Filtenna 1 can achieve simulated and measured -10 dB impedance fractional bandwidth (FBW) of 7.1% (27.14 - 29.13 GHz) and 8.6% (27.62 - 30.11 GHz), respectively. While filtenna 2 can achieve simulated and measured -10 dB FBW of 7.4% (27.86 - 29.99 GHz) and 10.1% (28.11 - 31.09 GHz), respectively. The filtennas features stable radiation patterns with an average gain of 5.0 dBi. The lower and upper sideband suppression levels for both filtennas exceed 18 dB. These filtennas are good candidates for 5G mmWave applications, as they simultaneously provide beam scanning and filtering capability with a low cost, and single layer structure.

eess.SP↗

arXiv2025

On Properties of Phase-Conjugation Focusing for Large Intelligent Surface Applications -- Part II: Horizontal Polarization

Near-field focusing (NFF) forms the basis for several applications of large intelligent surface (LIS) in sub-10 GHz bands, including wireless communications, wireless power transfer, positioning, and sensing. In this two-part paper, Part I analyzed the properties of phase conjugation NFF for vertically polarized antennas, in a circular array configuration. In Part II of this article, we continue to study phase conjugation NFF for circular arrays, but for horizontally polarized antennas. We investigate the focusing characteristics of a circular array in two distinct configurations. The numerical results show that the first configuration where all the antenna elements (including the user antenna) are aligned offers significant better performance in terms of peak gain, 3 dB focal width and sidelobe level, relative to the second configuration where the broadside of the elements faces the array center. This result points to the beneficial use of orthogonally oriented horizontally polarized antenna at the user to allow for flexible polarization alignment with the fixed array orientation. In addition, the vertical polarized circular array of Part I may be merged with the first configuration of Part II to provide optimal NFF to a randomly oriented user equipment with a polarization reconfigurable tripole antenna.

eess.SP↗

arXiv2025

On Properties of Phase-Conjugation Focusing for Large Intelligent Surface Applications -- Part I: Vertical Polarization

Large intelligent surface (LIS) is one promising path to leverage 6G performance in sub-10 GHz bands. This two-part paper explores the properties of phase-conjugation focusing for a simplified LIS setup with a two-dimensional (2D) circular antenna array and a user antenna located within the array aperture in the same plane. In Part I of this article, we assume vertical polarization for all antenna elements, whereas Part II assumes horizontal polarization. In Part I, we focus on the effect of array radius on the peak gain, 3 dB focusing width, and sidelobes for two types of circular arrays. The numerical results show that the gain minimum is located at the array center. The peak gain varies by less than 0.5 dB for focal points located within 2 from the array center. Similarly, the focal width and sidelobe level are also stable within this region, irrespective of array radius. From 2 from the center to the array edge, the closer proximity of the focal points to some array elements than other elements results in more drastic changes in these NFF properties. Finally, full-wave simulation using Ansys HFSS is used to partially validate the numerical results.

eess.SP↗

arXiv2025

Efficient Near-Field Beam Focusing Merging Orthogonal Matching Pursuit and CVX for Large Intelligent Surface Applications

In this paper, an efficient near-field beamforming method is proposed to support the large intelligent surfaces (LIS) that are expected to be widely deployed in 6G networks. This approach avoids directly applying convex (CVX) optimization for sparse selection in large-size array matrices, as such methods often lead to excessive computational time due to blind searching to satisfy a series of objective functions. First, based on the objective function, we prioritize a key component and employ the orthogonal matching pursuit (OMP) method to pre-select potential sparse target positions. To ensure focal symmetry, a coordinate mirror symmetry approach is adopted, meaning that selection is performed only in the first quadrant, while the remaining quadrants are determined through mirror symmetry relative to the first quadrant. This significantly reduces computational complexity at an early stage. Next, CVX is applied based on the pre-selected sparse array. Once a predefined threshold is met, a solution is obtained that satisfies the constraints of the beamfocusing. The results demonstrate that, compared with conventional methods, this approach improves efficiency by 15.12 times with 121 elements and 96.73 times with 441 elements. The proposed method demonstrates not only satisfactory performance but also considerable potential as a beam focusing technique for large-scale near-field array systems.

eess.SP↗

arXiv2025

Sparse Arrays Enable Near-Field Constant-Distance Focusing with Reduced Focal Shift

In near-field beam focusing for finite-sized arrays, focal shift is a non-negligible issue. The actual focal point often appears closer to the array than the predefined focal distance, significantly degrading the focusing performance of finite aperture arrays. Moreover, when the focus point is scanned across different locations, the degradation becomes even more pronounced, leading not only to positional deviation but also to substantial energy loss. To address this issue, we revisit the problem from the perspective of communication degrees of freedom. We demonstrate that a properly designed sparse array with optimized element spacing can effectively mitigate focal shift while enabling stable control of the focusing height during beam scanning. Simulation results based on dipole antennas with different polarizations and patch antennas validate our findings. Notably, with optimized inter-element distances, the energy distribution across focal points becomes nearly uniform, and highly accurate focusing positions are achieved.

eess.SP↗

arXiv2025

Near-Field Collinear Dipole Array Design with Dual-Polarization Operation for Wireless Power Transfer

A large intelligent surface (LIS) is a promising approach to enhancing 6G performance in sub-10 GHz frequency bands. An analysis of the horizontal dipole linear array reveals that when the array size is sufficiently large, employing the conjugate phase method to excite the antenna elements results in a focal point along the central line of the array that gradually stabilizes at a constant value. The study evaluates the 3dB focal resolution generated by the dipole array for two polarizations. Additionally, the feasibility of generating circularly polarized focal points and the focal displacement under a limited array size are explored. Previous near-field focal synthesis methods primarily considered only the line-of-sight (LOS) channel. To enhance practicality, this study adopts a two-ray channel model to analyze near-field focal synthesis results in the presence of a reflected path. Both two polarizations are discussed separately, with design guidelines provided for array placement and focal point positioning. Finally, electromagnetic simulations are conducted within the linear array to validate the proposed design. These simulations highlight the capability of the horizontal dipole array configuration to be directly applied without significant coupling effects between elements. The proposed design guidelines lay a solid foundation for the application of LIS in 6G technology.

eess.SP↗

arXiv2025

Properties of Near Field Focusing for Cylindrical Dipole Arrays in Enclosed Array Volume

Motivated by large intelligent surface applications, the electric field properties of near field focusing using phase conjugation method are analyzed for cylindrical dipole arrays. Firstly, for the transmitting antennas featuring vertical polarization, the polarization characteristic is decomposed along the x, y, and z directions. Three typical cases are studied when the focal points are at (xf , 0, 0), (0, yf , 0), and (0, 0, zf ). When the length of the cylindrical dipole array is significantly larger compared to its radius, certain unique insights emerge. When the focal point is positioned along (0, 0, zf ), apart from the region on both sides, the ratio between Ez and Ex/Ey remains {\pi}/2. Additionally, When the focal point is located within the cylinder, the electric field of each polarization is approximately the same everywhere. In other words, beam focusing does not incur losses due to different positions. The focusing resolution of Ez is the same in the transverse and longitudinal directions. Different from the situation where the 3 - dB focal beam depth is much smaller than the focal beam width for the most of arrays, the resolution in the longitudinal can be improved, respectively. Through a comprehensive grasp of these design principles, we can gain a deeper understanding of the specific areas with significant potential for practical applications.

eess.SP↗

arXiv2024

Unusual Pore Volume Dependence of Water Sorption in Monolithic Metal-Organic Framework

Monolithic metal-organic frameworks (MOFs), which have a continuous structure composed of small primary MOF particles and amorphous networks, are demonstrated to possess larger pore volume and thus better larger gas uptake capacity compared to their powder forms. Here, we systematically investigated the water vapor adsorption kinetics in a prototypical MOF, i.e., MOF-801. Our results show that the total pore volume (average pore diameter) of the monolithic MOF-801 is 0.831 cm3/g (5.20 nm) which is much larger than that of powder MOF-801, i.e., 0.488 cm3/g (1.95 nm). Unexpectedly, we find that the water uptake capacity of monolithic MOF-801 is much lower than that of powder MOF-801 when the RH ranges from 10% to 90%. Our molecular dynamics simulations further demonstrate that the unexpected water uptake capacity of monolithic MOF-801 at RH of 10%~90% is caused by the water film formed by the capillary condensation in these mesopores of monolithic MOF-801. The water molecules can overcome the capillary force when the RH is higher than 90%, and then leads to the increase of the corresponding water uptake capacity of monolithic MOF-801. Our findings reveal the underlying mechanisms for water adsorption kinetics in both powder and monolithic MOFs, which could motivate and benefit the new passive cooling or water harvesting system design based on MOFs.

physics.app-ph↗

arXiv2023

Direct Observation of Tunable Thermal Conductance at Solid/porous Crystalline Solid Interfaces Induced by Water Adsorbents

Improving interfacial thermal transport is crucial for heat dissipation in systems. Here, we design a strategy by utilizing the water adsorption-desorption process in porous metal-organic frameworks (MOFs) to tune the interfacial heat transfer. We observe a changeable thermal conductance across the solid/porous MOFs interfaces owing to the dense water channel formed by the adsorbed water molecules in MOFs. Our experimental results show that the interfacial thermal conductance of Au/Cu3(BTC)2 heterointerfaces is increased up to 7.1 folds by this strategy, where Cu3(BTC)2 is a typical porous MOF and usually referred to as HKUST-1. Our molecular dynamics simulations further show that the surface tension of Au layer will cause the adsorbed water molecules in HKUST-1 to gather at the interfacial region. The dense water channel formed at the interfacial region can activate the high-frequency lattice vibrations and act as an additional thermal pathway, and then enhance heat transfer across the interfaces significantly. Our findings provide a new concept for tailoring thermal transport at the solid/porous MOFs heterointerfaces which will largely benefit MOF-related applications.

cond-mat.mtrl-sci↗