Zhizhang David Chen

Dalhousie University | Dal · Department of Electrical & Computer Engineering

A three-dimensional FDTD subgridding method with the summation-by-parts simultaneous approximation term (SBP-SAT) technique is proposed to solve challenging multiscale problems. Its long-time stability is proved through the energy method by carefully enforcing properties of the SBP operators and selecting free parameters in SATs. It is found that t...

An ultrahigh-frequency (UHF) radio frequency identification (RFID) tag antenna that can be placed on a metal surface with stable performances is proposed. It is made of a modified dipole with two cylindrical metal grounds on both ends. The cylindrical metal grounds not only enable the antenna to roll freely on a metal surface with the stable readin...

Heart rate variability (HRV), indicating the variation in intervals between consecutive heartbeats, is a crucial physiological indicator of human health. However, detecting HRV using frequency-modulated continuous-wave (FMCW) radar is highly susceptible to interference from respiration, minor body movements, and environmental noise, especially in m...

Non-resonant partially reflective surface (PRS) antennas (PRSAs) offer high-gain performance, but their design typically requires extensive electromagnetic simulations to obtain near-field phase distributions, leading to time-consuming processes, especially for complex or large antennas. This letter presents a novel analytical design method that si...

It is important to estimate the global network traffic data from partial traffic measurements for many network management tasks, including status monitoring and fault detection. However, existing estimation approaches cannot well handle the topological correlations hidden in network traffic and suffer from limited imputation performance. This paper...

A finite-difference time-domain (FDTD) subgridding method is proposed to efficiently and accurately solve three-dimensional electromagnetic problems. Based on the reciprocal and symmetric interpolation operators between coarse and subgridding meshes, spatial coupling matrices are carefully designed to guarantee long-time stability. To further enhan...

One of the main factors that limit the practical applications of a wireless power transfer (WPT) system is its possible low-power transfer efficiency (PTE) against changes in distances and misalignments between the Rx and Tx coils, as well as changes in the loads. Although many methods have been proposed to address the issue, a systematic and optim...

The time-reversal method has been proven effective in reconstructing sources due to its spatiotemporal focusing properties. Much work so far on the topic has been conducted with sources of specific frequency bandwidths. However, most sources in practical situations are time-harmonic or single frequency. This paper proposes a novel approach to addre...

Face recognition for surveillance remains a complex challenge due to the disparity between low-resolution (LR) face images captured by surveillance cameras and the typically high-resolution (HR) face images in databases. To address this cross-resolution face recognition problem, we propose a two-stage dual-resolution face network to learn more robu...

A surface integral equation (SIE) formulation under the magneto-quasi-static assumption is proposed to efficiently and accurately model arbitrarily shaped interconnects in packages. Through decently transferring all electromagnetic quantities into circuit elements, the loop analysis is used to carefully construct matrix equations with an independen...

p>Electromagnetic time-reversal (TR) has been proven to be an effective and efficient method for solving source reconstruction problems. Different algorithms are proposed to deal with difficulties in real-world scenarios. In our previous work, we proposed the electromagnetic kurtosis methods and discussed the resolution in band-limited TR. However,...

p>Electromagnetic time-reversal (TR) has been proven to be an effective and efficient method for solving source reconstruction problems. Different algorithms are proposed to deal with difficulties in real-world scenarios. In our previous work, we proposed the electromagnetic kurtosis methods and discussed the resolution in band-limited TR. However,...

Like its outdoor counterpart (e.g., GPS), an indoor tracking system can bring about disruptive changes in how we live and work. This paper proposes a location and tracking system using a single WiFi link based on channel state information. The system can realize real-time, decimeter-level localization and tracking. In this system, phase calibration...

This article presents a wideband printed-monopole antenna pair with enhanced isolation. A clear two-step method based on even-mode (EM) and odd-mode (OM) analyses is developed for antenna decoupling and matching: first, the OM input impedance of the antenna pair is matched by adjusting the element spacing; next, the EM input impedance is matched by...

Human perturbation and its impact on vegetation and plant diversity during the Holocene have caused serious concerns for the biogeological and geoarchaeological communities. As a centre of origin of rice agriculture, the lower Yangtze region (LYR) has a long history of human occupation over most of the Holocene. However, early anthropogenic impacts...

Electrical impedance tomography (EIT) is a promising imaging technique that recovers the conductivity distribution inside a domain from noninvasive electrical measurements on the boundary. In this work, to accelerate the solving of EIT problems in arbitrarily-shaped domains and with a large number of unknowns ( N ), we propose a fast integral-equa...

A provably stable summation-by-parts simultaneous approximation term (SBP-SAT) finite-difference time-domain (FDTD) subgridding method with arbitrary grid ratios is proposed to accurately and flexibly solve 2-D electromagnetic problems. The interpolation matrix with the grid ratio 1: $n$ is first carefully designed, which satisfies the norm-compat...

In this paper, a context-based human target detection and position estimation algorithm as well as a position calibration algorithm based on radar irradiation angle are proposed to improve the positioning accuracy which is limited by the sparse and easily submerged characteristics of point cloud generated by millimeter-wave radar that leads to the...

A surface integral equation (SIE) formulation under the magneto-quasi-static assumption is proposed to efficiently and accurately model arbitrarily shaped interconnects in packages. Through decently transferring all electromagnetic quantities into circuit elements, the loop analysis is used to carefully construct matrix equations with an independen...

Subsidence of the ancient Zhejiang-Fujian Uplift (ZFU) is important to understand opening of the Chinese marginal seas and geomorphology of the east China coast. However, little was known about its subsidence history during the Quaternary. In this study we clarify three marine sedimentary layers constrained by luminescence (OSL) chronology in Ningd...

The transition from hunting and gathering to agricultural subsistence is a striking feature of the Neolithic revolution worldwide. Known as the cradle of a series of representative Neolithic cultures, south Hangzhou Bay (SHB) witnessed substantial changes in both landscape and human subsistence during the Holocene, yet the relationship between them...

p> The time reversal method has been shown to be capable of reconstructing electromagnetic sources. However, in the past, most sources reconstructed are point sources that may not be often seen in practical situations. This paper presents preliminary studies on the time-reversal reconstruction of the sources with arbitrary geometrical shapes and wi...

p>The time-reversal method has been applied to the source location due to its spatiotemporal focusing properties. Much work on the topic is to locate the impulse or wideband sources. However, most sources in practical situations are narrowband. Therefore, it is desirable to develop a single-frequency time-reversal method to reconstruct narrowband-s...

p> The time reversal method has been shown to be capable of reconstructing electromagnetic sources. However, in the past, most sources reconstructed are point sources that may not be often seen in practical situations. This paper presents preliminary studies on the time-reversal reconstruction of the sources with arbitrary geometrical shapes and wi...

p>The time-reversal method has been applied to the source location due to its spatiotemporal focusing properties. Much work on the topic is to locate the impulse or wideband sources. However, most sources in practical situations are narrowband. Therefore, it is desirable to develop a single-frequency time-reversal method to reconstruct narrowband-s...

p> Time-reversal (TR) is an effective method of solving inverse problems. However, the theory about the TR is still not clear. This work starts by examining fundamental laws in wave physics and electromagnetics and presents a revisit of the electromagnetic TR method. Then a cavity is employed, and the hardware experiments are performed to validate...

p> Time-reversal (TR) is an effective method of solving inverse problems. However, the theory about the TR is still not clear. This work starts by examining fundamental laws in wave physics and electromagnetics and presents a revisit of the electromagnetic TR method. Then a cavity is employed, and the hardware experiments are performed to validate...

A 19.5-m long Holocene sediment core (MZ-1) was retrieved from Manzala lagoon coast, the depocenter of the Nile Delta. AMS-dating shows the complete Holocene strata of MZ-1, mostly consisting of lagoon muds. The high-resolution dated geochemical elements (major and trace) and bio-markers (n-alkanes) of MZ-1 serve as effective hydroclimate proxies t...

In this letter, the convolutional implementation of the complex frequency shifted perfectly matched layer (CFS-PML) absorbing boundary condition (ABC) is proposed for the finite-difference time-domain (FDTD) method of solving wave equation. The FDTD solution time-marching formulations of the developed wave-equation convolutional CFS-PML (CPML) ABC...

The traditional finite-difference time-domain (FDTD) method is constrained by the Courant–Friedrich–Levy condition and suffers from the notorious staircase error in electromagnetic simulations. This article proposes a 3-D conformal locally one-dimensional FDTD (CLOD-FDTD) method to address the two issues for modeling perfectly electrical conducting...

To enhance the scalability and performance of the traditional finite-difference time-domain (FDTD) methods, a three-dimensional summation-by-parts simultaneous approximation term (SBP-SAT) FDTD method is developed to solve complex electromagnetic problems. It is theoretically stable and can be further used for multiple mesh blocks with different me...

A nonconformal domain decomposition method based on the hybrid surface integral equation partial differential equation (SIE-PDE) formulation is proposed to solve the transverse magnetic electromagnetic problems. In the hybrid SIE-PDE formulation, an equivalent model with only the electric current density is first constructed, and then is embedded i...

The Nile Delta in Egypt represents a valuable location to study the history of human societal development and agricultural advancement. However, the livelihood patterns of the earliest settlers – whether they were farmers or herders – remains poorly understood. Here we use non-pollen palynomorphs and pollen grains from a sediment core taken at Sais...

The increasing vulnerability of coastal megacities to storm surge inundation means both infrastructure and populations are subject to significant threat. Planning for further urban development should include consideration of the changing circumstances in coastal cities to ensure a sustainable future. A sustainable urban plan relies on sound prepare...

The Yangtze River originates from the Tuotuo on the southwestern slopes of the snow‐draped Geladandong Mountains on the Tibetan Plateau at about 6000 m elevation. The Yangtze Basin includes a complex variety of geological units, primarily sedimentary rocks of different ages. Limestone and terrigenous sedimentary rocks of Palaeozoic age with granite...

A summation-by-parts simultaneous approximation term (SBP-SAT) finite-difference time-domain (FDTD) subgridding method is proposed to model geometrically fine structures in this paper. Compared with our previous work, the proposed SBP-SAT FDTD method uses the staggered Yee's grid without adding or modifying any field components through field extrap...

The Yangtze River Tidal Reach (YRTR) is one of the most important land-sea transition zones in China. The YRTR is 640 km long and the adjacent land area of 220,000 km² has a population of ca. 150 million. Even though the effect of Three Gorges Dam (TGD) on its downstream has been extensively studied, a comprehensive understanding of natural and dam...

This study presents high-resolution multi-proxy analyses of a well-dated sediment core (BR-1, 19.20 m long) retrieved from the Nile Delta, aiming to reconstruct the Holocene evolution of the Burullus lagoon. In particular, we focus on the morphodynamical processes, sediment provenances and related hydro-climatic changes. The Holocene strata of the...

This article presents a unified description of numerical methods for solving electromagnetic field problems. Traditionally, these methods are considered to be independent and alternative ways of solving Maxwell's equations or equations derived therefrom. However, they all are projective approximations of the unknown solution by known expansion or b...

One of the major considerations for a short- or intermediate-distance wireless power transfer (WPT) system is its robustness against variations of the distance, displacement, misalignment between the transmitting and receiving coils, and variations of the loads. Designing such a robust system has not been easy since most methods require some forms...

The modern Nile delta developed in the Middle and Late Holocene, and at its most northern-central point is situated at the Burullus Lagoon, which is environmentally diverse, including salt marshes, mudflats, and sand plains, and separated from a sea by a sand barrier overtopped with high sand dunes. The lagoon has been fed since the Middle Holocene...

A summation-by-parts simultaneous approximation term (SBP-SAT) finite-difference time-domain (FDTD) subgridding method is proposed to model geometrically fine structures in this article. Compared with other works, the proposed SBP-SAT FDTD method uses the staggered Yee’s grid without adding or modifying any field components through field extrapolat...

To enhance the scalability and performance of the traditional finite-difference time-domain (FDTD) methods, a three-dimensional summation-by-parts simultaneous approximation term (SBP-SAT) FDTD method is developed to solve complex electromagnetic problems. It is theoretically stable and can be further used for multiple mesh blocks with different me...

An angle estimation algorithm for tracking indoor moving targets with WiFi is proposed. First, phase calibration and static path elimination are proposed and performed on the collected channel state information signals from different antennas. Then, the angle of arrival information is obtained with the joint estimation algorithm of the angle of arr...

With the development of wireless communication technology, indoor tracking technology has been rapidly developed. Wits presents a new indoor positioning and tracking algorithm with channel state information of Wi-Fi signals. Wits tracks using motion speed. Firstly, it eliminates static path interference and calibrates the phase information. Then, t...

We proposed a simple and efficient modular single-source surface integral equation (SS-SIE) formulation for electromagnetic analysis of arbitrarily connected penetrable and perfectly electrical conductor (PEC) objects in 2-D space. In this formulation, a modular equivalent model for each penetrable object consisting of the composite structure is fi...

Long-standing arguments regarding the early cultural transition, the domestication of plants and the impacts of climate change on past Egyptian societies remain contentious. In this paper, we demonstrate that grazing started at our study site, Kom El-Khilgan in the NE Nile Delta, ca. 7000 years ago, which was several hundred years prior to crop far...

A compact dual-band circularly polarized microstrip ring antenna for GPS L 1 and L 2 bands is proposed. The antenna comprises a two-ring radiator and a multiring-etched ground, forming two Yagi–Uda double-ring antennas, respectively. It operates in dual bands with a small ground size and unidirectional radiation. The proposed antenna yields rig...

A single-source surface integral equation (SS-SIE) for transverse electric (TE) scattering from cylindrical multilayered objects is proposed in this article. By incorporating the differential surface admittance operator (DSAO) and recursively applying the surface equivalence theorem from innermost to outermost boundaries, an equivalent model with o...

We demonstrate a broadband near-100%-efficiency ultrathin metasurface operating at microwave and millimeter-wave frequencies. We develop and employ two orthogonally polarized metallic gratings to form a Fabry-Perot cavity and incorporate a subwavelength metallic double-split-ring resonator at the center of each unit cell. It allows arbitrary amplit...

We proposed a provably stable FDTD subgridding method for accurate and efficient transient electromagnetic analysis. In the proposed method, several field components are properly added to the boundaries of Yee's grid to make sure that the discrete operators meet the summation-by-parts (SBP) property. Then, by incorporating the simultaneous approxim...

The traditional finite-difference time-domain (FDTD) method is constrained by the Courant-Friedrich-Levy (CFL) condition and suffers from the notorious staircase error in electromagnetic simulations. This paper proposes a three-dimensional conformal locally-one-dimensional FDTD (CLOD-FDTD) method to address the two issues for modeling perfectly ele...

The Holocene hydro-ecological and environmental change of the Nile Delta remains a knowledge gap. The study aimed to approach this objective by using multi-proxies of lipid biomarkers and bulk organic carbon (OC) in a well-dated MZ-1 core in Manzala lagoon off the Nile Delta. The data defined five distinctive stages at decadal-centennial scale. Sta...

Due to increasing water use, diversion and salinization, along with subsidence and sea-level rise, deltas in arid regions are shrinking worldwide. Some of the most ecologically important arid deltas include the Colorado, Indus, Nile, and Tigris-Euphrates. The primary stressors vary globally, but these deltas are threatened by increased salinization...

In our previous study, a concept of electromagnetic kurtosis for time-reversal source reconstruction was presented. From a two-step process, the electromagnetic kurtosis can first identify the excitation time instants then localize the sources locations with time-reversal technique. In this paper, we extend the concept of the electromagnetic kurtos...

In our previous study, a concept of electromagnetic kurtosis for time-reversal source reconstruction was presented. From a two-step process, the electromagnetic kurtosis can first identify the excitation time instants then localize the sources locations with time-reversal technique. In this paper, we extend the concept of the electromagnetic kurtos...

We proposed a simple and efficient modular single-source surface integral equation (SS-SIE) formulation for electromagnetic analysis of arbitrarily connected penetrable and perfectly electrical conductor (PEC) objects. In this formulation, a modular equivalent model for each penetrable object consisting of the composite structure is first independe...

This study reviews the monsoonal Yangtze and the arid Nile deltas with the objective of understanding how the process–response between river-basin modifications and delta-estuary ecological degradation are interrelated under contrasting hydroclimate dynamics. Our analysis shows that the Yangtze River had a long-term stepwise reduction in sediment a...