Feinian Wang

A Physics-Based Statistical Model for Wave Propagation Through Foliage

Accurate estimation of signal attenuation in highly scattering environments such as a forest medium has long been a challenging problem. The challenges arise from the fact that the incoherent power, which becomes dominant after some distance of wave propagation in the random medium, is difficult to model. In this paper, a statistical wave propagation model (SWAP) is developed for predicting the wave propagation path-loss in foliage. In this analysis, the forest is assumed to be statistically homogeneous along the direction of wave propagation, and the potentially large distance between the transmitter and receiver in the forest is divided into many statistically similar blocks of finite dimension. A fractal-based forest coherent scattering model (FCSM) is used as a foundation for predicting the characteristics of wave interaction with the foliage. By applying FCSM to a representative block of the forest, the statistical input-output field relationship including field attenuation and regeneration (due to scattering), is computed by a Monte Carlo simulation. These precomputed statistical quantities of the forest are then reused for all blocks using a network theory. The overall received power, and hence the path-loss, is estimated by following the coherent and incoherent power through all the forest blocks. Compared to a brute force approach, the computation time is significantly reduced while the prediction accuracy is maintained. Simulation results for path-loss as a function of propagation distance, frequency, and forest density are presented. The model is successfully validated by comparing its predictions against independent propagation measurements through foliage

An enhanced millimeter-wave foliage propagation model

In this paper, the behavior of wave propagation through coniferous forest stands at millimeter-wave frequencies is characterized both theoretically and experimentally. A coherent wave propagation model is used to simulate the propagation through foliage. The coherent model is composed of two components: a forest stand generator that makes use of a stochastic fractal model, and an electromagnetic model that makes use of Foldys approximation and single scattering. An outdoor measurement system is designed and used for characterizing the channel behavior for a pine tree stand at Ka-band (35 GHz). In this experiment, 84 independent spatial samples of transmitted signal through the pine stand were collected to obtain the path-loss statistics. The comparison between measurement and simulation results showed that single scattering theory overestimates the wave attenuation through foliage. To improve the accuracy of the coherent model, partial multiple scattering occurred among the needles of highly dense leaf clusters must be included for the estimation of the coherent attenuation. Distorted Born approximation is used to macromodel the scattering pattern from needle clusters. This technique has comparable accuracy and requires much less computational resources than a full-wave solution, such as method of moment. By including multiple scattering effects of needle clusters in the simulation model, much better agreement is obtained for both mean and standard deviation of the path-loss.

Estimation of coherent field attenuation through dense foliage including multiple scattering

Single-scattering theory is shown to be insufficient for the estimation of effective propagation constant in foliage at high microwave and millimetre-wave frequencies. Clusters of broad leaves and needles are treated as a unit scatterer whose ensemble forward scattering is used in Foldys approximation to estimate attenuation rate in foliage. It is shown that single-scattering approximation overestimates forward scattering as high as 3-4 dB at 35 GHz.

Study of millimeter-wave radar for helicopter assisted landing system

This paper discusses the development of an algorithm used to simulate the effectiveness of millimeter-wave radar in imaging a rough terrain, for the purpose of helicopter assisted landing. Using an externally generated terrain and the physical optics approximation, the algorithm computes the backscatter response of the terrain when illuminated by a real aperture antenna. Results are presented from simulating terrains with different macroscopic features, such as a hump, ditch or a slope. It shown that operating at millimeter-wave, more specifically at W-Band frequencies, is ideal for such an application where a compact sensor is required to achieve high resolution imaging.

A measurement system for ultrawide-band communication channel characterization

In this paper a novel wide-band propagation channel measurement system with high dynamic range and sensitivity is introduced. The system enables the user to characterize signal propagation through a medium over a very wide frequency band with fine spectral resolution (as low as 3 Hz) by measuring the attenuation and phase characteristics of the medium. This system also allows for the study of temporal, spectral and spatial decorrelation. The high fidelity data gathered with this system can also be utilized to develop empirical models or used as a validation tool for physics based propagation models which simulate the behavior of radio waves in different environments such as forests, urban areas or indoor environments. The mobility and flexibility of the system allows for site specific measurements in various propagation scenarios.

Study of Millimeter-Wave Radar for Helicopter Assisted-Landing System

Landing helicopters on rough unprepared terrain has always been considered hazardous, since the massive dust cloud generated by wind drafts from the helicopters rotors completely obstructs the pilots view of the landing area. This article presents a performance assessment of a proposed millimeter-wave (MMW) three-dimensional imaging radar system, specifically meant for helicopter assisted landing. The assessment includes simulation of radar backscattering from the underlying rough terrain, in addition to the signal attenuation and scattering from dust clouds generated by the helicopters rotors. Terrain scattering is simulated in two steps: 1) generation of a two-scale random rough surface according to prescribed statistics of a large-scale undulation and a small-scale surface roughness; and 2) simulation of the returned signal, including the effects of the real-aperture radar parameters and the terrain-backscatter response. Details of the three-dimensional imaging algorithm are presented. Single-scattering theory is used to simulate the effects of a dust cloud - including signal attenuation and backscatter clutter generation - on the radars performance. It is shown that operating in the upper millimeter-wave regime (70 GHz-220 GHz) is the most practical solution for a compact, high-resolution, three-dimensional imaging system for this problem.

Phased array of large reflectors for deep-space communication

In this paper the problem of uplink array calibration for deep-space communication is considered. A phased array of many modest-size reflectors antennas is used to drastically improve the uplink effective isotropic radiated power of a ground station. A radar calibration procedure for the array phase distribution is presented using a number of in-orbit targets. Design of optimal orbit and the number of calibration targets is investigated for providing frequent calibration opportunities needed for compensating array elements phase center movements as the array tracks a spacecraft. Array far-field focusing based on the near-filed in-orbit (low Earth orbit (LEO)) calibration targets is also presented and array gain degradation analysis based on the position error of the array elements and in-orbit targets has been carried out. It is shown that errors in the in-orbit targets positions significantly degrade the far-field array gain while the errors in array elements positions are not very important. Analysis of phase errors caused by thermal noise, system instability, and atmospheric effects show insignificant array gain degradation by these factors

Accurate estimation of electromagnetic wave extinction through foliage

In this paper a new statistical wave propagation (SWAP) model is introduced to model the wave propagation behavior through long distance forested environments. It divides the forest into statistically identical blocks along the wave propagation direction, By applying the existing single scattering wave theory model to one representative block of forest, it can precompute and store the statistical properties of the forest which can then be reused to compute the total power at the receiver. The computation intensity is significantly reduced while the modeling accuracy is enhanced. Three sets of simulation experiments are conducted to validate the SWAP model and the results are presented

Ground Array Calibration using Lunar InSAR Imagery

A new technique of phase calibrating the uplink of a ground array consisting of large reflector antennas is studied. The Moon is selected as a calibration target since it falls within the array far-field and avoids the positioning error problem encountered by low-Earth orbit (LEO) calibration targets. As a distributed radar target, the Moon cannot be directly used like point targets. A planetary synthetic aperture radar (SAR) imaging technique is employed to divide the antenna footprint on the lunar surface into many small pixels. Each array element can form its own SAR image of each pixel and the phase differences (interferograms) among these images can be used to perform phase calibration. Orthogonal pseudonoise (PN) codes are used at different array elements to distinguish their signals at a common receiver. A practical design of the calibration system parameters is illustrated. In order to evaluate the performance of this calibration technique, a high-fidelity 3-D lunar surface profile and scattering model is developed. Simulation results are presented to show the effects of multi-pixel averaging, surface undulation, baseline separation, and image misregistration on the proposed calibration performance.

Long distance path-loss estimation for wave propagation through a forested environment

Accurate modeling of wave propagation behavior through forested environments is of great interest for civilian and military communication applications. Existing foliage propagation models treat the forest as an effective lossy dielectric medium and predict an exponential increase of path-loss with respect to the propagation distance within the forest. Such a model captures only the coherent power of the signal and neglects the incoherent power from the field fluctuation due to the random distribution of scatterers, such as brranches and leaves, within th forest. For long distance communication, such incoherent power tends to dominate the overall received power after a certain distance. Experimental results show that the simple models grossly overestimate the path-loss. A new Statistical WAve Propagation (SWAP) model, based on coherent wave theory and a renormalization approach, is presented and is shown to provide a reasonably accurate and computationally efficient solution for the estimation of path-loss in a forested environment over long distances.