Helmken, Henry

Wireless communication has become a significant part of our life. Bluetooth(TM) Radio system is one example of wireless communication, Wireless Local Area Network (WLAN) is another example of wireless technology. Both of systems operate in the same frequency band, the Industrial Scientific Medical (ISM) band, which use a 2.4 GHz carrier frequency. According characteristic of system, multi-path effect are major concern for indoor propagation. A frequency sounding technique is introduced to determine characteristics of the multi-path signal. However, multi-path can not be evaluated directly; some parameters are measured to determine the effect of multi-path. Angles of arrival can be used to facilitate the effect of multi-path signal. Using MatLab programming, Spatial Filter Periodogram (SPF) is introduced to estimate angles of arrival.

Body-proximate telecommunications devices are examined in both direct and multipath propagation. The study begins with a characterization of standard field strength sensitivity measurement methods for body-proximate telecommunications devices. Original measurements on a group of anthropometrically diverse people reveal that human adults, in a standard pose, are remarkably similar with respect to belt worn sensitivity performance, which motivates and justifies the use of an existing and a newly introduced light weight simulated human body device for testing, analysis and optimization of body worn telecommunications devices. Standard measurement methods using standard open air test ranges are established and validated by international transfers of measurements. The study extends to optimization of telecommunications devices in multipath, and particularly to the diversity reception of signals. A novel signal simulation model is introduced which includes multipath and shadowing, and is validated against both theoretical statistics and measurements. The signal simulation model is extended to characterize polarization randomization and cross-coupling based on an urban generalization of building height to street width ratio. The model is used to analyze measurements of polarization randomization of signals originating from an airborne transmitter flying a path whose geometry is consistent with low earth orbiting communications satellites.

This dissertation is concerned with the development of a bandwidth extrapolation technique that performs maximum entropy estimations over wavelet subspaces. Bandwidth extrapolation techniques have been used in radar applications to improve range and cross range resolution of radar cross section (RCS) images. Comparisons are made of the performance of conventional maximum entropy estimation to maximum entropy estimation over wavelet subspaces. A least squares prediction error measure is used to compare original measured RCS data to extrapolated data. Then a relative error is defined as the ratio of prediction error using conventional maximum entropy to prediction error using maximum entropy over wavelet subspaces. Application of the bandwidth extrapolation technique is to measured RCS data of two objects. The first object consists of two 3/8" diameter conducting spheres placed 4" apart. Measurements used are for vertical polarization and 0 degree aspect angle covering a frequency range of 8.0 to 12.3827 GHz. The second object is a 1.6 meter aluminum cone. Measurements used are for vertical polarization and 0 degree aspect angle (nose on) covering a frequency range of 4.64 to 18.00 GHz. Results are shown for extrapolate measured data plus the original data with Gaussian white noise added to noise ratios of 25 dB, 20 dB, 15 dB, and 10 dB.

In this dissertation, new semi-analytical expressions for the diversity bit error rate performance of asynchronous direct sequence-code division multiple access (DS-CDMA) systems in multipath fading channels are derived. Also, the PN acquisition time of a DS system with offset frequency periods greater than the code period in a AWGN channel is measured via laboratory experiments. In Part I we consider DS-CDMA systems operating in a cellular environment with multipath reception. Multipath propagation is exploited through the use of RAKE receivers. Coherent, differentially coherent and noncoherent binary demodulation schemes are considered. The multiple access interference is modeled as AWGN, conditioned on the fading statistics of the received signal. The mobile radio channel introduces selective fading, and is modeled as a tapped delay line. The amplitude of each resolvable path is statistically described by the Nakagami distribution, which is a general solution to the random vector problem that causes rapid fading. However, we assume independent but nonidentical fading along the RAKE branches. Therefore our analysis supports unequal mean powers and different amount of fading in the multipath components combined by the receiver. Also, the results can be easily extended to account for diversity from multiple antennas or coding in the generalized Nakagami multipath fading environment. In Part II we consider a land mobile satellite channel. First, a laboratory experiment is setup to evaluate the PN acquisition performance of a digital IF receiver in a AWGN channel with large Doppler offset. The digital conversion receiver uses the inherent aliasing property of sampling to realize the baseband conversion using a single analog-to-digital converter. Thereafter, digital signal processing on the I and Q samples enable for design trade-offs in the acquisition of the PN code with Doppler periods greater than the code period. Two code phase selection criterions, namely the maximum criterion and the threshold crossing criterion, are investigated and their acquisition time is measured for different frequency offsets and input IF signal to noise ratios. We also derive semi-analytical expressions for the BER performance of coherent and differentially coherent systems operating in a mobile satellite channel. In this case the channel is modeled as a multipath nonselective channel, but diversity gain can still be obtained through path diversity. This is the scenario when a signal is transmitted to all satellites in view and the received replicas are independently demodulated and combined at the receiver. Our analysis extends previous results to the case of unequal mean powers and Rice factors in the combined signals; a valid assumption if we consider that the satellites are in view at different elevation angles. Furthermore, the effect of imperfect power control in such mobile satellite DS-CDMA systems is also considered.

In this dissertation, the Radar Cross Section (RCS) of a large periodic array of rectangular open-ended waveguide apertures is determined numerically using several methods. The aperture boundaries are presumed to be Perfect Electrical Conductors (PEC). Although the problems of radiation from such a waveguide array and of aperture array scattering have been treated in the literature, the problem of scattering from an array of waveguide apertures does not appear to have been solved before. Considering the case of an array with constituent guides of semi-infinite length, the RCS is computed by several numerical methods based on the Integral Equation (IE) method, a least-squared error minimization technique referred to as Squared Field Error (SFE) method, direct solution of a surface integral equation, the Spectral Domain Method, and by using waveguide modes computed via the Finite Element Method (FEM). The case of finite-length guides is also treated using the IE and SFE methods. The results of these methods are compared with experimental data obtained from an outdoor RCS range. In order to simulate the semi-infinite case, the finite-length waveguides were terminated with radar absorbing foam so that nearly all reflection occurred at the apertures impinged upon by the incident plane wave. For all the methods cited, the infinite array approximation (cell-to-cell field periodicity except for a linear progressive interelement phase shift) is assumed to hold. A derivation of Floquet modes which implement this "phase-periodic" boundary condition is provided in an appendix, where an incidental discussion concerning the scalar and vector Laplacian operators is also furnished. A description of the structure and user interface of the software which has been written to implement the various methods is also given. The purposes of major subroutines and data structures are also delineated and several control-flow diagrams are included. As a foundation to extend the present work to analysis of the electromagnetic fields within an absorber coated PEC waveguide, a brief survey and a discussion of related work is provided.

A theory of the circular loop antenna constructed from finite conductivity wire is developed via a Fourier series expansion of the currents in the loop. Models for a family of small loop antennas are also presented. A new high sensitivity and selectivity heterodyne fiber optic based electromagnetic field detector is developed compatible with open antenna range measurements made at low signal levels and in the presence of strong interfering signals. A new analytical solution pertaining to the response of a disk loaded dipole antenna representing a dipole configured on a lossy dielectric medium is developed using a field compensation theorem and a geometrical theory of diffraction. The multipole expansions for the scattered fields of a multilayered infinite cylinder illuminated by oblique incidence plane wave are formulated and programmed for numerical analysis. The response of cylinders with constitutive parameters reflecting those used in human phantoms are calculated. The response of a small antenna proximal to a multilayered cylinder is analyzed. The scattered fields from multilayered bodies are coupled to a small wire antenna using a combined methods induced electromagnetic force (EMF) technique. New results concerning the response of a loop antenna near a multilayered body obtained via a zero and first phase current model are presented. The new technique is applied in the analysis of human phantoms tested in an open field antenna range. Validation of the theory of multilayered human phantoms with measurements using the new detector is demonstrated.

Due to a continuous growth of demand in voice and data communications for wireless systems, there is an ongoing challenge to design improved radiowave communication links. Polarization is one of the key properties of electromagnetic waves used in wireless communication and is the least studied in a scattering environment. A detailed understanding of how signals become de-polarized can improve the propagation models and can lead to more accurate propagation predictions and possibly new applications in the use of polarization. Potential beneficiaries of a system design using multi-dimensional polarization constellations to provide maximum polarization separation might include inter-satellite links, WLAN, LEO satellites [BSPP92] [IPLJ81] [SIKA98] and other systems operating in an environment where depolarization is insignificant. To examine the applicability of polarization in wireless communication systems, polarization field measurements were conducted, and the results and analysis are presented in this dissertation. Based on the analysis, a statistical model that characterizes polarization in relation to Poincare sphere is developed and presented. Design and implementation of an N-constellation diversity scheme that provides maximum polarization separation distance is presented. Furthermore, a decision-making algorithm is utilized for detection of the received electric field that selects the minimum Euclidian distance between the transmitter and receivers in Stokes space. The scenario is simulated for an N-constellation scheme for N = 2, 3, 4 based on the design scheme utilizing the developed statistical model and decision-making algorithm and is used to evaluate the detectability and performance comparison for various values of standard deviations.

Time Division Multiple Access (TDMA) architecture is an established technology for digital cellular, personal and satellite communications, as it supports variable data rate transmission and simplified receiver design. Due to transmission bandwidth restrictions, increasing user demands and the necessity to operate at lower signal-to-noise ratio (SNR), the TDMA systems employ high order modulation schemes such as M-ary Quadrature Amplitude Modulation (M-QAM) and burst transmission. Use of such techniques in low SNR fading channels causes degradations of carrier frequency error, phase rotation error, and symbol timing jitter. To compensate for the severe degradation due to additive white Gaussian noise (AWGN) and channel impairments, precise and robust synchronization algorithms are required. This dissertation deals with the synchronization techniques for TDMA receivers using short burst mode transmission with emphasis on preamble-less feedforward synchronization schemes. The objective is to develop new algorithms for symbol timing, carrier frequency offset acquisition, and carrier phase tracking using preamble-less synchronization techniques. To this end, the currently existing synchronization algorithms are surveyed and analyzed. The performance evaluation of the developed algorithms is conducted through Monte-Carlo simulations and theoretical analyses. The statistical properties of the proposed algorithms in AWGN and fading channels are evaluated in terms of the mean and variance of the estimated synchronization errors and their Cramer-Rao lower bounds. Based on the investigation of currently employed feedforward symbol timing algorithms, two new symbol timing recovery schemes are proposed for 16-QAM land mobile signals operating in fading channels. Both schemes achieve better performance in fading channels compared to their existing counterparts without increasing the complexity of the receiver implementation. Further, based on the analysis of currently employed carrier offset and carrier phase recovery algorithms, two new algorithms are proposed for carrier acquisition and carrier tracking of mobile satellite systems utilizing short TDMA bursts with large frequency offsets. The proposed algorithms overcome some of the conventional problems associated with currently employed carrier recovery schemes in terms of capture range, speed of convergence, and stability.

The probability density function of the target boresight angle estimate of an amplitude comparison monopulse radar system is presented when the target in flight is obscured by dipole chaff, precipitation, or sea clutter. A maximum likelihood analysis is made to determine the form of the angle estimate in clutter plus receiver noise. The biasedness of the estimate, and the variance of that estimate is shown as a function of signal-to-clutter-plus-noise, SCNR. In order to improve the SCNR, the optimum transmit and receive polarization vectors for a given target and clutter scattering matrices are introduced. The optimum polarization vectors yield the maximum signal-to-clutter power ratio for a given scenario. The target model is a right circular cylinder with a hemispheric nose cone and the control surfaces attached. Included are the performance analysis of suboptimal polarization vector pair operations when the complex antenna cluster construction for the polarization diversity or agility is undersirable due to cost or the space restriction.