Neelakanta, Perambur S.

This research addresses a specific class of electromagnetic problems concerning the radiation and scattering of high frequency electromagnetic waves at the surfaces of composite materials. With the advent of need-based developments in electromagnetic material technology, a research niche has stemmed to analyze the interaction of electromagnetic energy with different versions of composite materials used mostly as surface materials such as in radar-stealth applications. Mixture-dielectrics, mixture magnetic materials, textured electromagnetic composites with matrix layers of lossy dielectric/magnetic materials, chiralic media, active surface materials etc. are a few emerging candidates of viable composites being considered in the state-of-the-art engineering electromagnetics. Specific to these materials, the analyses pertaining to electromagnetic radiation and scattering problems require a unique, approach vis-a-vis the heterogeneous properties of the composite material surfaces involved. Presently, the proximity of such surfaces is characterized and duly accounted for, by a mutual immittance formulation based on the Monteath's field compensation theorem. Using the relevant theoretical considerations, electromagnetic plane wave and/or focused beam radiation due to an aperture, conducting patch on flat and curved surfaces and scattering by an object coated with a composite material are elucidated. Also, an experimental method of evaluating the surface immittance is indicated. Theoretical computations are validated by comparing the results with those obtained via other methods. Some experimental results are furnished in support of the theoretical approaches presented.

By revisiting the popular framework of depicting neuronal (collective) activities as analogous to Ising's spin-glass theory of interacting magnetic spins, the contradictions that coexist with such an analogy are extracted and discussed. To alleviate such contradictions, an alternative strategy of equating the neuronal interactions to the partially anisotropic nematic phase of disorder pertaining to liquid crystals is proposed. Hence, the extent of anisotropy in the neuronal system, quantified in terms of an order-function, is specified to elucidate the nonlinear squashing action of the input-output relations in a neuronal cell. The relevant approach thereof, is based on Langevin's theory considerations as applied to dipole molecules. Further, in view of the stochastical properties due to the inherent disorder associated with the neuronal assembly, the progression of state-transitions across the interconnected cells is modeled as a momentum flow relevant to particle dynamics. Hence, corresponding wave mechanics attributions of such a collective movement of state-transition activity are described in terms of a probabilistic wave function. Lastly, the stochastical aspects of noise-perturbed neuronal dynamics are studied via Fokker-Planck equation representing the Langevin-type relaxational (nonlinear) process associated with the neuronal states. On each of these topics portraying the stochastical characteristics of the neuronal assembly and its activities, newer and/or more exploratory inferences are made, logical conclusions are enumerated and relevant discussions are presented along with the scope for future research to be pursued.

The research addressed in this study and deliberated in the dissertation is broadly tied to finding hybrid methods of diversity compatible for modern wireless applications. The hybrid strategy advocated thereof involves a combined use of space- (antenna and polarization), frequency-, and time-diversity schemes in a judicious manner so as to realize a desirable BER versus Eb/No performance across RF links that face multipath and other unwanted EM artifacts. The underlying reason for the hybrid scheme as above is to replace multiple-antenna based transreceive diversity. Such multiple antennas would otherwise require large base station real estate and may not be compatible for hand-held (space-constrained) RF units. On the contrary, use of hybrid schemes would restrict multiple number of antennas and conserves the space. After analyzing a set of plausible techniques of hybrid diversity compatible for modern wireless techniques, a focused study has been done on polarization-sense (PS) antenna diversity scheme. Its fruitful application for indoor systems (like Bluetooth(TM)/ZigBee(TM)) against multipath effects is demonstrated via simulation and experimental studies. Further, the PS-antenna diversity is shown to offer improved BER versus Eb/N o performance in pilot channels used in CDMA2000 systems. Also, such PS-diversity is shown to help improving the GPS receiver performance under RFI/jamming environment. The technique and heuristics proposed towards the PS-antenna diversity scheme imply novel and hitherto unexplored efforts in wireless communications. Lastly the dissertation concludes summarizing the results and offers open-questions for further studies.

The research addressed refers to a study on the electromagnetic performance aspects of body-worn radio units operating in the presence of scatterers in close proximity, using analytical, numerical, and experimental methods. The application potentials of such methods include evaluating the integrity of radio units such as cell phones. Consistent with the scope of the study above, considered in this research are specific details on analytical and numerical modeling of the effects of a nearby conducting cylindrical object on the electromagnetic field near a human-model phantom. Calculations are performed using the Finite Difference Time Domain (FDTD) method. Considered are various separations of the body wearing the test radio unit from the proximal object and polarization of the incident wave. An anechoic chamber and the test setup used for the measurement of EM field amplitudes near a saline-water phantom are described. Within the anechoic chamber, a small shielded loop is used as a field measurement probe and is positioned near the test phantom. The field probe orientation was in the vertical plane for characterizing the prevailing electromagnetic field intensity. This study indicates that variations in the field amplitude near the phantom occur, which are responsive to phantom rotation and measurement distance from the phantom. The electromagnetic field amplitude decreases rapidly with increasing distance between the probe and the surface of the phantom. The analysis is also extended to examine the electromagnetic field distribution in the gap between a human body phantom model and a nearby conducting cylinder. An appropriate three-dimensional FDTD method is presented and applied to a near-field problem of analyzing the influence of proximal conductive objects on fields near a phantom wearing an RF unit.

This research refers to studies on information-theoretic (IT) aspects of data-sequence patterns and developing thereof discriminant algorithms that enable distinguishing the features of underlying sequence patterns having characteristic, inherent stochastical attributes. The application potentials of such algorithms include bioinformatic data mining efforts. Consistent with the scope of the study as above, considered in this research are specific details on information-theoretics and entropy considerations vis-a-vis sequence patterns (having stochastical attributes) such as DNA sequences of molecular biology. Applying information-theoretic concepts (essentially in Shannon's sense), the following distinct sets of metrics are developed and applied in the algorithms developed for data-sequence pattern-discrimination applications: (i) Divergence or cross-entropy algorithms of Kullback-Leibler type and of general Czizar class; (ii) statistical distance measures; (iii) ratio-metrics; (iv) Fisher type linear-discriminant measure and (v) complexity metric based on information redundancy. These measures are judiciously adopted in ascertaining codon-noncodon delineations in DNA sequences that consist of crisp and/or fuzzy nucleotide domains across their chains. The Fisher measure is also used in codon-noncodon delineation and in motif detection. Relevant algorithms are used to test DNA sequences of human and some bacterial organisms. The relative efficacy of the metrics and the algorithms is determined and discussed. The potentials of such algorithms in supplementing the prevailing methods are indicated. Scope for future studies is identified in terms of persisting open questions.

What have been envisaged and focused in the research as deliberated in this dissertation, refer to certain state-of-the-art perspectives of modern wireless telecommunications. Specifically, the objectives and scope of the research efforts carried out include systematic analyses of two major wireless technologies, namely, the Bluetooth(TM) and the CDPD system. The relevant tasks undertaken refer to analytical modeling, experimental studies and simulations pertinent to the performance considerations of the test systems subjected to EMI/RFI influences. The following can be enumerated as the specific research endeavors pursued and completed: (1) Identification of performance implications in the Bluetooth(TM) and CDPD wireless transmissions subjected to EMI/RFI ambients. (2) A comprehensive presentation on the system description, performance requirements, protocol issues and operational environments of the two wireless technologies under study. (3) Development of analytically tractable models, which describe the extent of EMI/RFI on the Bluetooth(TM) operations and provide expressions to compute the transmission impairments involved in terms of the associated packet losses. (4) Modeling of CDPD performance and deducing the statistics of data packets seizing the free AMPS channels. The associated results offer compatible simulations to deduce the data packet blocking probability. The enhancement of such blocking caused by any impulse-like noise invading the system is also modeled. (5) Proposals for methods towards EMI solutions through adopting novel shielding in the wireless systems. The aforesaid efforts are comprehensively addressed supplemented with appropriate background details and literature survey. In conclusion, the scope of the present study is discussed and possible open-questions for future study are identified.