Thesis (Ph. D.)--University of Texas at Austin, 2001. / Vita. Includes bibliographical references. Available also from UMI/Dissertation Abstracts International.

Thesis (Doctor of Philosophy in Electrical Engineering)--Naval Postgraduate School, June 2004. / Thesis advisor(s): Clark Robertson. Includes bibliographical references (p. 85-87). Also available online.

Interference is fundamental to wireless networks. It is hard to achieve good performance when design routing metrics or algorithms without taking it into account. We study interference in wireless networks through empirical experiments and simulations. We find out that current routing protocols face difficulties in effectively managing it, which can lead to severe problems. For instance, a simple network of two links with one flow is vulnerable to severe performance degradation if interference is not properly accounted for. Motivated by these observations, we develop a simple and effective model to capture effects of interference in a wireless network. Different from the existing interference models, our model captures IEEE 802.11 DCF under both homogeneous and heterogeneous traffic and link characteristics, and is simple enough to be directly used as a basic building block for wireless performance optimization. Based on thismodel, we develop optimization algorithms for several objectives, such as network throughput and fairness. Given traffic demands as input, these algorithms compute rates at which individual flows must send to achieve these objectives. We implement these algorithms in Qualnet simulations and 19-node testbed. Our experiment and simulation results show that our methods can systematically account for and control interference to achieve good performance. More specifically, when optimizing fairness, our methods can achieve almost perfect fairness; when optimizing network throughput, they can lead to 100-200% improvement for UDP traffic and 10-50% for TCP traffic. / text

Heterogeneous cellular networks (HCNs) – comprising traditional macro base stations (BSs) and heterogeneous infrastructure such as microcells, picocells, femtocells and distributed antennas – are fast becoming a cost-effective and essential way of handling explosive wireless data traffic demands. Up until now, little basic research has been done on the fundamentals of managing so much infrastructure – much of it unplanned – together with the carefully planned macro-cellular network. This dissertation addresses the key technical challenges of inter-cell interference management in this new network paradigm. This dissertation first studies uplink femtocell access control in uncoordinated two-tier networks, i.e. where the femtocells cannot coordinate with macrocells. Closed access allows registered home users to monopolize their own femtocell and its backhaul connection, but also results in severe interference between femtocells and nearby unregistered macro users. Open access reduces such interference by handing over such users, at the expense of femtocell resource sharing. In the first analytical work on this topic, we studied the best femtocell access technique from the perspectives of both network operators and femtocell owners, and show that it is strongly contingent on parameters such as multiple access schemes (i.e. orthogonal vs. non- orthogonal) and cellular user density (in TDMA/OFDMA). To study coordinated algorithms whose success depends heavily on the rate and delay (vs. user mobility) of inter-cell overhead sharing, this dissertation develops various models of overhead signaling in general HCNs and derives the overhead quality contour – the achievable set of overhead packet rate and delay – under general assumptions on overhead arrivals and different overhead signaling methods (backhaul and/or wireless). The overhead quality contour is further simplified for two widely used models of overhead arrivals: Poisson and deterministic. Based on the overhead quality contour that is applicable to generic coordinated techniques, this dissertation develops a novel analytical framework to evaluate downlink coordinated multi-point (CoMP) schemes in HCNs. Combined with the signal-to-interference-plus-noise-ratio (SINR) characterization, this framework can be used for a class of CoMP schemes without user data sharing. As an example, we apply it to downlink CoMP inter-cell interference cancellation (ICIC), after deriving SINR results for it using the spatial Poisson Point Process (PPP) to capture the uncertainty in base station locations. / text

We demonstrate the existence of tunable bound-states in continuum (BIC) in a 1-dimensional quantum wire with two impurities by an intense monochromatic radiation field. We found that there is a new type of BIC due to the Fano interference between two optical transition channels, in addition to the ordinary BIC due to a geometrical interference between electron wave functions emitted by impurities. In both cases the BIC can be achieved by tuning the frequency of the radiation field. / text

published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

Not available / text

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The electronic structure and reaction dynamics of molecular and cluster anions in the gas phase has been investigated using negative ion velocity-map imaging photoelectron spectrometer. Photoelectron images provide important information about both energies and symmetries of the parent anion orbitals from which photoelectron originates. The symmetry and the ordering of several low-lying electronic states of neutral nitromethane (X¹A′, a³A″, b³A″, and A¹A″) are assigned based on a group theoretical analysis of the transitions angular distributions and the results of DFT calculations. The through-bond electronic coherence in meta- and para-dinitrobenzene anions is explored by recording a series of photoelectron images in 532-266 nm wavelength range. Photoelectron angular distributions for both isomers exhibit oscillatory behavior characteristic of the quantum interference effect, suggesting that dinitrobenzene anions retain their high symmetry electronic structures in the gas phase. Photoelectron imaging experiments on [O(N₂O)(n)]⁻, n =0–9 at 266 and 355 nm provide clear evidence of a switch from the cova)lent NNO₂⁻ cluster core to the atomic O⁻ core occurring between n = 3 and 4. The experimental results and theoretical modeling indicate that despite the greater stability of NNO₂⁻ relative to the O⁻ + N₂O⁻ dissociation limit, an O⁻ cluster core becomes energetically favored over NNO₂⁻ for n > 3, due to the more efficient solvation of the atomic anion. The photodissociation dynamics of I₂⁻ and IBr⁻ anions on the respective A' excited-state anion potentials is effectively unraveled in 780 nm pump - 390 nm probe time-resolve experiments. The time-dependent photoelectron spectra and classical trajectory calculations of the IBr⁻ dissociation provide the first rigorous dynamical test of the recently calculated A′ potential for this system. The photoelectron anisotropy cyclic variation observed in photodissociation of I₂⁻ is interpreted in the context of dual-center quantum interference model. The 390 nm pump – 390 nm probe experimental data reveal fast (≤100 fs) and delayed (~ 700 fs) appearance of the I⁻ channel in the photodissociation of I₂Cl⁻ and BrICl⁻ anions respectively. The difference in the reaction time-scales is attributed to the distinct dissociation pathways available for the anions to form I⁻ product.