Anti-multipath digital signal detector

Detector operates in conjunction with radio frequency portion of receiver to detect digital signals transmitted in known modulation formats. Signal is constructed by assigning known and distinct modulation waveforms to sequence of message symbols. It reconstructs transmitted digital sequence with minimum probability that any reconstructed digit will be in error

Mathematical models for chemotaxis and their applications in self-organisation phenomena

Chemotaxis is a fundamental guidance mechanism of cells and organisms, responsible for attracting microbes to food, embryonic cells into developing tissues, immune cells to infection sites, animals towards potential mates, and mathematicians into biology. The Patlak-Keller-Segel (PKS) system forms part of the bedrock of mathematical biology, a go-to-choice for modellers and analysts alike. For the former it is simple yet recapitulates numerous phenomena; the latter are attracted to these rich dynamics. Here I review the adoption of PKS systems when explaining self-organisation processes. I consider their foundation, returning to the initial efforts of Patlak and Keller and Segel, and briefly describe their patterning properties. Applications of PKS systems are considered in their diverse areas, including microbiology, development, immunology, cancer, ecology and crime. In each case a historical perspective is provided on the evidence for chemotactic behaviour, followed by a review of modelling efforts; a compendium of the models is included as an Appendix. Finally, a half-serious/half-tongue-in-cheek model is developed to explain how cliques form in academia. Assumptions in which scholars alter their research line according to available problems leads to clustering of academics and the formation of "hot" research topics.Comment: 35 pages, 8 figures, Submitted to Journal of Theoretical Biolog

Fourier space design of high-Q cavities in standard and compressed hexagonal lattice photonic crystals

Building upon the results of recent work [1], we use momentum space design rules to investigate high quality factor (Q) optical cavities in standard and compressed hexagonal lattice photonic crystal (PC) slab waveguides. Beginning with the standard hexagonal lattice, the results of a symmetry analysis are used to determine a cavity geometry that produces a mode whose symmetry immediately leads to a reduction in vertical radiation loss from the PC slab. The Q is improved further by a tailoring of the defect geometry in Fourier space so as to limit coupling between the dominant Fourier components of the defect mode and those momentum components that radiate. Numerical investigations using the finite-difference time-domain (FDTD) method show significant improvement using these methods, with total Q values exceeding 10^5. We also consider defect cavities in a compressed hexagonal lattice, where the lattice compression is used to modify the in-plane bandstructure of the PC lattice, creating new (frequency) degeneracies and modifying the dominant Fourier components found in the defect modes. High Q cavities in this new lattice geometry are designed using the momentum space design techniques outlined above. FDTD simulations of these structures yield Q values in excess of 10^5 with mode volumes of approximately 0.35 cubic half-wavelengths in vacuum

Mode Coupling and Cavity-Quantum-Dot Interactions in a Fiber-Coupled Microdisk Cavity

A quantum master equation model for the interaction between a two-level system and whispering-gallery modes (WGMs) of a microdisk cavity is presented, with specific attention paid to current experiments involving a semiconductor quantum dot (QD) embedded in a fiber-coupled, AlGaAs microdisk cavity. In standard single mode cavity QED, three important rates characterize the system: the QD-cavity coupling rate g, the cavity decay rate kappa, and the QD dephasing rate gamma_perpendicular. A more accurate model of the microdisk cavity includes two additional features. The first is a second cavity mode that can couple to the QD, which for an ideal microdisk corresponds to a traveling wave WGM propagating counter to the first WGM. The second feature is a coupling between these two traveling wave WGMs, at a rate beta, due to backscattering caused by surface roughness that is present in fabricated devices. We consider the transmitted and reflected signals from the cavity for different parameter regimes of {g,beta,kappa,gamma_perpendicular}. A result of this analysis is that even in the presence of negligible roughness induced backscattering, a strongly coupled QD mediates coupling between the traveling wave WGMs, resulting in an enhanced effective coherent coupling rate g = sqrt(2)*g0 corresponding to that of a standing wave WGM with an electric field maximum at the position of the QD. In addition, analysis of the second-order correlation function of the reflected signal from the cavity indicates that regions of strong photon antibunching or bunching may be present depending upon the strength of coupling of the QD to each of the cavity modes. Such intensity correlation information will likely be valuable in interpreting experimental measurements of a strongly-coupled QD to a bi-modal WGM cavity.Comment: rev4: updated references and added additional correlation function calculations; to appear in Phys. Rev. A in Feb 200

Convergence and Competition in Rules Governing Lawyers and Auditors

This Article examines problems (including information asymmetries, agency problems and cognitive biases) that auditors and lawyers (collectively, "gatekeepers") confront when they evaluate and respond to risk, as well as the various ways in which gatekeeper regulation addresses these problems. Then, using the analytical framework of New Institutional Economics, this Article recognizes that "optimal" solutions to gatekeeper problems are far from certain. Better solutions are thus more likely to emerge from experimenting with different rules and observing outcomes from those rules. If auditors and lawyers are governed by different rules, a jurisdiction can simultaneously experiment with two different regulatory approaches for these two professions, and then make adjustments accordingly. Private actors also can signal which rules they prefer in a particular context by choosing whether to use, or to insist that other private actors use, auditors or lawyers for a particular task. Similarly, if rules for both professions vary among jurisdictions, jurisdictions can learn not only from their own experimentation but from the experimentation of other jurisdictions with different rules. Private actors can also signal their rule preferences by choosing the jurisdiction in which they want professional services to be performed. Improvements to gatekeeper regulation should follow from the observed results of this experimentation.These purported benefits from regulatory competition could, however, be undermined if private actors are allowed unlimited choice among regulatory regimes (a "race to the bottom"). This article thus discusses the limits of regulatory competition, and mentions various strategies for reducing the likelihood of a race to the bottom. Finally, this article observes that, within a framework of controlled regulatory competition, market and social conditions may cause some rules governing auditors and lawyers to converge, as well as some rules governing both professions in the United States and Europe to converge. Profession-specific and jurisdiction-specific differences, however, will probably remain, unless experimentation and observation demonstrate that a particular rule is clearly superior across professional and/or jurisdictional boundaries.