2,257 research outputs found
A Hybrid Approach to Joint Estimation of Channel and Antenna impedance
This paper considers a hybrid approach to joint estimation of channel
information and antenna impedance, for single-input, single-output channels.
Based on observation of training sequences via synchronously switched load at
the receiver, we derive joint maximum a posteriori and maximum-likelihood
(MAP/ML) estimators for channel and impedance over multiple packets. We
investigate important properties of these estimators, e.g., bias and
efficiency. We also explore the performance of these estimators through
numerical examples.Comment: 6 pages, two columns, 6 figures. References update
Antenna Impedance Estimation at MIMO Receivers
This paper considers antenna impedance estimation based on training sequences
at MIMO receivers. The goal is to firstly leverage extensive resources
available in most wireless systems for channel estimation to estimate antenna
impedance in real-time. We assume the receiver switches its impedance in a
predetermined fashion during each training sequence. Based on voltage
observation across the load, a classical estimation framework is developed
incorporating the Rayleigh fading assumption. We then derive in closed-form a
maximum-likelihood (ML) estimator under i.i.d. fading and show this same ML
estimator is a method of moments (MM) estimator in correlated channels.
Numerical results suggest a fast algorithm, i.e., MLE in i.i.d. fading and the
MM estimator in correlated fading, that estimates the unknown antenna impedance
in real-time for all Rayleigh fading channels.Comment: 31 pages, 6 figure
Worst-Case Error Probability of a Spread-Spectrum System in Energy-Limited Interference
We consider a communication channel corrupted by thermal noise and by an unknown and arbitrary interference of bounded energy. For this channel, we derive a simple upper bound to the worst-case error probability suffered by a direct sequence (DS) communication system with error-correction coding, pseudorandom interleaving, and a correlation receiver. This bound is exponentially tight as the block length of the error correcting code becomes large. Numerical examples are given that illustrate the dependence of the bound on the choice of error correcting code, the type of interleaving used, and the relative energy of the Gaussian noise and arbitrary interferenc
Imaging topologically protected transport with quantum degenerate gases
Ultracold and quantum degenerate gases held near conductive surfaces can
serve as sensitive, high resolution, and wide-area probes of electronic current
flow. Previous work has imaged transport around grain boundaries in a gold wire
by using ultracold and Bose-Einstein condensed atoms held microns from the
surface with an atom chip trap. We show that atom chip microscopy may be
applied to useful purpose in the context of materials exhibiting topologically
protected surface transport. Current flow through lithographically tailored
surface defects in topological insulators (TI)---both idealized and with the
band-structure and conductivity typical of BiSe---is numerically
calculated. We propose that imaging current flow patterns enables the
differentiation of an ideal TI from one with a finite bulk--to--surface
conductivity ratio, and specifically, that the determination of this ratio may
be possible by imaging transport around trenches etched into the TI's surface.Comment: Extensively rewritten, better introduction. 12 pages, 10 figure
Optimal Antenna Selection Designs for Coupled MIMO Systems
We consider the impact of antenna mutual coupling on receive antenna selection systems. Prior work
on selection with mutual coupling has not considered the effects of the inactive (i.e., unselected) antenna
terminations and spatial noise correlation. In this work, we show that the presence of inactive antennas
can profoundly alter system performance when the antennas are strongly coupled. We also propose a new
antenna selection technique that seeks to exploit coupling to improve performance. Simulations suggest
that the new technique can significantly outperform traditional selection when coupling is present
Heat-flux footprints for I-mode and EDA H-mode plasmas on Alcator C-Mod
IR thermography is used to measure the heat flux footprints on C-Mod’s outer target in I-mode and EDA H-mode plasmas. The footprint profiles are fit to a function with a simple physical interpretation. The fit parameter that is sensitive to the power decay length into the SOL, λ[subscript SOL], is ~1–3× larger in I-modes than in H-modes at similar plasma current, which is the dominant dependence for the H-mode λ[subscript SOL]. In contrast, the fit parameter sensitive to transport into the private-flux-zone along the divertor leg is somewhat smaller in I-mode than in H-mode, but otherwise displays no obvious dependence on I[subscript p], B[subscript t], or stored energy. A third measure of the footprint width, the “integral width”, is not significantly different between H- and I-modes. Also discussed are significant differences in the global power flows of the H-modes with “favorable” ∇B drift direction and those of the I-modes with “unfavorable” ∇B drift direction.United States. Dept. of Energy (Cooperative Agreement DE-FC02-99-ER54512
Shallow stratigraphic control on pockmark distribution in north temperate estuaries
This paper is not subject to U.S. copyright. The definitive version was published in Marine Geology 329-331 (2012): 34-45, doi:10.1016/j.margeo.2012.09.006.Pockmark fields occur throughout northern North American temperate estuaries despite the absence of extensive thermogenic hydrocarbon deposits typically associated with pockmarks. In such settings, the origins of the gas and triggering mechanism(s) responsible for pockmark formation are not obvious. Nor is it known why pockmarks proliferate in this region but do not occur south of the glacial terminus in eastern North America. This paper tests two hypotheses addressing these knowledge gaps: 1) the region's unique sea-level history provided a terrestrial deposit that sourced the gas responsible for pockmark formation; and 2) the region's physiography controls pockmarks distribution. This study integrates over 2500 km of high-resolution swath bathymetry, Chirp seismic reflection profiles and vibracore data acquired in three estuarine pockmark fields in the Gulf of Maine and Bay of Fundy. Vibracores sampled a hydric paleosol lacking the organic-rich upper horizons, indicating that an organic-rich terrestrial deposit was eroded prior to pockmark formation. This observation suggests that the gas, which is presumably responsible for the formation of the pockmarks, originated in Holocene estuarine sediments (loss on ignition 3.5–10%), not terrestrial deposits that were subsequently drowned and buried by mud. The 7470 pockmarks identified in this study are non-randomly clustered. Pockmark size and distribution relate to Holocene sediment thickness (r2 = 0.60), basin morphology and glacial deposits. The irregular underlying topography that dictates Holocene sediment thickness may ultimately play a more important role in temperate estuarine pockmark distribution than drowned terrestrial deposits. These results give insight into the conditions necessary for pockmark formation in nearshore coastal environments.Graduate support for Brothers came from a Maine Economic Improvement
Fund Dissertation Fellowship
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