11,040 research outputs found
Shaping Current Waveforms for direct Modulation of Semiconductor Lasers
We demonstrate a technique for shaping current inputs for the direct
modulation of a semiconductor laser for digital communication. The introduction
of shaped current inputs allows for the suppression of relaxation oscillations
and the avoidance of dynamical memory in the physical laser device, i.e., the
output will not be influenced by previously communicated information. On the
example of time-optimized bits, the possible performance enhancement for high
data rate communications is shown numerically.Comment: 8 pages, 6 figures, to be published in IEEE Journal of Quantum
Electronic
A Hybrid (Active-Passive) VANET Clustering Technique
Clustering serves a vital role in the operation of Vehicular Ad hoc Networks (VANETs) by continually grouping highly mobile vehicles into logical hierarchical structures. These moving clusters support Intelligent Transport Systems (ITS) applications and message routing by establishing a more stable global topology. Clustering increases scalability of the VANET by eliminating broadcast storms caused by packet flooding and facilitate multi-channel operation. Clustering techniques are partitioned in research into two categories: active and passive. Active techniques rely on periodic beacon messages from all vehicles containing location, velocity, and direction information. However, in areas of high vehicle density, congestion may occur on the long-range channel used for beacon messages limiting the scale of the VANET. Passive techniques use embedded information in the packet headers of existing traffic to perform clustering. In this method, vehicles not transmitting traffic may cause cluster heads to contain stale and malformed clusters. This dissertation presents a hybrid active/passive clustering technique, where the passive technique is used as a congestion control strategy for areas where congestion is detected in the network. In this case, cluster members halt their periodic beacon messages and utilize embedded position information in the header to update the cluster head of their position. This work demonstrated through simulation that the hybrid technique reduced/eliminated the delays caused by congestion in the modified Distributed Coordination Function (DCF) process, thus increasing the scalability of VANETs in urban environments. Packet loss and delays caused by the hidden terminal problem was limited to distant, non-clustered vehicles. This dissertation report presents a literature review, methodology, results, analysis, and conclusion
Multiprocessing techniques for unmanned multifunctional satellites Final report,
Simulation of on-board multiprocessor for long lived unmanned space satellite contro
Fault and Defect Tolerant Computer Architectures: Reliable Computing With Unreliable Devices
This research addresses design of a reliable computer from unreliable device technologies. A system architecture is developed for a fault and defect tolerant (FDT) computer. Trade-offs between different techniques are studied and yield and hardware cost models are developed. Fault and defect tolerant designs are created for the processor and the cache memory. Simulation results for the content-addressable memory (CAM)-based cache show 90% yield with device failure probabilities of 3 x 10(-6), three orders of magnitude better than non fault tolerant caches of the same size. The entire processor achieves 70% yield with device failure probabilities exceeding 10(-6). The required hardware redundancy is approximately 15 times that of a non-fault tolerant design. While larger than current FT designs, this architecture allows the use of devices much more likely to fail than silicon CMOS. As part of model development, an improved model is derived for NAND Multiplexing. The model is the first accurate model for small and medium amounts of redundancy. Previous models are extended to account for dependence between the inputs and produce more accurate results
Final report on the evaluation of RRM/CRRM algorithms
Deliverable public del projecte EVERESTThis deliverable provides a definition and a complete evaluation of the RRM/CRRM algorithms selected in D11 and D15, and evolved and refined on an iterative process. The evaluation will be carried out by means of simulations using the simulators provided at D07, and D14.Preprin
Performance and structure of single-mode bosonic codes
The early Gottesman, Kitaev, and Preskill (GKP) proposal for encoding a qubit
in an oscillator has recently been followed by cat- and binomial-code
proposals. Numerically optimized codes have also been proposed, and we
introduce new codes of this type here. These codes have yet to be compared
using the same error model; we provide such a comparison by determining the
entanglement fidelity of all codes with respect to the bosonic pure-loss
channel (i.e., photon loss) after the optimal recovery operation. We then
compare achievable communication rates of the combined encoding-error-recovery
channel by calculating the channel's hashing bound for each code. Cat and
binomial codes perform similarly, with binomial codes outperforming cat codes
at small loss rates. Despite not being designed to protect against the
pure-loss channel, GKP codes significantly outperform all other codes for most
values of the loss rate. We show that the performance of GKP and some binomial
codes increases monotonically with increasing average photon number of the
codes. In order to corroborate our numerical evidence of the cat/binomial/GKP
order of performance occurring at small loss rates, we analytically evaluate
the quantum error-correction conditions of those codes. For GKP codes, we find
an essential singularity in the entanglement fidelity in the limit of vanishing
loss rate. In addition to comparing the codes, we draw parallels between
binomial codes and discrete-variable systems. First, we characterize one- and
two-mode binomial as well as multi-qubit permutation-invariant codes in terms
of spin-coherent states. Such a characterization allows us to introduce check
operators and error-correction procedures for binomial codes. Second, we
introduce a generalization of spin-coherent states, extending our
characterization to qudit binomial codes and yielding a new multi-qudit code.Comment: 34 pages, 11 figures, 4 tables. v3: published version. See related
talk at https://absuploads.aps.org/presentation.cfm?pid=1351
Gaussian Quantum Information
The science of quantum information has arisen over the last two decades
centered on the manipulation of individual quanta of information, known as
quantum bits or qubits. Quantum computers, quantum cryptography and quantum
teleportation are among the most celebrated ideas that have emerged from this
new field. It was realized later on that using continuous-variable quantum
information carriers, instead of qubits, constitutes an extremely powerful
alternative approach to quantum information processing. This review focuses on
continuous-variable quantum information processes that rely on any combination
of Gaussian states, Gaussian operations, and Gaussian measurements.
Interestingly, such a restriction to the Gaussian realm comes with various
benefits, since on the theoretical side, simple analytical tools are available
and, on the experimental side, optical components effecting Gaussian processes
are readily available in the laboratory. Yet, Gaussian quantum information
processing opens the way to a wide variety of tasks and applications, including
quantum communication, quantum cryptography, quantum computation, quantum
teleportation, and quantum state and channel discrimination. This review
reports on the state of the art in this field, ranging from the basic
theoretical tools and landmark experimental realizations to the most recent
successful developments.Comment: 51 pages, 7 figures, submitted to Reviews of Modern Physic
Optical instruments for measuring leaf area index in low vegetation : application in Arctic ecosystems
Author Posting. © Ecological Society of America, 2005. This article is posted here by permission of Ecological Society of America for personal use, not for redistribution. The definitive version was published in Ecological Applications 15 (2005): 1462–1470, doi:10.1890/03-5354.Leaf area index (LAI) is a powerful diagnostic of plant productivity. Despite the fact that many methods have been developed to quantify LAI, both directly and indirectly, leaf area index remains difficult to quantify accurately, owing to large spatial and temporal variability. The gap-fraction technique is widely used to estimate the LAI indirectly. However, for low-stature vegetation, the gap-fraction sensor either cannot get totally underneath the plant canopy, thereby missing part of the leaf area present, or is too close to the individual leaves of the canopy, which leads to a large distortion of the LAI estimate. We set out to develop a methodology for easy and accurate nondestructive assessment of the variability of LAI in low-stature vegetation. We developed and tested the methodology in an arctic landscape close to Abisko, Sweden.
The LAI of arctic vegetation could be estimated accurately and rapidly by combining field measurements of canopy reflectance (NDVI) and light penetration through the canopy (gap-fraction analysis using a LI-COR LAI-2000). By combining the two methodologies, the limitations of each could be circumvented, and a significantly increased accuracy of the LAI estimates was obtained. The combination of an NDVI sensor for sparser vegetation and a LAI-2000 for denser vegetation could explain 81% of the variance of LAI measured by destructive harvest. We used the method to quantify the spatial variability and the associated uncertainty of leaf area index in a small catchment area.This research was funded by U.S. National Science Foundation
grant DEB0087046
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