9,154 research outputs found
State-insensitive trapping of Rb atoms: linearly versus circularly polarized lights
We study the cancellation of differential ac Stark shifts in the 5s and 5p
states of rubidium atom using the linearly and circularly polarized lights by
calculating their dynamic polarizabilities. Matrix elements were calculated
using a relativistic coupled-cluster method at the single, double and important
valence triple excitations approximation including all possible non-linear
correlation terms. Some of the important matrix elements were further optimized
using the experimental results available for the lifetimes and static
polarizabilities of atomic states. "Magic wavelengths" are determined from the
differential Stark shifts and results for the linearly polarized light are
compared with the previously available results. Possible scope of facilitating
state-insensitive optical trapping schemes using the magic wavelengths for
circularly polarized light are discussed. Using the optimized matrix elements,
the lifetimes of the 4d and 6s states of this atom are ameliorated.Comment: 13 pages, 13 tables and 4 figure
Sensing and Sharing Schemes for Spectral Efficiency of Cognitive Radios
Increase in data traffic, number of users and their requirements laid to a necessity of more bandwidth. Cognitive radio is one of the emerging technology which addresses the spectrum scarcity issue. In this work we study the advantage of having collaboration between cognitive enabled small cell network and primary macrocell. Different from the existing works at spectrum sensing stage we are applying enhanced spectrum sensing to avoid probability of false alarms and missed detections which has impact on spectral efficiency. Later power control optimization for secondary users known as Hybrid spectrum sharing is used for further improvement of spectral efficiency. Furthermore, the failed packets of Primary users are taken care by high ranked relays which in turn decreases the average Primary user packet delay by 20% when compared between assisted Secondary user method and non-assisted Secondary user method.
Quality and Availability of spectrum based routing for Cognitive radio enabled IoT networks
With the recent emergence and its wide spread applicability Internet of Things (IoT) is putting pressure on network resources and most importantly on availability of spectrum. Spectrum scarcity is the issue to be addressed in networking within IoT. Cognitive radio is the technology which addresses the problem of spectrum scarcity in an efficient way. Equipping the IoT devices with cognitive radio capability will lead to a new dimension called cognitive radio enabled IoT devices. To achieve ON-demand IoT solutions and interference free communications cognitive radio enabled IoT devices will become an effective platform for many applications. As there is high dynamicity in availability of spectrum it is challenging for designing an efficient routing protocol for secondary users in cognitive device networks. In this work we are going to estimate spectrum quality and spectrum availability based on two parameters called global information about spectrum usage and instant spectrum status information. Enhanced energy detector is used at each and every node for better probability of detection. For estimating spectrum quality and availability we are introducing novel routing metrics. To have restriction on the number of reroutings and to increase the performance of routing in our proposed routing metric only one retransmission is allowed. Then, two algorithms for routing are designed for evaluating the performance of routing and we find that the bit error rates of proposed algorithms (nodes are dynamic) have decreased a lot when compared to conventional methods (Nodes are static) and throughput of proposed algorithm also improved a lot
Depletion of Nonlinearity in Magnetohydrodynamic Turbulence: Insights from Analysis and Simulations
We build on recent developments in the study of fluid turbulence [Gibbon
\textit{et al.} Nonlinearity 27, 2605 (2014)] to define suitably scaled,
order- moments, , of , where
and are, respectively, the vorticity and current density in
three-dimensional magnetohydrodynamics (MHD). We show by mathematical analysis,
for unit magnetic Prandtl number , how these moments can be used to
identify three possible regimes for solutions of the MHD equations; these
regimes are specified by inequalities for and . We then
compare our mathematical results with those from our direct numerical
simulations (DNSs) and thus demonstrate that 3D MHD turbulence is like its
fluid-turbulence counterpart insofar as all solutions, which we have
investigated, remain in \textit{only one of these regimes}; this regime has
depleted nonlinearity. We examine the implications of our results for the
exponents that characterize the power-law dependences of the energy
spectra on the wave number , in the inertial range of
scales. We also comment on (a) the generalization of our results to the case
and (b) the relation between and the order- moments
of gradients of hydrodynamic fields, which are used in characterizing
intermittency in turbulent flows.Comment: 14 pages, 3 figure
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