62 research outputs found
Adaptive Antenna Arrays for Ad-Hoc Millimetre-Wave Wireless Communications
New technologies that employ millimetre-wave frequency bands to achieve high speed wireless links are gaining more attention (Dyadyuk et. al., 2007, 2009b, 2010a; Hirata et. al., 2006; Lockie & Peck, 2009; Kasugi et. al., 2009; Wells, 2009) due to increasing demand for wideband wireless communications. Very wide uncongested spectrum is available in the E—bands (71-76 GHz and 81-86 GHz) recently allocated for wireless communications in USA, Europe, Korea, Russia and Australia. The E-band provides an opportunity for line-of – sight (LOS) links with higher data rates, well suited for fibre replacement and backhaul applications. Future mobile and ad-hoc communications networks will require higher bandwidth and longer range. An ad-hoc or mobile (e.g. inter-aircraft) network that relies on high gain antennas also requires beam scanning. Adaptive antenna arrays have found a wide rage of applications and are becoming essential parts of wireless communications systems (Abbaspour-Tamijani & Sarabandi, 2003; Do-Hong & Russer, 2004; Gross, 2005; Guo, 2004; Krim & Viberg, 1996; Mailloux, 2005, 2007; Rogstad et al., 2003; Singh et al., 2008). While the spectrum available in the millimetre-wave frequency bands enables multi-gigabit-per second data rates, the practically achievable communication range is limited by several factors. These include the higher atmospheric attenuation at these frequencies and limited output power of monolithic microwave integrated circuits (MMIC) (Doan et al., 2004; Dyadyuk et al., 2008a; Kasper et al., 2009; Floyd et al., 2007; Reynolds et. al., 2006; Vamsi et. al., 2005, Zirath et al., 2004) due to physical constraints. Therefore, the performance of the ad-hoc or mobile millimetre-wave networks requires enhancement by using spatial power combining antenna arrays
Spontaneous Hydrolyses in Sulfobetaine Micelles. Dependence of Micellar Charge Effects Upon Mechanism
Rate constants of spontaneous hydrolyses in zwitterionic micelles of N-hexadecyl-N,N-dimethyl-3-ammonio-propanesulfonate (SB3-16) are compared with those in cationic (n-C16H33NMe3X, X = Cl, Br, OMes; CTAC1, CTABr, CTAOMes) and anionic (n-C12H25OSO3Na, SDS) micelles. Substrates are methyl benzenesulfonate, 2-adamantyl and pinacolyl 4-nitrobenzenesulfonate, 4-bromo- and 4-nitrobenzoyl chloride, phenyl and 4-nitrophenyl chloroformate and bis(4-nitrophenyl) carbonate. Hydrolyses are micellar inhibited, except for the nitro substituted acid chlorides. Reactions with extensive bond-breaking in the transition state (SN1 hydrolyses) are faster in SDS than in cationic and sulfobetaine micelles, but the other hydrolyses, which involve significant bond-making, are slower in SDS. Rate constants are similar in cationic and sulfobetaine micelles. These micellar charge effects are ascribed to interactions of the polar transition states with the asymetrically charged interfacial region which complement effects of the lower polarities of micelles relative to water
A search for fast radio burst-like emission from Fermi gamma-ray bursts
We report the results of the rapid follow-up observations of gamma-ray bursts
(GRBs) detected by the Fermi satellite to search for associated fast radio
bursts. The observations were conducted with the Australian Square Kilometre
Array Pathfinder at frequencies from 1.2-1.4 GHz. A set of 20 bursts, of which
four were short GRBs, were followed up with a typical latency of about one
minute, for a duration of up to 11 hours after the burst. The data was searched
using 4096 dispersion measure trials up to a maximum dispersion measure of 3763
pc cm, and for pulse widths over a range of duration from 1.256 to
40.48 ms. No associated pulsed radio emission was observed above for any of the 20 GRBs.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical
Society Main Journa
The extraordinary linear polarisation structure of the southern Centaurus A lobe revealed by ASKAP
We present observations of linear polarisation in the southern radio lobe of
Centaurus A, conducted during commissioning of the Australian Square Kilometre
Array Pathfinder (ASKAP) telescope. We used 16 antennas to observe a 30 square
degree region in a single 12 hour pointing over a 240 MHz band centred on 913
MHz. Our observations achieve an angular resolution of arcseconds
(480 parsecs), a maximum recoverable angular scale of 30 arcminutes, and a
full-band sensitivity of 85 \muupJy beam. The resulting maps of
polarisation and Faraday rotation are amongst the most detailed ever made for
radio lobes, with of order 10 resolution elements covering the source. We
describe several as-yet unreported observational features of the lobe,
including its detailed peak Faraday depth structure, and intricate networks of
depolarised filaments. These results demonstrate the exciting capabilities of
ASKAP for widefield radio polarimetry.Comment: 10 pages, 6 figures. Accepted in "The Power of Faraday Tomography"
special issue of Galaxie
The Murchison Widefield Array: Design Overview
The Murchison Widefield Array (MWA) is a dipole-based aperture array
synthesis telescope designed to operate in the 80-300 MHz frequency range. It
is capable of a wide range of science investigations, but is initially focused
on three key science projects. These are detection and characterization of
3-dimensional brightness temperature fluctuations in the 21cm line of neutral
hydrogen during the Epoch of Reionization (EoR) at redshifts from 6 to 10,
solar imaging and remote sensing of the inner heliosphere via propagation
effects on signals from distant background sources,and high-sensitivity
exploration of the variable radio sky. The array design features 8192
dual-polarization broad-band active dipoles, arranged into 512 tiles comprising
16 dipoles each. The tiles are quasi-randomly distributed over an aperture
1.5km in diameter, with a small number of outliers extending to 3km. All
tile-tile baselines are correlated in custom FPGA-based hardware, yielding a
Nyquist-sampled instantaneous monochromatic uv coverage and unprecedented point
spread function (PSF) quality. The correlated data are calibrated in real time
using novel position-dependent self-calibration algorithms. The array is
located in the Murchison region of outback Western Australia. This region is
characterized by extremely low population density and a superbly radio-quiet
environment,allowing full exploitation of the instrumental capabilities.Comment: 9 pages, 5 figures, 1 table. Accepted for publication in Proceedings
of the IEE
Deep Investigation of Neutral Gas Origins (DINGO): HI stacking experiments with early science data
We present early science results from Deep Investigation of Neutral Gas
Origins (DINGO), an HI survey using the Australian Square Kilometre Array
Pathfinder (ASKAP). Using ASKAP sub-arrays available during its commissioning
phase, DINGO early science data were taken over 60 deg of the
Galaxy And Mass Assembly (GAMA) 23 h region with 35.5 hr integration time. We
make direct detections of six known and one new sources at . Using HI
spectral stacking, we investigate the HI gas content of galaxies at for different galaxy colours. The results show that galaxy morphology
based on optical colour is strongly linked to HI gas properties. To examine
environmental impacts on the HI gas content of galaxies, three sub-samples are
made based on the GAMA group catalogue. The average HI mass of group central
galaxies is larger than those of satellite and isolated galaxies, but with a
lower HI gas fraction. We derive a variety of HI scaling relations for physical
properties of our sample, including stellar mass, stellar mass surface density,
colour, specific star formation rate, and halo mass. We find that the
derived HI scaling relations are comparable to other published results, with
consistent trends also observed to 0.5 dex lower limits in stellar mass
and stellar surface density. The cosmic HI densities derived from our data are
consistent with other published values at similar redshifts. DINGO early
science highlights the power of HI spectral stacking techniques with ASKAP.Comment: 27 pages, 25 figures, 10 tables, accepted for publication in MNRA
Deep Investigation of Neutral Gas Origins (DINGO): HI stacking experiments with early science data
We present early science results from Deep Investigation of Neutral Gas Origins (DINGO), an H I survey using the Australian Square Kilometre Array Pathfinder (ASKAP). Using ASKAP subarrays available during its commissioning phase, DINGO early science data were taken over ∼60 deg2 of the Galaxy And Mass Assembly (GAMA) 23 h region with 35.5 h integration time. We make direct detections of six known and one new sources at z \u3c 0.01. Using H I spectral stacking, we investigate the H I gas content of galaxies at 0.04 \u3c z \u3c 0.09 for different galaxy colours. The results show that galaxy morphology based on optical colour is strongly linked to H I gas properties. To examine environmental impacts on the H I gas content of galaxies, three subsamples are made based on the GAMA group catalogue. The average H I mass of group central galaxies is larger than those of satellite and isolated galaxies, but with a lower H I gas fraction. We derive a variety of H I scaling relations for physical properties of our sample, including stellar mass, stellar mass surface density, NUV − r colour, specific star formation rate, and halo mass. We find that the derived H I scaling relations are comparable to other published results, with consistent trends also observed to ∼0.5 dex lower limits in stellar mass and stellar surface density. The cosmic H I densities derived from our data are consistent with other published values at similar redshifts. DINGO early science highlights the power of H I spectral stacking techniques with ASKA
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