8,925 research outputs found
An overview of data acquisition, signal coding and data analysis techniques for MST radars
An overview is given of the data acquisition, signal processing, and data analysis techniques that are currently in use with high power MST/ST (mesosphere stratosphere troposphere/stratosphere troposphere) radars. This review supplements the works of Rastogi (1983) and Farley (1984) presented at previous MAP workshops. A general description is given of data acquisition and signal processing operations and they are characterized on the basis of their disparate time scales. Then signal coding, a brief description of frequently used codes, and their limitations are discussed, and finally, several aspects of statistical data processing such as signal statistics, power spectrum and autocovariance analysis, outlier removal techniques are discussed
Gridded and direct Epoch of Reionisation bispectrum estimates using the Murchison Widefield Array
We apply two methods to estimate the 21~cm bispectrum from data taken within
the Epoch of Reionisation (EoR) project of the Murchison Widefield Array (MWA).
Using data acquired with the Phase II compact array allows a direct bispectrum
estimate to be undertaken on the multiple redundantly-spaced triangles of
antenna tiles, as well as an estimate based on data gridded to the -plane.
The direct and gridded bispectrum estimators are applied to 21 hours of
high-band (167--197~MHz; =6.2--7.5) data from the 2016 and 2017 observing
seasons. Analytic predictions for the bispectrum bias and variance for point
source foregrounds are derived. We compare the output of these approaches, the
foreground contribution to the signal, and future prospects for measuring the
bispectra with redundant and non-redundant arrays. We find that some triangle
configurations yield bispectrum estimates that are consistent with the expected
noise level after 10 hours, while equilateral configurations are strongly
foreground-dominated. Careful choice of triangle configurations may be made to
reduce foreground bias that hinders power spectrum estimators, and the 21~cm
bispectrum may be accessible in less time than the 21~cm power spectrum for
some wave modes, with detections in hundreds of hours.Comment: 19 pages, 10 figures, accepted for publication in PAS
Spectral and Energy Efficiency of Superimposed Pilots in Uplink Massive MIMO
Next generation wireless networks aim at providing substantial improvements
in spectral efficiency (SE) and energy efficiency (EE). Massive MIMO has been
proved to be a viable technology to achieve these goals by spatially
multiplexing several users using many base station (BS) antennas. A potential
limitation of Massive MIMO in multicell systems is pilot contamination, which
arises in the channel estimation process from the interference caused by
reusing pilots in neighboring cells. A standard method to reduce pilot
contamination, known as regular pilot (RP), is to adjust the length of pilot
sequences while transmitting data and pilot symbols disjointly. An alternative
method, called superimposed pilot (SP), sends a superposition of pilot and data
symbols. This allows to use longer pilots which, in turn, reduces pilot
contamination. We consider the uplink of a multicell Massive MIMO network using
maximum ratio combining detection and compare RP and SP in terms of SE and EE.
To this end, we derive rigorous closed-form achievable rates with SP under a
practical random BS deployment. We prove that the reduction of pilot
contamination with SP is outweighed by the additional coherent and non-coherent
interference. Numerical results show that when both methods are optimized, RP
achieves comparable SE and EE to SP in practical scenarios.Comment: 32 pages, 12 figures, 3 tables. Submitted in March 2017 to IEEE
Transactions on Wireless Communication
The KaVA and KVN Pulsar Project
We present our work towards using the Korean VLBI (Very Long Baseline
Interferometer) Network (KVN) and VLBI Exploration of Radio Astronomy (VERA)
arrays combined into the KVN and VERA Array (KaVA) for observations of radio
pulsars at high frequencies (22-GHz). Pulsar astronomy is generally
focused at frequencies approximately 0.3 to several GHz and pulsars are usually
discovered and monitored with large, single-dish, radio telescopes. For most
pulsars, reduced radio flux is expected at high frequencies due to their steep
spectrum, but there are exceptions where high frequency observations can be
useful. Moreover, some pulsars are observable at high frequencies only, such as
those close to the Galactic Center. The discoveries of a radio-bright magnetar
and a few dozen extended Chandra sources within 15 arc-minute of the Galactic
Center provide strong motivations to make use of the KaVA frequency band for
searching pulsars in this region. Here, we describe the science targets and
report progresses made from the KVN test observations for known pulsars. We
then discuss why KaVA pulsar observations are compelling.Comment: To appear in PASJ KaVA Special Issu
Measuring the dispersive frequency shift of a rectangular microwave cavity induced by an ensemble of Rydberg atoms
In recent years the interest in studying interactions of Rydberg atoms or
ensembles thereof with optical and microwave frequency fields has steadily
increased, both in the context of basic research and for potential applications
in quantum information processing. We present measurements of the dispersive
interaction between an ensemble of helium atoms in the 37s Rydberg state and a
single resonator mode by extracting the amplitude and phase change of a weak
microwave probe tone transmitted through the cavity. The results are in
quantitative agreement with predictions made on the basis of the dispersive
Tavis-Cummings Hamiltonian. We study this system with the goal of realizing a
hybrid between superconducting circuits and Rydberg atoms. We measure maximal
collective coupling strengths of 1 MHz, corresponding to 3*10^3 Rydberg atoms
coupled to the cavity. As expected, the dispersive shift is found to be
inversely proportional to the atom-cavity detuning and proportional to the
number of Rydberg atoms. This possibility of measuring the number of Rydberg
atoms in a nondestructive manner is relevant for quantitatively evaluating
scattering cross sections in experiments with Rydberg atoms
Five new real-time detections of Fast Radio Bursts with UTMOST
We detail a new fast radio burst (FRB) survey with the Molonglo Radio
Telescope, in which six FRBs were detected between June 2017 and December 2018.
By using a real-time FRB detection system, we captured raw voltages for five of
the six events, which allowed for coherent dedispersion and very high time
resolution (10.24 s) studies of the bursts. Five of the FRBs show temporal
broadening consistent with interstellar and/or intergalactic scattering, with
scattering timescales ranging from 0.16 to 29.1 ms. One burst, FRB181017, shows
remarkable temporal structure, with 3 peaks each separated by 1 ms. We searched
for phase-coherence between the leading and trailing peaks and found none,
ruling out lensing scenarios. Based on this survey, we calculate an all-sky
rate at 843 MHz of events sky day to a fluence
limit of 8 Jy-ms: a factor of 7 below the rates estimated from the Parkes and
ASKAP telescopes at 1.4 GHz assuming the ASKAP-derived spectral index
(). Our results suggest that FRB
spectra may turn over below 1 GHz. Optical, radio and X-ray followup has been
made for most of the reported bursts, with no associated transients found. No
repeat bursts were found in the survey.Comment: 13 pages, 11 figures, submitted to MNRA
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