10,280 research outputs found
Modulator noise suppression in the LISA Time-Delay Interferometric combinations
We previously showed how the measurements of some eighteen time series of
relative frequency or phase shifts could be combined (1) to cancel the phase
noise of the lasers, (2) to cancel the Doppler fluctuations due to non-inertial
motions of the six optical benches, and (3) to remove the phase noise of the
onboard reference oscillators required to track the photodetector fringes, all
the while preserving signals from passinggravitational waves. Here we analyze
the effect of the additional noise due to the optical modulators used for
removing the phase fluctuations of the onboard reference oscillators. We use a
recently measured noise spectrum of an individual modulator to quantify the
contribution of modulator noise to the first and second-generation Time-Delay
Interferometric (TDI) combinations as a function of the modulation frequency.
We show that modulator noise can be made smaller than the expected proof-mass
acceleration and optical-path noises if the modulation frequencies are larger
than MHz in the case of the unequal-arm Michelson TDI combination
, GHz for the Sagnac TDI combination , and
MHz for the symmetrical Sagnac TDI combination . These
modulation frequencies are substantially smaller than previously estimated and
may lead to less stringent requirements on the LISA's oscillator noise
calibration subsystem.Comment: 17 pages, 5 figures. Submitted to: Phys. Rev. D 1
Efficient Downlink Channel Reconstruction for FDD Multi-Antenna Systems
In this paper, we propose an efficient downlink channel reconstruction scheme
for a frequency-division-duplex multi-antenna system by utilizing uplink
channel state information combined with limited feedback. Based on the spatial
reciprocity in a wireless channel, the downlink channel is reconstructed by
using frequency-independent parameters. We first estimate the gains, delays,
and angles during uplink sounding. The gains are then refined through downlink
training and sent back to the base station (BS). With limited overhead, the
refinement can substantially improve the accuracy of the downlink channel
reconstruction. The BS can then reconstruct the downlink channel with the
uplink-estimated delays and angles and the downlink-refined gains. We also
introduce and extend the Newtonized orthogonal matching pursuit (NOMP)
algorithm to detect the delays and gains in a multi-antenna multi-subcarrier
condition. The results of our analysis show that the extended NOMP algorithm
achieves high estimation accuracy. Simulations and over-the-air tests are
performed to assess the performance of the efficient downlink channel
reconstruction scheme. The results show that the reconstructed channel is close
to the practical channel and that the accuracy is enhanced when the number of
BS antennas increases, thereby highlighting that the promising application of
the proposed scheme in large-scale antenna array systems
Time-Delay Interferometry
Equal-arm interferometric detectors of gravitational radiation allow phase
measurements many orders of magnitude below the intrinsic phase stability of
the laser injecting light into their arms. This is because the noise in the
laser light is common to both arms, experiencing exactly the same delay, and
thus cancels when it is differenced at the photo detector. In this situation,
much lower level secondary noises then set overall performance. If, however,
the two arms have different lengths (as will necessarily be the case with
space-borne interferometers), the laser noise experiences different delays in
the two arms and will hence not directly cancel at the detector. In order to
solve this problem, a technique involving heterodyne interferometry with
unequal arm lengths and independent phase-difference readouts has been
proposed. It relies on properly time-shifting and linearly combining
independent Doppler measurements, and for this reason it has been called
Time-Delay Interferometry (or TDI). This article provides an overview of the
theory and mathematical foundations of TDI as it will be implemented by the
forthcoming space-based interferometers such as the Laser Interferometer Space
Antenna (LISA) mission. We have purposely left out from this first version of
our ``Living Review'' article on TDI all the results of more practical and
experimental nature, as well as all the aspects of TDI that the data analysts
will need to account for when analyzing the LISA TDI data combinations. Our
forthcoming ``second edition'' of this review paper will include these topics.Comment: 51 pages, 11 figures. To appear in: Living Reviews. Added conten
The White Dwarf -- White Dwarf galactic background in the LISA data
LISA (Laser Interferometer Space Antenna) is a proposed space mission, which
will use coherent laser beams exchanged between three remote spacecraft to
detect and study low-frequency cosmic gravitational radiation. In the low-part
of its frequency band, the LISA strain sensitivity will be dominated by the
incoherent superposition of hundreds of millions of gravitational wave signals
radiated by inspiraling white-dwarf binaries present in our own galaxy. In
order to estimate the magnitude of the LISA response to this background, we
have simulated a synthesized population that recently appeared in the
literature. We find the amplitude of the galactic white-dwarf binary background
in the LISA data to be modulated in time, reaching a minimum equal to about
twice that of the LISA noise for a period of about two months around the time
when the Sun-LISA direction is roughly oriented towards the Autumn equinox.
Since the galactic white-dwarfs background will be observed by LISA not as a
stationary but rather as a cyclostationary random process with a period of one
year, we summarize the theory of cyclostationary random processes, present the
corresponding generalized spectral method needed to characterize such process,
and make a comparison between our analytic results and those obtained by
applying our method to the simulated data. We find that, by measuring the
generalized spectral components of the white-dwarf background, LISA will be
able to infer properties of the distribution of the white-dwarfs binary systems
present in our Galaxy.Comment: 36 pages, 15 figure
PTPsec: Securing the Precision Time Protocol Against Time Delay Attacks Using Cyclic Path Asymmetry Analysis
High-precision time synchronization is a vital prerequisite for many modern
applications and technologies, including Smart Grids, Time-Sensitive Networking
(TSN), and 5G networks. Although the Precision Time Protocol (PTP) can
accomplish this requirement in trusted environments, it becomes unreliable in
the presence of specific cyber attacks. Mainly, time delay attacks pose the
highest threat to the protocol, enabling attackers to diverge targeted clocks
undetected. With the increasing danger of cyber attacks, especially against
critical infrastructure, there is a great demand for effective countermeasures
to secure both time synchronization and the applications that depend on it.
However, current solutions are not sufficiently capable of mitigating
sophisticated delay attacks. For example, they lack proper integration into the
PTP protocol, scalability, or sound evaluation with the required
microsecond-level accuracy. This work proposes an approach to detect and
counteract delay attacks against PTP based on cyclic path asymmetry
measurements over redundant paths. For that, we provide a method to find
redundant paths in arbitrary networks and show how this redundancy can be
exploited to reveal and mitigate undesirable asymmetries on the synchronization
path that cause the malicious clock divergence. Furthermore, we propose PTPsec,
a secure PTP protocol and its implementation based on the latest IEEE 1588-2019
standard. With PTPsec, we advance the conventional PTP to support reliable
delay attack detection and mitigation. We validate our approach on a hardware
testbed, which includes an attacker capable of performing static and
incremental delay attacks at a microsecond precision. Our experimental results
show that all attack scenarios can be reliably detected and mitigated with
minimal detection time.Comment: Accepted at INFOCOM2
Sensitivity and parameter-estimation precision for alternate LISA configurations
We describe a simple framework to assess the LISA scientific performance
(more specifically, its sensitivity and expected parameter-estimation precision
for prescribed gravitational-wave signals) under the assumption of failure of
one or two inter-spacecraft laser measurements (links) and of one to four
intra-spacecraft laser measurements. We apply the framework to the simple case
of measuring the LISA sensitivity to monochromatic circular binaries, and the
LISA parameter-estimation precision for the gravitational-wave polarization
angle of these systems. Compared to the six-link baseline configuration, the
five-link case is characterized by a small loss in signal-to-noise ratio (SNR)
in the high-frequency section of the LISA band; the four-link case shows a
reduction by a factor of sqrt(2) at low frequencies, and by up to ~2 at high
frequencies. The uncertainty in the estimate of polarization, as computed in
the Fisher-matrix formalism, also worsens when moving from six to five, and
then to four links: this can be explained by the reduced SNR available in those
configurations (except for observations shorter than three months, where five
and six links do better than four even with the same SNR). In addition, we
prove (for generic signals) that the SNR and Fisher matrix are invariant with
respect to the choice of a basis of TDI observables; rather, they depend only
on which inter-spacecraft and intra-spacecraft measurements are available.Comment: 17 pages, 4 EPS figures, IOP style, corrected CQG versio
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