977 research outputs found
Noncoherent Space-Time Coding: An Algebraic Perspective
Cataloged from PDF version of article.The design of spaceātime signals for noncoherent
block-fading channels where the channel state information is
not known a priori at the transmitter and the receiver is considered.
In particular, a new algebraic formulation for the diversity
advantage design criterion is developed. The new criterion encompasses,
as a special case, the well-known diversity advantage
for unitary spaceātime signals and, more importantly, applies to
arbitrary signaling schemes and arbitrary channel distributions.
This criterion is used to establish the optimal diversity-versus-rate
tradeoff for training based schemes in block-fading channels.
Our results are then specialized to the class of affine spaceātime
signals which allows for a low complexity decoder. Within this
class, spaceātime constellations based on the threaded algebraic
spaceātime (TAST) architecture are considered. These constellations
achieve the optimal diversity-versus-rate tradeoff over
noncoherent block-fading channels and outperform previously
proposed codes in the considered scenarios as demonstrated by
the numerical results. Using the analytical and numerical results
developed in this paper, nonunitary spaceātime codes are argued
to offer certain advantages in block-fading channels where the appropriate
use of coherent spaceātime codes is shown to offer a very
efficient solution to the noncoherent spaceātime communication
paradigm
TIM: a time interval machine for audio-visual action recognition
Diverse actions give rise to rich audio-visual signals in
long videos. Recent works showcase that the two modalities of audio and video exhibit different temporal extents of
events and distinct labels. We address the interplay between
the two modalities in long videos by explicitly modelling the
temporal extents of audio and visual events. We propose
the Time Interval Machine (TIM) where a modality-specific
time interval poses as a query to a transformer encoder that
ingests a long video input. The encoder then attends to the
specified interval, as well as the surrounding context in both
modalities, in order to recognise the ongoing action.
We test TIM on three long audio-visual video datasets:
EPIC-KITCHENS, Perception Test, and AVE, reporting state-of-the-art (SOTA) for recognition. On EPICKITCHENS, we beat previous SOTA that utilises LLMs and
significantly larger pre-training by 2.9% top-1 action recognition accuracy. Additionally, we show that TIM can be
adapted for action detection, using dense multi-scale interval queries, outperforming SOTA on EPIC-KITCHENS-100
for most metrics, and showing strong performance on the
Perception Test. Our ablations show the critical role of integrating the two modalities and modelling their time intervals in achieving this performance. Code and models at:
https://github.com/JacobChalk/TIM
The Dependence of Star Formation Rates on Stellar Mass and Environment at z~0.8
We examine the star formation rates (SFRs) of galaxies in a redshift slice
encompassing the z=0.834 cluster RX J0152.7-1357. We used a low-dispersion
prism in the Inamori Magellan Areal Camera and Spectrograph (IMACS) to identify
galaxies with z<23.3 AB mag in diverse environments around the cluster out to
projected distances of ~8 Mpc from the cluster center. We utilize a
mass-limited sample (M>2x10^{10} M_sun) of 330 galaxies that were imaged by
Spitzer MIPS at 24 micron to derive SFRs and study the dependence of specific
SFR (SSFR) on stellar mass and environment. We find that the SFR and SSFR show
a strong decrease with increasing local density, similar to the relation at
z~0. Our result contrasts with other work at z~1 that find the SFR-density
trend to reverse for luminosity-limited samples. These other results appear to
be driven by star-formation in lower mass systems (M~10^{10} M_sun). Our
results imply that the processes that shut down star-formation are present in
groups and other dense regions in the field. Our data also suggest that the
lower SFRs of galaxies in higher density environments may reflect a change in
the ratio of star-forming to non-star-forming galaxies, rather than a change in
SFRs. As a consequence, the SFRs of star-forming galaxies, in environments
ranging from small groups to clusters, appear to be similar and largely
unaffected by the local processes that truncate star-formation at z~0.8.Comment: 5 pages, 3 figures, accepted for publication in ApJ
Deep Compact Person Re-Identification with Distractor Synthesis via Guided DC-GANs
We present a dual-stream CNN that learns both appearance and facial features in tandem from still images and, after feature fusion, infers person identities. We then describe an alternative architecture of a single, lightweight ID-CondenseNet where a face detector-guided DC-GAN is used to generate distractor person images for enhanced training. For evaluation, we test both architectures on FLIMA, a new extension of an existing person re-identification dataset with added frame-by-frame annotations of face presence. Although the dual-stream CNN can outperform the CondenseNet approach on FLIMA, we show that the latter surpasses all state-of-the-art architectures in top-1 ranking performance when applied to the largest existing person re-identification dataset, MSMT17. We conclude that whilst re-identification performance is highly sensitive to the structure of datasets, distractor augmentation and network compression have a role to play for enhancing performance characteristics for larger scale applications
Eddington-limited X-ray Bursts as Distance Indicators. I. Systematic Trends and Spherical Symmetry in Bursts from 4U 1728-34
We investigate the limitations of thermonuclear X-ray bursts as a distance
indicator for the weakly-magnetized accreting neutron star 4U 1728-34. We
measured the unabsorbed peak flux of 81 bursts in public data from the Rossi
X-Ray Timing Explorer (RXTE). The distribution of peak fluxes was bimodal: 66
bursts exhibited photospheric radius expansion and were distributed about a
mean bolometric flux of 9.2e-8 erg/cm^2/s, while the remaining (non-radius
expansion) bursts reached 4.5e-8 erg/cm^2/s, on average. The peak fluxes of the
radius-expansion bursts were not constant, exhibiting a standard deviation of
9.4% and a total variation of 46%. These bursts showed significant correlations
between their peak flux and the X-ray colors of the persistent emission
immediately prior to the burst. We also found evidence for quasi-periodic
variation of the peak fluxes of radius-expansion bursts, with a time scale of
approximately 40 d. The persistent flux observed with RXTE/ASM over 5.8 yr
exhibited quasi-periodic variability on a similar time scale. We suggest that
these variations may have a common origin in reflection from a warped accretion
disk. Once the systematic variation of the peak burst fluxes is subtracted, the
residual scatter is only approximately 3%, roughly consistent with the
measurement uncertainties. The narrowness of this distribution strongly
suggests that i) the radiation from the neutron star atmosphere during
radius-expansion episodes is nearly spherically symmetric, and ii) the
radius-expansion bursts reach a common peak flux which may be interpreted as a
standard candle intensity.Adopting the minimum peak flux for the
radius-expansion bursts as the Eddington flux limit, we derive a distance for
the source of 4.4-4.8 kpc.Comment: 9 pages, 7 figures, accepted by ApJ. Minor referee's revisions, also
includes 9 newly public X-ray burst
Semantically selective augmentation for deep compact person re-identification
We present a deep person re-identification approach that combines semantically selective, deep data augmentation with clustering-based network compression to generate high performance, light and fast inference networks. In particular, we propose to augment limited training data via sampling from a deep convolutional generative adversarial network (DCGAN), whose discriminator is constrained by a semantic classifier to explicitly control the domain specificity of the generation process. Thereby, we encode information in the classifier network which can be utilized to steer adversarial synthesis, and which fuels our CondenseNet ID-network training. We provide a quantitative and qualitative analysis of the approach and its variants on a number of datasets, obtaining results that outperform the state-of-the-art on the LIMA dataset for long-term monitoring in indoor livingĀ spaces
Electron--Electron Scattering in Quantum Wires and it's Possible Suppression due to Spin Effects
A microscopic picture of electron-electron pair scattering in single mode
quantum wires is introduced which includes electron spin. A new source of
`excess' noise for hot carriers is presented. We show that zero magnetic field
`spin' splitting in quantum wires can lead to a dramatic `spin'-subband
dependence of electron--electron scattering, including the possibility of
strong suppression. As a consequence extremely long electron coherence lengths
and new spin-related phenomena are predicted. Since electron bands in III-V
semiconductor quantum wires are in general spin-split in zero applied magnetic
field, these new transport effects are of general importance.Comment: 11 pages, LaTeX and APS-RevteX 2, Rep.No. GF66,Figures from author,
Physical Review Letters, scheduled for 7 June 199
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