229 research outputs found
Determination of the Local Standard of Rest using the LSS-GAC DR1
We re-estimate the peculiar velocity of the Sun with respect to the local
standard of rest using a sample of local stars within 600 pc of the Sun,
selected from the LAMOST Spectroscopic Survey of the Galactic Anti-centre
(LSS-GAC). The sample consists of 94332 FGK main-sequence stars with
well-determined radial velocities and atmospheric parameters. To derive the
LSR, two independent analyses are applied to the data. Firstly, we determine
the solar motion by comparing the observed velocity distribution to that
generated with the analytic formulism of Schonrich & Binney that has been
demonstrated to show excellent agreement with rigorous torus-based dynamics
modelling by Binney & McMillan. Secondly, we propose that cold populations of
thin disc stars, selected by applying an orbital eccentricity cut, can be
directly used to determine the LSR without the need of asymmetric drift
corrections. Both approaches yield consistent results of solar motion in the
direction of Galactic rotation, V_sun, that are much higher than the standard
value adopted hitherto, derived from Stromgren's equation. The newly deduced
values of V_sun are 1-2 km/s smaller than the more recent estimates derived
from the Geneva-Copenhagen Survey sample of stars in the solar neighbourhood
(within 100 pc). We attribute the small difference to the presence of several
well-known moving groups in the GCS sample that, fortunately, hardly affect the
LSS-GAC sample. The newly derived radial and vertical components of the solar
motion agree well with the previous studies. In addition, for all components of
the solar motion, the values yielded by stars of different spectral types in
the LSS-GAC sample are consistent with each other, suggesting that the local
disk is well relaxed and that the LSR reported in the current work is robust.
Our final recommended LSR is, (U,V,W)_sun = (7.01+/-0.20, 10.13+/-0.12,
4.95+/-0.09) km/s.Comment: MNRAS accepted, 13 pages, 11 figures, 7 table
Matrix Factorization Based Blind Bayesian Receiver for Grant-Free Random Access in mmWave MIMO mMTC
Grant-free random access is promising for massive connectivity with sporadic
transmissions in massive machine type communications (mMTC), where the
hand-shaking between the access point (AP) and users is skipped, leading to
high access efficiency. In grant-free random access, the AP needs to identify
the active users and perform channel estimation and signal detection.
Conventionally, pilot signals are required for the AP to achieve user activity
detection and channel estimation before active user signal detection, which may
still result in substantial overhead and latency. In this paper, to further
reduce the overhead and latency, we explore the problem of grant-free random
access without the use of pilot signals in a millimeter wave (mmWave) multiple
input and multiple output (MIMO) system, where the AP performs blind joint user
activity detection, channel estimation and signal detection (UACESD). We show
that the blind joint UACESD can be formulated as a constrained composite matrix
factorization problem, which can be solved by exploiting the structures of the
channel matrix and signal matrix. Leveraging our recently developed unitary
approximate message passing based matrix factorization (UAMP-MF) algorithm, we
design a message passing based Bayesian algorithm to solve the blind joint
UACESD problem. Extensive simulation results demonstrate the effectiveness of
the blind grant-free random access scheme
Efficient Rate-Splitting Multiple Access for the Internet of Vehicles: Federated Edge Learning and Latency Minimization
Rate-Splitting Multiple Access (RSMA) has recently found favour in the
multi-antenna-aided wireless downlink, as a benefit of relaxing the accuracy of
Channel State Information at the Transmitter (CSIT), while in achieving high
spectral efficiency and providing security guarantees. These benefits are
particularly important in high-velocity vehicular platoons since their high
Doppler affects the estimation accuracy of the CSIT. To tackle this challenge,
we propose an RSMA-based Internet of Vehicles (IoV) solution that jointly
considers platoon control and FEderated Edge Learning (FEEL) in the downlink.
Specifically, the proposed framework is designed for transmitting the unicast
control messages within the IoV platoon, as well as for privacy-preserving
FEEL-aided downlink Non-Orthogonal Unicasting and Multicasting (NOUM). Given
this sophisticated framework, a multi-objective optimization problem is
formulated to minimize both the latency of the FEEL downlink and the deviation
of the vehicles within the platoon. To efficiently solve this problem, a Block
Coordinate Descent (BCD) framework is developed for decoupling the main
multi-objective problem into two sub-problems. Then, for solving these
non-convex sub-problems, a Successive Convex Approximation (SCA) and Model
Predictive Control (MPC) method is developed for solving the FEEL-based
downlink problem and platoon control problem, respectively. Our simulation
results show that the proposed RSMA-based IoV system outperforms the
conventional systems
Full-Dimensional Rate Enhancement for UAV-Enabled Communications via Intelligent Omni-Surface
This paper investigates the achievable rate maximization problem of a
downlink unmanned aerial vehicle (UAV)-enabled communication system aided by an
intelligent omni-surface (IOS). Different from the state-of-the-art
reconfigurable intelligent surface (RIS) that only reflects incident signals,
the IOS can simultaneously reflect and transmit the signals, thereby providing
full-dimensional rate enhancement. To tackle such a problem, we formulate it by
jointly optimizing the IOS's phase shift and the UAV trajectory. Although it is
difficult to solve it optimally due to its non-convexity, we propose an
efficient iterative algorithm to obtain a high-quality suboptimal solution.
Simulation results show that the IOS-assisted UAV communications can achieve
more significant improvement in achievable rates than other benchmark schemes.Comment: 6 pages, 5 figure
Training Deeper Neural Machine Translation Models with Transparent Attention
While current state-of-the-art NMT models, such as RNN seq2seq and
Transformers, possess a large number of parameters, they are still shallow in
comparison to convolutional models used for both text and vision applications.
In this work we attempt to train significantly (2-3x) deeper Transformer and
Bi-RNN encoders for machine translation. We propose a simple modification to
the attention mechanism that eases the optimization of deeper models, and
results in consistent gains of 0.7-1.1 BLEU on the benchmark WMT'14
English-German and WMT'15 Czech-English tasks for both architectures.Comment: To appear in EMNLP 201
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