14,606 research outputs found
Flux Modulation from the Rossby Wave Instability in microquasars accretion disks: toward a HFQPO model
Context. There have been a long string of efforts to understand the source of
the variability observed in microquasars, especially concerning the elusive
High-Frequency Quasi-Periodic Oscillation. These oscillations are among the
fastest phenomena that affect matter in the vicinity of stellar black holes and
therefore could be used as probes of strong-field general relativity.
Nevertheless, no model has yet gained wide acceptance. Aims. The aim of this
article is to investigate the model derived from the occurrence of the Rossby
wave instability at the inner edge of the accretion disk. In particular, our
goal here is to demonstrate the capacity of this instability to modulate the
observed flux in agreement with the observed results. Methods. We use the
AMRVAC hydrodynamical code to model the instability in a 3D optically thin
disk. The GYOTO ray-tracing code is then used to compute the associated light
curve. Results. We show that the 3D Rossby wave instability is able to modulate
the flux well within the observed limits.We highlight that 2D simulations allow
us to obtain the same general characteristics of the light curve as 3D
calculations. With the time resolution we adopted in this work, three
dimensional simulations do not give rise to any new observable features that
could be detected by current instrumentation or archive data.Comment: 10 pages, 10 figures, accepted by A&
Finite-Blocklength Bounds for Wiretap Channels
This paper investigates the maximal secrecy rate over a wiretap channel
subject to reliability and secrecy constraints at a given blocklength. New
achievability and converse bounds are derived, which are shown to be tighter
than existing bounds. The bounds also lead to the tightest second-order coding
rate for discrete memoryless and Gaussian wiretap channels.Comment: extended version of a paper submitted to ISIT 201
-Learning: A Collaborative Distributed Strategy for Multi-Agent Reinforcement Learning Through Consensus + Innovations
The paper considers a class of multi-agent Markov decision processes (MDPs),
in which the network agents respond differently (as manifested by the
instantaneous one-stage random costs) to a global controlled state and the
control actions of a remote controller. The paper investigates a distributed
reinforcement learning setup with no prior information on the global state
transition and local agent cost statistics. Specifically, with the agents'
objective consisting of minimizing a network-averaged infinite horizon
discounted cost, the paper proposes a distributed version of -learning,
-learning, in which the network agents collaborate by means of
local processing and mutual information exchange over a sparse (possibly
stochastic) communication network to achieve the network goal. Under the
assumption that each agent is only aware of its local online cost data and the
inter-agent communication network is \emph{weakly} connected, the proposed
distributed scheme is almost surely (a.s.) shown to yield asymptotically the
desired value function and the optimal stationary control policy at each
network agent. The analytical techniques developed in the paper to address the
mixed time-scale stochastic dynamics of the \emph{consensus + innovations}
form, which arise as a result of the proposed interactive distributed scheme,
are of independent interest.Comment: Submitted to the IEEE Transactions on Signal Processing, 33 page
The Trade-off between Processing Gains of an Impulse Radio UWB System in the Presence of Timing Jitter
In time hopping impulse radio, pulses of duration are transmitted
for each information symbol. This gives rise to two types of processing gain:
(i) pulse combining gain, which is a factor , and (ii) pulse spreading
gain, which is , where is the mean interval between two
subsequent pulses. This paper investigates the trade-off between these two
types of processing gain in the presence of timing jitter. First, an additive
white Gaussian noise (AWGN) channel is considered and approximate closed form
expressions for bit error probability are derived for impulse radio systems
with and without pulse-based polarity randomization. Both symbol-synchronous
and chip-synchronous scenarios are considered. The effects of multiple-access
interference and timing jitter on the selection of optimal system parameters
are explained through theoretical analysis. Finally, a multipath scenario is
considered and the trade-off between processing gains of a synchronous impulse
radio system with pulse-based polarity randomization is analyzed. The effects
of the timing jitter, multiple-access interference and inter-frame interference
are investigated. Simulation studies support the theoretical results.Comment: To appear in the IEEE Transactions on Communication
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