8,037 research outputs found
Optimization Framework and Graph-Based Approach for Relay-Assisted Bidirectional OFDMA Cellular Networks
This paper considers a relay-assisted bidirectional cellular network where
the base station (BS) communicates with each mobile station (MS) using OFDMA
for both uplink and downlink. The goal is to improve the overall system
performance by exploring the full potential of the network in various
dimensions including user, subcarrier, relay, and bidirectional traffic. In
this work, we first introduce a novel three-time-slot time-division duplexing
(TDD) transmission protocol. This protocol unifies direct transmission, one-way
relaying and network-coded two-way relaying between the BS and each MS. Using
the proposed three-time-slot TDD protocol, we then propose an optimization
framework for resource allocation to achieve the following gains: cooperative
diversity (via relay selection), network coding gain (via bidirectional
transmission mode selection), and multiuser diversity (via subcarrier
assignment). We formulate the problem as a combinatorial optimization problem,
which is NP-complete. To make it more tractable, we adopt a graph-based
approach. We first establish the equivalence between the original problem and a
maximum weighted clique problem in graph theory. A metaheuristic algorithm
based on any colony optimization (ACO) is then employed to find the solution in
polynomial time. Simulation results demonstrate that the proposed protocol
together with the ACO algorithm significantly enhances the system total
throughput.Comment: 27 pages, 8 figures, 2 table
Modeling the hard states of three black hole candidates
Simultaneous multiwavelength observations were recently performed for three
black hole candidates --- SWIFT J1753.5-0127, GRO J1655-40 and XTE J1720-318.
In this paper we test the accretion-jet model originally proposed to XTE
J1118+480 by investigating the hard state of these three sources using this
model. The accretion flow in the model is composed of an inner hot accretion
flow and an outer truncated thin disk. We find that the model satisfactorily
explains the spectrum ranging from radio to X-rays, with the radio and X-ray
spectra dominated by the synchrotron and thermal Comptonization emissions in
the jet and the hot accretion flow respectively, while the infrared and optical
being the sum of the emissions from the jet, hot accretion flow, and the
truncated thin disk. Similar to the case of XTE J1118+480, the model can also
explain, although only qualitatively in some cases, the observed timing
features including QPO, and positive and negative time lags between the optical
and X-ray emissions detected in SWIFT J1753.5-0127. The origin of the ejection
events detected in XTE J1720-318 is also briefly discussed.Comment: 7 pages, 6 figures accepted for publication in ApJ, minor
modification after proo
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