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Effective video multicast over wireless internet
With the rapid growth of wireless networks and great success of Internet video, wireless video services are expected to be widely deployed in the near future. As different types of wireless networks are converging into all IP networks, i.e., the Internet, it is important to study video delivery over the wireless Internet. This paper proposes a novel end-system based adaptation protocol calledWireless Hybrid Adaptation Layered Multicast (WHALM) protocol for layered video multicast over wireless Internet. In WHALM the sender dynamically collects bandwidth distribution from the receivers and uses an optimal layer rate allocation mechanism to reduce the mismatches between the coarse-grained layer subscription levels and the heterogeneous and dynamic rate requirements from the receivers, thus maximizing the degree of satisfaction of all the receivers in a multicast session. Based on sampling theory and theory of probability, we reduce the required number of bandwidth feedbacks to a reasonable degree and use a scalable feedback mechanism to control the feedback process practically. WHALM is also tuned to perform well in wireless networks by integrating an end-to-end loss differentiation algorithm (LDA) to differentiate error losses from congestion losses at the receiver side. With a series of simulation experiments over NS platform, WHALM has been proved to be able to greatly improve the degree of satisfaction of all the receivers while avoiding congestion collapse on the wireless Internet
The principle of least action and the geometric basis of D-branes
We analyze thoroughly the boundary conditions allowed in classical non-linear
sigma models and derive from first principle the corresponding geometric
objects, i.e. D-branes. In addition to giving classical D-branes an intrinsic
and geometric foundation, D-branes in nontrivial H flux and D-branes embedded
within D-branes are precisely defined. A well known topological condition on
D-branes is replaced
Capacity of Compound MIMO Gaussian Channels with Additive Uncertainty
This paper considers reliable communications over a multiple-input
multiple-output (MIMO) Gaussian channel, where the channel matrix is within a
bounded channel uncertainty region around a nominal channel matrix, i.e., an
instance of the compound MIMO Gaussian channel. We study the optimal transmit
covariance matrix design to achieve the capacity of compound MIMO Gaussian
channels, where the channel uncertainty region is characterized by the spectral
norm. This design problem is a challenging non-convex optimization problem.
However, in this paper, we reveal that this problem has a hidden convexity
property, which can be exploited to map the problem into a convex optimization
problem. We first prove that the optimal transmit design is to diagonalize the
nominal channel, and then show that the duality gap between the capacity of the
compound MIMO Gaussian channel and the min-max channel capacity is zero, which
proves the conjecture of Loyka and Charalambous (IEEE Trans. Inf. Theory, vol.
58, no. 4, pp. 2048-2063, 2012). The key tools for showing these results are a
new matrix determinant inequality and some unitarily invariant properties.Comment: 8 pages, submitted to IEEE Transactions on Information Theor
Scheduling of Multicast and Unicast Services under Limited Feedback by using Rateless Codes
Many opportunistic scheduling techniques are impractical because they require
accurate channel state information (CSI) at the transmitter. In this paper, we
investigate the scheduling of unicast and multicast services in a downlink
network with a very limited amount of feedback information. Specifically,
unicast users send imperfect (or no) CSI and infrequent acknowledgements (ACKs)
to a base station, and multicast users only report infrequent ACKs to avoid
feedback implosion. We consider the use of physical-layer rateless codes, which
not only combats channel uncertainty, but also reduces the overhead of ACK
feedback. A joint scheduling and power allocation scheme is developed to
realize multiuser diversity gain for unicast service and multicast gain for
multicast service. We prove that our scheme achieves a near-optimal throughput
region. Our simulation results show that our scheme significantly improves the
network throughput over schemes employing fixed-rate codes or using only
unicast communications
Characterizing Human Mobility Patterns in a Large Street Network
Previous studies demonstrated empirically that human mobility exhibits Levy
flight behaviour. However, our knowledge of the mechanisms governing this Levy
flight behaviour remains limited. Here we analyze over 72 000 people's moving
trajectories, obtained from 50 taxicabs during a six-month period in a large
street network, and illustrate that the human mobility pattern, or the Levy
flight behaviour, is mainly attributed to the underlying street network. In
other words, the goal-directed nature of human movement has little effect on
the overall traffic distribution. We further simulate the mobility of a large
number of random walkers, and find that (1) the simulated random walkers can
reproduce the same human mobility pattern, and (2) the simulated mobility rate
of the random walkers correlates pretty well (an R square up to 0.87) with the
observed human mobility rate.Comment: 13 figures, 17 page
Update or Wait: How to Keep Your Data Fresh
In this work, we study how to optimally manage the freshness of information
updates sent from a source node to a destination via a channel. A proper metric
for data freshness at the destination is the age-of-information, or simply age,
which is defined as how old the freshest received update is since the moment
that this update was generated at the source node (e.g., a sensor). A
reasonable update policy is the zero-wait policy, i.e., the source node submits
a fresh update once the previous update is delivered and the channel becomes
free, which achieves the maximum throughput and the minimum delay.
Surprisingly, this zero-wait policy does not always minimize the age. This
counter-intuitive phenomenon motivates us to study how to optimally control
information updates to keep the data fresh and to understand when the zero-wait
policy is optimal. We introduce a general age penalty function to characterize
the level of dissatisfaction on data staleness and formulate the average age
penalty minimization problem as a constrained semi-Markov decision problem
(SMDP) with an uncountable state space. We develop efficient algorithms to find
the optimal update policy among all causal policies, and establish sufficient
and necessary conditions for the optimality of the zero-wait policy. Our
investigation shows that the zero-wait policy is far from the optimum if (i)
the age penalty function grows quickly with respect to the age, (ii) the packet
transmission times over the channel are positively correlated over time, or
(iii) the packet transmission times are highly random (e.g., following a
heavy-tail distribution)
Incentivizing High Quality Crowdwork
We study the causal effects of financial incentives on the quality of
crowdwork. We focus on performance-based payments (PBPs), bonus payments
awarded to workers for producing high quality work. We design and run
randomized behavioral experiments on the popular crowdsourcing platform Amazon
Mechanical Turk with the goal of understanding when, where, and why PBPs help,
identifying properties of the payment, payment structure, and the task itself
that make them most effective. We provide examples of tasks for which PBPs do
improve quality. For such tasks, the effectiveness of PBPs is not too sensitive
to the threshold for quality required to receive the bonus, while the magnitude
of the bonus must be large enough to make the reward salient. We also present
examples of tasks for which PBPs do not improve quality. Our results suggest
that for PBPs to improve quality, the task must be effort-responsive: the task
must allow workers to produce higher quality work by exerting more effort. We
also give a simple method to determine if a task is effort-responsive a priori.
Furthermore, our experiments suggest that all payments on Mechanical Turk are,
to some degree, implicitly performance-based in that workers believe their work
may be rejected if their performance is sufficiently poor. Finally, we propose
a new model of worker behavior that extends the standard principal-agent model
from economics to include a worker's subjective beliefs about his likelihood of
being paid, and show that the predictions of this model are in line with our
experimental findings. This model may be useful as a foundation for theoretical
studies of incentives in crowdsourcing markets.Comment: This is a preprint of an Article accepted for publication in WWW
\c{opyright} 2015 International World Wide Web Conference Committe
Determination of Wave Function Functionals: The Constrained-Search--Variational Method
In a recent paper [Phys. Rev. Lett. \textbf{93}, 130401 (2004)], we proposed
the idea of expanding the space of variations in variational calculations of
the energy by considering the approximate wave function to be a
functional of functions rather than a function. The
space of variations is expanded because a search over the functions can
in principle lead to the true wave function. As the space of such variations is
large, we proposed the constrained-search-- variational method whereby a
constrained search is first performed over all functions such that the
wave function functional satisfies a physical constraint such as
normalization or the Fermi-Coulomb hole sum rule, or leads to the known value
of an observable such as the diamagnetic susceptibility, nuclear magnetic
constant or Fermi contact term. A rigorous upper bound to the energy is then
obtained by application of the variational principle. A key attribute of the
method is that the wave function functional is accurate throughout space, in
contrast to the standard variational method for which the wave function is
accurate only in those regions of space contributing principally to the energy.
In this paper we generalize the equations of the method to the determination of
arbitrary Hermitian single-particle operators as applied to two-electron atomic
and ionic systems. The description is general and applicable to both ground and
excited states. A discussion on excited states in conjunction with the theorem
of Theophilou is provided.Comment: 26 pages, 4 figures, 5 table
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