228 research outputs found
Offloading in Software Defined Network at Edge with Information Asymmetry: A Contract Theoretical Approach
The proliferation of highly capable mobile devices such as smartphones and
tablets has significantly increased the demand for wireless access. Software
defined network (SDN) at edge is viewed as one promising technology to simplify
the traffic offloading process for current wireless networks. In this paper, we
investigate the incentive problem in SDN-at-edge of how to motivate a third
party access points (APs) such as WiFi and smallcells to offload traffic for
the central base stations (BSs). The APs will only admit the traffic from the
BS under the precondition that their own traffic demand is satisfied. Under the
information asymmetry that the APs know more about own traffic demands, the BS
needs to distribute the payment in accordance with the APs' idle capacity to
maintain a compatible incentive. First, we apply a contract-theoretic approach
to model and analyze the service trading between the BS and APs. Furthermore,
other two incentive mechanisms: optimal discrimination contract and linear
pricing contract are introduced to serve as the comparisons of the anti adverse
selection contract. Finally, the simulation results show that the contract can
effectively incentivize APs' participation and offload the cellular network
traffic. Furthermore, the anti adverse selection contract achieves the optimal
outcome under the information asymmetry scenario.Comment: 10 pages, 9 figure
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Many-Server Queues with Time-Varying Arrivals, Customer Abandonment, and non-Exponential Distributions
This thesis develops deterministic heavy-traffic fluid approximations for many-server stochastic queueing models. The queueing models, with many homogeneous servers working independently in parallel, are intended to model large-scale service systems such as call centers and health care systems. Such models also have been employed to study communication, computing and manufacturing systems. The heavy-traffic approximations yield relatively simple formulas for quantities describing system performance, such as the expected number of customers waiting in the queue. The new performance approximations are valuable because, in the generality considered, these complex systems are not amenable to exact mathematical analysis. Since the approximate performance measures can be computed quite rapidly, they usefully complement more cumbersome computer simulation. Thus these heavy-traffic approximations can be used to improve capacity planning and operational control. More specifically, the heavy-traffic approximations here are for large-scale service systems, having many servers and a high arrival rate. The main focus is on systems that have time-varying arrival rates and staffing functions.
The system is considered under the assumption that there are alternating periods of overloading and underloading, which commonly occurs when service providers are unable to adjust the staffing frequently enough to economically meet demand at all times. The models also allow the realistic features of customer abandonment and non-exponential probability distributions for the service times and the times customers are willing to wait before abandoning. These features make the overall stochastic model non-Markovian and thus thus very difficult to analyze directly. This thesis provides effective algorithms to compute approximate performance descriptions for these complex systems. These algorithms are based on ordinary differential equations and fixed point equations associated with contraction operators. Simulation experiments are conducted to verify that the approximations are effective.
This thesis consists of four pieces of work, each presented in one chapter.
The first chapter (Chapter 2) develops the basic fluid approximation for a non-Markovian many-server queue with time-varying arrival rate and staffing. The second chapter (Chapter 3) extends the fluid approximation to systems with complex network structure and Markovian routing to other queues of customers after completing service from each queue. The extension to open networks of queues has important applications. For one example, in hospitals, patients usually move among different units such as emergency rooms, operating rooms, and intensive care units. For another example, in manufacturing systems, individual products visit different work stations one or more times. The open network fluid model has multiple queues each of which has a time-varying arrival rate and staffing function.
The third chapter (Chapter 4) studies the large-time asymptotic dynamics of a single fluid queue. When the model parameters are constant, convergence to the steady state as time evolves is established. When the arrival rates are periodic functions, such as in service systems with daily or seasonal cycles, the existence of a periodic steady state and the convergence to that periodic steady state as time evolves are established. Conditions are provided under which this convergence is exponentially fast. The fourth chapter (Chapter 5) uses a fluid approximation to gain insight into nearly periodic behavior seen in overloaded stationary many-server queues with customer abandonment and nearly deterministic service times. Deterministic service times are of applied interest because computer-generated service times, such as automated messages, may well be deterministic, and computer-generated service is becoming more prevalent. With deterministic service times, if all the servers remain busy for a long interval of time, then the times customers enter service assumes a periodic behavior throughout that interval. In overloaded large-scale systems, these intervals tend to persist for a long time, producing nearly periodic behavior.
To gain insight, a heavy-traffic limit theorem is established showing that the fluid model arises as the many-server heavy-traffic limit of a sequence of appropriately scaled queueing models, all having these deterministic service times. Simulation experiments confirm that the transient behavior of the limiting fluid model provides a useful description of the transient performance of the queueing system. However, unlike the asymptotic loss of memory results in the previous chapter for service times with densities, the stationary fluid model with deterministic service times does not approach steady state as time evolves independent of the initial conditions. Since the queueing model with deterministic service times approaches a proper steady state as time evolves, this model with deterministic service times provides an example where the limit interchange (limiting steady state as time evolves and heavy traffic as scale increases) is not valid
On the Effectiveness of Spectral Discriminators for Perceptual Quality Improvement
Several recent studies advocate the use of spectral discriminators, which
evaluate the Fourier spectra of images for generative modeling. However, the
effectiveness of the spectral discriminators is not well interpreted yet. We
tackle this issue by examining the spectral discriminators in the context of
perceptual image super-resolution (i.e., GAN-based SR), as SR image quality is
susceptible to spectral changes. Our analyses reveal that the spectral
discriminator indeed performs better than the ordinary (a.k.a. spatial)
discriminator in identifying the differences in the high-frequency range;
however, the spatial discriminator holds an advantage in the low-frequency
range. Thus, we suggest that the spectral and spatial discriminators shall be
used simultaneously. Moreover, we improve the spectral discriminators by first
calculating the patch-wise Fourier spectrum and then aggregating the spectra by
Transformer. We verify the effectiveness of the proposed method twofold. On the
one hand, thanks to the additional spectral discriminator, our obtained SR
images have their spectra better aligned to those of the real images, which
leads to a better PD tradeoff. On the other hand, our ensembled discriminator
predicts the perceptual quality more accurately, as evidenced in the
no-reference image quality assessment task.Comment: Accepted to ICCV 2023. Code and Models are publicly available at
https://github.com/Luciennnnnnn/DualForme
Adaptive Preconditioned Gradient Descent with Energy
We propose an adaptive time step with energy for a large class of
preconditioned gradient descent methods, mainly applied to constrained
optimization problems. Our strategy relies on representing the usual descent
direction by the product of an energy variable and a transformed gradient, with
a preconditioning matrix, for example, to reflect the natural gradient induced
by the underlying metric in parameter space or to endow a projection operator
when linear equality constraints are present. We present theoretical results on
both unconditional stability and convergence rates for three respective classes
of objective functions. In addition, our numerical results shed light on the
excellent performance of the proposed method on several benchmark optimization
problems.Comment: 32 pages, 3 figure
Inter-generational consequences for growing Caenorhabditis elegans in liquid
In recent years, studies in Caenorhabditis elegans nematodes have shown that different stresses can generate multigenerational changes. Here, we show that worms that grow in liquid media, and also their plate-grown progeny, are different from worms whose ancestors were grown on plates. It has been suggested that C. elegans might encounter liquid environments in nature, although actual observations in the wild are few and far between. By contrast, in the laboratory, growing worms in liquid is commonplace, and often used as an alternative to growing worms on agar plates, to control the composition of the wormsâ diet, to starve (and synchronize) worms or to grow large populations for biochemical assays. We found that plate-grown descendants of M9 liquid medium-grown worms were longer than control worms, and the heritable effects were already apparent very early in development. We tested for the involvement of different known epigenetic inheritance mechanisms, but could not find a single mutant in which these inter-generational effects are cancelled. While we found that growing in liquid always leads to inter-generational changes in the wormsâ size, trans-generational effects were found to be variable, and in some cases, the effects were gone after one to two generations. These results demonstrate that standard cultivation conditions in early life can dramatically change the wormsâ physiology in adulthood, and can also affect the next generations. This article is part of the theme issue âDeveloping differences: early-life effects and evolutionary medicineâ.Fil: Lev, Itamar. Universitat Tel Aviv; IsraelFil: Bril, Roberta. Universitat Tel Aviv; IsraelFil: Liu, Yunan. Universitat Tel Aviv; IsraelFil: CerĂ©, Lucila InĂ©s. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico Conicet - Rosario. Instituto de FisiologĂa Experimental. Universidad Nacional de Rosario. Facultad de Ciencias BioquĂmicas y FarmacĂ©uticas. Instituto de FisiologĂa Experimental; Argentina. Universitat Tel Aviv; IsraelFil: Rechavi, Oded. Universitat Tel Aviv; Israel. Tufts University; Estados Unido
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