A Tutorial of 802.11 Implementation in NS-2

By analyzing the source codes of ns-2, we discuss the simulated implementations of wireless channels, network interfaces and mostly the 802.11 MAC protocol in ns-2. We also notice the "bugs" of the 802.11 simulation compared with the reality, and present an extension to fading channels as well

Applying Stochastic Network Calculus to 802.11 Backlog and Delay Analysis

Stochastic network calculus provides an elegant way to characterize traffic and service processes. However, little effort has been made on applying it to multi-access communication systems such as 802.11. In this paper, we take the first step to apply it to the backlog and delay analysis of an 802.11 wireless local network. In particular, we address the following questions: In applying stochastic network calculus, under what situations can we derive stable backlog and delay bounds? How to derive the backlog and delay bounds of an 802.11 wireless node? And how tight are these bounds when compared with simulations? To answer these questions, we first derive the general stability condition of a wireless node (not restricted to 802.11). From this, we give the specific stability condition of an 802.11 wireless node. Then we derive the backlog and delay bounds of an 802.11 node based on an existing model of 802.11. We observe that the derived bounds are loose when compared with ns-2 simulations, indicating that improvements are needed in the current version of stochastic network calculus

On Effectiveness of Backlog Bounds Using Stochastic Network Calculus in 802.11

Network calculus is a powerful methodology of characterizing queueing processes and has wide applications, but few works on applying it to 802.11 by far. In this paper, we take one of the first steps to analyze the backlog bounds of an 802.11 wireless LAN using stochastic network calculus. In particular, we want to address its effectiveness on bounding backlogs. We model a wireless node as a single server with impairment service based on two best-known models in stochastic network calculus: Jiang's and Ciucu's. Interestingly, we find that the two models can derive equivalent stochastic service curves and backlog bounds in our studied case. We prove that the network-calculus bounds imply stable backlogs as long as the average rate of traffic arrival is less than that of service, indicating the theoretical effectiveness of stochastic network calculus in bounding backlogs. From A. Kumar's 802.11 model, we derive the concrete stochastic service curve of an 802.11 node and its backlog bounds. We compare the derived bounds with ns-2 simulations and find that the former are very loose and we discuss the reasons. And we show that the martingale and independent case analysis techniques can improve the bounds significantly. Our work offers a good reference to applying stochastic network calculus to practical scenarios

Causal inference in degenerate systems: An impossibility result

Causal relationships among variables are commonly represented via directed acyclic graphs. There are many methods in the literature to quantify the strength of arrows in a causal acyclic graph. These methods, however, have undesirable properties when the causal system represented by a directed acyclic graph is degenerate. In this paper, we characterize a degenerate causal system using multiplicity of Markov boundaries. We show that in this case, it is impossible to find an identifiable quantitative measure of causal effects that satisfy a set of natural criteria. To supplement the impossibility result, we also develop algorithms to identify degenerate causal systems from observed data. Performance of our algorithms is investigated through synthetic data analysis

A Revised Incremental Conductance MPPT Algorithm for Solar PV Generation Systems

A revised Incremental Conductance (IncCond) maximum power point tracking (MPPT) algorithm for PV generation systems is proposed in this paper. The commonly adopted traditional IncCond method uses a constant step size for voltage adjustment and is difficult to achieve both a good tracking performance and quick elimination of the oscillations, especially under the dramatic changes of the environment conditions. For the revised algorithm, the incremental voltage change step size is adaptively adjusted based on the slope of the power-voltage (P-V) curve. An accelerating factor and a decelerating factor are further applied to adjust the voltage step change considering whether the sign of the P-V curve slope remains the same or not in a subsequent tracking step. In addition, the upper bound of the maximum voltage step change is also updated considering the information of sign changes. The revised MPPT algorithm can quickly track the maximum power points (MPPs) and remove the oscillation of the actual operation points around the real MPPs. The effectiveness of the revised algorithm is demonstrated using a simulation

The current density and transport coefficients in the fully ionized plasma with q-distributions in nonextensive statistics

We study the current density and transport coefficients in the fully ionized plasma with the q-distributions in nonextensive statistics and in strong magnetic field. By using the generalized Boltzmann transport equation in nonextensive statistics, we derive the current density and the expressions of the transport coefficients, including the conductivity, the thermoelectric coefficient, the Hall coefficient, and the Nernst coefficient. It is shown that these new transport coefficients has been generalized to the nonequilibrium complex plasmas with q-distributions in nonextensive statistics, which depend strongly on the q-parameters and when we take the limit q to 1, they perfectly return to those for the plasma based on a Maxwellian distribution.Comment: 10 pages,30 references. arXiv admin note: text overlap with arXiv:1807.0361

Finite-sample Analysis of Greedy-GQ with Linear Function Approximation under Markovian Noise

Greedy-GQ is an off-policy two timescale algorithm for optimal control in reinforcement learning. This paper develops the first finite-sample analysis for the Greedy-GQ algorithm with linear function approximation under Markovian noise. Our finite-sample analysis provides theoretical justification for choosing stepsizes for this two timescale algorithm for faster convergence in practice, and suggests a trade-off between the convergence rate and the quality of the obtained policy. Our paper extends the finite-sample analyses of two timescale reinforcement learning algorithms from policy evaluation to optimal control, which is of more practical interest. Specifically, in contrast to existing finite-sample analyses for two timescale methods, e.g., GTD, GTD2 and TDC, where their objective functions are convex, the objective function of the Greedy-GQ algorithm is non-convex. Moreover, the Greedy-GQ algorithm is also not a linear two-timescale stochastic approximation algorithm. Our techniques in this paper provide a general framework for finite-sample analysis of non-convex value-based reinforcement learning algorithms for optimal control.Comment: UAI 202

Spectrally enhancing near-field radiative transfer between gold gratings by exciting magnetic polariton in nanometric vacuum gaps

In the present work, we theoretically demonstrate that near field radiative transport between one dimensional periodic grating microstructures separated by nanometer vacuum gaps can be spectrally enhanced by exciting magnetic polariton. Fluctuational electrodynamics that incorporates scattering matrix theory with rigorous coupled wave analysis is employed to exactly calculate the near field radiative flux between two gold gratings. Besides the well known coupled surface plasmon polaritons, the radiative flux can be also spectrally enhanced due to magnetic polariton, which is excited in the gap between gold ridges. The mechanisms of magnetic polariton in the near field radiative transport are elucidated in detail, while the unusual enhancement cannot be predicted by either the Derjaguin or effective medium approximations. The effects of vacuum gap distance and grating geometry parameters between the two gratings are investigated. The findings will open up a new way to control near field radiative transfer by magnetic polariton with micro or nanostructured metamaterials

Revisiting the electronic phase diagram of YBa2Cu3Oy via temperature derivative of in-plane resistivity

We have re-examined the temperature-doping (T-p) phase diagram of YBa2Cu3Oy (YBCO) to address several issues therein by using the temperature derivative of in-plane resistivity, d\{rho}ab/dT. For p less than about 0.15, a temperature Tf has been defined to mark the onset of an upturn in d\{rho}ab/dT at T>Tc, which, in light of prior studies on another cuprate La2-xSrxCuO4, is attributed to the onset of superconducting fluctuations in normal state. The Tf exhibits a doping dependence similar to Tc, and the interval between Tf and Tc is about 5-30 K across the underdoped regime, showing agreement with a variety of other measurements to probe a restricted temperature range of superconducting fluctuations. Above Tf, the d\{rho}ab/dT increases linearly as T increases up to a value denoted as T2, which is about half the T* and falls roughly in parallel with T* as p rises up to about 0.13, indicating a prominent T^2-dependent \{rho}ab in this T-p region. The d\{rho}ab/dT helps reveal that, at Tf<T<T2, the \{rho}ab also involves an insulating-like component for p<0.08, or a T-linear component for p>0.10-0.11, thereby leaving a narrow window of doping for \{rho}ab to show a pure T^2 dependence. As T increases further, the d\{rho}ab/dT reaches a local maximum at a temperature T_R, signifying the known curvature change (inflection point) in \{rho}ab. With the derivatives, it is illustrated that, in the vicinity of T_R, the in-plane Hall coefficient R_H and thermopower Sab of YBCO display curvature changes as well, suggesting a correlation of the three transport properties. It is also found that the dSab/dT shows the onset of an upturn at a temperature coinciding with the Tf, which, backing the identification of Tf with the onset of superconducting fluctuations, demonstrates further the virtue of using the temperature derivative to unveil information helpful for the study of high-Tc cuprates.Comment: 22 pages, 13 figure

Refined Asymptotics for Minimal Graphs in the Hyperbolic Space

We study the boundary behaviors of solutions $f$ to the Dirichlet problem for minimal graphs in the hyperbolic space with singular asymptotic boundaries and characterize the boundary behaviors of $f$ at the points strictly located in the tangent cones at the singular points on the boundary. For $n=2$, we also obtain a refined estimate of $f.$Comment: Accepted by SCIENCE CHINA Mathematic