9,671 research outputs found
Inter-sensor propagation delay estimation using sources of opportunity
Propagation delays are intensively used for Structural Health Monitoring or
Sensor Network Localization. In this paper, we study the performances of
acoustic propagation delay estimation between two sensors, using sources of
opportunity only. Such sources are defined as being uncontrolled by the user
(activation time, location, spectral content in time and space), thus
preventing the direct estimation with classical active approaches, such as
TDOA, RSSI and AOA. Observation models are extended from the literature to
account for the spectral characteristics of the sources in this passive context
and we show how time-filtered sources of opportunity impact the retrieval of
the propagation delay between two sensors. A geometrical analogy is then
proposed that leads to a lower bound on the variance of the propagation delay
estimation that accounts for both the temporal and the spatial properties of
the sources field
A physics-based approach to flow control using system identification
Control of amplifier flows poses a great challenge, since the influence of environmental noise sources and measurement contamination is a crucial component in the design of models and the subsequent performance of the controller. A modelbased approach that makes a priori assumptions on the noise characteristics often yields unsatisfactory results when the true noise environment is different from the assumed one. An alternative approach is proposed that consists of a data-based systemidentification technique for modelling the flow; it avoids the model-based shortcomings by directly incorporating noise influences into an auto-regressive (ARMAX) design. This technique is applied to flow over a backward-facing step, a typical example of a noise-amplifier flow. Physical insight into the specifics of the flow is used to interpret and tailor the various terms of the auto-regressive model. The designed compensator shows an impressive performance as well as a remarkable robustness to increased noise levels and to off-design operating conditions. Owing to its reliance on only timesequences of observable data, the proposed technique should be attractive in the design of control strategies directly from experimental data and should result in effective compensators that maintain performance in a realistic disturbance environment
Is the metallicity of their hosts a good measure of the metallicity of Type Ia supernovae?
The efficient use of Type Ia supernovae (SNIa) for cosmological studies
requires knowledge of any parameter that can affect their luminosity in either
systematic or statistical ways. Observational samples of SNIa commonly use the
metallicity of the host galaxy, Z_host, as an estimator of the supernova
progenitor metallicity, Z_Ia, that is one of the primary factors affecting SNIa
magnitude. Here, we present a theoretical study of the relationship between
Z_Ia and Z_host. We follow the chemical evolution of homogeneous galaxy models
together with the evolution of the supernova rates in order to evaluate the
metallicity distribution function, MDF(Delta Z), i.e. the probability that the
logarithm of the metallicity of a SNIa exploding now differs in less than Delta
Z from that of its host. We analyse several model galaxies aimed to represent
from active to passive galaxies, including dwarf galaxies prone to experience
supernova driven outflows. We analyse the sensitivity of the MDF to uncertain
ingredients: IMF, star-formation law, stellar lifetime, stellar yields, and
SNIa delay-time distribution. There is a remarkable degree of agreement between
the mean Z_Ia in a galaxy and its Z_host when they both are measured as the CNO
abundance, especially if the DTD peaks at small time delays, while the average
Fe abundance of host and SNIa may differ up to 0.4-0.6 dex in passive galaxies.
The dispersion of Z_Ia in active galaxy models is quite small, meaning that
Z_host is a quite good estimator of the supernova metallicity. Passive galaxies
present a larger dispersion, which is more pronounced in low mass galaxies. We
discuss the use of different metallicity indicators: Fe vs. O, and gas-phase
metallicity vs. stellar metallicity. The results of the application of our
formalism to a galactic catalogue (VESPA) are roughly consistent with our
theoretical estimates. (abridged)Comment: 15 pages, 10 figures, 1 table, accepted for MNRA
Real-Time Localization Using Software Defined Radio
Service providers make use of cost-effective wireless solutions to identify, localize, and possibly track users using their carried MDs to support added services, such as geo-advertisement, security, and management. Indoor and outdoor hotspot areas play a significant role for such services. However, GPS does not work in many of these areas. To solve this problem, service providers leverage available indoor radio technologies, such as WiFi, GSM, and LTE, to identify and localize users. We focus our research on passive services provided by third parties, which are responsible for (i) data acquisition and (ii) processing, and network-based services, where (i) and (ii) are done inside the serving network. For better understanding of parameters that affect indoor localization, we investigate several factors that affect indoor signal propagation for both Bluetooth and WiFi technologies. For GSM-based passive services, we developed first a data acquisition module: a GSM receiver that can overhear GSM uplink messages transmitted by MDs while being invisible. A set of optimizations were made for the receiver components to support wideband capturing of the GSM spectrum while operating in real-time. Processing the wide-spectrum of the GSM is possible using a proposed distributed processing approach over an IP network. Then, to overcome the lack of information about tracked devices’ radio settings, we developed two novel localization algorithms that rely on proximity-based solutions to estimate in real environments devices’ locations. Given the challenging indoor environment on radio signals, such as NLOS reception and multipath propagation, we developed an original algorithm to detect and remove contaminated radio signals before being fed to the localization algorithm. To improve the localization algorithm, we extended our work with a hybrid based approach that uses both WiFi and GSM interfaces to localize users. For network-based services, we used a software implementation of a LTE base station to develop our algorithms, which characterize the indoor environment before applying the localization algorithm. Experiments were conducted without any special hardware, any prior knowledge of the indoor layout or any offline calibration of the system
Evaluation of an End-to-End Delay Estimation in the Case of Multiple Flows in SDN Networks
International audienceThough SDN (Software Defined Network) provides the executive building blocks for programming data-plane appliances , controller decisions must be grounded in an accurate outlook on the network topology and performance. In this context, we focus on the possibility of providing accurate measurements for the end-to-end (E2E) delay in SDN networks. In practice, like many variable quantities, a good description of the E2E delay requires characterizing its first two moments, i.e., expectation and variance. We propose to estimate the E2E delay by making use only of measurements collected locally on each node of the network. We extend a procedure that has been proposed to estimate the E2E delay in the case of one flow to handle the case of multiple competing flows. We compare its accuracy using several scenarios, with different types of traffic following real traces, different topologies and bandwidth. Also, an analysis of the computational and networking costs of our solution is proposed
Evaluation of an End-to-End Delay Estimation in the Case of Multiple Flows in SDN Networks
International audienceThough SDN (Software Defined Network) provides the executive building blocks for programming data-plane appliances , controller decisions must be grounded in an accurate outlook on the network topology and performance. In this context, we focus on the possibility of providing accurate measurements for the end-to-end (E2E) delay in SDN networks. In practice, like many variable quantities, a good description of the E2E delay requires characterizing its first two moments, i.e., expectation and variance. We propose to estimate the E2E delay by making use only of measurements collected locally on each node of the network. We extend a procedure that has been proposed to estimate the E2E delay in the case of one flow to handle the case of multiple competing flows. We compare its accuracy using several scenarios, with different types of traffic following real traces, different topologies and bandwidth. Also, an analysis of the computational and networking costs of our solution is proposed
Optimizing the delivery of multimedia over mobile networks
Mención Internacional en el título de doctorThe consumption of multimedia content is moving from a residential environment to mobile
phones. Mobile data traffic, driven mostly by video demand, is increasing rapidly and wireless
spectrum is becoming a more and more scarce resource. This makes it highly important to operate
mobile networks efficiently. To tackle this, recent developments in anticipatory networking
schemes make it possible to to predict the future capacity of mobile devices and optimize the
allocation of the limited wireless resources. Further, optimizing Quality of Experience—smooth,
quick, and high quality playback—is more difficult in the mobile setting, due to the highly dynamic
nature of wireless links. A key requirement for achieving, both anticipatory networking
schemes and QoE optimization, is estimating the available bandwidth of mobile devices. Ideally,
this should be done quickly and with low overhead.
In summary, we propose a series of improvements to the delivery of multimedia over mobile
networks. We do so, be identifying inefficiencies in the interconnection of mobile operators with
the servers hosting content, propose an algorithm to opportunistically create frequent capacity estimations
suitable for use in resource optimization solutions and finally propose another algorithm
able to estimate the bandwidth class of a device based on minimal traffic in order to identify the
ideal streaming quality its connection may support before commencing playback.
The main body of this thesis proposes two lightweight algorithms designed to provide bandwidth
estimations under the high constraints of the mobile environment, such as and most notably
the usually very limited traffic quota. To do so, we begin with providing a thorough overview
of the communication path between a content server and a mobile device. We continue with
analysing how accurate smartphone measurements can be and also go in depth identifying the
various artifacts adding noise to the fidelity of on device measurements. Then, we first propose
a novel lightweight measurement technique that can be used as a basis for advanced resource
optimization algorithms to be run on mobile phones. Our main idea leverages an original packet
dispersion based technique to estimate per user capacity. This allows passive measurements by
just sampling the existing mobile traffic. Our technique is able to efficiently filter outliers introduced
by mobile network schedulers and phone hardware. In order to asses and verify our
measurement technique, we apply it to a diverse dataset generated by both extensive simulations
and a week-long measurement campaign spanning two cities in two countries, different radio
technologies, and covering all times of the day. The results demonstrate that our technique is effective even if it is provided only with a small fraction of the exchanged packets of a flow. The
only requirement for the input data is that it should consist of a few consecutive packets that are
gathered periodically. This makes the measurement algorithm a good candidate for inclusion in
OS libraries to allow for advanced resource optimization and application-level traffic scheduling,
based on current and predicted future user capacity.
We proceed with another algorithm that takes advantage of the traffic generated by short-lived
TCP connections, which form the majority of the mobile connections, to passively estimate the
currently available bandwidth class. Our algorithm is able to extract useful information even if the
TCP connection never exits the slow start phase. To the best of our knowledge, no other solution
can operate with such constrained input. Our estimation method is able to achieve good precision
despite artifacts introduced by the slow start behavior of TCP, mobile scheduler and phone hardware.
We evaluate our solution against traces collected in 4 European countries. Furthermore, the
small footprint of our algorithm allows its deployment on resource limited devices.
Finally, in an attempt to face the rapid traffic increase, mobile application developers outsource
their cloud infrastructure deployment and content delivery to cloud computing services
and content delivery networks. Studying how these services, which we collectively denote Cloud
Service Providers (CSPs), perform over Mobile Network Operators (MNOs) is crucial to understanding
some of the performance limitations of today’s mobile apps. To that end, we perform
the first empirical study of the complex dynamics between applications, MNOs and CSPs. First,
we use real mobile app traffic traces that we gathered through a global crowdsourcing campaign
to identify the most prevalent CSPs supporting today’s mobile Internet. Then, we investigate how
well these services interconnect with major European MNOs at a topological level, and measure
their performance over European MNO networks through a month-long measurement campaign
on the MONROE mobile broadband testbed. We discover that the top 6 most prevalent CSPs
are used by 85% of apps, and observe significant differences in their performance across different
MNOs due to the nature of their services, peering relationships with MNOs, and deployment
strategies. We also find that CSP performance in MNOs is affected by inflated path length, roaming,
and presence of middleboxes, but not influenced by the choice of DNS resolver. We also
observe that the choice of operator’s Point of Presence (PoP) may inflate by at least 20% the
delay towards popular websites.This work has been supported by IMDEA Networks Institute.Programa Oficial de Doctorado en Ingeniería TelemáticaPresidente: Ahmed Elmokashfi.- Secretario: Rubén Cuevas Rumín.- Vocal: Paolo Din
Analytical and experimental stability investigation of a hardware-in-the-loop satellite docking simulator
The European Proximity Operation Simulator (EPOS) of the DLR-German Aerospace
Center is a robotics-based simulator that aims at validating and verifying a
satellite docking phase. The generic concept features a robotics tracking
system working in closed loop with a force/torque feedback signal. Inherent
delays in the tracking system combined with typical high stiffness at contact
challenge the stability of the closed-loop system. The proposed concept of
operations is hybrid: the feedback signal is a superposition of a measured
value and of a virtual value that can be tuned in order to guarantee a desired
behavior. This paper is concerned with an analytical study of the system's
closed-loop stability, and with an experimental validation of the hybrid
concept of operations in one dimension (1D). The robotics simulator is modeled
as a second-order loop-delay system and closed-form expressions for the
critical delay and associated frequency are derived as a function of the
satellites' mass and the contact dynamics stiffness and damping parameters. A
numerical illustration sheds light on the impact of the parameters on the
stability regions. A first-order Pade approximation provides additional means
of stability investigation. Experiments were performed and tests results are
described for varying values of the mass and the damping coefficients. The
empirical determination of instability is based on the coefficient of
restitution and on the observed energy. There is a very good agreement between
the critical damping values predicted by the analysis and observed during the
tests...Comment: 16 page
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