Ben Lauwens

Performance analysis of unslotted CSMA/CA in wireless networks

In this paper a novel analytical model for the saturation throughput of unslotted Carrier Sensing Multiple Access with Collision Avoidance (CSMA/CA) in wireless networks is proposed. A fixed point procedure is developed based on the interaction of the Physical layer (PHY) and the Medium Access Control sub-layer (MAC). The output of the Clear Channel Assessment (CCA), i.e. idle or busy medium in the neighborhood of a node, serves as a feedback mechanism for the dynamical scheduling rate controlled by the back-off procedure. The PHY is described by a renewal process between successful transmissions with failed attempts and collided packets in between. A semi-Markov process of the internal states of a node is used as a model for the MAC. An event-driven simulator for the non-beacon enabled IEEE Std 802.15.4™MAC is developed to verify the numerical results of the analytical method. A detailed analysis of the idle period after a transmission is carried out based on the proposed analytical approach. The probability that the CCA senses the channel idle depends clearly on the actual back-off stage and the first back-off expiration after a transmission cannot be modeled by a exponential distribution when a finite number of nodes are in contention. The output of the event-driven simulations confirms both statements in great detail and the saturated throughput evaluated with the analytical procedure is verified by event-driven simulations.

Design Considerations for an Electromagnetic Railgun Firing Intelligent Bursts to Be Used Against Antiship Missiles

Railguns can reach higher muzzle velocities and fire rates than conventional guns. Muzzle velocities up to 2400 m/s and fire rates of more than 50 Hz have already been demonstrated with projectiles having a mass of 140 g and a square caliber of 25 mm. We investigated if a close-in weapon system (CIWS) based on a railgun performs better against incoming antiship missiles than a conventional CIWS such as the goalkeeper and propose solutions to optimize the performance of such a railgun. CIWSs are operational systems that defend a ship against incoming subsonic antiship missiles. However, the future antiship missiles will be supersonic and more difficult to defeat with conventional gun systems. Railguns are expected to perform better against these future threats thanks to their higher muzzle velocity and fire rate. Furthermore, the muzzle velocity within a single burst can be varied easily from shot to shot, generating a so-called intelligent burst. It allows varying the velocity of each projectile such that all projectiles arrive on the target at the same time. The number of projectiles, and thus the electrical energy required to achieve a target kill with an intelligent burst is expected to be lower than for railguns firing at constant muzzle velocity. In the first part, the performance of an electromagnetic CIWS is discussed using simulation models calculating the hit probability of a burst of projectiles fired with muzzle velocities ranging from 1200 to 2400 m/s and fire rates ranging from 75 to 300 rounds/s. The geometry of the target is that of a typical antiship missile, its velocity ranges from subsonic (300 m/s) to supersonic (600 m/s). The influence of the projectile mass on the performance of the system and the required electric energy was also investigated. We confirmed that the concept of intelligent burst reduces the required electric energy, especially against supersonic targets. The second part deals with some technical aspects of high fire rate railguns. We have shown experimentally that an automatic loading system allows increasing the fire rate of a medium caliber railgun from 50 to 75 Hz.

Design considerations for an electromagnetic railgun firing intelligent bursts to be used against anti-ship missiles

Railguns can reach higher muzzle velocities and fire rates than conventional guns. Muzzle velocities up to 2400 m/s and fires rates of more than 50 Hz have already been demonstrated with projectiles having a mass of 140 g and a square calibre of 25 mm. We investigated if a Close In Weapon System (CIWS) based on a railgun performs better against incoming anti-ship missiles than a conventional CIWS such as the Goalkeeper and propose solutions to optimize the performance of such a railgun. CIWS are operational systems that defend a ship against incoming subsonic anti-ship missiles. However, future anti-ship missiles will be supersonic and more difficult to defeat with conventional gun systems. Railguns are expected to perform better against these future threats thanks to their higher muzzle velocity and fire rate. Furthermore, the muzzle velocity within a single burst can be varied easily from shot to shot, generating a so-called intelligent burst. It allows varying the velocity of each projectile such that all projectiles arrive on the target at the same time. The number of projectiles and thus the electrical energy required to achieve a target kill with an intelligent burst is expected to be lower than for railguns firing at constant muzzle velocity. In the first part, the performance of an electromagnetic CIWS is discussed using simulation models calculating the hit probability of a burst of projectiles fired with muzzle velocities ranging from 1200 m/s to 2400 m/s and fire rates ranging from 75 rounds per second to 300 rounds per second. The geometry of the target is that of a typical anti-ship missile, its velocity ranges from subsonic (300 m/s) to supersonic (600 m/s). The influence of the projectile mass on the performance of the system and the required electric energy was also investigated. We confirmed that the concept of intelligent burst reduces the required electric energy, especially against supersonic targets. The second part deals with some technical aspects of high fire rate railguns. We have shown experimentally that an automatic loading system allows increasing the fire rate of a medium calibre railgun from 50 Hz to 75 Hz.

Letter to the editor: On the non-singularity of the quasi-Newton-least squares method

We show that, for an affine problem, the approximate Jacobian of the Quasi-Newton-Least Squares method cannot become singular before the solution has been reached.

Equivalence of QN-LS and BQN-LS for affine problems

Previously, we studied methods to solve the coupled system of non-linear equations F ( g ) = p and S ( p ) = g . In this paper we take a closer look at two of them, the Quasi-Newton method with Least Squares Jacobian (QN-LS) and the Block Quasi-Newton method with Least Squares Jacobian (BQN-LS). We show that both are algebraically equivalent if one of the operators ( F or S ) is affine. This implies that for this type of problem there is no reason to use BQN-LS, as the results will be the same but for a higher computational cost.

Pattern formation in a two-component reaction–diffusion system with delayed processes on a network

Reaction–diffusion systems with time-delay defined on complex networks have been studied in the framework of the emergence of Turing instabilities. The use of the Lambert W-function allowed us to get explicit analytic conditions for the onset of patterns as a function of the main involved parameters, the time-delay, the network topology and the diffusion coefficients. Depending on these parameters, the analysis predicts whether the system will evolve towards a stationary Turing pattern or rather to a wave pattern associated to a Hopf bifurcation. The possible outcomes of the linear analysis overcome the respective limitations of the single-species case with delay, and that of the classical activator–inhibitor variant without delay. Numerical results gained from the Mimura–Murray model support the theoretical approach.

Applicability of compressive sensing on three-dimensional terahertz imagery for in-depth object defect detection and recognition using a dedicated semisupervised image processing methodology

Abstract. The quality control of composite multilayered materials and structures using nondestructive tests is of high interest for numerous applications in the aerospace and aeronautics industry. One of the established nondestructive methods uses microwaves to reveal defects inside a three-dimensional (3-D) object. Recently, there has been a tendency to extrapolate this method to higher frequencies (going to the subterahertz spectrum) which could lead to higher resolutions in the obtained 3-D images. Working at higher frequencies reveals challenges to deal with the increased data rate and to efficiently and effectively process and evaluate the obtained 3-D imagery for defect detection and recognition. To deal with these two challenges, we combine compressive sensing (for data rate reduction) with a dedicated image processing methodology for a fast, accurate, and robust quality evaluation of the object under test. We describe in detail the used methodology and evaluate the obtained results using subterahertz data acquired of two calibration samples with a frequency modulated continuous wave system. The applicability of compressive sensing within this context is discussed as well as the quality of the image processing methodology dealing with the reconstructed images.

Buffer Occupation Probability of Trace-Driven Background Streams in Hybrid Simulation

In this paper, numerical calculation schemes of the buffer occupation probability for a buffer which is fed by a large number of independent and stationary sources are evaluated. The resulting buffer occupation probability density function can be sampled in order to model the impact of a background traffic stream on the foreground traffic in a hybrid fluid-flow packet-event simulation. The calculation methods are based on the large deviations asymptotic or on the central limit theorem. An estimate combining both approaches is also studied. The so called moderate deviations scaling gives a large deviations result for a Gaussian source. The paper concludes with a quantitative comparison of the calculation schemes with simulations. Periodic on/off-sources with a heavy tailed distribution are used as arrival processes for performance evaluations. The methods are also applied to the well known Bellcore LAN traffic traces.

On the similarities between the quasi-Newton least squares method and GMRes

We show how the quasi-Newton least squares method (QN-LS) relates to Krylov subspace methods in general and to GMRes in particular.

Stochastic hybrid simulation with applications to queuing networks

This paper deals with an extension to the hybrid simulation paradigm, i.e. the combination of event-driven simulation and analytical modelling, applied to packet telecommunication networks. In order to speed up the simulation only a small part of all packets, the foreground traffic, is processed in an event-driven way. On each arrival of a foreground packet, the waiting time of the packet is sampled from the virtual waiting time distribution function of the combined foreground and background traffic. This distribution function is stochastically modelled by the exact large deviations asymptotic of the virtual waiting time in a many sources regime. This novel methodology is not only valid for wired point-to-point queueing networks having a fixed transmission capacity, but it can also be applied to queueing networks for which the transmission capacity varies with the traffic load of all the elements in the network. The results obtained by the stochastic hybrid simulator are compared to full-blown event-driven simulations. An important reduction in simulation run-time is gained without sacrificing accuracy.