Cross-correlation analyses of nonlinear systems with spatiotemporalinputs [visual neurons]

Methods are presented for analyzing the low-order stimulus-response cross-correlation functions (or kernels) of visual neurons studied with spatiotemporal white noise. In particular, formulas are derived that relate the low-order kernels of a cell to its responses to single-drifting, double-drifting, and counterphase gratings. The harmonic response terms contributed by the low-order kernels include a mean response term, first- and second-harmonic terms, and sum- and difference-harmonic terms. Using the formulas given, one can obtain kernel-based predictions for the spatiotemporal-frequency tuning of each harmonic. These kernel-based predictions can then be compared with harmonic tuning data obtained in experiments with real grating stimuli. The methods are illustrated using data recorded from one simple and one complex cell from the primary visual cortex of the monkey. The approach of transforming low-order kernels into predicted harmonic tuning functions provides a useful data reduction technique as well as providing insight into the interpretation of kernels     

Random-access systems with a time varying channel

A random multiple-access system which employs a collision resolution algorithm for accessing a common time-varying channel is considered. The channel is assumed to be in one of two states. In each state, the channel is characterized by a general discrete memoryless channel and the transitions between the two states are assumed to be Markovian. A set of sufficient conditions for the ergodicity (stability) of the system is established. As an example, the case when the Capetanakis tree algorithm operates with a time-varying noisy channel is examined. The stability of this system is analyzed, and the throughput is evaluated     

Multiple time scales for nonlinear systems

In this paper we extend the results on the multiple time-scale structure for linear autonomous systems of the form $$\dot x = A( \in )x$$ (cf. Coderchet al. [1]) to nonlinear autonomous systems. Our main result is in obtaining conditions under which the linearized system and the nonlinear system around an equilibrium point have the same time-scale structure.     

Index two linear time varying singular systems of differential equations

Linear time varying singular systems of differential equations of the formA(t)x(t) + B(t)x(t)=f(t) whereA(t) is singular and the system has index at most two are considered. Recent results on their analytic solution are improved on. Examples are given that show these results are not easily extended.This work was supported in part by the National Science Foundation under Grant No. DMS-8318026 and the Air Force Office of Scientific Research under Grant No. 84-0240.     

Stability of nonlinear systems with three time scales

We study the asymptotic stability of a singularly perturbed nonlinear time-invariant systemS _v , which has three vastly different time scales. The systemS _v is approximated by three simpler systems over different time intervals. We give a straightforward proof of the fact that the asymptotic stability ofS _v is guaranteed when the equilibrium points of the three simpler systems are exponentially stable and when the parameters and are sufficiently small.Research sponsored by the Joint Services Electronics Program, Contract Number F4962084-C-0057, and NASA, Grant NAG2-243.     

Transfer functions attached to linear systems with time varying parameters

To obtain transfer functions attached to linear time varying (Ltv) systems, a new method for getting the poles and residia of linear time invariant (Lti) continuous or discrete systems is proposed. The explored method is superior to others known, because it can be extended to systems with time varying parameters. With the poles and residia so obtained, the transfer function attached to the ltv systems, both continuous and discrete, results easily.     

Absolute stability of nonlinear control systems with time lag

A nonlinear control system with time lag is transformed into a nonlinear control system with no time lag by means of certain types of transformations. If absolute stability for this transformed nonlinear control system is obtained by Popov's method, absolute stability in the nonlinear control system with time lag is realized.     

Computer-assisted applications [telephone systems]

Capabilities of computer-to-telephone system linkages are discussed. The intelligent answering, coordinated voice/data transfer, preview dialing, and predictive dialing features of computer-assisted telephone systems are described. The CallPath Services Architecture, which provides an application program interface that masks out the differences between telephone systems, is also discussed     

Earthquake [earthquake warning systems]

Accurate earthquake warnings are, at last, within reach. Instead of coming from the mechanical phenomena that have been the focus of decades of study, however, they will come from electromagnetic phenomena. And, remarkably, these predictions will come from signals gathered not only at the earth's surface but also in the ionosphere. It has been observed that electromagnetic disturbances happen during the days that precede an earthquake. These disturbances happen when crystalline rocks are deformed by the slow grinding of the earth that occurs just before an earthquake. The cracking creates tremendous electric currents in the ground, which travel to the surface and into the air. These currents alter the magnetic field surrounding the earthquake zone and these electromagnetic effects can easily be detected. While such an earthquake warning system would be useful, there are a number of technical and financial problems that need first to be addressed before they can be installed worldwide.     

Tighter controls for busier systems [power systems]

A variety of new technologies are becoming available that will help utilities maintain power system reliability while handling the larger volume of transactions. For example, power electronic systems can provide unprecedented control over electricity flow on transmission networks, preventing or containing cascading disturbances. In addition, new sensor technology, faster communications between control centers, and advanced software tools can enable utilities to monitor system conditions in real time, letting them respond more quickly to disturbances and minimize their impact. Over the next decade, these technologies will facilitate increased power transfers through power delivery systems that are presently constrained, providing power at lower cost to a greater number of customers. It is a question of how to optimize power flow through an entire network by integrating multiple technologies and coordinating control over wide areas. Such integration and coordination will be necessary for deregulation to proceed smoothly toward its twin goals: reducing electricity costs to US consumers, while maintaining the high levels of reliability they have come to expect. The authors discuss the following technologies to help achieve these goals including: flexible AC transmission systems (FACTS), the next generation of FACTS controllers, hierarchical control of FACTS, on-line power systems analysis, control centre communications, and wide area measurement system     

Channel characterization of time reversal UWB communication systems

An ultra wideband (UWB) communications system that applies time reversal to transmit the desired signal is investigated. Exact expressions for the first- and second-order moments, cross-correlation, intersymbol interference metric, and correlation coefficient of time reversal (TR) UWB equivalent channel are derived in terms of the physical channel parameters such as delay spread and mean excess delay. These expressions are verified by simulated and experimental results. It is shown that TR-UWB excess delay is very smaller than UWB and its delay spread decreases as signaling bandwidth increases. Semi-analytical results show that the time reversal UWB delay spread is approximately the same as UWB. Furthermore, an ISI metric is derived for TR-UWB channel based on transmitted signal and UWB channel parameters. Moreover, correlation coefficient of two TR-UWB received signals with different power delay profile is computed analytically. Simulation and analytical results show that for τ?>?0.3T _w correlation coefficient is below 0.25 and for τ?>?T _w correlation coefficient is zero, where T _w is the transmitted pulse width. Finally, theoretical performance of a receiver with one tap matched filter is computed and compared with measured and simulated result.     

Transient and frequency responses of systems with time delays

For systems containing time delays, root locus methods can locate any Laplace transform closed-loop poles. L'H?pital's rule is shown to give exact residue values for any of the infinity of closed-loop poles. The transient or frequency response can then be estimated from dominant poles and their residues.     

Explicit formulae for completions of linear time varying singular systems of differential equations

Completions of linear time varying singular systems of the formE(t)x(t)+F(t)x(t)=f(t) are explicitly computed using recent results on rational matrix functions. The algorithm and the theory behind it are carefully described. Computational issues are discussed.Research supported in part by the U.S. Army Research Office under DAAL03-89-D-0003, and the National Science Foundation under ECS-9012909.     

Method for nonlinear transfer function approximation [mobilecommunication]

The application of the genetic algorithm to the approximation of nonlinear transfer functions is considered. It is shown that the GA approximation method gives better accuracy than the classical Chebyshev approximation, which is sometimes considered to be the best available method for the minimax criterion. Other advantages of the proposed method include the ability to carry out a global search for the optimal solution and easy implementation of various approximation criteria     

Clear skies ahead [navigation systems]

Several projects aimed squarely at upgrading airplane travel, either by enhancing navigation or by improving communications between pilots and air traffic controllers, drew closer to full implementation in 2001. In mid-June, the US Federal Aviation Administration (FAA), Washington, DC, outlined its 10-year operational evolution plan to improve air travel by rolling out new technology to air traffic control facilities. All the systemwide projects target congestion in the skies and on the runways. For too long a time, too many airports were scheduling too many takeoffs or landings for any given period, driving the numbers of delays up. After 11 September, safer air travel became the burning issue. But congestion will no doubt return to prominence once airlines resume full schedules. Meantime, the global positioning system (GPS) satellite constellation and improved situational awareness tools are beginning to enhance safety now, and will alleviate congestion when it returns     

The millennium bug [computer systems]

There is a time bomb ticking away within the world's computers. Come the next millennium and many computer systems will, quite literally, not know what year it is. Here, the author describes the origins of this problem and suggests some solutions