Reconstruction of chaotic signals with application to channel equalization in chaos‐based communication systems

A number of schemes have been proposed for communication using chaos over the past years. Regardless of the exact modulation method used, the transmitted signal must go through a physical channel which undesirably introduces distortion to the signal and adds noise to it. The problem is particularly serious when coherent‐based demodulation is used because the necessary process of chaos synchronization is difficult to implement in practice. This paper addresses the channel distortion problem and proposes a technique for channel equalization in chaos‐based communication systems. The proposed equalization is realized by a modified recurrent neural network (RNN) incorporating a specific training (equalizing) algorithm. Computer simulations are used to demonstrate the performance of the proposed equalizer in chaos‐based communication systems. The Hénon map and Chua's circuit are used to generate chaotic signals. It is shown that the proposed RNN‐based equalizer outperforms conventional equalizers as well as those based on feedforward neural networks for noisy, distorted linear and non‐linear channels. Copyright © 2004 John Wiley & Sons, Ltd.     

Cross-polarization characteristics of a probe-fed hemisphericaldielectric resonator antenna

The cross-polarization characteristics of a probe-fed hemispherical dielectric resonator antenna are studied theoretically. The mode-matching method was used to find the exact antenna Green's functions rigorously. The effects of the dielectric constant, probe length, and probe displacement on the cross-polarization level are examined     

An improved wavelet approach for finding steady-state waveforms of power electronics circuits using discrete convolution

Due to the switching action and the presence of parasitics, waveforms arising from power electronics circuits often contain high-frequency ringings embedded in slowly varying segments. Such a feature is consistent with the localization property of wavelets which has previously been exploited for fast approximations of steady-state waveforms. This paper proposes an improved and more robust approach for calculating the wavelet coefficients, exploiting the orthogonal property of the Chebyshev polynomials. Simulation results demonstrate the effectiveness of the new algorithm.     

Effective interference and effective bandwidth of linear multiuserreceivers in asynchronous CDMA systems

The performance of linear multiuser receivers in terms of the signal-to-interference ratio (SIR) achieved by the users has been analyzed in a synchronous CDMA system under random spreading sequences. In this paper, we extend these results to a symbol-asynchronous but chip-synchronous system and characterize the SIR for linear receivers-the matched-filter receiver the minimum mean-square error (MMSE) receiver and the decorrelator. For each of the receivers, we characterize the limiting SIR achieved when the processing gain is large and also derive lower bounds on the SIR using the notion of effective interference. Applying the results to a power controlled system, we derive effective bandwidths of the users for these linear receivers and characterize the user capacity region: a set of users is supportable by a system if the sum of the effective bandwidths is less than the processing gain of the system. We show that while the effective bandwidth of the decorrelator and the MMSE receiver is higher in an asynchronous system than that in a synchronous system, it progressively decreases with the increase in the length of the observation window and is asymptotic to that of the synchronous system, when the observation window extends infinitely on both sides of the symbol of interest. Moreover, the performance gap between the MMSE receiver and the decorrelator is significantly wider in the asynchronous setting as compared to the synchronous case     

Autonomous mobile target tracking system based on grey-fuzzycontrol algorithm

This paper presents a new position-based tracking system for autonomous mobile target tracking task. A grey-fuzzy controller (GFC) is developed for motion control of the tracker, in which dynamics models of the target and tracker are not required a priori. The target detection is based on the adaptive visual detector (AVD), which can online adjust the histogram model based on the change of surrounding conditions, such as light variation, in a natural environment. The AVD and GFC are integrated together for mobile target-tracking applications. There are several advantages of the integrated system, in particular: (1) it can rapidly learn the target appearance model for the detection involved with the tracking task; (2) the temporal dynamics model of the target motion can be approximated for the predictive localization of the moving target; and (3) the system can deal with the uncertain environmental conditions to ensure the tracking performance by GFC. Three mobile robots in the authors' laboratory have been used to demonstrate the success of this integrated system experimentally. They also conduct target tracking experiments, in which Chung Cheng-I tracks various moving targets. The results demonstrate the robustness and flexibility of the overall system in dealing with mobile target-tracking problems under varied natural environment conditions     

Synchronization: An Obstacle to Identification of Network Topology

Identification of network topology is an important issue for physics, biology, engineering, and other science disciplines. Under the application of an adaptive-feedback control algorithm, we explore a process through which the topology of an unknown dynamical network can be identified. Analytical results show that the key to guaranteeing successful topology identification is the existence of the condition that all coupling terms of the unknown network are linearly independent of the synchronization manifold between the drive (unknown) network and a response network. When complete synchronization is achieved in the unknown network, the linearly independent condition is no longer satisfied, making its topology unidentifiable. We also find that partial synchronization in the unknown network implies a part of topology being unidentifiable. The results can be extended to projection synchronization and some generalized synchronization. Furthermore, we show that when the network dynamical equation is stable, its topology can be identified subject to some limitations. Finally, a method of avoiding synchronization for a network is presented and verified by numerical simulations.     

Capacity and mutual information of wideband multipath fadingchannels

We investigate the capacity and mutual information of a broadband fading channel consisting of a finite number of time-varying paths. We show that the capacity of the channel in the wideband limit is the same as that of a wideband Gaussian channel with the same average received power. However, the input signals needed to achieve the capacity must be “peaky” in time or frequency. In particular, we show that if white-like signals are used instead (as is common in spread-spectrum systems), the mutual information is inversely proportional to the number of resolvable paths L˜ with energy spread out, and in fact approaches 0 as the number of paths gets large. This is true even when the paths are assumed to be tracked perfectly at the receiver. A critical parameter L˜_crit is defined in terms of system parameters to delineate the threshold on L over which such overspreading phenomenon occurs     

Spectral characteristics of randomly switched PWM DC/DC convertersoperating in discontinuous conduction mode

This paper addresses a comparative study of the spectral characteristics of four random-switching schemes that apply to the basic pulsewidth-modulation (PWM) DC/DC converters operating in discontinuous conduction mode (DCM). They include randomized pulse position modulation, randomized pulsewidth modulation, and randomized carrier frequency modulation with fixed duty cycle and with fixed duty time, respectively. Mathematical models that characterize the input current and output voltage of the three basic PWM converters operating in DCM are derived. In particular, the effectiveness of spreading the dominant switching harmonics in the input current that normally exist in the standard PWM scheme and the introduction of low-frequency harmonics in the output voltage with respect to the randomness level are investigated. The validity of the models and analyses are confirmed experimentally by using a DC/DC buck converter     

Outage Capacity of the Fading Relay Channel in the Low-SNR Regime

In slow-fading scenarios, cooperation between nodes can increase the amount of diversity for communication. We study the performance limit in such scenarios by analyzing the outage capacity of slow fading relay channels. Our focus is on the low signal-to-noise ratio (SNR) and low outage probability regime, where the adverse impact of fading is greatest but so are the potential gains from cooperation. We showed that while the standard Amplify-Forward protocol performs very poorly in this regime, a modified version we called the Bursty Amplify-Forward protocol is optimal and achieves the outage capacity of the network. Moreover, this performance can be achieved without a priori channel knowledge at the receivers. In contrast, the Decode-Forward protocol is strictly suboptimal in this regime. Our results directly yield the outage capacity per unit energy of fading relay channels     

Circuit Theoretic Classification of Parallel Connected DC–DC Converters

This paper describes a classification of paralleling schemes for dc-dc converters from a circuit theoretic viewpoint. The purpose is to provide a systematic classification of the types of parallel converters that can clearly identify all possible structures and control configurations, allowing simple and direct comparison of the characteristics and limitations of different paralleling schemes. In the proposed classification, converters are modeled as current sources or voltage sources, and their connection possibilities, as constrained by Kirchhoff's laws, are categorized systematically into three basic types. Moreover, control arrangements are classified according to the presence of current sharing and voltage-regulation loops. Computer simulations are presented to illustrate the characteristics of the various paralleling schemes