Search

The relation between local quantities and the uncertainty principle is quantitatively investigated to find out the possible limitations uncertainty principle places on local quantities. The results contradicted the notion that the uncertainty principle limits local quantities.

Research is presented concerning the development of a multiwindow technique for the generation of nonstationary signals with a time-varying spectrum. The computation of a time-varying spectrum is discussed.

A positivity constrained iterative weighted least-squares method for designing non-negative joint time-frequency distributions, or Cohen-Posch time-frequency distributions, is presented. The Cohen-Posch time-frequency distributions satisfy marginal, joint moment, conditional moment and generalized marginal constraints and solve the leakage problem of the least-squares approach. The proposed algorithm is also computationally faster and efficient as reflected in numerical results.

Improvements were made on the least-squares method of T. Sang et al. for establishing nonnegative joint time-frequency distributions (TFDs) satisfying the time and frequency marginals. The proposed method is a positively constrained iterative weighted least-squares algorithm to modify an initial TFD to derive a Cohen-Posch TFD. The algorithm provides a solution for the leakage problem of the least-squares technique and is computationally faster.

The existence of positive time-frequency distributions (TFD) having correct marginal densities provide a method for the construction of any signal. TFDs possess several properties of positive distribution and exhibit a strong finite support. The troughs and peaks in the spectra of TFDs correspond to those of instantaneous spectral and power density. The spectrograms appear like conditional distributions and do not resemble joint distributions.

The Cohen-class time-frequency representations (TFR) used in the processing of multicomponent signals are studied. Generalized bilinear formulations are employed to describethe TFRs' interference properties. The kernel constraints used in obtaining these properties are also derived and presented in tabular form for easier reference. Results show that the time-varying spectral content of nonstationarysignals can be visually interpreted provided the TFRs have good interference localization and attenuation properties.

Recently, Amin et al. introduced a projection filtering method for excising constant amplitude FM jammers from DSSS communications, with minimal distortion to the PN sequence. In this paper, we show that this approach can be applied to AM-FM jammers as well, with a simple modification. Theoretical performance measures (correlator SNR) of the AM-FM projection method are derived, and demonstrate that near ideal performance is achieved for unbiased estimates of the jammer parameters. Results showing the effects of estimation errors in the AM and FM of the jammer on SNR are also provided. In general, FM errors cause greater performance degradation than the same level of error in estimating the AM.

Exact results derived by Cohen and Lee are used to study the distortion induced by the window in the computation of instantaneous bandwidth via the spectrogram. These concepts have been recently used in an interesting study regarding lesion-induced blood flow disturbances, where an approximation was made to compensate for the window effects. We show that this compensation is accurate for stationary signals, but becomes increasingly poorer as the signal becomes less stationary (e.g., large frequency modulations). We propose an alternative technique to reduce the window distortions, and point out the use of other time-frequency distributions that do not suffer such distortions. Index Terms--Instantaneous bandwidth, spectrogram, time-frequency analysis.

This work considers the design of optimal, energy-constrained transmit signals for active sensing for the case when the designer has incomplete or uncertain knowledge of the target and/or environment. The mathematical formulation is that of a multi-objective optimization problem, wherein one can incorporate a plurality of potential targets, interference, or clutter models and in doing so take advantage of the wide range of results in the literature related to modeling each. It is shown, via simulation, that when the objective function of the optimization problem is chosen to maximize the minimum (i.e., maxmin) probability of detection among all possible model combinations, the optimal waveforms obtained are advantageous. The advantage results because the maxmin waveforms judiciously allocate energy to spectral regions where each of the target models respond strongly and each of the environmental models affect minimal detection performance degradation. In particular, improved detection performance is shown compared to linear frequency modulated transmit signals and compared to signals designed with the wrong target spectrum assumed. Additionally, it is shown that the maxmin design yields performance comparable to an optimal design matched to the correct target/environmental model. Finally, it is proven that the maxmin problem formulation is convex. [ABSTRACT FROM AUTHOR]

Vibrotactile feedback (VTF) has been shown to improve balance performance in healthy people and people with vestibular disorders in a single-task experimental condition. It is unclear how age-related changes in balance affect the ability to use VTF and if there are different attentional requirements for old and young adults when using VTF. Twenty younger and 20 older subjects participated in this two-visit study to examine the effect of age, VTF, sensory condition, cognitive task, duration of time, and visit on postural and cognitive performance. Postural performance outcome measures included root mean square of center of pressure (COP) and trunk tilt, and cognitive performance was assessed using the reaction time (RT) from an auditory choice RT task. The results showed that compared with younger adults, older adults had an increase in COP in fixed platform conditions when using VTF, although they were able to reduce COP during sway-referenced platform conditions. Older adults also did not benefit fully from using VTF in their first session. The RTs for the secondary cognitive tasks increased significantly while using the VTF in both younger and older adults. Older adults had a larger increase compared with younger adults, suggesting that greater attentional demands were required in older adults when using VTF information. Future training protocols for VTF should take into consideration the effect of aging. [ABSTRACT FROM AUTHOR]