Prof. Luis F. Chaparro Profile

Prof. Luis F. Chaparro

at Univ of Pittsburgh

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Proceedings Article | 23 May 2014 Paper

Asynchronous signal-dependent non-uniform sampler

Azime Can-Cimino, Luis Chaparro, Ervin Sejdić

Proceedings Volume 9109, 910907 (2014) https://doi.org/10.1117/12.2050568

KEYWORDS: Signal processing, Analog electronics, Modulation, Modulators, Quantization, Reconstruction algorithms, Statistical analysis, Electroencephalography, Error analysis, Biomedical optics

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Analog sparse signals resulting from biomedical and sensing network applications are typically non–stationary with frequency–varying spectra. By ignoring that the maximum frequency of their spectra is changing, uniform sampling of sparse signals collects unnecessary samples in quiescent segments of the signal. A more appropriate sampling approach would be signal–dependent. Moreover, in many of these applications power consumption and analog processing are issues of great importance that need to be considered. In this paper we present a signal dependent non–uniform sampler that uses a Modified Asynchronous Sigma Delta Modulator which consumes low–power and can be processed using analog procedures. Using Prolate Spheroidal Wave Functions (PSWF) interpolation of the original signal is performed, thus giving an asynchronous analog to digital and digital to analog conversion. Stable solutions are obtained by using modulated PSWFs functions. The advantage of the adapted asynchronous sampler is that range of frequencies of the sparse signal is taken into account avoiding aliasing. Moreover, it requires saving only the zero–crossing times of the non–uniform samples, or their differences, and the reconstruction can be done using their quantized values and a PSWF–based interpolation. The range of frequencies analyzed can be changed and the sampler can be implemented as a bank of filters for unknown range of frequencies. The performance of the proposed algorithm is illustrated with an electroencephalogram (EEG) signal.

Proceedings Article | 25 August 2006 Paper

An evolutionary time-frequency approach to multi-carrier wireless communications

Seda Şenay, Luis Chaparro, Aydin Akan

Proceedings Volume 6313, 63130W (2006) https://doi.org/10.1117/12.681638

KEYWORDS: Orthogonal frequency division multiplexing, Time-frequency analysis, Doppler effect, Fourier transforms, Modulation, Telecommunications, Receivers, Wireless communications, Signal to noise ratio, Optical spheres

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This paper provides a time-frequency approach to modeling and estimating the communication channel, and to symbol transmission in multi-carrier wireless systems. The method is based on the evolutionary spectral theory of signals and systems. Multi-carrier systems, such as orthogonal frequency division multiplexing (OFMD) and multi-carrier spread spectrum (MCSS), are very efficient in fast fading channels. However, the basic pulse used in the modulation causes dispersion in time-frequency, complicating inter-symbol and inter-channel interferences. Our time-frequency approach deals separately with the channel modeling and estimation, and with the symbol transmission. Using the properties of the response of the channel to a linear chirp signal it is shown that the typical linear time-variant channel model, needed to characterize multi-path and Doppler, is simplified into a linear time-invariant model of minimal order. Time and Doppler shifts are represented by equivalent time-shifts, and are estimated in a blind fashion from the evolutionary kernel of the received signal. Thus, the linear chirp signal is used as a pilot sequence to characterize the channel. A coherent receiver uses such information to detect the sent symbols. A multi-user OFDM system is obtained using a linear chirp as the modulating signal for basic pulses shifted in time, and by choosing the instantaneous frequency of the linear chirp to be of unity slope for an optimal time-frequency lattice. This optimal lattice increases the transmission rate, diminishes the inter-symbol and inter-channel interference, and provides a new way of looking at OFDM. This approach is extended to MCSS. To illustrate the concepts, simulations, with different signal to noise ratios, are performed. The results are encouraging and worth of further investigation.

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