New stimulation pattern design to improve P300-based matrix speller performance at high flash rate.

OBJECTIVE: We propose a new stimulation pattern design for the P300-based matrix speller aimed at increasing the minimum target-to-target interval (TTI). APPROACH: Inspired by the simplicity and strong performance of the conventional row-column (RC) stimulation, the proposed stimulation is obtained by modifying the RC stimulation through alternating row and column flashes which are selected based on the proposed design rules. The second flash of the double-flash components is then delayed for a number of flashing instants to increase the minimum TTI. The trade-off inherited in this approach is the reduced randomness within the stimulation pattern. MAIN RESULTS: We test the proposed stimulation pattern and compare its performance in terms of selection accuracy, raw and practical bit rates with the conventional RC flashing paradigm over several flash rates. By increasing the minimum TTI within the stimulation sequence, the proposed stimulation has more event-related potentials that can be identified compared to that of the conventional RC stimulations, as the flash rate increases. This leads to significant performance improvement in terms of the letter selection accuracy, the raw and practical bit rates over the conventional RC stimulation. SIGNIFICANCE: These studies demonstrate that significant performance improvement over the RC stimulation is obtained without additional testing or training samples to compensate for low P300 amplitude at high flash rate. We show that our proposed stimulation is more robust to reduced signal strength due to the increased flash rate than the RC stimulation.

Investigation of multilevel amplitude modulation for a dual-wavelength free-space optical communications system using realistic channel estimation and minimum mean-squared-error linear equalization.

Fog is a highly dispersive medium at optical wavelengths, and the received pulse waveform may suffer significant distortion. Thus it is desirable to have the impulse response of the propagation channel to recover data transmitted through fog. The fog particle density and the particle size distribution both strongly influence the channel impulse response, yet it is difficult to estimate these parameters. We present a method using a dual-wavelength free-space optical system for estimating the average particle diameter and the particle number density and for approximating the particle distribution function. These parameters serve as inputs to estimate the atmospheric channel impulse response using simulation based on the modified vector radiative transfer theory. The estimated channel response is used to design a minimum mean-square-error equalization filter to improve the bit error rate by correcting distortion in the received signal waveform due to intersymbol interference and additive white Gaussian noise.

Microembolus Sizing in a Blood-Mimicking Fluid Using a Novel Dual-Frequency Pulsed Doppler.

Sizing of emboli in a blood-mimicking fluid is explored with the application of a theoretical description of emboli to embolic signatures obtained in a phantom flow loop. The theoretical predictions for the backscattered power versus embolus size are based on the embolus to blood power ratio (EBR) model. The experimental setup utilizes a customized ultrasound pulsed Doppler that is capable of interrogating a sample volume with two different frequencies concurrently. Polystyrene "emboli" having nominal diameters of 161 &mgr;m are placed in a specially constructed flow loop and scattering signatures are recorded. These signatures are investigated and the effect of beam refraction is discussed, which, combined with the EBR theory, yields a method to determine embolus size from an embolic signal. Embolus size determined experimentally is in close agreement with manufacturer's reported size. (ECHOCARDIOGRAPHY, Volume 13, September 1996)