The time-domain signals representing the heart rate variability (HRV) in the presence of an ectopic beat exhibit a sharp transient at the position of the ectopic beat, which corrupts the signal, particularly the power spectral density (PSD) of the HRV. Consequently, there is a need for correction of this type of beat prior to any HRV analysis. This paper deals with the PSD estimation of the HRV by means of the heart timing (HT) signal when ectopic beats are present. These beat occurrence times are modeled from a generalized, continuous time integral pulse frequency modulation model and, from this point of view, a specific method for minimizing the effect of the presence of ectopic beats is presented to work together with the HT signal. By using both, a white noise driven autoregressive model of the HRV signal with artificially introduced ectopic beats and actual heart rate series including ectopic beats, the more usual methods of HRV spectral estimation are compared. Results of the PSD estimation error function of the number of ectopic beats are presented. These results demonstrate that the proposed method has one order of magnitude lower error than usual ectopic beats removal strategies in preserving PSD, thus, this strategy better recovers the original clinical indexes of interest.

The heart rate variability (HRV) is an extended tool to analyze the mechanisms controlling the cardiovascular system. In this paper, the integral pulse frequency modulation model (IPFM) is assumed. It generates the beat occurrence times from a modulating signal. This signal is thought to represent the autonomic nervous system action, mostly studied in its frequency components. Different spectral estimation methods try to infer the modulating signal characteristics from the available beat timing on the electrocardiogram signal. These methods estimate the spectrum through the heart period (HP) or the heart rate (HR) signal. The authors introduce a new time domain HRV signal, the Heart Timing (HT) signal. They demonstrate that this HT signal, in contrast with the HR or HP, makes it possible to recover an unbiased estimation of the modulating signal spectra. In this estimation the authors avoid the spurious components and the low-pass filtering effect generated when analyzing HR or HP.

Our main goal is to provide a comprehensive explanation of the existing differences in bending losses arising from having step-index multimode plastic optical fibers with different cladding thicknesses and under different types of conditions, namely, the variable bend radius R, the number of fiber turns, or the fiber diameter. For this purpose, both experimental and numerical results of bending losses are presented for different cladding thicknesses and conditions. For the measurements, two cladding thicknesses have been considered: one finite and another infinite. A fiber in air has a finite cladding thickness, and rays are reflected at the cladding-air interface, whereas a fiber covered by oil is equivalent to having an infinite cladding, since the very similar refractive index of oil prevents reflections from occurring at the cladding-oil interface. For the sake of comparison, numerical simulations based on ray tracing have been performed for finite-cladding step-index multimode waveguides. The numerical results reinforce the experimental data, and both the experimental measurements and the computational simulations turn out to be very useful to explain the behavior of refracting and tunneling rays along bent multimode waveguides and along finite-cladding fibers.

We propose to characterize optical power transmission in step-index plastic optical fibers by estimating fiber diffusion and attenuation as functions of the propagation angle. We assume that power flow is described by Glogeś differential equation and find a global solution that was fitted to experimental far field patterns registered using a CCD camera as a function of fiber length. The diffusion and attenuation functions obtained describe completely the fiber behavior and thus, along with the power flow equation, can be used to predict the optical power distribution for any condition.

We present a method to obtain the frequency response of step index (SI) plastic optical fibers (POFs) based on the power flow equation generalized to incorporate the temporal dimension where the fibre diffusion and attenuation are functions of the propagation angle. To solve this equation we propose a fast implementation of the finite-difference method in matrix form. Our method is validated by comparing model predictions to experimental data. In addition, the model provides the space-time evolution of the angular power distribution when it is transmitted throughout the fibre which gives a detailed picture of the POFs capabilities for information transmission. Model predictions show that angular diffusion has a strong impact on temporal pulse widening with propagation.