2010/2

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Wave Propagation in Lossy and Superconducting Circular Waveguides
(Společnost pro radioelektronické inženýrství, 2010-06) Yeap, Kim Ho; Tham, Choy Yoong; Yeong, Kee Choon; Woo, Haw Jiunn
We present an accurate approach to compute the attenuation of waves, propagating in circular waveguides with lossy and superconducting walls. A set of transcendental equation is developed by matching the fields at the surface of the wall with the electrical properties of the wall material. The propagation constant kz is found by numerically solving for the root of the equation. The complex conductivity of the superconductor is obtained from the Mattis-Bardeen equations. We have compared the loss of TE11 mode computed using our technique with that using the perturbation and Stratton’s methods. The results from the three methods agree very well at a reasonable range of frequencies above the cutoff. The curves, however, deviate below cutoff and at millimeter wave frequencies. We attribute the discrepancies to the dispersive effect and the presence of the longitudinal fields in a lossy waveguide. At frequencies below the gap, the superconducting waveguide exhibits lossless transmission behavior. Above the gap frequency, Cooper-pair breaking becomes dominant and the loss increases significantly.
A Study of Fog Characteristics using Free-Space Optical Wireless Links
(Společnost pro radioelektronické inženýrství, 2010-06) Awan, Muhammad Saleem; Leitgeb, Erich; Marzuki; Plank, Thomas
A technique for modeling the fog droplet size distributions using modified gamma distribution has been demonstrated by considering two separate radiation fog events recorded in Graz (Austria) and Prague (Czech Republic). The measurement of liquid water content (LWC) and the optical attenuations at visible wavelength are used to form equations to obtain the three parameters of the modified gamma distribution i.e., the slope (Λ), the intercept (N0 ) and the shape parameter (m). Calculated attenuation or LWC from the retrieved parameters are in excellent agreement with attenuation or LWC obtained from the measurement. Hence this method is useful in the study of fog microphysics and in modeling the fog attenuations for terrestrial FSO links in situations when our measurement data contains values of attenuations only, or liquid water content only or both at a particular location. For the two case studies, Graz and Prague, we obtained the DSD parameters Λ=3.547 ± 1.935, N0 =3.834 ± 2.239, m=6.135 ± 2.692 and Λ=5.882 ± 2.889, N0 =13.41 ± 3.875, m=5.288 ± 3.113, respectively. It is evident that the observed behavior of computed modified gamma distribution parameters for Graz and Prague is closely the same and is consistent with the previous literature for the radiation (continental) fog. Moreover, we observed the variation of the computed DSD parameters at the different stages of fog (formation, maturity and dissipation phases) indicating different microphysical processes at each stage.
Modeling of 830 nm FSO Link Attenuation in Fog or Wind Turbulence
(Společnost pro radioelektronické inženýrství, 2010-06) Pesek, Jiri; Fiser, Ondrej; Svoboda, Jaroslav; Schejbal, Vladimir
This study is focused on the experimental research of the atmosphere impact on FSO link attenuation. Experiment is performed on a mountain observatory Milesovka of severe weather conditions. The empirical relationships on 830 nm only between FSO attenuation A on one hand and atmospheric visibility V or wind turbulent energy ET on the other hand are presented and discussed. In the fog case it was found A= 401.4V-0.5738- 1.462 dB/60m (V in meters) while for the attenuation due to the wind turbulence ET it holds A = 2.112 - 2.213 exp(-0.2867ET) dB/60m (ET in m2s-2, it was derived for 830 nm wave length). A discussion concerning problems of the “clear air attenuation,” atmospheric turbulences and sun shine impact, which was observed as a maximum 2 dB attenuation for a few minutes with a period of 24 hours is added.
Kalman Filter Aided Cooperative Optical Beam Tracking
(Společnost pro radioelektronické inženýrství, 2010-06) Soysal, Gokhan; Efe, Murat
In free-space optical communication between mobile terminals relative motion of the terminals requires an active mechanism to maintain optical alignment between the stations. Cooperative optical beam tracking could be used to address this problem. In this alignment scheme, each station tracks the arrival direction of its impinging beam to employ it as a guide to precisely point its own beam toward the opposite station. Tracking is achieved at each station by a quadrant photodetector which generates simultaneous azimuth and elevation error voltages. In this study a Kalman filtering assisted cooperative optical beam tracking has been proposed and its suitability to cooperative beam tracking is discussed. In this method, Kalman filter is employed to predict the alignment error that is used to produce appropriate control signals for re-alignment. Performance of the proposed method has been demonstrated through simulations.
PDF Estimation and Liquid Water Content Based Attenuation Modeling for Fog in Terrestrial FSO Links
(Společnost pro radioelektronické inženýrství, 2010-06) Muhammad, Sajid Sheikh; Awan, Muhammad Saleem; Rehman, Abdul
Terrestrial Free-space optical communication (FSO) links have yet to achieve a mass market success due to the ever elusive 99.999% availability requirement. The terrestrial FSO links are heavily affected by atmospheric fog. To design systems which can achieve high availability and reliability in the presence of fog, accurate and better models of fog attenuation need to be developed. The current article puts forth appropriate probability density function estimates for received signal strength (hereafter RSS) under fog conditions, where variations in the RSS during foggy events have been statistically characterized. Moreover, from the surface observations of fog density, liquid water content (hereafter LWC) of fog is estimated. The actual measured optical attenuations are then compared with the optical attenuations estimated from LWC. The results presented suggest that fog density measurements carried out are accurate representation of the fog intensity and the attenuation predictions obtained by the LWC estimate match the actual measured optical attenuations. This suggests that the LWC is a useful parameter besides visibility range to predict optical attenuations in the presence of hydrometeors.

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