Otakar Wilfert

Statistical model of free-space optical data link

The paper deals with a statistical model of free-space optical (FSO) data link that takes into account the duration of individual fade events. It is well known that a general data transmission using the Internet protocol (IP) is almost insensitive to link interruptions for hundreds of milliseconds while real-time video services will be badly affected. The statistical model is based on the knowledge of the probability density function of random attenuations and the probability density function of durations of the fades. The paper shows the results from our test site obtained from 1999 to 2001.

Optical communication in free space

The success of free-space optical links operating indoors or in the atmosphere or deep space shows a good perspective of the technology. There is no doubt that free-space technology is ready for wide practical application. Optical links have several advantages in comparison with radio links, namely high transmission rates and high security. Advance in systems for the detection and tracking of moving objects allowed the development of mobile optical wireless links. This paper is focused on satellite and mobile optical links. Included are basic characteristics of such links and current results of international research projects.

Propagation study of 850nm/58 GHz hybrid municipal system

The paper deals with the results of a propagation study on a fixed hybrid Free Space Optical (FSO) and Radio Frequency (RF) system operating in 850 nm / 58 GHz bands. Propagation models for the availability assessment of both FSO and RF links were examined against a comprehensive database of meteorological attenuation events. The influences of individual hydrometeors were analyzed and the availability performances of the simulated FSO/MMW hybrid link were evaluated. The study pointed out that visibility and rainfall measurements can be only used for the raw assessment of availability performance due to the concurrent occurrence of different attenuation effect.

Application of nondiffracting beams to wireless optical communications

In wireless optical communications, information is carried by coherent laser beams propagating through the free space. In realized communication channels, the standard beams created directly in the laser resonator are utilized. In recent time, an increasing attention has been focused on the so-called nondiffracting beams generated by auxiliary optical systems. In this paper, the theoretical and experimental aspects of the nondiffracting propagation of light are discussed and geometrical parameters and physical properties of nondiffracting beams promising for wireless communications reviewed.

Analytical expression of FSO link misalignments considering Gaussian beam

In the paper, an optical wireless link for point-to-point communication is analyzed. Its main focus is the analysis of the influence of various misalignments on the received power. Analysis includes influence of the tilt of the transmitter and the receiver as well as their mutual misalignment respecting the Gaussian intensity distribution of the transmitted beam in terms of geometrical optics. As a result of the analysis, expression for influence of the various misalignments on the received power are presented. For the simulation of the link, a MATLAB environment was used. The conclusion contains synthesis of the simulations results. The main contribution is in consideration of the Gaussian beam distribution of the transmitted beam and its analysis for precise prediction of geometrical losses for individual free-space optical links.

Link budget for high-speed short-distance wireless optical link

The paper discusses analyses of high-speed wireless optical link where various effects influence the detected optical power. For purpose of the analyses, Gaussian optical intensity distribution is considered. Further mathematical analysis leads to analytical expressions of attenuation of detected optical power under all common conditions (e.g. misalignment, tilts). All analyses are based on exact mathematical derivation and are modeled using a computer. In conclusion, results of simulations are described and individual influences of various effects are discussed and compared. Emphasis on exact mathematical analysis leads to thorough understanding of a system and allows us to design optical links more effectively with respect to the performance and also the price.

The influence of atmospheric turbulence on the rangefinder laser beam

This paper deals with the influence of atmospheric turbulence on the rangefinder laser beam. Analogous to the optical wireless links, turbulent flow causes optical intensity variation at the receiver plane. In optical communication, most problems namely at longer distances, are caused by beam scintillation and by beam wandering. In range finding systems, a strong influence from irradiated surfaces occurs. Since the laser beam in range finding systems is reflected by the target surface the fluctuation of optical intensity at the receiver depends on surface properties too.

Attenuation measurement on dual-wavelength optical free-space link

Institute of Atmospheric Physics measures atmospheric attenuation on experimental FSO link on 850 and 1550 nm for more than one year. Experimental site is located at meteorological observatory on the isolated mountain with frequent fog, low clouds and strong wind occurrence. Measured attenuation is correlated with the wind turbulence intensity, visibility and LWC. Simple empirical models estimating attenuation on FSO link from meteorological parameters are formulated and verified through the experiment described. The paper shows also basic statistical behavior of the longterm FSO signal level in extreme conditions.

Dual optical wireless test link

A dual optical wireless test link works with two different optical wavelengths in spectral windows of 850 nm and 1550 nm. The transceivers of the link are placed on the highest peak of the Czech Central Mountains (Milesovka Mountain) so that the transmission path is oriented almost in a vertical direction. The installation site is situated in a locality with the harshest climate in the Czech Republic with extreme attenuation conditions. The almost vertically oriented path of the link allows analyzing the impact of the different atmospheric layers on the signal transmission. The monitoring of the received power and the archiving of the appropriate data are constantly provided. The relationship between the link attenuation and the atmospheric visibility has been investigated. The results of this experiment are presented.

Complex model of terrestrial FSO links

The complex model of terrestrial FSO link is based on two models: the power-budget model of a given link (steady model) and the installation site model (statistical model). The steady model is represented by the power balance equation and the power level diagram. The statistical model consists in the knowledge of cumulative exceedance probability of the random atmospheric attenuation coefficient. The parameters of the statistical model depend only on the atmospheric phenomena on installation site. Data from more than 200 sites in Italy, France and Germany are presented.