Published in IET Microwaves, Antennas & Propagation Received on 30th December 2010 Revised on 11th March 2011 doi: 10.1049/iet-map.2010.0630 In Special Section on Selected Papers from Mosharaka International Conference on Communications, Propagation and Electronics (MIC-CPE2010) ISSN 1751-8725 Influences of turbulences in near vicinity of buildings on free-space optical links J. Libich S. Zvanovec Department of Electromagnetic Field, Czech Technical University, Technicka 2, Prague 166 27, Czech Republic E-mail: xzvanove@fel.cvut.cz Abstract: Free-space optical systems offer many advantages for modern wireless communication compared to classical radio- band systems. Nevertheless, unlike fibre optics, their availability is highly affected by atmospheric disturbances. This study discusses the influences of temperature gradients around buildings that cause turbulent areas within the atmosphere, leading to the spatial change of the refractive index and consequently resulting in the bending or widening of transmitted optical beams. The influences of a nearby building on free-space optical links are discussed on the basis of measured data. 1 Introduction Free-space optics (FSO) introduce a line-of-sight optical wireless communication technology which uses narrow optical beams for the transmission of information [1]. FSO offer many advantages for modern communication, including larger frequency bandwidths and substantially higher available data rates, immunity to interference, free license, higher safety of transmission due to narrow optical beams and so on [1]. Nevertheless, many factors may affect an optical beam causing the extinction or fluctuation of the received optical signal. Some of the most important effects – including absorption by atmospheric gases, scattering of small particles, beam wandering and scintillation caused by thermal turbulence within the transmission medium and the non-homogeneities of the refractive index – are described in [2–5]. Most FSO links are nowadays deployed in dense urban areas, where thermal influences (owing to building heating, air-conditioning, wind circulation etc.) can be substantial. It would therefore be beneficial to investigate the statistical influence of turbulences on FSO links not only in open areas but also in the vicinity of buildings. To support system performance analyses, a measurement campaign has been set up at the Czech Technical University in Prague (CTU). The goal of this paper is to demonstrate the relationship between fluctuation of the received optical power and temperature gradients along the path of the optical beam caused by turbulence around buildings. The paper is organised along the following pattern. It opens with a discussion of the performance of the optical link, based on a measurement of the free-space optical link reception statistics and turbulent atmosphere effects and providing essential background information. After this the measurement campaign performed on the CTU campus is introduced in detail. In the next part, measured results from the FSO link, meteorological stations and a special thermal sensor line are analysed and compared to typical turbulence statistics. A discussion follows on the specific dependence of the turbulence features on the distance from the building. This paper concludes with a brief summary. 2 Scintillation due to turbulences Scintillation and wandering of optical beams are mainly caused by thermal turbulence within the transmission medium and non-homogeneities of the refractive index. As air circulates within the urban environment, assuming temporal gradients from buildings and the ground, thermal characteristics can change dramatically along the optical link. For instance, temporal differences of up to 138C/ 100 m were observed during a study of airflow distribution inside street canyons in [6]. Such thermal divergences result in a divergence of the beam and the fluctuation of received optical power over time. The frequency of received signal fluctuations can reach up to 200 Hz [2]. The variance of scintillation (s 2 x ) can be expressed by Rytov variance [5, 7] s 2 x = 1.23C 2 n   2p l   7 L 11 6  ⎛ ⎝ ⎞ ⎠ (1) where l represents the wavelength and L is the length of the link. C n 2 introduces the so-called refractive index structure parameter – a vital measure of turbulence strength. Extensive research has been carried out, especially in cases of vertical variations of the structure parameter. The IET Microw. Antennas Propag., 2011, Vol. 5, Iss. 9, pp. 1039–1044 1039 doi: 10.1049/iet-map.2010.0630 & The Institution of Engineering and Technology 2011 www.ietdl.org