FSO Proceedings of EuCAP2011 - Dipartimento di Ingegneria ...

Fig. 1: I) FSO optical head; II) FSO site in southern Rome.Previous studies on the local micro-climate, withmeasurements made at the meteorological station located in A,confirmed the peculariatiy of the chosen site for urbanwireless communications experiments (see Fig. 2). The FSOexperiment should last no less than two solar years, a periodof interest from a scientific perspective.Fig. 2: Temperature and humidity histogram in Rome.The fSONA-SONAbeam ® 1250-E optical transceiverdevice has been recently installed in A and B (see Fig. 3). Itsmain characteristics are: weight of about 10 kg; structureoperating in the presence of wind gusts over 120 km/h andresistant to over 160 km/h; dimensions of 25×33×46 cm 3 ;power supply with 22÷57 VDC or 85÷260 VAC; safety withstandard IP66 (1550-nm is safe for human eyes); laser with 4transmitters of 160-mW peak power at 1550 nm; aperturediameter of 20 cm; nominal maximum range in clear air up to5300 m; transmission rates (transparent and reclocked) up to1448 Mbs with transmission standards OC-3/STM-1, OC-12/STM-4, 270 Mbps, 1064 Mbps, Fast/Gig Ethernet.Together with the optical transreceiver, there will be alsoavailable at the site A: meteorological station (pressure,temperature, wind), raingauge, particle disdrometer, opticalscintillometer, millimetre-wave radiometer, and visibilimeter.Fig. 3: fSONA-SONAbeam® 1250-E optical transceiver device.III. CHANNEL MODELLINGFSO channel modeling requires to deal with the extinctionof the laser beam power due to optical interactions withatmospheric particle distributions, mainly aerosols, snow, fogand rain [5]. Only the latter two will be treated below. Ingeneral, the “extinction” law for any electromagnetic radiationintensity I (W m -2 sr -1 Hz -1 ) can be written as [8]:Irke( r')dr', 000 (0, r)r, I0,, e I e I t(0,r) (1)where r (km) is the range, () are the incidence angles, I 0 isthe incident intensity, k e (1/km) is the wavelength- andmeteorological-dependent extinction coefficient due toatmospheric gases and particle polydispersions, the opticalthickness and t the intensity transmittance. The relation (1) isalso referred to as the “Beer–Lambert–Bouguer” law withinthe radiative transfer theory [8].A. Atmospheric attenuation due fog and rainThe main uncertainty to design an outdoor short-rangeoptical wireless link is the atmospheric attenuation, caused bythe absorption and scattering [5]-[7]. Water particles andcarbon dioxide mainly cause the absorption of optical signals,whereas fog, rain, snow and clouds cause the scattering ofoptical signals transmitted in free space. This scatteringprocesss causes the emitted light beam to deflect away fromthe intended receiver.Fog is the most deterrent attenuating factor for FSO links[6]. Fog causes significant path attenuation of optical signalsfor considerable amount of time due to the fact that the size offog particles is comparable to the transmission wavelengths ofoptical and near infrared waves [7]. Optical attenuations incase of spherical fog droplets can be accurately predicted byapplying the “Mie scattering” theory [8]. The latter, however,involves requires a detailed information of fog parameters,like particle size, refractive index, particle size distribution,which may not be available at a particular site of installation.An alternate approach is to predict fog attenuation, basedon the visibility range information [9]-[12]. The visibilityrange r V is the distance r to an object where the image contrastdrops to a percentage of what it would be if the object werenearby. The 550-nm wavelength is commonly used tomeasure the reference visibility range as and routinelyavailable at meteorological stations and airports as at thiswavelength the atmosphere has maximum transmittance.To predict fog attenuation, based on visibility rangeestimate, three models are widely used: the Kruse et al. [10],Kim et al. [11] and Al-Naboulsi et al. [12] models. As anexample, from (1) the fog specific attenuation coefficient Fog(dB/km) for both the Kim and Kruse FSO models, assuming auniform medium, is given by [10], [11]:qln 1/ t0th Fog 4.343ker (2)V 0