FSO Proceedings of EuCAP2011 - Dipartimento di Ingegneria ...

Free-Space Optical High-Speed Link in the UrbanArea of Southern Rome: Preliminary ExperimentalSet Up and Channel ModellingFrank S. Marzano 1 , Saverio Mori 1 , Fabrizio Frezza 1 , Paolo Nocito 2 , Giorgio M. Tosi Beleffi 2 , Gabriele Incerti 2 , ElioRestuccia 2 , Fernando Consalvi 31Dipartimento di Ingegneria dell’Informazione, Elettronica e Telecomunicazioni, Sapienza Università di RomaVia Eudossiana 18, 00184 Rome (Italy){marzano, mori, frezza}@die.uniroma1.it2 Ministero dello Sviluppo Economico - Dipartimento per le Comunicazioni, Istituto Superiore C.T.I.Viale America 201 – 00144 Roma{paolo.nocito,giorgio.tosibeleffi,gabriele.incerti,elio.restuccia}@sviluppoeconomico.gov.it3 Fondazione Ugo BordoniViale America 201 – 00144 Romafconsalvi@fub.itAbstract— The activity devoted to the deployment of a new freespace optics (FSO) link set up in Rome (Italy) is illustratedtogether with a preliminary FSO channel modelling and coupledoptical/meteorological measurements. The two-way optical linkat 1550 nm is about 1 km long and has a transmission rate up 1.5Gbps. Among dust, precipitation, turbulence, fog, and any flyingobjects affecting the capacity of the optical channel, a focus isplaced on water particle effects (fog and rain). A semi-empiricalmodel evaluation of these atmospheric effects is illustrated.I. INTRODUCTIONFree space optics (FSO) communications, also known aswireless optical communication (WOC), refer to thetransmission of modulated near-infrared (NIR) beams throughthe atmosphere to obtain optical communications [1]. Asoptical fibers, FSO techniques uses lasers to transmit data, butinstead of enclosing the data stream in a glass fiber, it istransmitted through the air. It can thus be fully considered as a“wireless way” of transmitting information like ordinarywireless systems [2]. The idea, over which FSO is based, isthe transmission of collimated light beams from one telescopeto another using low power infrared lasers. The light from aFSO channel is intercepted by a lens (or system of lenses)capable of focusing photons on a highly sensitive detectorreceivers. Furthermore a new conception of Optical Wirelessinvolves the use of single-mode fibers (SMF) directly as lightlaunchersand light-collectors. Since this technique bypassesthe conversion into the electrical domain, an all-opticaltreatment of the information can be realized so that highperformances achieved by optical fibers can be exploited [3].Optical wireless systems can work over distances of severalkilometers [4]-[7]. As long as there is a clear line of sightbetween the source and the destination, and enoughtransmitter power the communication is guaranteed.Communications are demonstrated to be possible up to 5 km;however the shorter the link range, the higher the “quality ofservice” which can be reached. FSO data rates, comparable tooptical fiber transmission, can be carried with very low errorrates, while the extremely narrow laser beam widths ensure: i)there is almost no practical limit to the number of separateFSO links that can be installed in a given location; ii) thecommunication is intrinsically secure as intrusion is quiteimprobable [1]-[3].In this evolving scenario this work is devoted to illustratethe deployment of a new FSO set up in Rome (Italy) and thepreliminary FSO channel modelling, based on semi-empiricalattenuation prediction and radiative transfer theory.II. EXPERIMENTAL SET UPThe FSO set up in southern Rome (Italy) has been realizedthrough a connection between the roof of the CommunicationsDepartment building (point A, Viale America, height 25 m),the “EUR Mushroom” tower (point B, Piazza Pakistan, height50 m), and the headquarters of the Department of ForeignTrade (point C, Viale Boston, Rome, height 20 m), asschematically shown in Fig. 1. The distance between A and Bis about 800 m, whereas between B and C about 1100 m.An optical link can be successfully established if itstrajectory is not interrupted by obstacles. The quality of thelink is directly related to the characteristics of the medium thelaser beam crosses. Dust, precipitation, turbulence, fog, andany flying objects, may affect the capacity of the channel, thuscausing the deterioration of the received signal. Within theforeseen FSO experiment, the optical link at 1550 nm crossesdifferent urban scenarios . The sections between the sites Aand B and B and C (see Fig. 1) are set up above a high-flowvehicle road, a stretch of small lake water and, finally, a parksparse vegetation with meadows and trees, close to one of thebusiest arteries of Rome.

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

where r V (km) is the visibility range, t 0th stands for powertransmittance threshold (set to 2% or 5%, typically), λ(nm)stands for wavelength, λ 0 as visibility range referencewavelength (set to 550 nm) and q is the size-distributioncoefficient of scattering, empirically tuned, as shown in Fig. 4for r V

A fast analytical approximated method of solution for RTE,based on the generalization of the Eddington approximationand capable to unfold the azimuthal dependence of theradiance field, has been developed [17], [18]. Performances ofthis new radiative model, named GERM (GeneralizedEddington Radiative Model), in simulating the backscattered(reflected) and forwarded (transmitted) intensity field due to aplane- parallel homogeneous medium, have been evaluated bycomparison with the “exact” DISORT numerical model. Themain theoretical limitations of GERM are related to theassumption of: i) stratified medium geometry; ii) unpolarizedradiation. A way to apply GERM to a 3-D problem is to resortto a 1-D slant geometry along the viewing angle, that is toconstruct an equivalent 1-D problem from the given 3-D one.The limitation due to the hypothesis of unpolarized radiancecan be only removed if the elements of the rotated phasematrix can be approximated by means of a Sobolev phasefunction [16]. The validity of this approximation would, ofcourse, depend on the properties of the particle distributionPreliminary GERM numerical results are shown in Fig. 6 interms of percentage fractional error, with respect to DISORToutputs, as a function of optical thickness and volumetricalbedo for asymmetry scattering factor g. The percentageerror is less than 10% on average, being higher for largeralbedo and optical thickness.IV. PRELIMINARY MEASUREMENTSPreliminary measurements, carried out in Rome using theFSO link illustrated in Sect. II, are shown here. The temporaltrend of temperature and humidity for an event with moderatestratiform rain with some embedded convection is shown inFig. 7. The coupled FSO/meteorological measurements spanbetween December 14 and 15, 2010 and clearly evidences theanti-correlation between thermal and wet characteristics of theatmosphere.For the considered winter event Fig. 8 shows the sametemporal trend of Fig. 7, but for the measured rainfall andreceived power at 1550 nm. The received power is anticorrelatedwith rain intensity: when the latter increases, thereceived power decreases and viceversa. Fig. 9 shows themeasured path attenuation, derived from the received power inFig. 8, and the predicted rain specific attenuation, derivedfrom the semi-empirical model (3). There is a good agreementin terms of value range, even though a certain dispersion isnoted probably due to the raingauge sensitivity (about 0.2mm/h). Further analysis is needed to investigate themeasurement quality and test attenuation prediction models.2520Precipitationrain [mm/h]1510500 5 10 15 20 25 30 35 40 45 50time [h]90Received power [W]80706050403020100 5 10 15 20 25 30 35 40 45 50time [h]Fig. 8. Same as in Fig. 7, but for rain (mm/h) and received power (W).Fig. 7. Temporal trend of temperature (K) and relative humidity (%) overa period of 50 hours on Dec. 14-15, 2010 in Rome (Italy).V. CONCLUSIONSThe activity devoted to the deployment of a new free spaceoptics (FSO) link set up in Rome (Italy) has been illustrated.

The two-way optical link at 1550 nm is about 1 km long andcan operate with a transmission rate up 1.5 Gbps. Among dust,precipitation, turbulence, fog, and any flying objects affectingthe capacity of the optical channel, a focus has been placed onwater particle effects (fog and rain). A numerical evaluation ofthese atmospheric effects has been illustrated, together with apreliminary FSO channel modelling based on semi-empiricalprediction models and the radiative transfer theory. The latterframework can, in principle, take into account multiplescattering effects along the FSO path.Further activity will be devoted to assess data quality,collect long time series in various meteorological scenariosand optimize the attenuation prediction models.Path attenuation [dB]Specific attenuation Rain[dB/km]54.543.532.521.510.5FSO Measured average path attenuation at 1550 nm00 5 10 15 20 25 30 35 40 45 50time [h]654321Rain specific attenuation prediction at 1550 nm00 5 10 15 20 25 30 35 40 45 50time [h]Fig. 9. Same as in Fig. 7, but for FSO measured path attenuation (dB) andrain predicted specific attenuation (dB/km), from expression (3).ACKNOWLEDGMENTThe authors deeply thank the staff of CommunicationsDepartment ISCTI (Italy) for the technical support. ISCTI isgreatly acknowledged for sponsoring the fellowship of one ofthe authors (SM).REFERENCES[1] X. Zhu and J. M. Kahn, “Free-space optical communication throughatmospheric turbulence channels,” IEEE Trans. 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