FSO-Relevant New Measurement Results under Moderate Continental Fog Conditions at Graz and Milan

Transcription

1 FSO-Relevat New Measuremet Results uder Moderate Cotietal Fog Coditios at Graz ad Mila Muhammad Saleem Awa, Carlo Capsoi, Erich Leitgeb, Roberto Nebuloi, Marzuki, Farukh Nadeem, Muhammad Saeed Kha Abstract Iterest i Free Space Optics (FSO) has gaied eormous popularity over the last few years, maily due to two reasos, firstly deliverig data rates that ca meet future broadbad telecommuicatio requiremets ad secodly as a alterate techology to solve the last mile access gap. However, optical sigal trasmitted i free space is subjected to deterioratio while propagatig due to its iteractios with the particles of differet sizes ad shapes. The presece of hydrometeors i the atmosphere, for example, fog, mist, clouds, sleet, rai, sow ad hail presets a very severe challege to the optical sigal propagatio; especially fog leads to sigal atteuatios up to a order of few hudreds of db/km. I this paper, the behaviour of FSO liks istalled i Mila ad Graz i the presece of moderate cotietal fog coditios is aalyzed aalytically. Based o measuremets made of sigal atteuatio at the two metioed locatios, comparisos are made for the peak, mea ad media values of measured specific atteuatio values ad predicted sigal atteuatio values calculated aalytically usig stadard modified gamma drop size distributio parameters ad Mie scatterig theory. Moreover, usig Mie scatterig theory a ew set of modified gamma drop size distributio parameters are proposed for the measured peak, media ad average values of specific atteuatios for moderate cotietal fog coditios at Mila ad Graz. Keywords: Drop size distributio (DSD), free space optics (FSO), specific atteuatio, visibility. P I. INTRODUCTION ossible applicatio scearios for Free Space Optics (FSO) are iter-satellite liks, liks for deep space missios, liks betwee umaed aerial vehicles (UAV), high altitude platforms (HAP), data liks from earth to satellites ad itersatellite liks. However, today FSO is beig see as a complemet to the existig commuicatio techologies (RF, mmw ad Fiber Optics etc.) ad a possible solutio to the last mile access problem due to its iheret advatages like very arrow beam, ease of maiteace, quick istallatio, licese free badwidth ad moreover almost o accumulatio ed to ed delays [1]. Muhammad Saleem Awa, Erich Leitgeb, Marzuki, Farukh Nadeem ad Muhammad Saeed Kha are with the Istitute for Broadbad Commuicatio, Graz Uiversity of Techology, Graz, Austria (phoe: ; fax: ; ( m.saleemawa@ studet.tugraz.at, erich.leitgeb@tugraz.at). Carlo Capsoi ad Roberto Nebuloi are with the Politecico di Milao, 0133 Milao, Italy ( capsoi@elet.polimi.it, ebuloi@elet.polimi.it). With the curret moder day telecommuicatio requiremets like broadbad to the ed user, secure ad always-o coectios, FSO provides a solutio to all these eeds ad suits well besides maitaiig a cotiuous flow of iformatio without ay loss of iformatio betwee trasmitter ad receiver [1]. The space applicatios of FSO are divided ito liks i the troposphere (e.g., terrestrial FSO liks ad up-dow liks betwee the satellites ad earth statios), ad liks above the troposphere (e.g., optical itersatellite liks). FSO liks through troposphere are substatially iflueced by differet weather coditios like fog, rai, smog, sleet, clouds ad scitillatios [, 3]. However, the real challege to these FSO liks arises i the presece of differet fog coditios, sice the size of the fog particles is comparable to the wavelegths used for optical wireless trasmissio. Uder dese maritime fog coditios the specific atteuatios reported are up to 480 db/km [4] ad i case of moderate fog the atteuatios are up to 130 db/km [5]. This severe atteuatio caused by fog limits FSO usage as a desirable last mile access solutio ad a competitive complemetary techology for future terrestrial ad space applicatios. Hece, i order to miimize the trasmitted optical wavelegth loss effects especially due to fog for optical wireless liks, the optimum scalig of the relevat fog drop size distributio (DSD) is ecessary that leads to the fietuig of the methods used for the predictio of atteuatios uder certai fog coditios ad thereby improvig the FSO liks availability. This paper is orgaized as follows: After the itroductio part, Mie scatterig ad modified gamma distributio aalysis is discussed for the spherical shape fog droplets. The optical sigal atteuatios measuremet setup at Graz ad Mila is described. Next, we preset the aalysis of fog particles size distributio (PSD) based o Mie scatterig ad Modified Gamma Distributio. Based o this aalysis for moderate cotietal fog coditios at Graz ad Mila we propose a ew set of modified gamma distributio parameters for the mea, media ad peak values of the measured specific atteuatio values. Fially the coclusios are draw. A. Mie Scatterig II. THEORETICAL APPROACH If light passes through a perfectly homogeous medium, it is ot scattered. Scatterig takes place more profoudly if ihomogeeities exist i a particular medium like i the earth /08/$ IEEE

2 atmosphere [6, 7]. I priciple, the scatterig or extictio measuremet studies uder certai fog coditios allow us to estimate ad measure the size ad distributio for a particular fog type droplets preset per uit volume, their shape ad orietatio, ad their chemical compositio. The type of scatterig that govers i case, if the size of the particles i cotact is comparable with the trasmitted wavelegth, is the Mie scatterig [6, 7]. This type of scatterig is domiat i smog, smoke, mist, haze ad especially fog due to the fact that phase of the wave is ot uiform over the particle, these phase differeces give rise to the observed scatterig [3]. I this case forward scattered part of the icidet optical sigal is more tha the backscattered part thereby prevetig the receiver of detectig the miimum required power [8]. To estimate the expected sigal atteuatios over a atmospheric path usig Mie scatterig, few assumptios are made which does ot seem to have a large impact o the accuracy of the results calculated [6, 7]. They are: the scattered light has the same wavelegth as of the icidet light, the particles are spherical i shape ad are actig idepedetly with a complex refractive idex i space, ad a assumptio that oly sigle scatterig occurs while the multiple scatterig effects are eglected [6]. The trasmissio of light i the atmosphere is best explaied by famous Beer-Lambert law, equated i Eq. 1 below: trasmissio wavelegth i µm, Q d the Mie scatterig efficiecy, is real part of the complex refractive idex ad (r) is volume cocetratio (umber of fog particles per uit volume (cm -3 ) per uit icremet of radius (µ) or umber desity of the droplets. Mie scatterig cross sectio (C d ) ad absorptio cross sectio (C a ) are: C W I d d ad i Wa Ca Ii Where W d (W) ad W a (W) is the electromagetic eergy scattered ad absorbed across the surface of a imagiary sphere cetred o the particle ad I i is the itesity (W/m ) of the icidet radiatio. The extictio cross sectio (C e ) is the sum of scatterig ad absorptio cross sectios. The eergy removed from the icidet optical beam by the fog particles is proportioal to the extictio cross sectio (C e ), which accordig to the eergy coservatio priciple is equal to the sum of scatterig (C d ) ad absorptio (Ca) cross sectios: Ce Cd+ Ca sice Qe Qd + Qa The extictio, scatterig ad absorptio efficiecies ca be obtaied by ormalizig the correspodig cross sectios ad are strogly depedet o the parameter r/λ. P( λ, L) τλ (, L) exp[ γλ ( ). L] P( λ,0) (1) C C C Q Q Q πr πr πr a d e a, d, e Where τ(λ, L) is the total trasmittace, L is the legth of the propagatio medium, while P(λ, 0) ad P(λ, L) are the emitted ad received powers at distace L respectively. γ(λ) is the atteuatio of the total extictio coefficiet per uit legth ad is: γ( λ ) αm( λ ) +αa( λ ) +βm( λ ) +βa( λ ) Where α m (λ) ad α a (λ) are the molecular ad aerosol absorptio coefficiets ad β m (λ) ad β a (λ) are the molecular ad aerosol scatterig coefficiets respectively. The effects of molecular scatterig coefficiet, molecular ad aerosol absorptio coefficiets are egligible due to the fact that Rayleigh scatterig is egligible ad also imagiary part of the refractive idex is also very small for the fog particles, therefore above equatio reduces to: or, γ( λ) βa( λ ) 5 πr γ ( λ ) β a( λ ) 10 Qd, π r ( r) dr () 0 λ Here, γ(λ) is the specific atteuatio measured i db/km ad calculated by summig up the atteuatio effects of all idividual fog droplets preset per uit volume per uit icrease of radius. Radius of a fog particle is r (cm), λ the Fig. 1: Predicted specific atteuatios at 10 ºC for moderate cotietal fog coditios. The ormalized cross sectios are complex fuctio of size parameter ad refractive idex. If we already kow the scatterig or extictio cross-sectios ad the correspodig atteuatios, the cocetratio of the fog particles ca be determied by the followig relatioship [6]: γλ ( ) NC. ex (3) Here N is the cocetratio or umber of fog particles preset per uit volume. The full Mie solutio of scatterig due to sigle ad idepedetly actig fog particles is a legthy ad difficult process ivolvig Riccatti-Bessel fuctios with

3 complex argumet [6, 7]. The extictio ad scatterig cross sectios ca be obtaied by: C e x + a + b 1 C ( 1)Re( ) d a b x ( 1)( ) Here x is the size parameter while a ad b are the Mie scatterig coefficiets whose values for a magetic permeability equal to uity are: m j ( mx)[ xj ( x)] j ( x)[ mxj ( mx)] a m j mx xh x h x mxj mx b ' ' (1) ' (1) ' ( )[ ( )] ( )[ ( )] ' ' j( mx)[ xj( x)] j( x)[ mxj( mx)] (1) ' (1) ' ( )[ ( )] ( )[ ( )] j mx xh x h x mxj mx B. Modified Gamma Distributio The size distributio of atmospheric particulates is commoly represeted by two well kow aalytical size distributio models, the logormal distributio ad the Deirmedijia s modified gamma distributio. Modified gamma distributio model is believed a best fit for differet kids of fog ad clouds [9, 10] ad is represeted by Eq. 4 below : α γ (r) ar exp(-br ), 0 r < (4) Here α, γ, a, b are parameters that characterize the PSD of moderate cotietal fog ad are positive costats. Typically, α is the shape parameter, a the itercept ad is also the ormalizatio costat, while b is the slope or gradiet. The value of γ for differet fog ad clouds types is ormally cosidered uity [9, 10]. Usig stadard values of modified gamma distributio the values of its parameters for the moderate cotietal fog ad dese cotietal fog coditios are tabulated i table 1 below: TABLE 1 STANDARD VALUES FOR MODIFIED GAMMA DISTRIBUTION PARAMETERS FOR CONTINENTAL FOG CONDITIONS [4, 9, 10]: Type α a b Moderate cotietal fog Dese cotietal fog N (cm -1 ) M (g/m 3 ) r γ c (µm) The total particle desity (N) measured i cm -3 ca be determied by the relatio [3] below: ( α+ 1) 1 γ α+ 1 N aγ b Γ γ Ad, the modal radius (r γ c) by: γ α r c b γ Fially at modal radius ( r c ), the value of the distributio fuctio is: α γ r ( c) arc α e III. EXPERIMENTAL MEASUREMENT SETUP, MEASURED ATTENUATIONS AND THEIR ANALYSIS A. Setup The measuremets at Graz, Austria were carried out at wavelegths of 850 m ad 950 m for a lik distace of 79.8 m ad 650 m. The optical trasmitter havig two idepedet LED based light sources, oe operatig at 850 m ceter wavelegth ad 50 m spectral width at a full divergece of.4 deg which emits 8 mw average optical power; average emitted power i this case after the les is about 3.5 mw. Secod source operated at 950 m ceter wavelegth ad 30 m spectral widths at a beam divergece of 0.8 deg usig four LEDs each emittig 1 mw to produce the same average power at the receiver. The data was collected ad sampled at every 1 s. The measuremet campaigs were carried out i the witer moths from 004 to 005 ad 005 to 006. The optical lik at Mila, Italy is istalled withi the campus Leoardo of Politecico di Milao at about 0 m above groud ad about 10 m above sea level. The experimetal set up cosist of a commercial optical lik Terescope 3000 formed by two idetical trasceivers, which ca trasmit both data up to 155 Mbps ad a sigle carrier at 785 m. Terescope 3000 system was maufactured by Optical Access ow MRV. The trasmitters o each side of the lik are assembled i triagular shape ad use three idetical ad idepedet semicoductor laser diodes havig omial output power of 10 mw ad the beam divergece is.5 mrad. The receiver has a receivig area of 0.01 mm ad a field of view of.8 mrad while the data is sampled every 1 s. The lik distace is 319 m. The visibility is measured by a optical trasmissiometer (Model 6100) maufactured by Belfort Istrumet while the meteorological quatities temperature, relative humidity, solar radiatio ad pressure are measured by a weather statio maufactured by Davis. The visibilimeter ad the weather statio are placed ear oe of the two termials of the optical lik. Almost cotiuously sice 003, this setup has bee collectig data ad the represetative data for our aalysis is collected for oe full day (6:00 P.M Ja 11 th 005 to 6:00 P.M Ja 1 th 005) ad cotais oe complete fog evet that is aalysed for this cotributio work. B. Measured Atteuatios The atteuatio at millimetre wavelegths is isesitive to fog particle drop size distributio, thus closely followig the Rayleigh predictios, but this is ot the case for the optical ad IR waves where the fog atteuatio is strogly depedet o the drop size distributio. The represetative moderate cotietal fog evets selected for our aalysis are

4 of 0 d February 006 for Graz ad 11 th Jauary 005 to 1 th Jauary 005 for Mila. The atteuatios measured due to moderate cotietal fog were calculated by the followig relatioship: 3.91 λ λ β λ V 550 γ( ) a( ). q I Eq. 5, V is the visibility ad λ is the trasmissio wavelegth. q is a size distributio coefficiet of scatterig, The value of q is reported i literature [4, 6] ad is ot beig metioed ad dealt here. Fig. shows a represetative fog evet measured o at Graz, Austria. The specific atteuatio values for Graz cotai peak value of 59.5 db/km, a mea value of 130 db/km rages ad a media value of 80.4 db/km. O the average, of all the 14 moderate cotietal fog evets measured the specific atteuatio values ever exceed 130 db/km. Fig. : Time series of a moderate cotietal fog evet at Graz (5) Depedig upo the visibility values the specific atteuatio values were estimated for the whole fog evet i parallel. Estimated specific atteuatios from visibility cotai a peak value of db/km, a mea value of db/km ad a media value of db/km. While for specific atteuatios measured from optical lik path, a peak value of 66 db/km is measured sice values exceedig this limit are ot measurable, a mea atteuatio value of db/km ad a media atteuatio value of db/km is measured. C. Measured Atteuatios Aalysis It is importat to metio here that the time variatios of temperature for the Mila s whole fog evet were from 3. ºC to 9.4 ºC with a average temperature of about 5.5 ºC. Geerally, the temperature variatios for Graz i witer are from 0 ºC to 10 ºC. It is also worthwhile to metio that ormally for moderate cotietal fog coditios the liquid water cotet is below 0.1 g/m 3 [10]. To better uderstad the relatioship betwee temperature ad correspodig atteuatios uder particular fog coditios, a plot of complex refractive idex for differet optical wavelegths is show i Fig. 4; the plot is draw by the method give by P. S. Ray [11]. Lookig at the plot show i Fig. 4, o sigificat variatios i the real part of the complex refractive idex of water show; imagiary part of the refractive idex beig very small idicates egligible atteuatio due to absorptio. Secodly, almost stable value of real part of the complex refractive idex of water idicates that there will be o sigificat additioal atteuatios due to the observed chages of temperature of about 0 ºC, besides major atteuatio cotributios due to Mie scatterig, durig a moderate cotietal fog evet i Mila; this same priciple holds true also for Graz. Fig. 3 represets a fog evet measured o durig at Mila, Italy. The maximum dyamic rage for atmospheric losses cosidered for this lik istalled at Mila was about 66 db/km. That meas the system goes i outage wheever the specific atteuatio values exceeds this limit. COMPLEX INDEX OF REFRACTION Real part at 0C Imagiary part 0C Real part at 10C Imagiary part 10C Real part at -10C Imagiary part -10C WAVELENGTH (MICRONS) Fig. 4: Complex refractive idex at differet temperatures for wavelegths betwee 0.5 µm to 10 µm Fig. 3: Time series of a moderate cotietal fog evet at Mila The estimated modified gamma distributio parameters calculated by Eq. 4, which model our measured average, media ad peak atteuatio values for Graz ad Mila i compariso with stadard model values are plotted i Figs. 5 & 6 respectively. The peak specific atteuatio values estimated by Eq. usig stadard parameters of modified

5 gamma distributio for moderate cotietal fog coditio are much less tha the measured peak atteuatio values calculated i Graz ad are much higher tha those measured i Mila. Therefore, a ew set of modified gamma DSD are required that model our observed peak, media ad mea specific atteuatio values which are either less or are eve greater tha those specific atteuatio values that are estimated by usig stadard modified gamma distributio parameters [9, 10]. stadard ad proposed set of modified gamma distributio parameters ad the Mie scatterig procedure for fog particles at a mea temperature of 5.5 ºC. Similarly, Fig. 8 shows a represetative plot of the estimated specific atteuatio values for Graz at 10 ºC but with optical trasmissio at 950 m by adoptig the same calculatio procedure as used for Mila. It was also foud out there are slight idicatios of wavelegth depedet atteuatio for optical liks uder moderate cotietal fog coditios cosiderig the optical wireless trasmissios from 650 m to 1550 m. At loger wavelegths the atteuatios are smaller as compared to shorter wavelegths, but their effects will be better clear if trasmissios are made at 10 micro ad 650 m wavelegths. However, this idicates the preferred usage of loger wavelegths for optical trasmissio as compared to shorter wavelegths. Fig. 5: Estimated values for modified gamma distributio parameters for peak, stadard, mea ad media values of specific atteuatio at Graz Fig. 7: Predicted specific atteuatio for peak, stadard, mea ad media values of specific atteuatio for Mila at 5.5 ºC Fig. 6: Estimated values for modified gamma distributio parameters for peak, stadard, mea ad media values of specific atteuatio at Mila The size rage of moderate cotietal fog droplets ivestigated for our whole aalysis is from 0.17 micro to 50 micro. So keepig the mode value of for the cosidered fog droplets radius [6, 7, 8] uder moderate fog coditios, a ew set of modified gamma distributio parameters were estimated for the peak, media ad mea values of specific atteuatio at the two metioed locatios ad are summarised i table. I case of moderate cotietal fog, it was also foud out that the cotributio from 1-5 micro radius size particles towards the predicted specific atteuatio values is about 96 % ad from 5-50 micro radius size particles the cotributio is oly 4 % of the total specific atteuatio measured. Fig. 7 shows predicted values of specific atteuatio values at 785 m wavelegth at Mila usig Fig. 8: Predicted specific atteuatio for peak, stadard, mea ad media values of specific atteuatio for Graz at 10 ºC Sice durig the life cycle of a fog evet, there are temporal variatios as well as spatial variatios at ay give time. It is importat to metio here that idetical visibilities durig fog formatio ad dissipatio might correspod to differet size distributios. Therefore, the values reported may differ for differet locatios sice the fog drop size distributio ca chage with the locatio ad, eve at a same locatio with time. Also due to the impact of atmospheric

6 factors like wid, temperature, humidity etc. the particle cocetratio may chage drastically which ca further chage ad icrease the sigal atteuatio levels. Table here summarises the proposed modified gamma distributio parameters for the moderate cotietal fog coditios at Mila ad Graz. TABLE PROPOSED MODIFIED GAMMA DISTRIBUTION PARAMETERS FOR MILAN AND GRAZ Moderate Cotietal Fog Graz Mila Sp. Atteuatio (db/km) α A b α a b Stadard Value Peak Value [] S. Sheikh Muhammad, P. Kohldorfer, E. Leitgeb: Chael Modelig for Terrestrial Free Space Optical Liks, ICTON 005. [3] Fredrick G. Smith: Atmospheric Propagatio of Radiatio Volume, The ifrared ad electro-optical systems hadbook, [4] M. Gebhart, E. Leitgeb, S. Sheikh Muhammad, B. Flecker, C. Chlestil, M. Al Naboulsi, H. Sizu, F. De Forel: Measuremet of light atteuatio i dese fog coditios for optical wireless liks, SPIE proceedigs, Vol. 589, 005. [5] Sajid S. Muhammad, B. Flecker, E. Leitgeb ad M. Gebhart: Characterizatio of fog atteuatio i terrestrial free space liks, Joural of OE 46(4) Jue 007. [6] Va de Hulst: Light scatterig by small particles, Wiley, New York, [7] C. F. Bohre ad D. R. Huffma: Absorptio ad scatterig of light by small particles (Joh Wiley ad sos, New York, 1983). [8] M. Achour: Simulatig atmospheric free space optical propagatio, part II: Haze, fog ad low clouds atteuatios, SPIE Proceedigs Vol. 4873, 00. [9] D. Harris: The atteuatio of electromagetic waves due to atmospheric fog, Iteratioal joural of ifrared ad millimeter waves 16(6) (1995). [10] E.P. Shettle: Models of aerosols, clouds ad precipitatio for atmospheric propagatio studies, AGARD coferece 454(15), [11] P. S. Ray, Broadbad Complex refractive idices of ice ad water, Applied Optics, Vol. 11, No. 8, August 197. Mea Value Media Value IV. CONCLUSIONS This cotributio provides ewly estimated values of modified gamma distributio parameters for the moderate cotietal fog coditios at Graz ad Mila. Geerally, it was cocluded that the moderate cotietal fog coditios are stable at both locatios although there are small variatios i the measuremet values of specific atteuatios but that may be due to differet types of optical trasmissiometer systems used to measure the atteuatios of the optical sigal that is trasmitted through the moderate cotietal fog. This paper also gives a compariso betwee measured specific atteuatio values ad the predicted specific fog atteuatio values obtaied from Mie scatterig ad the modified gamma drop size distributio for moderate cotietal fog coditios at Mila ad Graz. It would be of further iterest if the correlatio betwee the atteuatio values ad prevailig atmospheric coditios for differet kids of fog eviromets could be studied ad foud out by fidig the all possible implicatios of the fog temperature, complex refractive idex, relative humidity, particle cocetratio ad wid speed. The kowledge of these parameters implicatios o the optical sigal trasmitted ca be useful to model the atmospheric chael for terrestrial as well as terrestrial to space applicatios. This kowledge would be useful i improvig the predictios of atteuatios ad improvig the availability of these FSO liks especially uder harsh eviromets like fog. REFERENCES [1] C. C. Davis, I. I. Smolyaiov, S. D. Miler: Flexible optical wireless liks ad etworks, IEEE Commuicatio Magazie, pg , March 003.