FREE SPACE OPTICS - MEASUREMENT OF TRANSMISSION QUALITY LINK PARAMETERS

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1 FREE SPACE OPTICS - MEASUREMENT OF TRANSMISSION QUALITY LINK PARAMETERS Pavel Hovořák, Otaka Wilfet Depatment of Radioelectonics, Bno Univesity of Technology, Faculty of Electical Engineeing and Communication, Pukyňova 118, 612, Bno, Czech Republic, phone: hovoak@mendelu.cz This contibution deals with the measuement of tansmission quality of fee space optical (FSO) link. The example of measuement esults made by test FSO link is shown. Steady and statistical model of the FSO link is pesented. The calculation of the link unavailability and pobability of bust eo ae shown. Keywods: Fee space optics, FSO, tansmission quality paametes, atmosphee 1. INTRODUCTION FSO is a full duplex link that uses optical caie wave in the atmospheic tansmission medium, whose output powe is concentated to one o moe naow beams and which is designed fo infomation tansmission with digital modulation IM-OOK. FSO systems epesent good altenative of optical fibe systems fo high bit ate. In case when an undegound laying of an optical cable is inconvenient (tempoay usage, advese teain, etc.) FSO is acceptable. Howeve, FSO ae affected by atmospheic effects those can educe link availability and may intoduce bust eos, which does not occu in fibe tansmission. The main atmospheic phenomena effecting beam popagation ae: - An attenuation of the optical iadiance caused by absoption, scatteing and efaction on gas molecules and aeosols (fog, snow, ain) - An attenuation of the eceived powe caused by beam deflection (tempeatue o mechanical defomation of consoles) - Shot-tem inteuptions of the beam caused by bids- Fluctuations of optical iadiance caused by ai tubulence - Backgound adiation The test FSO link is placed between two univesity buildings in Bno (Pukynova 118 and Technicka 2). The distance between tansceives (heads of the FSO link) is 75m, test link opeates with bit ate of 155 Mb/s and maximum bit eo ate is 1-9. A single beam tansmission and also multi beam tansmission in the test FSO link ae used. The pepaation of methodology of the link availability detemination in whicheve locality the FSO link is placed is goal of this eseach wok. 2. STEADY MODEL OF THE FSO LINK The powe balance of the FSO link expessed by the aveage powe level is called steady model of FSO link. It could be expessed by the powe balance equation o by using gaphical epesentation.

2 Steady model involves the output tansmitte powe P t, eceive sensitivity P, popagation losses L pop, tansmitte and eceive optical system losses L opt,t, L opt,, eceive optical system gain G opt,, magin of the link M and standad atmospheic attenuation α atm,1. The powe balance equation in decibels is [1]: whee P = Pt + Gopt Latm Lopt M, L = L +α. V, atm pop atm,1 atm L opt Lopt, t + Lopt, =, and V atm is the distance between the tansmitte and the eceive in kilometes. The example of gaphical epesentation of the powe balance equation is in Fig. 1. [1] Fig.1. Gaphical epesentation of the powe balance equation (P - eceived powe) 3. STATISTICAL MODEL OF THE FSO LINK Statistical model of the FSO link involves the pobability density function of shot-tem losses (ode of 1 milliseconds) p shot (L), the pobability density function of long-tem losses (ode of 1 minutes) p long (P ) and fomulas by which could be calculated pobability of bust eos PBE and link unavailability P un fo cetain link magin M. Sometimes it happens that in cetain time inteval ae the andom losses in the atmosphee highe than the link magin M o the eceiving optical powe P is highe than maximum detecting powe. In the fist case the value of the eceiving optical powe P is lowe than sensitivity of the eceive P. In the second case satuation of the eceive is coming up. It means that the value of the eceived optical powe P is highe than the eceived powe satuation level P sat. See Fig. 2. [1]

3 Fig. 2. Gaphical epesentation of andom losses effect on powe link balance Whee: L base aveage losses (see steady model), L min losses deceased by constuctive intefeence (eceived optical powe level equals to the eceive satuation level), L high losses, which coesponds to the eceive sensitivity and the eceive dynamic ange. δ = M L min = L base δ L high = L base + M In the both cases, that ae descibed ove, the bit eo ate BER will incease. The pobability of density function of this situation could be deteminated. Long-tem losses ae evaluated by the link unavailability P un. The expeimentally obtained histogam of eceived powe is pesented in Fig. 3. p long (P ) [%.dbm -1 ] P t = 1 dbm P un [% ] M P [dbm] P Fig. 3. Received optical powe P histogam The unavailability P un is given by expession (see Fig. 4.): P P = p ( P ) dp. un long

4 Long-tem andom losses ae caused by beam efaction, scatteing and efaction on gas molecules and aeosols and tempeatue defomation of consoles. The shot-tem inteuption pobability PBE (pobability of bust eo caused only by events cetain length ). The shot-tem inteuption pobability PBE caused only by events whose length is moe than a and less than b.see Fig. 4. is given by [3]: Lhigh PBE = 1 plong ( L) dl L min b i pdf ( i) di a. pdf ( ) d i i i 1 ] y [% b i lit a P ob [s] Fig. 4. Estimation of the pobability density function of fade duations pdf 4. MEASUREMENTS In the Fig. 5 ae shown measued values on testing FSO link [3]. a) b) c) Fig. 5. The example of measuement of the quality link paametes (single beam) a) Bit eo ate, b) pecent of time eo fee seconds, c) eceived optical powe

5 In the Fig. 5 some examples of measuement that wee made by the testing link ae shown. This pictue shows the measuement at the beginning of Novembe last yea. Bit eo ate, pecentage of eo fee seconds and level of eceived powe wee ecoded. On the ight side of the gaph the effect of heavy fog is pesented. On the left side of the gaph incease of BER is pesented which is affected by bids. 5. CONCLUSIONS This contibution shows the poblems concening the FSO link design optimalization. Pepaation of the methodology of link availability detemination in any locality of the link installation is the goal of these measuements. Theefoe it is necessay to measue the tansmissions quality paametes. Thee is also vey impotant to know the weathe conditions like: local tempeatue, atmosphee pessue and humidity. The esults of measuements should give us answe which of FSO system (single beam o multi beam) fo the cetain application and the locality is the optimum solution. 6. ACKNOWLEDGEMENTS This eseach has been suppoted by the eseach pogamme FRVS 2246/25 and by the Gant Agency of the Czech Republic unde the contacts No. 12/4/28, No. 12/5/571 and No 15/5/ REFERENCES [1] Wilfet, O., Kolka, Z., Hlaváč, D., Biolková, V.: Statistický model optického směového spoje - Optické komunikace 21, Paha 21. [2] Santamaía, A., Lopez-Henández, F.J.: Wieless LAN System - Atech House, London, 1994 [3] Wilfet, O., Kolka, Z.: Statistical model of fee-space optical data link. In: Poc. of The Intenational Symposium on Optical Science and Technology. Confeence 555. Denve: SPIE. 24, p (ISBN ) [4] Shimau, A.: Wave Popagation and Scatteing in Random Media -Academic Pess, New Yok, 1978 [5] Andews, L., Phillips, R., Hopen, C.: Lase Beam Scintillation with Applications - Spie Pess, Washington, 21