Effect of Weather Conditions on FSO link based in Islamabad

Effect of Weather Conditions on FSO link based in Islamabad Nauman Hameed * , Tayyab Mehmood Jatoi ** , Habib Ullah Manzoor * Department of Teleco mmunication, University of E ngineering & Technology, Taxi la, Pakistan, nauman112te@gmail.co m, ** NUST, School of Electr ical Engineering & Co mputer Science, Isla m abad, Pakistan. Abstract —Free space optics (FSO) is a field of curiosity and importance for the scientists because of its numerous applications and advantages like low cost FSO systems, easy dep loyment, high data rate, secure FSO links and license free bands. Very high bandwidth FSO link can be effectively established between t he skyscrapers of the Islamabad Pakistan for the purpose high capacity applications i n these skyscrapers. FSO links are badly affected by the weather condition s especially rain and fog because of high attenuation factors. OPTI-System is used to study the effect of rain and fog on the performance of FSO links. Keywords—Free Space Optics; Weather conditions; BER; Atmospheric attenuation. I. I NTRODUCTI ON In today’s Internet community, demand of services consuming high data rates is incre asing day b y da y . Research work is being carried out in the field of communicatio n technology to fulfill high data rate demand with reliable quality of service and lowest cost possible. Security is a top priority in between communication of two or more par ties. By taking all the facts into mind, Free Space Optics (FSO) is one of the choice to fulfill these demands. FSO provides data rates in Gbps in wireless scenario w ith the m os t secure communication because light bea m is prone to eavesd ropping. FSO is having many advantages over other wireless technolo gies like free lice nsing, ease of installation, low capital equip m ent cost and ver y high data rates. FSO was first used for militar y purposes. FSO has found fascinating applications in access net w or ks (last mile solution), airbo rne & inter-satellite communicatio n, disaster recovery, inter-building co nn ect ions (point to point or multipoint) and short term installatio n for certain events [1] . FSO havi ng larger bandwidth can be a solution to growing capacity hungr y applicatio ns. Despite of its several merits, FSO may not be a good choice for many locations because FSO link is susceptib le to bad weather conditions. FSO utilizes air as an interface for establishing link betw ee n transmitter and receiver. Therefore, weather conditions must be examined bef ore practical implementation o f FSO link. Factors which affect the FSO link are absor ption, scintillation & scattering [2, 3] . Certain weather conditions lik e haze, rain, fog & snow have a different e ffect on optica l trans m issio n. The main factor which affects the most is fog. For maxi mum availab ility of an FSO link, it is necessary to evaluat e these w eat her conditions ove r a large period of time before setting up a link. In this paper work, we have tried to estimate the performance of an FSO link bas ed on weather conditions of Islamabad city. Optisystem is utilized to perform simulative a nalysis. Fig. 1 Future of w ireless comm un ication, c on ceptual FSO lin ks established in Blue Area Islamabad Pakistan II. S I MU LA T I ON S E TU P The system which is set up in Opti system is sho wn in Fig . 2 . First bloc k s ho ws a la ser sou r ce having op erating freque ncy of 15 50 nm because at m o spheric attenuation pro duces less effect at this freque ncy. Second bloc k shows a subsystem wh ic h consists of PRBS (P seudo Rando m Bit Seque nce) generat or, NRZ pulse generato r, lo w pa ss besse l filter and M achze n der m od ulator as shown in Fig. 3 . After that FSO chan nel is pre sent which co m prise s o f a link range of 1K m , atten uation factor , Tx and Rx aper ture dia m eter and beam di vergence. Fourth bloc k is an APD (Avala nche Photo Det ector ) having io nizatio n ratio 0.9 and 10 nA dar k current. Fifth bloc k is a lo w pass bessel filter wi th c utoff frequency= 0.7 5*Bit rate and ord er equals to 4. At the e nd, BER anal y zer is used to co m p ute e ye dia gra m , minimum BER and Q - factor o f the desig nated s ystem. Fig. 2 Reference simulation model of FSO Link Fig. 3 Subsystem containing Bit sequence generator, NRZ pulse driver and Mach-zender modulator. 1) Reference system ch aracteristics Design Param eters Values Data Rate 10Gbps Input Power 5dBm Link Range 1Km Operating Fre quenc y 1550nm Modulator Machzender Mod ulator Sequence L ength 128 bits Samples/bit 64 Number of Sampl es 8192 Beam Diverge nce 2mrad Optical Detector Avalanche Photodiode Filter Ty pe Low Pass Bessel filter Filter O rder 4 III. Q UALITY OF FSO L INK There are certain factors which decide how the FSO link works in specific environ ment. Detailed discussion is gi ven as: A. Link Ma rgin Link margin can be calculat ed by obser ving recei ved signal power at the receiver side [4] . It is an important factor to be observed that can affect the quality of an FSO link. Mathematical expression for link margin ( LM) is given as: LM= 10 log P R ∕ s (1) In the abo ve equation, P R is a received signal po wer and s is receiver sensitivit y. At receiver side, for signal to be detected its po w er should be greater than receiver sensitivity. Receiver sensitivity is a constant value in dB m given b y manufacturer and ranging from -20 to -40 dBm. So, received signal po wer must be evaluated for q uality check of an FSO li nk. B. Geometric A ttenuation Divergence of op tical beam is ca used due to geo metric attenuation while pro pagating through air interface. It can be calculated for performance evaluation of an FSO link. The expression used for geometrical attenuation is give n in [4] as:   =         (2) Where d RX and d TX are dia m et ers of apertures for receiver and transmitter respec tively. While ‘  ’ represents the diver gence angle and ‘l’ denote s link length. C. Atmospheric Attenu ation Attenuation which occurs in attenuation channel due to presence of aerosols is termed as atmospheric atten uation. As a result of atmospheric attenuation, light bea m is partially distorted resulting in scatteri ng, absor ption and diffractio n. From [5], e xpression is noted as: ∝=   (3) Whereas ‘l’ is d istance between tra nsmitter and rece iver, ‘  ’ is attenuation coe fficient per unit length. Further value o f ‘  ’ can be calculated usin g Kim and Kruse relations. Atmospheric attenuation produced due t o the phenomenon of sca ttering a nd absor ption of light beam can be calculated using Beer ’s Law [6] which is:  =    () (4) Whereas ‘  ’ and ‘   ’are dete cted and initial intensities at certain location ‘x’. While ‘  ’ is atten uation coefficient. IV. W EATH ER INFLUEN CE ON FSO LINK Free Space Optics links are ope rated in op en atmosphere, so local w eather conditions and microphysics of at m osph ere highly affect the pro pagating light sig nal. Certai n para m et ers play their role in degrad ation of signal quality. Visibilit y determines that ho w far an optical signal can travel in open a ir. Various elements pre sent in air can limit the visibilit y . Dust particles, smoke, rain, haze, fog and snow atte nuate the sig nal at different intensities. Fog is the major attenuation facto r because the size of its par ticles is similar to the wavelength of light used as a signal carrier. The size of sno w particles is a bit larger therefore less atte nuation is induced. In [7] the impact of different weather conditions like rain, fog and sno w was investigated. A. Rain Attenuation Rain is o ne of the facto r for inducing atte nuation in a FSO system. Rain has less impact than fog because wavelength of optical signal is very small a s compared to rain dro p [6]. The attenuation of optical signal due to rain is b y scatteri ng phenomenon. Specific optical attenuation increases linear ly with increa se in rain rate. Rainy sea son in Isla mabad starts at the end of Ju ne and remains till the e nd of September with average monsoon rainfall of 790 .8mm [8 ]. In the months of December, Januar y a nd February, mostly modera te rain is observed. The rain rate of 25 mm/hr induces atte nuation factor of 6dB/Km for 1K m of link length. FSO systems having 25dB of link margin are able to penetrate rain unobstructed [6]. Fig. 4 shows the attenuation factor vs. rain rate computed at Milan, Italy reported in [9]. For rain rates of 25, 50, 100, 150mm/hr, optical attenuation values are 7.3, 14. 6, 23.8 and 30.38dB/K m respectively. Statistics in [10 ] show that avera ge rainfall in the month of Jul y and August is 100 mm /hr which y ields the attenuation value of 30.38dB/Km. After performing simulatio n, Q-factor value of 25 an d BER value o f 1.3 3e-144 at attenuation factor of 30. 38dB/Km is achieved. The eye diagra m is s hown in fig. 5: Fig. 4 Estimated optical attenuation at Milan, Italy. Fig. 5 Eye diagram for simulation setup of rain attenuation. B. Fog Effect Fog is the most hostile facto r to FSO link. Fo g particles are having nearly the same wavelength as wav e length of light. Therefore, the a ttenuation caused by fog is extremel y large with reference to other weather condition s. It is the most alarming weather condition for an FSO link. Fog events in Islamab ad occur mostly in the months of November, Dece m ber , January and Februa ry. In [11], four fog events are studied by using Kim, Kruse and Al Naboulsi model. Peak attenuation val ues for four events are 88 .43, 77.88 and 110dB/K m for Kim, Kr use a nd Al Naboulsi m od el respectivel y . At such high value of attenuation, possible link length ca n be 5 00m for reliab le co mmunication. By takin g link length of 50 0m with atte nuation factor of 100dB/Km, si mu lation result s show that Q-factor of 9.17 is achieved at BER value o f 2.28e-20. Eye diagra m for this scenario is shown in fig. 6. Fig. 6 Eye diagram for simulation setup of fog attenuation. V. P OSSI B L E S CENARIOS OF FSO LINKS I N I SLAMABAD Some possible scenarios for setting up FSO co mm un ication links are shown in table. 2 in the city of Islamabad. Google maps is used for capturing images and calculatin g distances between b uildings. Aerial view o f certain li nks in I slamabad is shown in fig. 7. 2) Proposed FS O links VI. C ONCLUSION Free Space Optics is a feasible solution for day-by-day increasing demands of ba ndwidth hungry applicatio ns. Exposure to bad weather conditions make its imple mentation nearly impo ssible for certain regions. For cities like Islamabad Link # Transmitting building Receiver building Distance (Km) 1 Cisco Systems KPMG Taseer Hadi & Co 0.047 2 Cisco Systems Shaheed-e- Millat Secretariat 0.132 3 Shaheed-e- Millat Secretariat OGDCL 0.153 4 OGDCL United Bank L imited 0.068 5 OGDCL Green Trust Tow er 0.103 6 Green Trust Tow er HR Consultants 0.522 7 Green Trust Tow er NIC building 1.452 8 NIC building Huawei technol ogies Pakistan 0.191 9 NIC building State Life Tow er 0.064 10 Huawei technolo gies Pakistan Ufone Towe r 0.659 11 Ufone Towe r Islamabad Stock Exchange 0.050 12 Ufone Towe r Centaurs 0.851 13 Centaurs ZTBL 1.728 where weather conditions ar e reasonably bad for FSO links spanning certai n kilometers, short links ca n be setup by calculating link margin and link budget with accep table attenuation. R EFERENCES [1] F orin , Davide M., et al. "Free space optical technol ogies." Trends in Telecom muni cations Tech nologies (2010): 257-296. [2] Manor, Haim, and Shlomi Arnon. "Performance of an optical wireless communication system as a function of wavele ngth. " Applied optics 42.21 (2003): 4285-42 94. [3] A .S. Mohd Supa’at, A.B. Mohammad, Y.T. Tong, S.M. I drus, Unguided optical comm unication: design and ev aluation in Malaysian weather, in: Proc. Research Seminar on Electronics, Aerospace, Information Technology an d Tel ecommunications, UTM, 1998, p p. 268 - 271. [4] Al ma, Husagic, and Wajdi Al-Khateeb. "Effect of weather conditions on quality of Free Space Optics li nk s (w ith focus on Malaysia)." Computer and Communication Engineering, 2008. ICCCE 2008. International Conference on . I EEE, 2008. [5] Naboulsi, M., H. Sizun, and F. Fornel. "Propagation of optical and infrared waves in the atmospher e." Proceedings of the union radio scientifique inter nationale (2005). [6] W illebrand, Heinz, and Bakshees h S. Ghuman. Free space optics: enabling optical connect ivity in today's networks . Sams Publis hing, 2002. [7] Aw an, Muhammad Saleem, et al. "Weather effects impact on the optical pulse propagation in free space." Vehicular Technology Conference, 2009. VTC Sprin g 2009. IEEE 69th . IEEE, 2009. [8] http://en.w ikipedia.org/wiki/Climate_o f_ I slamabad [9] T. Oguchi (1983). Electromagnetic wave propagation and scattering in rain and other hy drometers. Proceedings of the I EEE, 71, 9, 1029-1079. [10] http://www.holiday-w eather.com/islamabad/average s/ [11] Khan, M. S., et al. "Optical attenuation estimation from measured visibility data in Islamabad, Pakista n." Netwo rk and Optical Communications (NOC), 2013 18th European Confer ence on and Optical Cabling an d Infrastructure (OC&i), 2013 8th Conference on. IEEE, 2013. Fig. 7 Aerial view of the FSO links established between skyscrapers in Blue Area Islamabad