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2011 4th

International Conference on Mechatronics (ICOM), 17-19 May 2011, Kuala Lumpur, Malaysia

978-1-61284-437-4/11/$26.00 ©2011 IEEE

Project sponsored by E-Science Project (MOSTI)

Rain Fade Analysis for Practical Free Space OpticLink in Tropical Region

Wajdi Al-Khateeb , Md Rafiqul Islam, Myat Tun Oo

Department of Electrical and Computer Engineering, Faculty of Engineering,International Islamic University Malaysia,

Jalan Gombak, 53100 Kuala Lumpur, MalaysiaE-mail: [email protected]

Abstract — Attenuation due to rainfall can severely degrade the

FSO links operating under tropical weather. It restricts the

distance of FSO communication systems and limits the

availability for line-of-sight terrestrial link. Practical FSO link

has been setup, monitored and analyzed. A commercially

available FSO system was installed at 800 m length together

with rain fall intensity measurement facility at IIUM KualaLumpur campus. This paper is focused on the effect of rain on

the FSO link. The rain attenuation on link has been

synchronized in real time with rain fall intensity meter to

enable link measurement. The measured rain intensity data

was used to compare with ITU-R rain attenuation prediction

model to validate the model. The measured rain attenuation on

link was compared with that predicted by ITU-R prediction

model recommended for the microwave link

Keywords-FSO,Aailability prediction; Rain intensity.

I. I NTRODUCTION

Free-space optical communication link include the use of optical links across the space between two points. It isestimated that in metropolitan areas, optical fiber links cancost as much as US$200,000.00 per kilometer, and 85% of this amount is spent on excavation and installation. On theother hand, the installation cost of an FSO system is about20% of that of a fiber-based system[3].The FSO systems canoperate at rates up to 1 to 100 Gb/s depending on the wavelength and modulation technique; it has the low systemcomplexity. The commercial FSO systems are designed tooperate in the infrared region of the electromagneticspectrum, at wavelengths of 850 nm and 1550nm depend theapplication.

The availability of free space optics (FSO) systems

depends on weather conditions especially rain in tropicalweather. Attenuation due to rainfall intensity has the greatimpact on free space optic system as inferred laser that usein FSO system is vulnerable to weather effect. It is affectedmostly due to rainfall, snow, scattering and scintillation. In atropical region, like Malaysia, where excessive rainfall is acommon phenomenon throughout the year, the knowledgeof the rain attenuation is extremely required for the design of a reliable terrestrial and earth space communication link at a

particular location.

There was not much analysis that has been done aboutrain affect on FSO link attenuation in tropical regions andhence there is no suitable model for predicting availabilityof FSO link due to rain impact [7]. There was a Trial-basedStudy of Free Space Optics Systems in Singapore in 2002 inwhich various means that can attenuate the FSO link was

tested and analyzed including the attenuation of the FSOlink due to rain. It was found that the overall FSO link attenuation was due to rain impact. However, there are twomain parameters that determine the link performance; fademargin and transmission distances are design parameterscritical to ensure the reliability of a FSO link. Study wasdone using 3 vendor with 6 different types of FSO devicesfor the period of three months where only 4 Rain Eventswith Rainfall Rate exceeding 60mm/hr and 2 Rain Eventexceeding 96mm/hr were recorded and tested [6].

II. R AIN STATISTIC

The rain fall rate has been measured in USM for 5 years.The measurements were taken continuously from 1st January2002 to 31st December 2006. Five years of rain ratedistributions are obtained at USM [1] and it is shown inFigure1.

Figure 1: Annual rain rate distribution of rain rate exceededmeasured in Malaysia [1].



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III. ATTENUATION DUE TO RAIN FALL PREDICTION MODEL

When the size of the irregularities due to precipitation becomes important compared to the wavelength, the wave isattenuated by reflection and refraction. Attenuation due torain, independent of the wavelength, is a function of the rainintensity R (in mm/h) according to the following equation[3].

Arain attenuation (dB/km) = aR b

(1)

A is rain attenuation and R is rain intensity and a and b arefrequency factors and value of a at 1.076 and b at 0.67 iswidely recommended for FSO which has very highfrequency and that is out of a and b allocation graph.Attenuation A is meant for only 1 km and it is needed tomultiply with effective path length in order to calculate theattenuation due to rain that has the range of more or lessthan one km distances [5].

A total path attenuation = A * Deff (2a)

Deff = d* r (2b)

Where A is attenuation for 1 km and Deff is effective pathlength. d is actual path length and r is reduction factor and

defined as followed [5];

(3a)and

do = 35e- 0.015 R 0.01 (3b)

Path attenuation exceeded for other time percentages can becalculated by multiplying A0.01 by factors of 0.12, 0.39 and2.14 for 1%, 0.1% and 0.001% respectively according tofollowing equation[5];

(4)

Where has been shown above, A p is desired attenuation and p is also desired degree of unavailability. By using the aboveformula, we can calculate the other rail fall rate as follow.

IV. FREE SPACE OPTIC LINK MARGIN CALCULATION

There is a need to calculate the link budget of the free spaceoptic link system. The link margin analysis for FSO systemsis the most important factor in FSO systems to look at thesignal power available at the receiver after the signal reachesthe opposite location. It is required to know the fade marginof the system in order to predict the attenuation due to rain.

A. Propogation of the link

Beam area at intended destination can be calculated by usingthe formula of the area of circle and also the receiver lens

area because infrared laser light will travel and graduallyincreasing its size in the form of circular shape. In figure 2,the beam area is the total transmitted beam area at thereceiver side.

Figure 2: The beam divergence

(5a)

dB = θ *D (5b)

where θ is divergence angle of transmitter and D is distance between transmitter and receiver. Receiver len’s Area isdone with simple calculation of area of a circle.

(6)

dR is the diameter of the receiver aperture. For microwave,Free Space Path loss (FSL) is necessary to calculate the lossand the same principle will be applied for the FSO whereFSL will be replaced by the Geometrical Loss. Thetransmitted Beam area at receiver side is larger than receiver aperture. Out of big beam area at receiver side, receivedsignal will be only part of it according to the receiver lensarea aperture.

(7)

B. Link Margin of the free space optic

. Knowing how well the receiver utilizes this power, andmore importantly, how reliable the communication link will

be over the anticipated distance is also important. It isdesirable to have a larger link margin than the minimumnominal received power level to make the link more reliableso that fade margin is available.

Link Margin (FM) = TotalsysGain - TotalsysLost (8a)

TotalsysGain= Tx power + Rx sensitivity (8b)

TotalsysLost = Equipment loss + Geometric loss +

Additional loss (8c)



2011 4th


Additional loss is consider in this paper as loss dut to rain

and equipment lost includes fiber cable lost that connects to

link head to optical management device, management deviceitself and some of electrical lost inside link head [2].

V. EXPERIMENTAL SET UP

The experiment includes of the two major part;communication link and rain meter. In communication, freespace optical communication link has been set up for 800 mdistance and rain meter has been set up at the location near to FOS link.

A. Optical system set up

The optical transmitter and receiver at both sides use thelight point flight strata 155 link head. Light Pointe wirelessoptical systems communicate using multiple beams of infrared light (invisible to the human eye). The systemsrequire true line-of-sight between locations. The system

operate at 850 nm optical wavelengths so that spectrum or Radio Frequency licensing is not required [4].Table 1summarized the operational and general manufacturer data.

Table 1: Characteristic of FSO device

Figure 3:Setup 1 flight-manager (SNMP)

Figure 4: Setup 2 Direct PC (mobile application)

Figure 5: Installation of the FSO link

Figure 3 to Figure 5 show the FSO system set up at thesender and receiver side. We set up the system in twodifferent ways; one is SNMP setup and second is direct Pc.Flight manager is fix application that both link heads will befixed at certain place and monitoring of the link will be done

by using the interface device like modem call flightmanager. Optical management cable from link head will gointo flight manager and Ethernet cable form flight manager will be connected to network. Orion SNMP network manager software will be using as user inter face for thedevices. as for setup 2,direct pc, optical management cable

from link head will directly go to optical managementinterface and it converters the optical signal to data signal .

Name Type

light point Flight Strata 155E

Description four Tx,four Rx system

Dimensions 11.8 * 11.8 * 25 inches30.0 *30.0 * 64.0 cm

Power consumption MaX.20 W

Bandwidth 1.5 to 155 Mbps

Output wavelength 850 nm

Beam divergence 2.0 mrad

Laser Output Power 4*6 (24) mW

Optical receive power SM -8 to -31 dBmMM -14 to -30 dBm

Optical transmit power SM -8 to -15 dB mMM -14 to -22 dBm



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Direct pc application can be set up anywhere since it doesnot need the network.

Figure 6: Map for two building

The primary link’s set up will be between two buildings; oneis E1 engineering and the other is hostel at IIUM Gombak campus. The distance is 800 m long.

B. Rain gauge system set up

In order to measure the rain attenuation, there is a needto measure the rain intensity. “Young Bucket” rain gaugeand Nexsens data logger hardware and software are beingused to measure rain fall. The Young Tipping Bucket RainGauge has been used to measure the rain fall rate and thatmeets the specifications of the world meteorologicalorganization (WMO) The design uses a proven tipping

bucket mechanism for simple and effective rainfallmeasurement. The bucket geometry and material arespecially selected for maximum water release, therebyreducing contamination and errors. Figure 7, Figure 8 andFigure 9 show the complete rain gauge system.

Figure 7: Rain gauge

Figure 8: Data logger

Figure 9: User inerface for rain gauge

VI. R ESULT AND ANALYSIS

Three days monitoring of the FSO links and rain meter from 26 November 2010 to 29 November 2010 has beendisplayed in figure 8 and Figure 9. As it can be seen in bothfigures that attenuation of the link and increment of rain rateoccurs at the same time which confirm that FSO link hasattenuated due to rain fall. There are two events of highattenuation; 7:30 pm 26 November and 11 pm 28

November.



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Figure 8: FSO link’s receive power level status from 26 to 29

November

Figure 9: Daily rain rate graph from 26 to 29 November, 2010

Rain data from the Nexsen rain meter and link attenuationdata from FSO link collected at UIA Malaysia and has beenanalyzed. As it can be observed above, when rain intensityincreases, attenuation also increases accordingly. In Figure10 and Figure 11, upper part of the graph is the rain intensityand lower part of the graph is the attenuation of the FSOcommunication link. At the highest rain intensity that is 90mm/h, the receive signal level reaches to -38 dBm and thatis 18 dB of attenuation as shown in Figure 10. At the highestrain intensity that is 48 mm/h, the receive signal levelreaches to -29 dBm that is 8 dB of attenuation as shown inFigure 11.

Although there is attenuation in FSO link, there is nosignificant reduce in bit error rate which is always 10E-10.This is because the FSO link is completely error-free as longas the amount of signal attenuation does not exceed thereceiver threshold level since it is optical wireless and link will be down as soon as signal attenuation reaches thereceiver threshold. Only at that point BER was showedwhich 10E-4 is then immediately link down and sometimeeven before BER result, the link is down. It was also madethe same conclusion about the bit-errors rate test for FSOlink in Trial-based Study of Free Space Optics Systems inSingapore [6].

Figure 10: Link attenuation at peak one according to rain intensity

at that moment collected form Nexsen rain meter

Fig.11: Link attenuation at peak two according to rain intensity atthat moment collected form Nexsen rain meter

Two raining events have been analyzed for verification

of ITU-R rain attenuation prediction model. Based onmeasured rain rate and regression co-efficients proposed byITU-R, the predicted rain attenuation has been compared

with measured value. It has been found that ITU-R rain

prediction model can be used for FSO link communicationwith modification of regression coefficients for tropicalweather. In this ITU-R model, the path reduction factor is0.91 for path length of 800m. It is shown in Figure 12 andFigure 13 the distance vs. availability vs. rain intensity from100 m to 1 km and 1 km to 10 Km.



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Figure 12: Rain fade margin for different percentage of time in ayear with path lengths from 0.1 to 1 km.

Figure 13: Rain fade margin for different percentage of time in ayear with path lengths from 1.5 to 5 km.

Based on measured rain attenuation with link margin of 23 dB according to our FSO transmitter and receiver, theavailability of 800 m distance is 99.993% and it is extendedto determine the availability of FSO link for longer distances. The availability of current experimental FSOsetup is 99.9 % for 3 km distance, 99.8% for 5 km distanceand 99.5% for 10 km distance.

VII. CONCLUSION

Rain is the main factor in the design of FSO systemsoperating in tropical regions. Free space optical link systemhas been established over 800 m distance and attenuationdue to rain has been measured and analyzed. The rainintensity of 1 minute integration time has been recordedwhich is time synchronized with measured attenuation data.Two raining events have been analyzed for validation of ITU-R rain attenuation prediction model. It has been foundthat ITU-R rain prediction model can be used for FSO withmodification of regression coefficients for tropical weather.

Rain fade margins are also investigated for FSO links up to10 km length. It is observed that margin of 25 dB is needed

for 1 km but more than 80 dB is required for 10 km link dueto rain fade in order to achieve 99.99% availability.

ACKNOWLEDGMENT

The authors are very grateful to MIMOS BERHAD,Research Management Centre, International Islamic

University Malaysia and Ministry of Science, Technologyand Innovation (MOSTI) for supporting the project byresearch grant.

R EFERENCE

1. M.S.J. Singh, S.I.S. Hassan and M. F. Ain (2007), RainfallAttenuation and Rainfall Rate Measurements in MalaysiaComparison with Prediction Models, American Journal of Applied Sciences 4 (1): 5-7, 2007

2. O. Bouchet, H. Sizun ,C. B.,Frédérique de Fornel,Pierre-NoëlFavennec(2006), Free-Space Optics:Propagation andCommunication, 2004 by Hermes Science/Lavoisier Frenchand 2006 by ISTE Ltd, USA

3. P. B. Harboe and J. R. Souza, Free Space OpticCommunication Systems: A Feasibility Study For Deployment In Brazil, Journal of Microwaves andOptoelectronics, Vol. 3, N.o 4, April 2004.

4. Light point (2005), FlightStrata 155 E Manual5. International Telecommunication Union (2003),

Characteristics of precipitation for propagation modeling,2003 Recommendation ITU-R P.837- 4.

6. J. T. Ong., Trial-Based Study of Free-Space Optics Systems inSingapore,2002 Info-Communications DevelopmentAuthority of Singapore,( Oct 2002)

7. J.T. Ong, K.I. Timothy, J.H. Chong, and S.V.B. Rao, “HeavyRain Effects on the Propagation of Free Space Optical Linksin Singapore”, Twelfth International Conference on Antennas

and Propagation 2003 (ICAP 2003), 2003, Vol 1, pp. 365- 368

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Azhar Ready to Submit PID1808985-1