JOURNAL OF TELECOMMUNICATIONS, VOLUME 29, ISSUE 1, JANUARY 2015

1

Improving the Performance of DWDM Free Space Optics System under Worst Weather

Conditions Ibraheem Abdulah, Hayder Ali

Abstract—Multiple transmitters/receivers (TX/RX) with travelling wave Semiconductor Optical Amplifier (SOA) is one of

technologies that used to eliminate the effect of weather conditions such as dense fog and heavy dry snow on free space optical

communication link. This technology is used to improve the quality of Free Space Optics (FSO) communication system and

increase the maximum transmission range.In this paper we used simulation tool Optisystem toimplemente the architecture of

multiple TX/RX system with travelling wave Semiconductor Optical Amplifier (SOA) under dense fog atmospheric condition. By

evaluating the maximum Q factor and the maximum transmission range of FSO link we can compare between used multiple

transmitters/receivers (TX/RX) with travelling wave Semiconductor Optical Amplifier (SOA) and without travelling wave

Semiconductor Optical Amplifier (SOA).

Index Terms—FSO; DWDM; Dense Fog; Heavy dry snow; Semiconductor Optical Amplifier(SOA); Multiple TX/RX.

————————————————————

1 INTRODUCTION

he global increase in demand for broadband commu-nications has led to the development of new and enabling technologies to support the conventional

methods such as the coaxial cable, copper, microwave and radio frequency (RF) systems. Optical Wireless Commu-nications(OWC) has benefited from the developments in optoelectronics and can be a key technology for achieving cost-effective high-speed optical links. The Optical Wire-less Communications (OWC) is the technology in which modulated optical signal is propagated over free space without using optical fiber medium [1,2].

There are several significant advantages offered by FSO technology: 1)FSO technology is very high band-width availability. 2) low cost per bit 3) Not require dig-ging and permissionfrom authorities for installation. 4)No licensing or frequency allocating is required. 5) FSO terminals are portable and quickly deployable. 6) The narrow beam of the laser makes detection, interception and jamming verydifficult. 7) free-space optical commu-nication is faster than fiber-optic communication because the light travels faster through air (at approximately 300,000 km/s) than it does through glass (approximately 200,000 km/s). 8) low power consumption [2,3,4,5].

The factors limiting the performance of FSO com-munication systems are:1)FSO has a very crucial draw-back which is highly attenuated by weather condition like fog ,rain and snow that causes scattering and absorption . 2)FSO communication systems can also be affected by pointing errors due to weak earthquakes, strong winds, and thermal expansion and cooling. 3)The radiated power must not exceed the limits established by the Internation-al Electro-technical Commission Standards.4)The light beam of FSO communications can't penetrate trees, hills or buildings. A clear line-of-sight path must exist between the light transmitter and the receiver. 5)Heated air rising from the ground or rooftops creates temperature varia-tions among different air pockets. As a consequence, the

refractive index may vary in a time dependent and ran-domness manner along the line of sight of the link, giving rise to scintillations over the beam cross section. These scintillations appear as power fluctuation in the receiver [6,7,8,9,10].

Fog and snow are the most important atmospheric attenuators influencing the optical signals transmission in free space . In fog, the attenuation is nearly independent of the wavelength.Generally, the severe attenuation caused by dense fog results in poor availability of the FSO links [11,12].

Another important attenuating factor is snow that causes reduced the link availability. The amount of light attenuation is proportional to number and size of snow particles .A large snowflake can cause link failure if laser beam is narrow[13].

Multiple transmitters/receivers (TX/RX) system is the proposed architecture that can be used to improve the quality of FSO link . In case of heavy atmospheric attenu-ation such as fog attenuation in the path of data processing, multiple TX/RX system with travelling wave semiconductor optical amplifier (SOA) is better perfor-mance of FSO link [14].

This technology can be used to improve the per-formance of FSO link under heavy dry snow weather condition.

Commercially available FSO equipment tends to operate in two frequency bands; 780- 900 nm and 1500-1600 nm. Lasers in the 780-900 nm band are less expen-sive and therefore usually selected for applications over moderate distances, while the 1550 nm wavelength is ar-guably the most attractive for reasons including the low absorption characteristic of air in this wavelength, and the availability of more transmittable optical power com-pared to the 800 nm wavelength under the eye safety standards [2,15].

T

2

2 OPTICAL AMPLIFIER

Optical amplifiers have become essential compo-nents in optical communication systems and have suc-cessfully replaced electrical repeaters as a means of compensating for optical signal loss. An advantage of-fered by optical amplifiers over repeaters include the fairly large gain bandwidth offered by the optical am-plifier which makes it practical for Wavelength Division Multiplexing (WDM) whereby a singleamplifier can am-plify multiple signals on different wavelengths simulta-neously, while without the optical amplifier separate repeaters would be needed for each wavelength. Addi-tionally, optical amplifiers are easily adaptable for many bit rates and signal modulation formats without a need to replace the amplifier, while the repeaters are designed to work at a particular bit rate (or at around only one wavelength) and modulation format[16,17] .

Similarly FSO communication systems can benefit from using an optical amplifier in various ways. The opt-ical preamplifier configuration can be used to boost opti-cal signal strength which has been degraded due to vari-ous atmospheric phenomena, to overcome the eye-limit restrictions on transmitted laser power, to suppress the limiting effect of the receiver thermal noise generated in the electronic amplifier, as well as to effectively improve receiver sensitivity[18,19].

Basically there are two types of optical amplifiers that can be used in wireless optical communication sys-tems: semiconductor optical amplifier (SOA) and am-plifier Erbium doped fiber (EDFA) . SOA is better than EDFA for small area networks due to its various proper-ties 1) SOA is compact and easily integrated with other devices. 2) The much smaller size and weight of the SOAs than the EDFAs. 3) High-speed capability, and low switching energy. 4) SOA amplifiers have Large BW. 5) SOA can operate at 800, 1300, and 1500 nm wavelength regions(unlike the EDFAs which are restricted mainly to the C-band . 6)The cost of SOA is also low compared with the EDFAs. 7). The pump power requirement of SOA is also low it is less than 400 ma[20,21].

3 THE FOG ATTENUATION

Fog can be described as a cloud of small particles of water, smoke, ice or combination of these near the earth surface thereby scattering the incident light and hence reducing the visibility [11] . The commonly ways to calculate attenuation in case of fog droplets are usedvi-sibility data for finding specific attenuation due to fog. The atmospheric visibility is defined as a distance where the 550 nm collimated light beam is attenuated to a frac-tion (5% or 2%) of an original power[12]. The visibility is measured at 550 nm which is the wavelength correspond-ing to maximum intensity of solar spectrum.

The transmissiometer usually operate at 550 nm center wavelengths with 250 nm bandwidth to collect visibility data in meteorological stations or at airports. The FSO systems operate at wavelengths, mostly at 850, 950 or 1550 nm center wavelength. The wavelength 550 nm is used as visibility reference [13]. The two most wide-ly used models are the Kruse model and the Kim model. According to these models, the Fog attenuation (𝐴𝐹𝑜𝑔 )can

be calculated from equation (1)[22]:

𝐴𝐹𝑜𝑔 𝑑𝐵

𝑘𝑚 =

10 log 𝑉℅

𝑉𝑘𝑚(𝜆 𝜆° )−𝑞 ≈

13𝑑𝐵

𝑉𝑘𝑚

𝜆𝑛𝑚

550𝑛𝑚 −𝑞

(1)

Where: 𝑉=Visibility, 𝑞=particle size distribution cient, 𝜆°=wavelength.

The wavelength dependency in this expression is expressed by q, which is in the a) Kruse’s Model:Particle size distribution coefficient inKruse’s Model is given by equation (2):

𝑞 =

1.6 𝑖𝑓 𝑉 > 50𝑘𝑚1.3 𝑖𝑓 6𝑘𝑚 > 𝑉 > 50𝑘𝑚

0.585𝑉1/3 𝑖𝑓 𝑉 < 6𝑘𝑚

(2)

b) Kim’s Model:Kim’s Model is applicable at very high

attenuations. Particle size distribution coefficient in Kim’s

Model is given by equation (3):

𝑞 =

1.6 𝑖𝑓 𝑉 > 50𝑘𝑚1.3 𝑖𝑓 6𝑘𝑚 < 𝑉 < 50𝑘𝑚0.16 𝑉 + 0.34 𝑖𝑓 1𝑘𝑚 < 𝑉 < 6𝑘𝑚𝑉 − 0.5 𝑖𝑓 0.5𝑘𝑚 < 𝑉 < 1𝑘𝑚0 𝑖𝑓 𝑉 < 0.5𝑘𝑚

3

The main difference between these models is that the Kruse model assumes a wavelength dependency and the Kim model assumes wavelength independent attenu-ation for dense fog conditions.Measurements of light at-tenuation at different wavelengths in dense fog condi-tions [23] .

4 THE SNOW ATTENUATIONS The snowflakes are generally larger than rain

drops and cause much larger received signal strength fluctuation and significant snow attenuation consequent-ly. The snowflakes as large as 20 mm have been reported and such a large snowflake can cause link failure if laser beam is narrow. When a snowflake crosses the laser beam, the receive signal level depends on the diameter of the snowflake and distance from the transmitter, as well as the position of the snowflake relative to the cross sec-tion of the beam [22] .

The optical signal attenuation due to snow weather condition as a function of wavelength to the snowfall in-tensity(S) in (mm/h). The attenuations due to snowfall are modeled based on categorization into dry and wet snow types and the specific attenuation is given by the follow-ing equation (4)[24]:

𝐴𝑠𝑛𝑜𝑤 = 𝑎𝑆𝑏 𝑑𝐵

𝑘𝑚 (4)

Here parameters 𝑎 and 𝑏 categorize snowfall type and for the two types of snow have values as: • For wet snow : 𝑎 = 1.023 ∗ 10−4𝜆 + 3.7855466 , 𝑏 = 0.72 • For dry snow : 𝑎 = 5.42 ∗ 10−5𝜆 + 5.4958776 , 𝑏 = 1.38

From equation (4) the specific attenuation by dry snow is much higher than specific attenuation caused by wet snow. 5 RESULTS AND DISCUSSIONS

Using Optisystem software, an implementation of the proposed multiple TX/RX system architecture with travelling wave SOA in an FSO link has been simulated. Parameters in table (1) were used to analyze the perfor-mance. The final Link construction has been represented in Figure(1).

3

TABLE 1 PARAMETERS OF MULTIPLE TX/RX SYSTEM ARCHITECTURE WITH

TRAVELLINGWAVE SOA IN FSO LINK

parameters value parameters value

Transmission Bit Rate 2.5 Gb/s Receiver aperture diame-

ter

15cm

Number of WDM

channel

16 Transmitter optics effi-

ciency

0.8

Frequencies of WDM

channels

(193.1-194.6)THz Receiver optics efficiency 0.8

Frequency spacing 100GHz Signal Attenuation (Fog) 260 dB/km

Modulation Type NRZ(Non Return

to Zero)

Signal Attenuation (Snow) 260 dB/km

Transmitter Wave-

length

850 nm Additional losses 12 dB

Transmitter aperture

diameter

15cm Photodetector Type PIN diod

Injection Current of travellingWave SOA 0.209 A

Figure(1)The block diagram of the 16-channel DWDM FSO with

multiple TX/RX system and using travelling wave SOA.

The results that obtained from simulated shown in fig-

ures(2-5): i) Figures(2,3) show the maximum transmission

range of 16-channel DWDM FSO10TX/10RX system withthe increasing of transmitter powerby using travel-ling wave SOA and withouttravelling wave SOA under dense fog and heavy dry snow weather condition respec-tively. These figures shown that maximum transmission range of system increases for increasing of transmitter power under these conditions. The transmission range of system increasing from 0.284km at transmitter pow-er(10dBm) to 0.332km at transmitter power(24dBm) with-out travelling wave SOA and increasing from 0.577km at transmitter power(10dBm) to 0.628 km transmitter pow-er(24dBm)with travelling wave SOA under dense fog weather condition and from 0.513km at transmitter pow-er(10dBm) to 0.607km at transmitter power(24dBm) with-out travelling wave SOA and increasing from 1.080km at transmitter power(10dBm) to 1.179 km transmitter pow-er(24dBm)with travelling wave SOA under heavy dry snow weather condition.

Figure(2) shows the maximum transmission range of 16-channel

DWDM FSO 10TX/10RX system with the increasing of transmitter

power by using travelling wave SOA and without travelling wave

SOA under dense fog weather condition.

Figure(3) shows the maximum transmission range of 16-channel

DWDM FSO 10TX/10RX system with the increasing of transmitter

power by using travelling wave SOA and without travelling wave

SOA under heavy dry snow weather condition.

ii) Figures(4,5) present the maximum transmission

range of 16-channel DWDM FSO system withthe incre-ment of TX/RX number by using travelling wave SOA and withouttravelling wave SOA at transmitter pow-er(24dBm) for each under dense fog and heavy dry snow weather condition, respectively .

These figures show that maximum transmission range of system increases for incrementing of TX/RX number . The transmission range of system increasing from 0.263km at 1TX/1RX to 0.332km at 10TX/10RX with-out travelling wave SOA and increasing from 0.555km at 1TX/1RX to 0.628 km at 10TX/10RX with travelling wave SOA under dense fog weather condition and from 0.513km at 1TX/1RX to 0.607km at 10TX/10RX without travelling wave SOA and increasing from 1.081km at 1TX/1RX to 1.179 km at 10TX/10RX with travelling wave SOA under heavy dry snow weather condition.

10 12 14 16 18 20 22 240.25

0.35

0.45

0.55

0.65

Transmitter power[dBm]

Max.

transm

issio

n r

ange[k

m]

Dense fog weather condition

Without SOA

With SOA

10 12 14 16 18 20 22 240.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

Transmitter power[dBm]

Max.

transm

issio

n r

ange[k

m]

Heavy dry snow weather condition

Without SOA

With SOA

4

Figure(4) shows the maximum transmission range of 16-channel

DWDM FSO system with the incrementing of TX/RX number by

using travelling wave SOA and without travelling wave SOA under

dense fog weather condition.

Figure(5) shows the maximum transmission range of 16-channel DWDM FSO system with the incrementing of TX/RX number by

using travelling wave SOA and without travelling wave SOA under heavy dry snow weather condition.

6 CONCLUSION

The results of implementation and simulation of our system proved the performance improvement of 16-channel DWDM FSO system by using multiple TX/RX system with travelling wave semiconductor optical am-plifier under dense fog and heavy dry snow weather conditions.

This paper investigated that maximum transmis-sion range and Q factor increases for both increasing of transmitter power or number of TX/RX of system under these weather conditions.

This study provides designers with a wide range of system conditions that can be applied practically and maybe failed the system.

7 References

[1] D. Killinger, "Free Space Optics For Laser Com-munication Through The Air", Optics and Pho-tonics News, vol. 13, pp. 36-42, Oct. 2002.

[2] A. K. Majumdar, "Free-Space Laser Communication Performance Inthe Atmospheric Channel" ,Journal of Optical and Fiber Communications Research, vol. 2, pp. 345-396, 2005.

[3] Merrill Lynch, "Free Space Optics", 8 May 2001. [4] Heinz Willebrand and Baksheesh S. Ghuman ,"Free-

Space Optics: Enabling Optical Connectivity in To-day’s Networks" 2002.

[5] A. Acampora, .Last Mile by Laser. (Scientific Ameri-can, 2002).

[6] S. Bloom, E. Korevaar, J. Schuster, and H. A. Wille-brand, "Understanding the performance of free-space optics," Journal of Optical Networking, vol. 2, pp. 178-200, June 2003.

[7] H. G. Sandalidis, T. A. Tsiftsis, G. K. Karagiannidis, and M. Uysal, "BER performance of FSO links over strong atmospheric turbulence channels with point-ing errors," IEEE Communications Letters, vol. 12, no. 1, pp. 44-46, 2008.

[8] Isaac I. Kim, Ron Stieger, Joseph A. Koontz, Carter Moursund, Micah Barclay, Prasanna Adhikari, John Schuster, Eric Korevaar, Richard Ruigrok, Casimer DeCusatis," Wireless optical transmission of Fast Ethernet, FDDI, ATM, and ESCON protocol data us-ing the Terra Link laser communication system," Optical Engineering, Vol. 37 No. 12, December 1998.

[9] Norhanis Aida M.Nor, Md. Rafiqul Islam, Wajdi Al-Khateeb, Suriza A. Z.," Atmospheric Effects On Free Space Earth-To-Satellite Optical Link In Tropical Climate", International Journal of Com-puter Science, Engineering and Applications (IJCSEA) Vol.3, No.1, February 2013.

[10] David A. Johnson, P.E." Optical Through-the-Air Communications" Handbook.

[11] M. S. Awan, E. Leitgeb, S. Sheikh Muhammad, Mar-zuki, F. Nadeem, M. Saeed Khan, C. Capsoni," Dis-tribution Function For Continental and Maritime Fog Environments For Optical Wireless Communi-cation", IEEE, 2008.

[12] M. S. Khan, M. R. Naqvi, M. A. Khan, R. D. Khan, R. WaliM. Latif, K. Ullah, E. Leitgeb," Optical Attenua-tion Estimation from Measured Visibility Data in Is-lamabad, Pakistan", 18th European Conference on Network and Optical Communications & 8th Confe-rence on Optical Cabling and Infrastructure - NOC/OC&I 2013 ISBN: 978-1-4673-5822-4, Graz, Austria, July 10-12, 2013.

[13] F. Nadeem, B. Flecker, E. Leitgeb, M. S. Khan, M. S. Awan, T. Javornik," Comparing The Fog Effects on Hybrid Network Using Optical Wireless and GHz Links", IEEE, 2008.

[14] Farhana Hossain, Zeenat Afroze." Eliminating the Effect of Fog Attenuation on FSO Link by Multiple TX/RX System with Travelling Wave Semiconductor Optical Amplifier", Proceedings of 2013 2nd International Conference on Ad-vances in Electrical Engineering (ICAEE2013), Dhaka, Bangladesh, December, 2013.

[15] L. C. Andrews and R. L. Phillips, Laser beam propagation through random media, Second Edi-tion, SPIE Press, Bellingham, Washington, 2005.

1 2 3 4 5 6 7 8 9 100.25

0.3

0.35

0.4

0.45

0.5

0.55

0.6

0.65

Number of TX/RX

Max.t

ransm

issio

n r

ange[k

m]

Dense fog weather condition

Without SOA

With SOA

1 2 3 4 5 6 7 8 9 100.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

Number of TX/RX

Max.t

ransm

issio

n r

ange[k

m]

Heavy dry snow weather condition

Without SOA

With SOA

5

[16] R. Ramaswami and K. N. Sivarajan, Optical net-works- a pratical perspective, Second Edition, Academic Press, London, 2002.

[17] M. J. Yadlowsky, E. M. Deliso, and V. L. Da Silva, "Optical fibers and amplifiers for WDM systems," Proceedings of the IEEE, vol. 85, no. 11, pp. 1765-1779, Nov. 1997.

[18] M. Razavi and J. H. Shapiro, "Wireless optical communications via diversity reception and opt-ical preamplification," IEEE Transactions on Wireless Communications, vol. 4, no. 3, pp. 975-983, May 2005.

[19] M. Abtahi, P. Lemieux, W. Mathlouthi, and L. A. Rusch, "Suppression of turbulence-induced scin-tillation in free-space optical communication sys-tems using saturated optical amplifiers," Journal of Lightwave Technology, vol. 24, no. 12, pp. 4966-4973, Dec. 2006.

[20] Juan-de-Dios Sanchez- L َ pez, Arturo Arvizu M, Francisco J. Mendieta and Ivan Nieto Hip َ lito," Trends of the Optical Wireless Communications", InTech, February, 2011.

[21] Charu Sharma, Sukhbir Singh,"Investigations on Bit error rate performance of DWDM Free space optics system using Semiconductor Optical Am-plifier in Rain Environment",International jour-nal of computers & technology Vol 8, no 3,2013.

[22] F. Nadeem, E. Leitgeb , M. S. Awan, G. Kandus," Optical Wavelengths Comparison for Different Weather Conditions", IEEE, 2009.

[23] Michael Gebhart, Erich Leitgeb ,Maher Al Na-boulsi, Herve Sizun, Frederique de Fornel," Mea-surements of light attenuation at different wave-lengths in dense fog conditions for FSO applica-tions", COST 270 Short Term Scientific Mission 7 Report (2004).

[24] M. S. Awan," Statistical-Dynamical Channel Modeling of Outdoor Optical Wireless Links", PhD. Thesis, Graz University of Technology, Austria,May 2010.