Study of the Atmospheric Turbulence in Free Space Optical Communications

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Study of the Atmospheric Turbulence in FreeSpace Optical Communications

M. Ijaz, Shan Wu, Zhe Fan, W.O. Popoola and Z. Ghassemlooy

Muhammad Ijaz PGNET2009

Optical Communications Research Group, NCRLab, School of Computing, Engineering and Information

Sciences, Northumbria University, UKhttp://soe.unn.ac.uk/ncrlab/

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Contents

Introduction Free Space Optical Communication Atmospheric Turbulence Refractive Index Fluctuations Experimental Work and Procedure Results and Discussions Conclusions


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Introduction- Research Aim • Free space optical communications is currently seen as a promising alternative

technology for bandwidth hungry applications, particularly within the last mile access networks.

• The applications of FSO includes base station to base station in cellular networks, building to building, multicampus university networks, airports, hospitals, a high-speed, high-capacity back up link and disaster recovery links

• FSO systems offer rapid deployment with no need for trenches and its spectrum is licence free unlike the radio communication spectrum

• Despite the absorption and scattering from the constituents of the atmosphere, FSOC can be severely affected by the inhomogenities in the temperature(Turbulence) on the clear day

• In this research work the affect of atmospheric turbulence on FSOC link is studied experimentally under controlled environment.


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Introduction-Free Space Optical Communication


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Atmospheric Turbulence

• Atmospheric turbulence results from thermal gradients within the optical path caused by the variation in air temperature and density

• Random distributed cells are formed.• They have variable size (10 cm - 1 km) and

different temperature. • These various cells have different refractive

indexes thus causing scattering, multipath variation of the arriving signal


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Refractive Index fluctuations

• Refractive index is highly dependent on the small scale temperature fluctuations in air defined by

• Where no is mean index of refraction (no = 1) and n1(R,t) is the random deviation of index from its mean value.

• Where R is the vector position in three dimension and t is the time.

• n1(R,t) is dependent on the temperature and pressure and is given by

n(R,t) = no + n1(R,t)


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Refractive Index Fluctuations-cont

• The differentiation of the above equation tell us about the dependence of temperature small variations in the temperature gives us large change in the refraction index

• Where P and T are absolute temperature and Pressure respectively.


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Laser Beam Deformation

• Laser beam wander due to turbulence cells which are larger than the beam diameter.

• Scintillation or fluctuations in beam intensity at the receiver due to turbulent cells that are smaller than the beam diameter.

Isaac I. Kim et al (1998)


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Experimental Work and Procedure


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Experimental Work and Procedure-cont

Main simulation parameters used in the experiment

Parameter Value

Optical source laser diode (Beta Tx)

Class IIIb

Optical Beta-Tx wavelength

850nm

Maximum optical power 3mWMaximum data rate 1MbpsPIN photo detector SFH203PFA switching time

0.5μs

Modulation type OOK

Optical band-pass filter 800nm-1100nmTurbulence simulation chamber

140×30×30cm

Temperature range 20℃-80℃


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Results and Discussion- The Strength of the Turbulence

• In order to characterize the strength of turbulence generated

within the simulated turbulence chamber, the received average signal with and without the turbulence was studied.

• The signal distribution without and with scintillation are fitted to a Gaussian distributions and log normal respectively

• The turbulence model discussed thus far is valid for the weak turbulence with small values of

X

Strength of Fluctuations Rytov VarianceWeak σx

2 < 0.3Intermediate σx

2 ≈1Strong σx

2>>1

σx2


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The Strength of the Turbulence-cont

0 0.005 0.01 0.015 0.02 0.0250

10

20

30

40

50

60

Value of Sample

Num

ber o

f Occ

uren

ces

ExperimentalTheoretical

0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.60

5

10

15

20

25

30

35

40

45

50

Value of Sample

Num

ber o

f occ

uren

cs

ExperimentalTheoretical

The value of the log intensity variance was calculated to be 0.002 Results in weak turbulence while without scintillation; the noise variance was 10-5.

The received average signal (without scintillation) The received average signal (with scintillation)


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BER Evaluation

Binary signal with additive noise and PDFs for the binary signal with the threshold

• where a0 and a1 are probabilities of transmission for binary ones and zeros respectively and P0 and P1 are given by .


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BER Evaluation-cont

The received signal distribution(without scintillation) Dotted lines -Theoretical fit solid line –experimental data

The received signal distribution(with scintillation) Dotted lines -Theoretical fit solid line –experimental data

Temperature (°C) BER

T4 T1

36 30 6.84×10 -4

39 34 3.94×10 -4

45 39 3.24×10 -4

55 49 2.74×10 -4

59 53 6.63×10 -5

60 54 1.93×10 -4


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Optical Power Loss vs. Temperature

• The measured variance of the power fluctuation was 0.012• This also confirms that the turbulence generated was

indeed very weak during our study.


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Conclusion

• In this research work the effect of turbulence in FSOC is studied experimentally.

• The experimental data showed that if scintillation effect is not mitigated, it can cause a serious impairment to the performance and availability of an FSO link.

• From an error free link, the simulated turbulence (weak in strength) caused the BER to degrade to about 10-4.


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Special Thanks for Prof. Z. Ghassemlooy

Mr. W. Popoola

All colleagues in NCRL

&

Your Attention

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Thank You ! Question, please ?

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Study of the Atmospheric Turbulence in Free Space Optical Communications