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Study of the Atmospheric Turbulence in Free Space Optical Communications M. Ijaz, Shan Wu, Zhe Fan, W.O. Popoola and Z. Ghassemlooy Muhammad IjazPGNET2009 Optical Communications Research Group, NCRLab, School of Computing, Engineering and Information Sciences, Northumbria University, UK http://soe.unn.ac.uk/ncrlab/ 1

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Contents Introduction Free Space Optical Communication Atmospheric Turbulence Refractive Index Fluctuations Experimental Work and Procedure Results and Discussions Conclusions Muhammad IjazPGNET2009 2

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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. Muhammad IjazPGNET2009 3

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Introduction-Free Space Optical Communication Muhammad IjazPGNET2009 4

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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 Muhammad IjazPGNET2009 5

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Refractive Index fluctuations Refractive index is highly dependent on the small scale temperature fluctuations in air defined by Where n o is mean index of refraction (n o = 1) and n 1 (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. n 1 (R,t) is dependent on the temperature and pressure and is given by n(R,t) = n o + n 1 (R,t) Muhammad IjazPGNET2009 6

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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. Muhammad IjazPGNET2009 7

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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) Muhammad IjazPGNET2009 8

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Experimental Work and Procedure Muhammad IjazPGNET2009 9

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Experimental Work and Procedure-cont Main simulation parameters used in the experiment ParameterValue Optical source laser diode (Beta Tx) Class IIIb Optical Beta-Tx wavelength 850nm Maximum optical power3mW Maximum data rate1Mbps PIN photo detector SFH203PFA switching time 0.5μs Modulation typeOOK Optical band-pass filter800nm-1100nm Turbulence simulation chamber 140×30×30cm Temperature range20 ℃ -80 ℃ Muhammad IjazPGNET2009 10

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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 FluctuationsRytov Variance Weakσ x 2 < 0.3 Intermediateσx2 ≈1σx2 ≈1 Strongσ x 2 >>1 σx2σx2 Muhammad IjazPGNET2009 11

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The Strength of the Turbulence-cont 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) Muhammad IjazPGNET2009 12

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BER Evaluation Binary signal with additive noise and PDFs for the binary signal with the threshold where a 0 and a 1 are probabilities of transmission for binary ones and zeros respectively and P 0 and P 1 are given by. Muhammad IjazPGNET2009 13

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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 T4T4 T1T1 3630 6.84×10 -4 3934 3.94×10 -4 4539 3.24×10 -4 5549 2.74×10 -4 5953 6.63×10 -5 60541.93×10 -4 Muhammad IjazPGNET2009 14

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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. Muhammad IjazPGNET2009 15

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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. Muhammad IjazPGNET2009 16

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Special Thanks Special Thanks for Prof. Z. Ghassemlooy Mr. W. Popoola All colleagues in NCRL & Your Attention 17

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Thank You ! Question, please ? 18

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