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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 22, Iss. 3 — Feb. 10, 2014
  • pp: 3208–3218
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Experimental characterization and mitigation of turbulence induced signal fades within an ad hoc FSO network

Joaquin Perez, Stanislav Zvanovec, Zabih Ghassemlooy, and Wasiu O. Popoola  »View Author Affiliations


Optics Express, Vol. 22, Issue 3, pp. 3208-3218 (2014)
http://dx.doi.org/10.1364/OE.22.003208


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Abstract

Optical beams propagating through the turbulent atmospheric channel suffer from both the attenuation and phase distortion. Since future wireless networks are envisaged to be deployed in the ad hoc mesh topology, this paper presents the experimental laboratory characterization of mitigation of turbulence induced signal fades for two ad hoc scenarios. Results from measurements of the thermal structure constant along the propagation channels, changes of the coherence lengths for different turbulence regimes and the eye diagrams for partially correlated turbulences in free space optical channels are discussed. Based on these results future deployment of optical ad hoc networks can be more straightforwardly planned.

© 2014 Optical Society of America

1. Introduction

In practice, assessment of the FSO network depends on gathering measured signal data over a long period of time. The cost and time implication of this process explains why there are few such real-life data published in the literature. One such long-term measurements was carried out at the Czech Technical University, Prague as outlined in [19

19. J. Libich, S. Zvanovec, and M. Mudroch, “Mitigation of time-spatial influence in free-space optical networks utilizing route diversity,” Proc. SPIE 8246, 82460O (2012). [CrossRef]

]. Their measurements show that the route diversity scheme can further enhance the availability of FSO links.

The rest of the paper is organized as follows: Section II gives the theoretical background of the atmospheric turbulence and its impact on the FSO topologies. In Section III, the experimental set-up that is used in the laboratory to study the atmospheric turbulence effect on an FSO based ad hoc network is presented. The key findings are introduced and discussed in Section IV with concluding remarks given in Section V.

2. Atmospheric turbulence theoretical background

2.1 Statistical models

2.2 Network topologies

3. Laboratory experimental set-up

The controlled indoor atmospheric channel test-bed is made from combinations of modular 5.5 m × 0.3 m × 0.3 m glass chambers. The turbulence strength within the chamber is defined by the thermal fluctuations inside the chamber. Moreover, a number of air vents located along the length of the chamber (i.e. line-of-sight path) allow us to blow hot and cold air from external sources in directions that are perpendicular to the propagating optical beam within the chamber. A variation in the turbulence strength is achieved by adjusting the temperature and air flow of the fan heater sources, thus ensuring a temperature gradient of >6° C. The turbulence strength is adjusted by the strength of fan heaters and how far they are positioned from the air vents. In these experiments we considered a range of temperatures from 20°C to 60°C. The temperature at different locations along the chamber was measured and recorded every second by a network of 19 temperature sensors positioned within the chamber; see Fig. 2(b), each with a measurement span of −55°C to + 125°C and a resolution of 0.1°C. The measured temperatures are used to determine the temperature structure constant, as depicted in Fig. 2(b).

The first experimental set-up depicted by Fig. 3(a) uses an 830 nm laser diode with built-in circular collimating lens, 50 MHz electrical bandwidth and 10 mW optical power. For the second set-up, shown in Fig. 3(b), a 632 nm laser diode with 10 mW optical power and 50 MHz electrical bandwidth is employed to allow configuration of two FSO channels. The emitted optical beams from laser sources are intensity modulated using OOK-NRZ signal format with 1 Mbit/s line-rate and 250 mVpp electrical signal amplitude. The receiver front-end consists of a Silicon PIN photodiode with 150 MHz bandwidth, 0.8 mm2 active area and responsivities of 0.38 A/W and 0.39 A/W at 830 nm and 670 nm, respectively. The regenerated electrical signal is then captured using a high-frequency digital sampling oscilloscope. The captured signal thereafter is processed off-line using mathematical analysis software to obtain the following performance metrics: BER, Q-factor, SNR and eye diagrams.

4. Results and discussions

4.1 The first scenario

The first measurement was performed for the configuration of split beam transmission, as described in Fig. 3(a). The laser beam at 830 nm broaden in the first half of the chamber is passed through the plexiglass perpendicularly placed slab (shown in blue in Fig. 3) and then split into two beams by the lengthwise plexiglass. The split beams propagate through two ~1 m length channels, one with turbulence and one without. This is followed by a common ~1 m long path with turbulence.

4.2 The second scenario - channels with partially intersect path

In the second case, the channel 2 was under the influenced of a constant weak turbulence regime (measured Rytov variance in the channel was kept at <0.16), while the turbulent flow circulating from channel 1 to the area of intersection (and partially to the path of channel 1) was gradually increased from 10−13 up to 10−11 m-2/3 (values enumerated from temperature measurement via Eq. (1)). The measurement environment is shown in Fig. 3(b). This deployment corresponds to the blue depicted network segment from Fig. 1, where the interconnecting Node B serves as a routing hub to several nodes. At first, the data captured using the temperature sensors was analyzed in order to obtain the relationship between measured turbulence regimes and temperature changes along propagating optical paths. From these statistics, the correlation of Cn2 profile within the channels was observed to change from 0.1068 up to 0.4896 in line with the increase in the turbulence level in the channel 1.

ρ0=[1.45k20LpCn2(x)dx]3/5.
(8)

Figure 5
Fig. 5 Derived empirical histograms of spatial coherence distance determined from measured CT2 profiles for cases: (a) similar Cn2 in channels and (b) one order different Cn2 in the channels
shows typical example of derived empirical histograms against the spatial coherence distances for both channels with similar Cn2, see Fig. 5(a) and for channels with different values of Cn2 (i.e. one order higher in on channel) see, Fig. 5(b).

5. Conclusion

Acknowledgments

This research is supported by the EU COST ICT Action IC1101 - Optical Wireless Communications - An Emerging Technology (OPTICWISE) and by the MEYS Czech Republic grant LD12058.

References and links

1.

Z. Ghassemlooy, W. Popoola, and S. Rajbhandari, Optical Wireless Communications: System and Channel Modelling with MATLAB (CRC Press., Boca Raton, 2012).

2.

“fSONA unveils 2.5-Gbps free-space optical systems,” in Lightwave Online, (2012).

3.

S. Nauerth, F. Moll, M. Rau, C. Fuchs, J. Horwath, S. Frick, and H. Weinfurter, “Air-to-ground quantum communication,” Nat. Photonics 7(5), 382–386 (2013). [CrossRef]

4.

E. Ciaramella, Y. Arimoto, G. Contestabile, M. Presi, A. DErrico, V. Guarino, and M. Matsumoto, “1.28 terabit/s (32x40 Gbit/s) WDM transmission system for free space optical communications,” IEEE J. Sel. Areas Comm. 27(9), 1639–1645 (2009). [CrossRef]

5.

E. Leitgeb, M. Gebhart, and U. Birnbacher, “Optical networks, last mile access and applications,” J. Opt. Fiber Commun. Rep. 2, 56–85 (2005).

6.

M. N. Smadi, S. C. Ghosh, A. A. Farid, T. D. Todd, and S. Hranilovic, “Free-space optical gateway placement in hybrid wireless mesh networks,” J. Lightwave Technol. 27(14), 2688–2697 (2009). [CrossRef]

7.

A. O. Aladeloba, A. J. Phillips, and M. S. Woolfson, “Improved bit error rate evaluation for optically pre-amplified free-space optical communication systems in turbulent atmosphere,” IET Optoelectron. 6(1), 26–33 (2012). [CrossRef]

8.

L. Dordova and O. Wilfert, “Calculation and comparison of turbulence attenuation by different method,” Radioengineering 19, 162–163 (2010).

9.

L. C. Andrews and R. L. Phillips, Laser Beam Propagation through Random Media, II ed. (SPIE Press, Washington, 2005).

10.

W. Gappmair, “Further results on the capacity of free-space optical channels in turbulent atmosphere,” IET Commun. 5(9), 1262–1267 (2011). [CrossRef]

11.

M. A. Khalighi, N. Schwartz, N. Aitamer, and S. Bourennane, “Fading reduction by aperture averaging and spatial diversity in optical wireless systems,” J. Opt. Commun. Netw. 1(6), 580–593 (2009). [CrossRef]

12.

T. A. Tsiftsis, H. G. Sandalidis, G. K. Karagiannidis, and M. Uysal, “Optical wireless links with spatial diversity over strong atmospheric turbulence channels,” IEEE Trans. Wirel. Comm. 8(2), 951–957 (2009). [CrossRef]

13.

G. Yang, M.-A. Khalighi, S. Bourennane, and Z. Ghassemlooy, “Approximation to the sum of two correlated gamma-gamma variates and its applications in free-space optical communications,” IEEE Wireless Commun. Lett. 1(6), 621–624 (2012). [CrossRef]

14.

S. Kaneko, T. Hamai, and K. Oba, “Evaluation of a free-space optical mesh network communication system in the Tokyo metropolitan area,” J. Opt. Netw. 1, 414–423 (2002).

15.

M. A. Kashani, M. Safari, and M. Uysal, “Optimal relay placement and diversity analysis of relay-assisted free-space optical communication systems,” J. Opt. Commun. Netw. 5(1), 37–47 (2013). [CrossRef]

16.

X. Yang, “Availability-differentiated service provisioning in free-space optical access networks,” J. Opt. Netw. 4(7), 391–399 (2005). [CrossRef]

17.

Z. Hu, P. Verma, and J. J. Sluss Jr., “Improved reliability of free-space optical mesh networks through topology design,” J. Opt. Netw. 7(5), 436–448 (2008). [CrossRef]

18.

S. D. Milner, J. Llorca, and C. C. Davis, “Autonomous reconfiguration and control in directional mobile ad-hoc networks,” IEEE Circuits Syst. Mag. 9(2), 10–26 (2009). [CrossRef]

19.

J. Libich, S. Zvanovec, and M. Mudroch, “Mitigation of time-spatial influence in free-space optical networks utilizing route diversity,” Proc. SPIE 8246, 82460O (2012). [CrossRef]

20.

S. Hippler, F. Hormuth, D. J. Butler, W. Brandner, and T. Henning, “Atmosphere-like turbulence generation with surface-etched phase-screens,” Opt. Express 14(22), 10139–10148 (2006). [CrossRef] [PubMed]

21.

C. Wilcox and S. Restaino, “A New Method of Generating Atmospheric Turbulence with a Liquid Crystal Spatial Light Modulator,” in New Developments in Liquid Crystals, G. V. Tkachenko, ed. (InTech, 2009), pp. 71–92.

22.

Z. Ghassemlooy, H. Le Minh, S. Rajbhandari, J. Perez, and M. Ijaz, “Performance analysis of ethernet/fast-ethernet free space optical communications in a controlled weak turbulence condition,” J. Lightwave Technol. 30(13), 2188–2194 (2012). [CrossRef]

23.

W. K. Pratt, Laser Communication Systems, I ed. (John Wiley & Sons, Inc., New York, 1969).

24.

A. Kolmogorov, ed., Turbulence, Classic Papers on Statistical Theory (Wiley-Interscience, New York, 1961).

25.

A. M. Obukhov, “Structure of the temperature field in turbulent flow,” Izv. Akad. Nauk. SSSR Ser.Ser. 13, 58–96 (1949).

26.

S. Corrsin, “On the spectrum of isotropic temperature fluctuations in an isotropic turbulence,” J. Appl. Phys. 22(4), 469–473 (1951). [CrossRef]

27.

G. R. Osche, Optical Detection Theory for Laser Applications, I ed. (Wiley-Interscience, 2002).

28.

Z. Ghassemlooy, W. O. Popoola, S. Gao, J. I. H. Allen, and E. Leitgeb, “Free-space optical communication employing subcarrier modulation and spatial diversity in atmospheric turbulence channel,” IET Optoelectron. 2(1), 16–23 (2008). [CrossRef]

29.

M. Uysal, J. T. Li, and M. Yu, “Error rate performance analysis of coded free-space optical links over gamma-gamma atmospheric turbulence channels,” IEEE Trans. Wirel. Comm. 5(6), 1229–1233 (2006). [CrossRef]

30.

H. Moradi, H. H. Refai, and P. G. LoPresti, “Switch-and-stay and switch-and-examine dual diversity for high-speed free-space optics links,” IET Optoelectron. 6(1), 34–42 (2012). [CrossRef]

31.

S. Zvanovec, J. Perez, Z. Ghassemlooy, S. Rajbhandari, and J. Libich, “Route diversity analyses for free-space optical wireless links within turbulent scenarios,” Opt. Express 21(6), 7641–7650 (2013). [CrossRef] [PubMed]

32.

Y. Guowei, M. Khalighi, and S. Bourennane, “Performance of receive diversity FSO systems under realistic beam propagation conditions,” in Proc. of the 2012 8th Int. Symp. Commun. Syst. Netw. Digital Signal Process. (CSNDSP), (2012), pp. 1–5.

33.

J. Jaeshin and W. Sunghong, “Comparative study on cooperative communications in the upper layers at ad-hoc networks,” in 2013 15th Int. Conf. Adv. Commun. Technolo. (ICACT) (2013), pp. 133–137.

34.

J. Perez, Z. Ghassemlooy, S. Rajbhandari, M. Ijaz, and H. L. Minh, “Ethernet FSO communications link performance study under a controlled fog environment,” IEEE Commun. Lett. 16(3), 408–410 (2012). [CrossRef]

35.

J. A. Louthain and J. D. Schmidt, “Anisoplanatism in airborne laser communication,” Opt. Express 16(14), 10769–10785 (2008). [CrossRef] [PubMed]

OCIS Codes
(010.1330) Atmospheric and oceanic optics : Atmospheric turbulence
(060.4510) Fiber optics and optical communications : Optical communications
(060.2605) Fiber optics and optical communications : Free-space optical communication

ToC Category:
Optical Communications

History
Original Manuscript: October 3, 2013
Revised Manuscript: December 20, 2013
Manuscript Accepted: December 21, 2013
Published: February 4, 2014

Citation
Joaquin Perez, Stanislav Zvanovec, Zabih Ghassemlooy, and Wasiu O. Popoola, "Experimental characterization and mitigation of turbulence induced signal fades within an ad hoc FSO network," Opt. Express 22, 3208-3218 (2014)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-22-3-3208


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References

  1. Z. Ghassemlooy, W. Popoola, and S. Rajbhandari, Optical Wireless Communications: System and Channel Modelling with MATLAB (CRC Press., Boca Raton, 2012).
  2. “fSONA unveils 2.5-Gbps free-space optical systems,” in Lightwave Online, (2012).
  3. S. Nauerth, F. Moll, M. Rau, C. Fuchs, J. Horwath, S. Frick, H. Weinfurter, “Air-to-ground quantum communication,” Nat. Photonics 7(5), 382–386 (2013). [CrossRef]
  4. E. Ciaramella, Y. Arimoto, G. Contestabile, M. Presi, A. DErrico, V. Guarino, M. Matsumoto, “1.28 terabit/s (32x40 Gbit/s) WDM transmission system for free space optical communications,” IEEE J. Sel. Areas Comm. 27(9), 1639–1645 (2009). [CrossRef]
  5. E. Leitgeb, M. Gebhart, U. Birnbacher, “Optical networks, last mile access and applications,” J. Opt. Fiber Commun. Rep. 2, 56–85 (2005).
  6. M. N. Smadi, S. C. Ghosh, A. A. Farid, T. D. Todd, S. Hranilovic, “Free-space optical gateway placement in hybrid wireless mesh networks,” J. Lightwave Technol. 27(14), 2688–2697 (2009). [CrossRef]
  7. A. O. Aladeloba, A. J. Phillips, M. S. Woolfson, “Improved bit error rate evaluation for optically pre-amplified free-space optical communication systems in turbulent atmosphere,” IET Optoelectron. 6(1), 26–33 (2012). [CrossRef]
  8. L. Dordova, O. Wilfert, “Calculation and comparison of turbulence attenuation by different method,” Radioengineering 19, 162–163 (2010).
  9. L. C. Andrews and R. L. Phillips, Laser Beam Propagation through Random Media, II ed. (SPIE Press, Washington, 2005).
  10. W. Gappmair, “Further results on the capacity of free-space optical channels in turbulent atmosphere,” IET Commun. 5(9), 1262–1267 (2011). [CrossRef]
  11. M. A. Khalighi, N. Schwartz, N. Aitamer, S. Bourennane, “Fading reduction by aperture averaging and spatial diversity in optical wireless systems,” J. Opt. Commun. Netw. 1(6), 580–593 (2009). [CrossRef]
  12. T. A. Tsiftsis, H. G. Sandalidis, G. K. Karagiannidis, M. Uysal, “Optical wireless links with spatial diversity over strong atmospheric turbulence channels,” IEEE Trans. Wirel. Comm. 8(2), 951–957 (2009). [CrossRef]
  13. G. Yang, M.-A. Khalighi, S. Bourennane, Z. Ghassemlooy, “Approximation to the sum of two correlated gamma-gamma variates and its applications in free-space optical communications,” IEEE Wireless Commun. Lett. 1(6), 621–624 (2012). [CrossRef]
  14. S. Kaneko, T. Hamai, K. Oba, “Evaluation of a free-space optical mesh network communication system in the Tokyo metropolitan area,” J. Opt. Netw. 1, 414–423 (2002).
  15. M. A. Kashani, M. Safari, M. Uysal, “Optimal relay placement and diversity analysis of relay-assisted free-space optical communication systems,” J. Opt. Commun. Netw. 5(1), 37–47 (2013). [CrossRef]
  16. X. Yang, “Availability-differentiated service provisioning in free-space optical access networks,” J. Opt. Netw. 4(7), 391–399 (2005). [CrossRef]
  17. Z. Hu, P. Verma, J. J. Sluss., “Improved reliability of free-space optical mesh networks through topology design,” J. Opt. Netw. 7(5), 436–448 (2008). [CrossRef]
  18. S. D. Milner, J. Llorca, C. C. Davis, “Autonomous reconfiguration and control in directional mobile ad-hoc networks,” IEEE Circuits Syst. Mag. 9(2), 10–26 (2009). [CrossRef]
  19. J. Libich, S. Zvanovec, M. Mudroch, “Mitigation of time-spatial influence in free-space optical networks utilizing route diversity,” Proc. SPIE 8246, 82460O (2012). [CrossRef]
  20. S. Hippler, F. Hormuth, D. J. Butler, W. Brandner, T. Henning, “Atmosphere-like turbulence generation with surface-etched phase-screens,” Opt. Express 14(22), 10139–10148 (2006). [CrossRef] [PubMed]
  21. C. Wilcox and S. Restaino, “A New Method of Generating Atmospheric Turbulence with a Liquid Crystal Spatial Light Modulator,” in New Developments in Liquid Crystals, G. V. Tkachenko, ed. (InTech, 2009), pp. 71–92.
  22. Z. Ghassemlooy, H. Le Minh, S. Rajbhandari, J. Perez, M. Ijaz, “Performance analysis of ethernet/fast-ethernet free space optical communications in a controlled weak turbulence condition,” J. Lightwave Technol. 30(13), 2188–2194 (2012). [CrossRef]
  23. W. K. Pratt, Laser Communication Systems, I ed. (John Wiley & Sons, Inc., New York, 1969).
  24. A. Kolmogorov, ed., Turbulence, Classic Papers on Statistical Theory (Wiley-Interscience, New York, 1961).
  25. A. M. Obukhov, “Structure of the temperature field in turbulent flow,” Izv. Akad. Nauk. SSSR Ser.Ser. 13, 58–96 (1949).
  26. S. Corrsin, “On the spectrum of isotropic temperature fluctuations in an isotropic turbulence,” J. Appl. Phys. 22(4), 469–473 (1951). [CrossRef]
  27. G. R. Osche, Optical Detection Theory for Laser Applications, I ed. (Wiley-Interscience, 2002).
  28. Z. Ghassemlooy, W. O. Popoola, S. Gao, J. I. H. Allen, E. Leitgeb, “Free-space optical communication employing subcarrier modulation and spatial diversity in atmospheric turbulence channel,” IET Optoelectron. 2(1), 16–23 (2008). [CrossRef]
  29. M. Uysal, J. T. Li, M. Yu, “Error rate performance analysis of coded free-space optical links over gamma-gamma atmospheric turbulence channels,” IEEE Trans. Wirel. Comm. 5(6), 1229–1233 (2006). [CrossRef]
  30. H. Moradi, H. H. Refai, P. G. LoPresti, “Switch-and-stay and switch-and-examine dual diversity for high-speed free-space optics links,” IET Optoelectron. 6(1), 34–42 (2012). [CrossRef]
  31. S. Zvanovec, J. Perez, Z. Ghassemlooy, S. Rajbhandari, J. Libich, “Route diversity analyses for free-space optical wireless links within turbulent scenarios,” Opt. Express 21(6), 7641–7650 (2013). [CrossRef] [PubMed]
  32. Y. Guowei, M. Khalighi, S. Bourennane, “Performance of receive diversity FSO systems under realistic beam propagation conditions,” in Proc. of the 2012 8th Int. Symp. Commun. Syst. Netw. Digital Signal Process. (CSNDSP), (2012), pp. 1–5.
  33. J. Jaeshin and W. Sunghong, “Comparative study on cooperative communications in the upper layers at ad-hoc networks,” in 2013 15th Int. Conf. Adv. Commun. Technolo. (ICACT) (2013), pp. 133–137.
  34. J. Perez, Z. Ghassemlooy, S. Rajbhandari, M. Ijaz, H. L. Minh, “Ethernet FSO communications link performance study under a controlled fog environment,” IEEE Commun. Lett. 16(3), 408–410 (2012). [CrossRef]
  35. J. A. Louthain, J. D. Schmidt, “Anisoplanatism in airborne laser communication,” Opt. Express 16(14), 10769–10785 (2008). [CrossRef] [PubMed]

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