FSO networks: understanding route diversity under turbulence phenomena towards reliable FSO mesh networks design. In last mile extensions of MANs, wireless mesh networks are multi-hop networks being used as backbone networks connecting end-users with the access points connected to the Internet. Wireless mesh networks are an attractive option over optical fibres because of their ease of installation and cost effectiveness of deployment[1]. Moreover, Free Space Optics (FSO) technology is an attractive option for use in mesh networks [2, 3]. However, time-variant influence of the atmosphere in FSO links that introduces one of the main drawbacks [4]. In order to overcome the turbulence induced fading in FSO systems, several techniques have been proposed These include: spatial transmitter/receiver diversity [5] [6]; adaptive beam forming [7]; wavelength diversity [8], multiple-beam communication [9], novel modulation techniques and hybrid RF/optical link scheme. Moreover, topology design and routing are essential tools for FSO mesh networks performance. The turbulence phenomena also influences in the topology and routing design of complex FSO networks, then route diversity techniques will improve the mesh network reliability [14]. For example, route diversity application within mesh optical networks deployed Tokyo provided interesting experiment results in [15]. This presentation will offer an overview of turbulence phenomena on FSO mesh networks from route diversity point of view. References [1] I. F. Akyildiz, X. Wang, and W. Wang, "Wireless mesh networks: a survey," Computer Networks, vol. 47, pp. 445-487, 2005. [2] Z. Hu, P. Verma, and J. J. Sluss, "Improved reliability of free-space optical mesh networks through topology design," J. Opt. Netw., vol. 7, pp. 436-448, 2008. [3] A. Kashyap, K. Lee, M. Kalantari, S. Khuller, and M. Shayman, "Integrated topology control and routing in wireless optical mesh networks," Computer Networks, vol. 51, pp. 4237-4251, 2007. [4] Z. Ghassemlooy, W. Popoola, and S. Rajbhandari, Optical Wireless Communications : System and Channel Modelling with MATLAB: CRC Press 2012. [5] S. M. Navidpour, M. Uysal, and M. Kavehrad, "BER performance of free-space optical transmission with spatial diversity," IEEE Trans. Wireless Commun., vol. 6, pp. 2813-2819, Aug 2007. [6] 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, vol. 6, pp. 34-42, 2012. [7] R. K. Tyson, "Bit-error rate for free-space adaptive optics laser communications," J. Opt. Soc. Am. A:, vol. 19, pp. 753-758, Apr 2002. [8] V. Weerackody and A. R. Hammons, "Wavelength Correlation in Free Space Optical Communication Systems," in Proceedings of IEEE Military Communications Conference 2006, 2006, pp. pp. 1-6.

1. FSO networks: understanding route diversity
under turbulence phenomena towards reliable
FSO mesh networks design
Joaquin Perez Soler
Northumbria University 2013 Research Conference
15th- 16th May 2013
City Campus East

2. Northumbria Communications Research Lab
Faculty of Engineering and Environment
Northumbria University
Newcastle upon Tyne, United Kingdom
Northumbria University 2013 Research Conference
15th- 16th May 2013
City Campus East

3. FSO applications
• Sports events (F1)
• Campus, MAN networks
(Cablefree, LightPointe ...)
Northumbria University 2013 Research Conference ‐ 15th/16th May 2013
• Security and defence
communications
(CASSIDIAN, EADS, ...)

4. Air-to-Ground FSO
• Air-to-ground FSO
quantum-key distribution
communication
(Nature Photonics 7, 382–386 (2013))
(Photos: DLR)
Northumbria University 2013 Research Conference ‐ 15th/16th May 2013
Speed of 290 km/h
at a distance of 20 km
or 4 mrad/s

5. Space-ground FSO
• Laser Communications Relay
Demonstration LCRD
• High bandwidth geo-sync to
ground optical link
– Downlink: 1.2 Gbps
– Uplink: 1.2 Gbps
– LCRD Payload Flight in 2016 or
2017 on Loral Commercial Satellite
Northumbria University 2013 Research Conference ‐ 15th/16th May 2013
• Bidirectional low Earth orbit-to-
ground optical link
– 2 Mb/s UL and 50 Mb/s DL
M.W. Wright et al. , 26 September 2011, SPIE Newsroom.
(Photos: NASA and SPIE)

6. • Intra-satellite
• Campus networks
Northumbria University 2013 Research Conference ‐ 15th/16th May 2013
• FSO link needs line
of sight (LOS)
• Link Obstacles??
– Buildings, fog,
turbulence...
– No connection!
• A mesh topology as
a solution??
FSO networks
(Photos: ESA)

7. • Node in a mesh
topology:
– capture and disseminate
its own data,
– serve as a relay for other
– Collaboration on routing
• Backup, flexibility, end-to-
end quality of service
assured, ...
Northumbria University 2013 Research Conference ‐ 15th/16th May 2013
• FSO and mesh topology
– Blocking link
• Node (transceiver)
intelligence to find different
route
FSO mesh topology
[1]
[1] J. Tapolcai, et al. “Switching/merging node placement in survivable optical networks with SSP”,
Computer Communications, Volume 33, Issue 3, 26 February 2010, Pages 381-389, ISSN 0140-3664

8. Route diversity in FSO
• FSO link non-availability affected by:
– Turbulence
– Fog
– Blocking
• Test Diversity techniques
• Impact on route diversity?
• Diversity techniques as Maximal Ratio Combining
(MRC), Equal Gain Combining (EGC) and Selection
Combining (SelC) [2]
[2] W. O. Popoola, Z. Ghassemlooy, J. I. H. Allen, E. Leitgeb, and S. Gao, "Free-space optical communication employing
subcarrier modulation and spatial diversity in atmospheric turbulence channel," IET Optoelectron 2, 16-23 (2008).
Northumbria University 2013 Research Conference – 15th/16th May 2013

9. Turbulence in FSO
• Appears due to temperature gradient in the channel,
movement of air particles and other eddies.
• Turbulence measurement in FSO
– Related with atmospheric scintillation and defining coefficient
structure index Cn
2
• From signal received (Rytov variance)
• Directly from scintillometers
• Indirectly from temperature variance
(temperature gradient)
P T T C
    n
  86  10 2  
2  
2 i   1.23 C 2 7/6 11/6 n k L p , (Photo: Wikipedia)
Northumbria University 2013 Research Conference – 15th/16th May 2013
2  2
2 6 1 2
T R

10. Measurements
• Outdoor FSO turbulence – Cn
Northumbria University 2013 Research Conference – 15th/16th May 2013
2 [3]
• {}
0.08
0.06
0.04
0.02
0
-0.02
-0.04
-0.06
fit error [-]
[3] J. Libich et al “ Influences of Turbulences in Near Vicinity of Buildings on
Free-space Optical Links. IET Microwaves, Antennas & Propagation. 2011,
vol. 9, no. 5, p. 1039-1044. ISSN 1751-8725.
• Indoor lab turbulence – NCR Lab Proposal
lognormal distribution fit error
gamma-gamma distribution fit error
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
normalized scintillation index [-]
– In-line 20 thermal sensors inside chamber monitoring temperature

11. FSO Diversity study
• Indoor chamber to replicate turbulence
• Inside - Route diversity paths affected by turbulence
Northumbria University 2013 Research Conference – 15th/16th May 2013

12. FSO Diversity study
• Indoor chamber to replicate turbulence
• Inside - Route diversity paths affected by turbulence
• Measured different path configurations
Northumbria University 2013 Research Conference – 15th/16th May 2013

13. Measurements indep Route
• FSO links topology:
– Allow redundancy in case of failure of one FSO link, another
path is used.
– Route diversity improves availability of the network.
• Case under study:
– Same network end, different paths
– Independent paths, how to calculate turbulence effect?
– Needed to compare with correlated paths
Northumbria University 2013 Research Conference – 15th/16th May 2013

14. Measurements indep Route
100
10-1
10-2
2 (a) from FSO [-]
2 (b) [K2 m-2/3]
(a) Rytov variances derived from received optical signal and from thermal sensors measurements
(symbols, red circles - channel 1, blue crosses - channel 2)
(b) Cn
2 from sensors [-]
Northumbria University 2013 Research Conference ‐ 15th/16th May 2013
14
10-2 10-1 100 101 10-3
 R
 R
10-8
10-10
10-12
10-2 100 102 10-14
2 [m-2/3]
Cn
CT
2 theoretical relations (black dotted lines) derived from measured thermal distributions (channel
1 red and channel 2 blue lines) andCn
2 derived from measured of optical power on CT
2 measured by
the sensor line (symbols; red circles - channel 1, blue crosses – channel 2)
Good agreement

15. Measurements correl Route
• FSO network redundancy
• Case under study :
– two links terminating at the same point
– passing the common volume with similar turbulence characteristic
– one optical links is along the distant part influenced by non-correlated
turbulent flow
Northumbria University 2013 Research Conference – 15th/16th May 2013

16. Measurements correl Route
• Diversity gain defined as the difference between
attenuation of single link and minimum attenuation in
case of joint diversity links.
Diversity gain [dB]isolated channels
partially correlated turbulences
0 0.2 0.4 0.6 0.8 0.9
8
6
4
2
0
• Two isolated channels, enhancement if we can change
to the unaffected channel = Diversity technique
Selection Combining (SelC)
Northumbria University 2013 Research Conference – 15th/16th May 2013
Qch1/Qch2 [-]

17. Measurements correl Route
Case of increased turbulence level from low towards moderate in Ch2:
route diversity scheme is efficient of both channels experience different turbulences along
their links compare to the case when both links pass through a common turbulent channel.
6
5
4
3
2
1
0
-1
Northumbria University 2013 Research Conference – 15th/16th May 2013
17
100 101 102
channels Cn 2
ratio [-]
Diversity gain [dB]
isolated channels
partially corelated turbulences
channel 1
weak
channel 2
mid to
strong
turbulences
both
channels
weak
turbulences

18. Conclusion
• Valuable information
– Physical parameters  diversity scheme  ROUTING
– Influence on FSO networks design
• Maximum number of hops between nodes
• Prediction of routes
• From physical to network layer
• Next steps
– More route diversity cases
– Implement the turbulence measurement in to network
routing algorithms
– Simulate FSO mesh networks behaviour
Northumbria University 2013 Research Conference – 15th/16th May 2013

19. Conclusion
• Route diversity measured under several deployments
• Joint journal publications with European Institutions (Fresnel
Institute Marseille, CTU Prague) [5, 6]
• This will allow us to transfer knowledge on other institutions :
– E.g. CTU developed an indoor FSO chamber
Northumbria University 2013 Research Conference – 15th/16th May 2013
[5] S. Zvanovec, J. Perez, Z. Ghassemlooy, S. Rajbhandari, J. Libich,
"Route diversity analyses for free-space optical wireless links within
turbulent scenarios," Opt. Express, vol. 21 (6), pp. 7641-7650,
2013.
[6] J. Perez, S. Zvanovec, Z. Ghassemlooy, W. O. Popoola,
"Experimental characterization and mitigation of turbulence induced
signal fades within an ad hoc FSO network(Link)," Opt. Express, vol
22 (3), pp. 3208-3218, 2014.
19

20. ACKNOWLEDGEMENTS
 OCRG and NCRLab teams @
Northumbria University
 Prof Zabih Ghassemlooy
 Czech Technical University, Prague
 Dr Stanislav Zvanovec
 EU COST action IC1101 and IC0802

21. NCRLab ‐ Weekly Research Meeting ‐ 01 December 2014

22. Thank you
www.linkedin.com/in/joaquinperezsoler/en
http://joaquinperezsoler.blogs.upv.es