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