fso networks under turbulence - northumbria university 2013 research conference
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FSO networks: understanding route diversity under turbulence phenomena towards reliable
FSO mesh networks design
Joaquin Perez Soler
Northumbria University 2013 Research Conference15th- 16th May 2013City Campus East
Northumbria Communications Research LabFaculty of Engineering and Environment
Northumbria UniversityNewcastle upon Tyne, United Kingdom
Northumbria University 2013 Research Conference15th- 16th May 2013City Campus East
Northumbria University 2013 Research Conference ‐ 15th/16th May 2013
FSO applications• Sports events (F1)• Campus, MAN networks
(Cablefree, LightPointe ...)
• Security and defence communications (CASSIDIAN, EADS, ...)
Northumbria University 2013 Research Conference ‐ 15th/16th May 2013
Air-to-Ground FSO
• Air-to-ground FSO quantum-key distribution communication (Nature Photonics 7, 382–386 (2013))
(Photos: DLR)Speed of 290 km/h
at a distance of 20 km or 4 mrad/s
Northumbria University 2013 Research Conference ‐ 15th/16th May 2013
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
• 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)
Northumbria University 2013 Research Conference ‐ 15th/16th May 2013
• Intra-satellite• Campus networks
• FSO link needs line of sight (LOS)
• Link Obstacles??– Buildings, fog,
turbulence...– No connection!
• A mesh topology as a solution??
FSO networks
(Photos: ESA)
Northumbria University 2013 Research Conference ‐ 15th/16th May 2013
• 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, ...
• FSO and mesh topology– Blocking link
• Node (transceiver) intelligence to find different route
FSO mesh topology
[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
[1]
Northumbria University 2013 Research Conference – 15th/16th May 2013
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
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 Cn2
• From signal received (Rytov variance)
• Directly from scintillometers• Indirectly from temperature variance
(temperature gradient)
221 22 6
2 286 10n
T TPCT R
2 2 7/6 11/61.23 ,i n pC k L (Photo: Wikipedia)
Northumbria University 2013 Research Conference – 15th/16th May 2013
Measurements• Outdoor FSO turbulence – Cn
2 [3]
• {}
• Indoor lab turbulence – NCR Lab Proposal– In-line 20 thermal sensors inside chamber monitoring temperature
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5-0.06
-0.04
-0.02
0
0.02
0.04
0.06
0.08
normalized scintillation index [-]
fit e
rror [
-]
lognormal distribution fit errorgamma-gamma distribution 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.
Northumbria University 2013 Research Conference – 15th/16th May 2013
FSO Diversity study • Indoor chamber to replicate turbulence• Inside - Route diversity paths affected by turbulence
Northumbria University 2013 Research Conference – 15th/16th May 2013
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
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
Measurements indep Route
14
10-2 10-1 100 10110-3
10-2
10-1
100
R2 from FSO [-]
R2
from
sen
sors
[-]
10-2 100 10210-14
10-12
10-10
10-8
Cn2 [m
-2/3
]
CT2 [K2 m-2/3]
(b)(a)
(a) Rytov variances derived from received optical signal and from thermal sensors measurements(symbols, red circles - channel 1, blue crosses - channel 2)
(b) Cn2 theoretical relations (black dotted lines) derived from measured thermal distributions (channel
1 red and channel 2 blue lines) and Cn2 derived from measured of optical power on CT
2 measured bythe sensor line (symbols; red circles - channel 1, blue crosses – channel 2)
Good agreement
Northumbria University 2013 Research Conference – 15th/16th May 2013
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
Measurements correl Route• Diversity gain defined as the difference between
attenuation of single link and minimum attenuation in case of joint diversity links.
• Two isolated channels, enhancement if we can change to the unaffected channel = Diversity technique Selection Combining (SelC)
0 0.2 0.4 0.6 0.8 0.9
0
2
4
6
8
Qch1/Qch2 [-]
Div
ersi
ty g
ain
[dB
]
isolated channelspartially correlated turbulences
Northumbria University 2013 Research Conference – 15th/16th May 2013
Measurements correl RouteCase 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.
17
100 101 102-1
0
1
2
3
4
5
6
channels Cn2 ratio [-]
Div
ersi
ty g
ain
[dB
]
isolated channelspartially corelated turbulences
channel 1weakchannel 2mid tostrongturbulences
bothchannels
weakturbulences
Northumbria University 2013 Research Conference – 15th/16th May 2013
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
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
19
[5] S. Zvanovec, J. Perez, Z. Ghassemlooy, S. Rajbhandari, J. Libich,"Route diversity analyses for free-space optical wireless links withinturbulent 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 inducedsignal fades within an ad hoc FSO network(Link)," Opt. Express, vol22 (3), pp. 3208-3218, 2014.
OCRG and NCRLab teams @ Northumbria University Prof Zabih Ghassemlooy
Czech Technical University, Prague Dr Stanislav Zvanovec
EU COST action IC1101 and IC0802
ACKNOWLEDGEMENTS
NCRLab ‐Weekly Research Meeting ‐ 01 December 2014
Thank you
www.linkedin.com/in/joaquinperezsoler/enhttp://joaquinperezsoler.blogs.upv.es