fso networks under turbulence - northumbria university 2013 research conference

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, ...)


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


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)


• 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)


• 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).


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)


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.


FSO Diversity study • Indoor chamber to replicate turbulence• Inside - Route diversity paths affected by turbulence


FSO Diversity study • Indoor chamber to replicate turbulence• Inside - Route diversity paths affected by turbulence• Measured different path configurations


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


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


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


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


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


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


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