Free Space Optical (FSO) Communications in Next Generation Cellular Networks

1
Survey of Free Space Optical (FSO) Communications
Opportunities in Next Generation Cellular Networks
Frédéric Demers, Halim Yanikomeroglu Marc
St-Hilaire Presented at the Communication
Networks and Services Research Conference 4 May
2011
2
Outline

• Motivation Key Characteristics of FSO systems
• Channel model and path loss overview
• Recent advances in FSO communications
• Full Optical FSO systems
• Hybrid RF/FSO systems
• Mobile FSO systems
• Indoor diffuse FSO systems
• Applications within Next Generation Cellular
  Networks
• Conclusions

3
Motivation key characteristics

• RF spectrum scarcity vs increasing throughput
  requirements
• A single FSO channel can offers Tb/s throughput
  wirelessly
• Free space optical spectrum is license free and
  nearly unlimited (very dense reuse)
• FSO systems are generally very difficult to
  intercept
• Effective range limited by weather and eye-safety
  considerations

4
Channel model

• Factors affecting light propagation through the
  atmosphere
• Physical composition of atmosphere
• Changes in refractive indices
• Aerosol particles

5
Channel model
850 nm
1550 nm
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Channel model

• Channel effects
• Absorption
• Diffraction
• Rayleigh scattering (atmospheric gases molecules)
• Mie scattering (aerosol particles)
• Atmospheric (refractive) turbulence
• Scintillation
• Beam wander

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Channel model
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Path loss, RF

• Typical RF attenuation (e.g. 2 GHz, 15 dBi
  antenna gains)
• Avg path loss in free space -gt 68 dB _at_ 1km , 118
  dB _at_ 10 km
• Avg path loss in mobile radio (n3.4, d0100 m)
  -gt 82 dB/km, 146 dB _at_ 10 km

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Path loss, FSO
M. Bass, "Atmospheric optics," in Handbook of
Optics ,Third Edition ed., vol. 5, M. Bass, Ed.
McGraw-Hill, pp. 3.3., 2010.
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Path loss, FSO
Refractive index of air
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Path loss, RF vs FSO

• Typical RF attenuation (e.g. 2 GHz, 15 dBi
  antenna gains)
• Avg path loss in free space -gt 68 dB _at_ 1km , 118
  dB _at_ 10 km
• Avg path loss in mobile radio (n3.4, d0100 m)
  -gt 82 dB/km, 146 dB _at_ 10 km
• Typical optical attenuation (e.g. 1550 nm or 194
  THz)
• clear atmospheric conditions -gt 0.2 dB/km
• urban (because of dust) -gt 10 dB/km
• Rain -gt 2-35 dB/km
• Snow -gt 10-100 dB/km
• light fog -gt 120 dB/km
• dense fog -gt 300 dB/km
• maritime fog -gt 480 dB/km

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Full Optical FSO

• No requirement for electrical-optical conversion
• Easy extension of RF-over-fibre links
• Wavelength division multiplexing

K. Kazaura, K. Wakamori, M. Matsumoto, T.
Higashino, K. Tsukamoto and S. Komaki, "RoFSO A
universal platform for convergence of fiber and
free-space optical communication networks,"
Communications Magazine, IEEE, vol. 48, pp.
130-137, 2010.
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Hybrid RF/FSO

• FSO is most affected by fog, RF by rain
• RF links complements FSO to achieve carrier class
  availability (99.999)
• Lower throughput in adverse weather

I. I. Kim and E. Korevaar, "Availability of free
space optics (FSO) and hybrid FSO/RF systems,"
Optical Wireless Communications IV, EJ Korevaar,
Eds. , Proc. SPIE, vol. 4530, pp. 84-95, 2001.
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Mobile FSO Systems

• Tightly packed LED transceivers around spherical
  device
• Able to maintain optical link in motion
• Experiment rather simplistic

J. Akella, C. Liu, D. Partyka, M. Yuksel, S.
Kalyanaraman and P. Dutta, "Building blocks for
mobile free-space-optical networks," in Wireless
and Optical Communications Networks, 2005. WOCN
2005. Second IFIP International Conference on,
pp. 164-168, 2005.
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Indoor Diffuse Optical Wireless

• Non Line-of-Sight optical communications
• Multipath interference an issue, limiting
  throughput
• Hybrid narrow-beam designs provide both bandwidth
  and coverage

R. J. Green, H. Joshi, M. D. Higgins and M. S.
Leeson, "Recent developments in indoor optical
wireless systems," IET Communications, vol. 2,
pp. 3, 2008
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Next Generation Cellular Networks

• Densification of access points (eNodeB)
• Shorter hops
• Suitability to mesh connectivity
• Heterogeneous access points
• Relaying
• Distributed antennas
• Coordinated Multi-Point Transmission Reception
  (CoMP)
• Self-Organizing Networks

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Next Generation Cellular Networks
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Conclusions

• Radio frequencies alone will not suffice to
  provide the required throughput to the end-users

• Next generation networks will require a denser
  infrastructure to cater to mobile user needs

This denser infrastructure will shorten hops
between base stations and ease the establishment
of mesh connectivity
These architectural changes open the door to an
increased reliance upon FSO communication systems
PHY layer is not dead!
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Main references

• J. Akella, C. Liu, D. Partyka, M. Yuksel, S.
  Kalyanaraman and P. Dutta, "Building blocks for
  mobile free-space-optical networks," in Wireless
  and Optical Communications Networks, 2005. WOCN
  2005. Second IFIP International Conference on,
  2005, pp. 164-168. Available http//citeseerx.ist
  .psu.edu/viewdoc/download?doi10.1.1.143.6352rep
  rep1typepdf
• M. Bass, "Atmospheric optics," in Handbook of
  Optics ,Third Edition ed., vol. 5, M. Bass, Ed.
  McGraw-Hill, 2010, pp. 3.3.
• R. J. Green, H. Joshi, M. D. Higgins and M. S.
  Leeson, "Recent developments in indoor optical
  wireless systems," IET Communications, vol. 2,
  pp. 3, 2008. Available http//www.ieeexplore.ieee
  .org.proxy.library.carleton.ca/stamp/stamp.jsp?tp
  arnumber4446618
• K. Kazaura, K. Wakamori, M. Matsumoto, T.
  Higashino, K. Tsukamoto and S. Komaki, "RoFSO A
  universal platform for convergence of fiber and
  free-space optical communication networks,"
  Communications Magazine, IEEE, vol. 48, pp.
  130-137, 2010. Available http//www.ieeexplore.ie
  ee.org.proxy.library.carleton.ca/stamp/stamp.jsp?t
  parnumber5402676
• I. I. Kim and E. Korevaar, "Availability of free
  space optics (FSO) and hybrid FSO/RF systems,"
  Optical Wireless Communications IV, EJ Korevaar,
  Eds. , Proc. SPIE, vol. 4530, pp. 84-95, 2001.
  Available http//www.ece.mcmaster.ca/hranilovic/
  woc/resources/local/spie2001b.pdf

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Other considerations

• FSO mesh topologies have been shown to provide
  carrier class reliability in spite of low FSO
  link availability
• Relay-assisted transmission is a powerful fading
  mitigation tool for FSO systems operating in
  channels with atmospheric turbulence
• Redundant links and MIMO can be used to mitigate
  alignment challenges (turbulence, building sway)

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Path loss, FSO
Structure function
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Channel model (8)
Diffraction
Scattering
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Channel model (9)
Refraction
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FOS Systems

• Unaffected by EMI interference

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Conclusion
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Whatever
Propagation of light beams through the atmosphere
Absorption and Scattering
Tilt
Diffraction
Scintillation

• Index of refraction of the air has a constant
  and a variable part

• At optical wavelengths the index of refraction
  is much more affected by temperature than
  pressure fluctuations
• The equation that relates temperature
  fluctuations to index of refraction fluctuations
  is

Where ? Cp/Cv 1.4 for air p air pressure
in millibars T temperature in Kelvin
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Link Budget Examples

• Tx Power 20 dBm (100 mW)
• Rx Sensitivity 35 dBm
• Beam divergence 2 mrad
• Distance 1 km
• Receiver size 50 mm
• Geometric loss 21 dB
• Margin 10 dB
• Able to handle dense fog at 120 dB/km

• Tx Power 0 dBm (1 mW)
• Rx Sensitivity 35 dBm
• Beam divergence 2 mrad
• Distance 1 km
• Receiver size 50 mm
• Geometric loss 21 dB
• Margin 10 dB
• Able to handle light fog 16 dB/km