SKA and Optical Fibre Links

1
SKA and Optical Fibre Links

• R.E. Spencer JBO Dec 2001

• Fibre links
• Fibre optics and link design
• Array configurations
• Cost implications

2
Fibre Links

• Microwave links and satellite links limited to
  lt200 Mbps. Fibres have Terabit capability, using
  multiple wavelength channels (WDM wavelength
  division multiplexing).
• Current fibre optic technology has 10 Gbps
  components per channel available in 2001, 40 Gbps
  in 2 years time (but expensive and dispersion
  effects are more severe).
• A conservative approach would use 10 Gbps per
  wavelength launched. 100 wavelengths possible
  per fibre.
• Commercial internet provision costs for virgin
  sites are exhorbitant!

3
Link Design

• The link design is constrained by fibre
  transmission characteristics such as attenuation
  and dispersion, which are dependant on length.
• Error rates depend on signalnoise and distortion
  due to dispersion.
• Non-linear effects restrict total input power to
  lt few mW. In WDM four wave mixing etc. gives
  cross-talk between channels.

4
Properties of links

• Typical loss of fibre 0.25 dB per km at 1550 nm
  l.
• Dispersion (NZDSF) 4 ps/km/nm.
• Power output of 10 Gbps laser diodemodulators is
  1 mW
• Typical receiver sensitivity for 10-10 error rate
  is 10-2 mW.
• These limit maximum span of fibre to 50 km
  before amplification required.
• Amplifiers can be Erbium doped fibre amplifiers
  (EDFAs) or Raman effect amplifiers (just becoming
  available).
• Multiple wavelengths on a link require optical
  multiplexers and de-multiplexers with extra
  loss.
• E.g ALMA has 12 ls and needs EDFAs for 20 km
  links.

5
Effect of losses and dispersion on error rates

• Error rate given by the complementary error
  function

Electrical SNR
6
Limits of 10Gbps transmission over SMF fibre

• Attenuation limit at 80km
• Can be overcome using Erbium Doped Fibre
  Amplifiers (EDFAs) to reach spans of hundreds kms
• Dispersion limit at 80km
• Can be overcome using NZ-DSF and dispersion
  compensation methods to reach spans of hundreds
  of kms
• Polarisation Mode Dispersion limit at 400km
• Cannot easily be compensated. Regeneration
  required at this limit.

7
Effects of signal/noise ratio
-Eye diagram for 10 Gbps Data transmission
(phase switched 5 GHz signal). -Using
Multiplex pin diode Detector as for ALMA
8
Some current prices (10 Gbps link per antenna)
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Some Possible SKA Configurations

• O Why ?

Circle Y (cf. VLA)
Spiral rq2
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A Random Array

• Antenna position chosen at random
• 100 antennas
• Max spacing 900 km
• Gives low, noise-like sidelobes for snapshot
  mapping

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Random array connections to centre and beam
Compass diagram
Array Beam of antenna positions
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Connecting a random array
Another random array, The
travelling salesman solution 25000 km radial
dig 7500 km dig
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Costs of Links in k
random
Circle Spiral Y
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(No Transcript)
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Llano de Chajnantor
Simon Radfords photo
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Conclusion

• Optical fibres only sensible solution for high
  data rate systems.
• Costs depend heavily on the dig cost (average
  100/m in UK) -- choosing the terrain can help
  e.g. plough in for sand.
• Small change in configuration could lead to major
  changes in costs of fibre links links should be
  considered when deciding the configuration.