Transcript Disperror trial - Jodrell Bank Observatory

SKA and Optical Fibre Links
R.E. Spencer JBO Dec 2001
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Fibre links
Fibre optics and link design
Array configurations
Cost implications
Fibre Links
• Microwave links and satellite links limited to
<~200 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!
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 signal:noise and distortion
due to dispersion.
• Non-linear effects restrict total input power to <
few mW. In WDM four wave mixing etc. gives
cross-talk between channels.
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 diode+modulators 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 l’s and needs EDFAs for ~20 km links.
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• Error rate given by the
complementary error
function:
BER  12 erfc ( 2 S 2 )
s
2
s
2
n
i
i
erfc( x ) 
=Electrical SNR
2



x
e  u du
2
Optical input power (W), Electrical SNR and bit error rates
Effect of losses and dispersion on error
rates
10
10
10
4 ps/nm/km fibre 40 ps risetime
10
5
SNR
0
Worse case dispersion broadened
Opt input power
10
-5
Ideal
10
10
-10
-15
0
20
40
60
Link Length km
80
100
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.
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
Some current prices (10 Gbps link per antenna)
SKA Array configurations - fibre costs
N
1  300
7/11/01
Number of antennas
B 1000
Max baseline in km
A 10
Cost per antenna of Tx and Rx etc. in k$
D 10
Cost of dig k$ per km
F 0.1
Cost of fibre k$ per km
R 50
Spacing of repeaters in km
C
Cost of repeater in k$
30
Some Possible SKA
Configurations
• O Why ?
400
400
200
400
200
0
200
0
-200
0
-200
-400
-200
-400
-600
-400
-600
-400
-200
0
200
400
-600
600
-600
Circle
Costc ( N )
N  A L D
-400
-200
0
200
400
2
2
1 F
L
R
C
Costy ( N )
N  A L D
L N
2 3
-200
0
200
400
600
Spiral r~q2
Y (cf. VLA)
L N
-400
600
1 F
L
R
C Costs ( N )
NA
L D
L
2
(N
1)  F
L
R
C
A Random Array
400
300
200
100
km
• Antenna position chosen
at random
• 100 antennas
• Max spacing ~900 km
• Gives low, noise-like
sidelobes for snapshot
mapping
Random Array, N=100
0
-100
-200
-300
-400
-600
-400
-200
0
km
200
400
Random array: connections to centre and beam
90
Beam in x direction
5000
544.1922
120
60
435.3538
4000
326.5153
150
30
217.6769
3000
108.8384
180
0
2000
1000
210
330
0
240
300
270
Compass diagram
of antenna positions
-1000
-50
0
angle in units of 10 mas
Array Beam
50
Connecting a random array
Random Array, N=100
Random Array, N=100
300
300
200
200
100
100
0
-100
km
km
0
-100
-200
-200
-300
-300
-400
-400
-500
-500
-600
-400
-200
0
200
400
-600
-400
-200
km
Another random array,
25000 km radial dig
0
200
400
km
The travelling salesman solution
7500 km dig
costr ( N )
NA
L D
N
100
L N  F
L
R
C
Costs of Links in k$:
 05
3 .6 241
6
1 1 0
random
Co stc( N )
Co sty( N )
Co sts( N )
5
1 1 0
costr( N )
Circle:
Spiral:
Y:
 04  4
1 .2 761
1 10
1
0
50
1 00
1 50
N
2 00
2 50
3 00
3 00
Llano de Chajnantor
Simon Radford’s photo
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.