SKA and Optical Fibre Links

R.E. Spencer JBO Dec 2001

• 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 .• 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 ’s and needs EDFAs for ~20 km links.

Effect of losses and dispersion on error rates

• Error rate given by the complementary error function:

2

12 2 2

2

2

( )

2( )

S

s

n

u

x

BER erfc

is

i

erfc x e du

=Electrical SNR

0 20 40 60 80 10010

-15

10-10

10-5

100

105

1010 4 ps/nm/km fibre 40 ps risetime

Link Length km

Opt

ical

inpu

t pow

er (W

), E

lect

rical

SN

R a

nd b

it er

ror r

ates

SNR

Opt input power

Ideal

Worse case dispersion broadened

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 diodeDetector as for ALMA

Some current prices (10 Gbps link per antenna)

SKA Array configurations - fibre costs 7/11/01

N 1 300 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 30 Cost of repeater in k$

Some Possible SKA Configurations

• O Why ?

-600 -400 -200 0 200 400 600

-400

-200

0

200

400

-600 -400 -200 0 200 400 600

-400

-200

0

200

400

-600 -400 -200 0 200 400 600

-600

-400

-200

0

200

400

Circle Y (cf. VLA) Spiral r~2

Costc N( ) N A L DL

2

N

21 F

L

RC Costy N( ) N A L D

L

2

N

31 F

L

RC Costs N( ) N A L D

L

2N 1( ) F

L

RC

A Random Array

• Antenna position chosen at random

• 100 antennas

• Max spacing ~900 km

• Gives low, noise-like sidelobes for snapshot mapping

-600 -400 -200 0 200 400

-400

-300

-200

-100

0

100

200

300

400

Random Array, N=100

km

km

Random array: connections to centre and beam

Compass diagram Array Beam of antenna positions

108.8384

217.6769

326.5153

435.3538

544.1922

30

210

60

240

90

270

120

300

150

330

180 0

-50 0 50-1000

0

1000

2000

3000

4000

5000Beam in x direction

angle in units of 10 mas

Connecting a random array

-600 -400 -200 0 200 400

-500

-400

-300

-200

-100

0

100

200

300

Random Array, N=100

km

km

-600 -400 -200 0 200 400

-500

-400

-300

-200

-100

0

100

200

300

Random Array, N=100

km

km

Another random array, The travelling salesman solution 25000 km radial dig 7500 km dig

costr N( ) N A L DN

100 L N F

L

RC

Costs of Links in k$:

Circle:Spiral: Y:

0 50 100 150 200 250 3001 104

1 105

1 106

3.624105

1.276104

Costc N( )

Costy N( )

Costs N( )

costr N( )

3001 N

random

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.