Pathfinders for the SKA:
Nlog(N) vs. N2 Imaging
Kristian Zarb Adami
Instruments:
N log N Astronomy
In fact...
 Japan designed the SKA in 1994
 8x8 Images in 1994 with Waseda
telescope
 Extrapolating with Moore’s Law
 (doubling every 18 months) 
 2016 is 1x106 antennas
 Which is equivalent to SKA-phase-1
Remit of the talk
 Science Justification for SKA1-Low
 Science and Technical simulations towards implementation of
the SKA
 Physical Implementation on Medicina as a flexible DSP test-bed
and a comparison between spatial-FFT and N2 imaging
 Industrial Engagement
SKA Phase-1 Specifications
Memo 125
Sensitive (-ity) Issues..
[SKA Memo 100]
Roadmap to the SKA-lo
SKA-1
N
LOFAR
Super Terp
MWA-512
(400,50x2xNbeams)
(8,50x2xNbeams)
PAPER
(32, 1024)
(1,768)
LOFAR UK
LWA
(100,128)
MWA-32
(25,192)
(78,100)
GMRT
(32, 64)
(16,60)
Medicina
MITEOR
(16,32)
(25,16)
BW
H1-Power Spectrum (z≈8)
Theoretical 21-cm Power Spectrum @ 150 MHz
Power Spectrum from a (100,256) instrument
Foregrounds suppressed by frequency/angle
differencing
NlogN vs. N2
Super-Terp
LOFAR
2011
2010
SKA-Phase 1
SKA-Phase 2
HI Power Spectra (SKA-Phase-II)
Blue: HI > 108
Green: HI > 20’
Linear bias = 0.8
Co-moving Volume =
(500MPc/h)3
Linear Bias = 1.0
SKA1 Low Layout
100km
200m
The numbers game (SKA1-low)

Bandwidth 70 – 450 MHz (Instantaneous B/W 380 MHz)

ADC Sampling at 1 GSa/s @ 8-bit

Antenna Spacing ~ 2.6m

Array Configuration:

50 stations

11,200 antennas per station (~10,000)

Output beams of 2-bit real; 2-bit imag
Numbers cont...
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SKA-1 ~ 50 stations of 10,000 antennas each
Station diameter ≈ 200m
Station beam @ 70 MHz ≈ 1○, @ 450 MHz ≈ 0.2○
Nbaselines = 5,000 (50^2/2 *4)


Input data rate to station 160 Tb/s (total data rate 8 Pb/s for the SKA-1 lo)
Output rate?


Output beams 2+2 bits, ~100kHz channels (1.6Mbps per beam-channel)


6.25 million beam-channels – by DFT need 0.1 Pop/s (6250 beams @ 1000 channels)
Equalise sky coverage so N(f) ~f2 – 100 beams in lowest (70 – 70.1 MHz) channel


Assume 10 Tb/s off station = 100 x 100Gb/s fibres
100 sq deg instantaneous coverage.
Correlator has to do 1,000 baselines for each 1 kHz beam-channel

(for a total ~ 10 Pop/s)
Station Architecture
Station Layout
Tile
Processor
Tile
Processor
Copper
Optical Fibre
Station
Processor
Tile
Processor
Tile
Processor
Richard Armstrong – [email protected]
Optical Fibre
Hierarchical Architecture
Hierarchical
Sub-Station
Direct
Beam
Cross-correlation
Station
Forming
Beam(tiles
Forming
(calibration)?
then station)
Antennas
Sub-Station
Tile
Station
LevelWeights
Weights
Weights
Tile level
Electronic Calibration
Field or Strong Source
Calibration
~CAS-A
Station Level Weights
Polarisation
Calibration
Source & Polarisation
Calibration
Multiply and add by weights
Multiply and add by weights
Cross correlation of sub-arrays (for station calibration and ionospheric calibration)
Tile processor box
RF in (coax)
16 x dual pol
Fibre:
Data out
Clock and control in
DC in
Multi-chip module
Reg
Tile Processor
Tile Processor
Inputs: 16 dual-pol antennas
ADC @ 1GSA/s @ 8-bit
A
D
C
Coarse frequency splitting
Into 4 channels
A
D
C
Outputs: dual-pol beams
@ 1GSA/s @ 4-bit re/4-bit imag
A
D
C
Output is optical
A
D
C
Coarse freq
splitting
1st
Level
Beamforming
RFI
Mitigation
&
4-bit
Quantisation
Control and Calibration Interface
Space-Frequency Beamforming
Time-delay beamforming is now an
option…
Dense mid-freq array: Antenna sep ~ 20cm
Time step ~ 1ns ~ 30 cm
Angle step > 45 deg
Sparse low-freq array: Antenna sep ~2 m
Time step ~ 1ns ~ 30 cm
Angle step ~10 deg – less if interpolate
Front end unit can combine space-freq
beamforming in a single FIR-like structure
Golden Rule: throw away redundant
data before spending energy
processing/transporting it
Station processor
Opticalelectro
Heirarchical
processor
Electrooptical
M&C
Opticalelectro
Multi-chip module
Heirarchical
processor
Electrooptical
M&C
Multi-chip module
Opticalelectro
Heirarchical
processor
Electrooptical
M&C
Opticalelectro
Multi-chip module
Heirarchical
processor
M&C
Multi-chip module
Opticalelectro
Heirarchical
processor
M&C
Multi-chip module
Electrooptical
Clock & control
Electrooptical
Station Processor
Inputs: 64-dual pol 1st stage beams
Outputs: selectable dual-pol beams
@ 1GSA/s @ 2-bit re/2-bit imag
Channelisation to 4096 channels
With a 1024 channeliser
2nd Level
Beamforming
2nd Level
Channelisation
Corner
Turner
Station Calibration and station
correlator
Output is optical and correlator ready
Station Calibration and Correlator
Simulations
Multi-Level Beamforming
 Split the problem to be
hierarchical and parallel.
 Station divided into tiles (can be
logical).
 Dump as much unwanted data as
we can early on.
Station beams
Tile beam
Simple Beam Patterns
80 x 80 degrees:
Tile beam at zenith.
Station beam at (45, 87) degrees.
Visualisation of beams
Elevation 85 - 90 degrees
1000 MHz
65536 antennas, 256 tiles
Station beams 0.20 degrees apart
Tile beams 2 degrees apart
27 tile beams, 8005 station beams
Station beams 0.05 degrees apart
Tile beams 2 degrees apart
27 tile beams, 31707 station beams
Run time: 2.18 seconds
Run time: 5.67 seconds
Dynamic Range Simulation
Courtesy: S. Schediwy & Danny Price
This is the reason a correlator is required for a beamformer
Array station sparsed x3
Auto-power beam Peak power 0 dB
Cross-power beam 3deg rotation
Peak power -20dB
Cross-power beam 30 deg rotation
Peak power -50dB
Examples of Implementation
Medicina Radio Telescopes
Introduction
564m
24 segments
32m dish
BEST-3Lo
BEST-2
640m
64 cylinders
BEST-2 specs
N cylinders
8
N receivers
32
Total collecting
1357.98 m2
area
Total effective
964.17 m2
area
Central Freq.
408 MHz
Frequency BW 16 MHz
IF
Longest
baseline N/S
E/W
30MHz
70m
17.04m
Marco Bartolini, IRA - INAF
Primary FOV
37.65 deg2
Sensitivity /
0.363 K/Jy
Antenna Gain
11.651
Aeff / Tsys
m2/K
Transit time at
2353.3 sec.
delta = 45 deg
X - ROACH
S - ROACH
F - ROACH
64- Channel ADC
B - ROACH
Medicina Radio Telescopes
Jack
Richard
Griffin
Jack
1Gb-E
PCI-X
GPU
Transient
Alessio
GPU
Imaging & Calibration
Dickie
OeRC
10 Gb-e
HOST - PC
Medicina Backend: Spatial FFT
Danny Price – Jack Hickish
Medicina Fringes…
Medicina Fringes (Cas. A.)
Cas. A. Image
Industrial Engagement

It is NOT the intention of the SKA community to deliver 'finished' chip
designs yet.

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

There are basic engineering processes that have to be done to
enable meaningful sizing, cost & power estimation
IP identification and development – potential industrial involvement
Development of strategic technology partnerships



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Aiming for detailed device specifications ready to start prototype
manufacture when NRE money available
ADC design
IP macros for eg FFT, switch fabric
Embedded controllers
Non-packaged device mounting
Identification of key architectural features
Identify appropriate optimisation opportunities and trade-offs.
Development of accurate models for cost and power analysis at the
wider system level.
Identify key interface 'Hot Spots' and apply effort accordingly
Industrial Engagement
 Multi-Chip Module (One Chip to Rule them all!)
 4 x 4 antenna array (currently) – easily extended to 8x8
RF
IN
10mW/
channel
10mW/FFT
ADC
 Can also be used for Phased Array feeds for dishes
FIR-FFT
Processor
UWB 16-8 bit
1GS/s
RX
1024 channel
splitter
4mW/Beam
Beam Combiner
&
Calibrator
16 element
Beam combiner
 Current Chip RFI protection shows -57dB/m (in air)
??
Optical I/O
Optical
Chip
Optical
OUT
Requirement Specifications