SKA 1 AA-low system design considerations

AAVP WorkshopDecember 2010 SKA1 AA-low design

SKA1 AA-low system design

considerations

Andrew Faulkner


SKA1 AA-low is a major System:>10x LOFAR sensitivity

>1 km2 collecting area

>6:1 frequency range 70 – 450MHz

~50 deg2 Field of View

Very quiet site

Required in SKA Memo 125


But........ It is also part of a bigger vision:

SKA2

Some implications....


Priority

SKA1 AA-low System requirements

• Stand alone instrument meeting SKA1 science goals• Deployable 2016 onwards

• Specification will mesh with SKA2 system design e.g.

– Frequency range ̶ Data rates

– Calibration capability ̶ Correlation requirements etc...

• Scaleable architecture for 2018+ commonality with AA-mid

• Architecture will support upgraded processing devices– Phase 2 expansion ̶ Retro-fit Phase 1 (if good pay-back)

SKA1 performance must not be compromised?


Element requirements

Feature Requirements CommentsElectromagnetics Frequency range, beampattern,

part of an arrayIt must work properly!

Quickly deployable Simple 10,000 per array.....

Manufacturable Easy, repeatable, cheap Essential for cost and reliability

Transportable Ideally “flat pack” The volume (m3) is huge

Reliable Failure rate must be very low MTBF of 100,000 hrs → ~1 element/array/day

“Low” cost From above Of course...

Low RFI Negligible radiation

Low Power ~100-200mW Essential for solar

Upgrading Deployed elements cannot be upgradedAdditions functionally equivalent

Ideally avoid any modifications


One or two elements? One element Two elements

Cost Minimised HigherDoubles # RF chains, ground planes and interconnects

Element performance Some compromise across band

Less compromise across band

Station design Forces highly sparse design at top-end of band

Reduces sparcity while maintaining sensitivity

Power Minimises power Increases powerFilling factor Good filling factor at

low fPoor filling factor unless multiple cores

Matching Challenging Easier


LWA element: mechanical example

Simple “skeleton” elements (delivered flat)

Cheap mesh groundplane

Single pole fixing – just sunk into ground

Electronics at top – well away from floods etc.

Clamp type rotational adjustment

Easy and quick deployment

Buried cables


Integrated AA-low element possibility

Processing

e/o

e/o

e/oPower

conditioning

Solar panel

Energy storage

Elements:70-450MHz

Analogue

ADC: 1GS/s

Data

Control

Sync.

50-100m all optical

Benefits:Integrated single unitNo copper connectionEasy to deployMinimum RFILightning “immunity”

Challenges:Low total powerIntegration ManufacturabilityPackaging

Stand-alone powerOptical links only

Elements only connected by glass....


Where to digitise?At element At processor

RFI Maximised MinimisedPhase stability LNA & Filters +

Clock distributionLNA + Filters + Second stage Gain + Cables

Data transport Digital possibly over fibre

RF over copper

Power At element or over copper

Over RF cable or at element

Lightning protection Can be good if link is fibre

Can be challenging if link is copper

Bandpass Very good Equalization after transport

Cross talk Minimised between elements and polarizations

Dependent on screening and design of RF boards

Can be done – safe optionLikely to require distributed digitisation around station. (Cable cost and range)

Requires good, stable analogue design

More advanced – more stableRequires: low power, quiet digitisers. Low cost short range optical drivers.

Likely, requires custom chips

No option to upgrade digitisers.


Tile Digitisation

Tile Digitisation

Element

Digitisation

RFI Shielded......

......

......

...

Power Distribution

Element Digitisation

...

Cooling

Element Digitisation

......

Station Processing

RFI shielded

System clock

Control &Monitoring

StationBeams

PowerGrid

Element Data

C & M

Clock

To Correlator

& Services

2x 500MHzAnalogue+ power

FibreCopper

2-PolElements

Front-end

Potential AA-low station:Safe solution:Single elementAnalog to clustered digitisationCentrally powered elements


Station processor

PrimaryStation

ProcessorBoard

0

…..

PrimaryStation

ProcessorBoard

1

…..

PrimaryStation

ProcessorBoard

(max 35)

…..

…..

SecondaryStation

ProcessorBoard

0

SecondaryStation

ProcessorBoard

1

SecondaryStation

ProcessorBoard

(max 35)

…..

…..

…..

…..

…..

…..

…..

012

35

ToElement

Digitisation

Long distance drivers

…..

012


…..


…..

Optical linksTo Correlator

Each link is 12 fibre lanes@10Gb/s

“All to All”Connections


…..

…..

…..

Station Processor

o/eo/eo/eo/eo/eo/e

o/eo/eo/eo/eo/eo/e

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o/eo/eo/eo/eo/eo/e

o/eo/eo/eo/eo/eo/e

o/eo/eo/eo/eo/eo/e

e/oe/oe/oe/oe/oe/o

e/oe/oe/oe/oe/oe/o

e/oe/oe/oe/oe/oe/o

e/oe/oe/oe/oe/oe/o

e/oe/oe/oe/oe/oe/o

e/oe/oe/oe/oe/oe/o

PChip

PChip

PChip

PChip

PChip

PChip

PChip

PChip

PChip

PChip

PChip

PChip

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To Element digitisationor Primary

Station Processors

To Secondary Station Processors or long distance fibre drivers


Each link is 12 diff. copper lanes@10Gb/s

12-channel Rx module. e.g Avago

AFBR-820BXXZ

12-channel Tx module. e.g Avago AFBR-810BXXZ

Total Raw input data rate: 4.32Tb/s

Total Raw output data rate: 4.32Tb/s max

“All to All”Connections

ControlProcessor

LineTx/R

x

Station Control

Requirements:• High bandwidth in • High bandwidth out• Largely cross connected• Scaleable at various levels• Programmable beamforming


Station size

• For a given total Aeff the collector cost is roughly constant

– Increasing overhead with many smaller stations– Possibly increasing station processor complexity with large arrays

• Total data rate to correlator for a fixed survey speed remains constant whatever number of stations

BUT• Correlator and central processor demands become more

challenging

Station size largely determined by

central processing costs


Core Design

Core for SKA1 AA-low becomes virtually fully filled.

More so for SKA2.

Core “stations” are not separated – there is a “sea” of elements

Design options/considerations to be made:

• Non-circular “stations” easier? e.g. Square or hexagonal?

• Maximising the sensitivity from each element:

• overlapping “stations”?

• smaller “stations” (how small) with more correlation?

• Apodising element density within areas of the core:

• Benefit? Save money?

Implies interconnected “station processing”, especially for SKA2

Correlation goes up as n2, but incoming data rate is constant


AA-low: SKA1 to SKA2

Fixed Aspects• Frequency range

– Intrinsic to the design

• Element & array electromagnetic design

• Front end functionality• Analogue implementation

– Unlikely to change

• Interfaces– Could bypass a level of

interconnect

Possible Variables• Station size & configuration• Core layout (getting bigger)

– Move element positions.....?– Trenching etc.

• Processing technology level– Essential for economical

deployment

• Upgraded calibration methods– Needs flexible processing system


..

Sparse AA

Dense AA

..

Central Processing Facility - CPF

User interfacevia Internet

...

To 250 AA Stations

DSP

...

DSP

To 1200 Dishes

...15m Dishes

16 Tb/s

10 Gb/s

Data

Time

Control

70-450 MHzWide FoV

0.4-1.4 GHzWide FoV

1.2-10 GHzWB-Single Pixel feeds

Tile &Station

Processing

OpticalData links

... AA slice

... AA slice

... AA slice

...Dish &

AA+Dish Correlation

ProcessorBuffer

ProcessorBuffer

ProcessorBuffer

ProcessorBuffer

ProcessorBuffer

ProcessorBuffer

ProcessorBuffer

ProcessorBuffer

ProcessorBuffer

ProcessorBuffer

ProcessorBuffer

ProcessorBuffer

ProcessorBuffer

ProcessorBuffer

ProcessorBuffer

ProcessorBuffer

ProcessorBuffer

ProcessorBuffer

ProcessorBuffer

ProcessorBuffer

ProcessorBuffer

ProcessorBuffer

Data switch ......

Data Archive

ScienceProcessors

Tb/s Gb/s Gb/s

...

...

TimeStandard

Imaging P

rocessors

Control Processors & User interface

Pb/s

Correlator UV Processors Image formation Archive

Aperture Array Station

SKA2 System

A reminder for AA-mid!

A tremendous step forward in SKA2

There must be ongoing development:

which will also benefit AA-low


Top design decisions......

Urgent (by CoDR April 2011)

• Confirm specification in Memo 125:– Frequency range, sensitivity ....

Immediate (by CDR, end 2011):

• Single or dual element? • Demonstrable element in small array

Medium Term (by AAVS1, end 2013)

• Station size and configuration• Core configuration• Station system design

– digitisation, processing location, data flow

SKA 1 AA-low system design considerations

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