low-frequency aperture array (lfaa)

Technical Description

Low Frequency Aperture Array

Technical Description Reference : AADC-TEL.LFAA.SE.MGT-AADC-PL-002

Issue - Revision : 1 - 0

Date : 2013-06-08

Author(s) : A.J. Faulkner

Checked : J.G. bij de Vaate

M. Gerbers

Approval : M. Gerbers

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Authorisation : J.G. bij de Vaate

Customer Approval : N.A.

Reference : AADC-TEL.LFAA.SE.MGT-AADC-PL-002 of 56 Issue – Revision : 1 - 0

Date : 2013-06-08 RESTRICTED

© 2013 AADC Consortium This document shall only be reproduced for the agreed purpose for which it has been supplied.


Record of Changes Issue Rev Date Change Affected Area Author

0 A 2013-05-06 Initial version Entire document A. J. Faulkner

0 B 2013-05-13 Updated version comments from Marchel Gerbers, Peter Hall and Jan Geralt Bij de Vaate

Entire Document A. J. Faulkner

0 C 2013-05-17 Updated version after discussion on document review and input from other people


0 D 2013-06-02

Updated version after discussion on document review and input following Consortium meeting


0 E 2013-06-04 Updates from Peter Hall Entire Document A. J. Faulkner

0 F 2013-06-04 Station processing mod Section 5.1.2.1 A. J. Faulkner

0 G 2013-06-08 Template and distribution list Distribution list M. Gerbers

1 0 2013-06-09 Formal release Entire document A. J. Faulkner





Table of Contents

1 Introduction ................................................................................................................................................... 7

1.1 Scope .................................................................................................................................................... 7 1.2 Document Structure .............................................................................................................................. 7

2 Applicable and Reference Documents............................................................................................................. 8

2.1 Applicable Documents ........................................................................................................................... 8 2.2 Reference Documents ........................................................................................................................... 8

3 LFAA description ............................................................................................................................................ 9

3.1 LFAA Concept .......................................................................................................................................10

4 LFAA science considerations ..........................................................................................................................12

4.1 EoR Science ..........................................................................................................................................13 4.2 High Band science ................................................................................................................................14

4.2.1 Pulsars .............................................................................................................................................15 4.2.2 High band imaging ...........................................................................................................................15 4.2.3 Observing flexibility..........................................................................................................................16

4.3 Section references ...............................................................................................................................16

5 Detailed technical description .......................................................................................................................17

5.1 Implementation options .......................................................................................................................19 5.1.1 LFAA design choices already made ...................................................................................................19 5.1.2 Ongoing design options for Stage 1 decisions ...................................................................................20 5.1.3 Existing experience ..........................................................................................................................21 5.1.4 LFAA design presented in this document ..........................................................................................23

5.2 Antenna and LNA .................................................................................................................................23 5.3 Power for the LNA and antenna receiver ..............................................................................................25 5.4 Receiver ...............................................................................................................................................26

5.4.1 Antenna front-end ...........................................................................................................................26 5.4.2 Signal Transport ...............................................................................................................................26 5.4.3 Analogue processing in the bunker ...................................................................................................27

5.5 Digital Signal Processing .......................................................................................................................27 5.5.1 Tile processing .................................................................................................................................27 5.5.2 Station beamforming .......................................................................................................................29

5.6 Implementation of bunker receiver and tile processor ..........................................................................29 5.6.1 Tile Processing .................................................................................................................................31 5.6.2 Tile output data rate ........................................................................................................................32

5.7 Station processing ................................................................................................................................32 5.8 Processing Rack - construction .............................................................................................................33 5.9 Monitoring and Control ........................................................................................................................35 5.10 Bunker system and SKA implementation ..............................................................................................36

5.10.1 Integration with LFAA correlator and post processing...................................................................36 5.11 Transient buffering ...............................................................................................................................37

5.11.1 Element level data storage ...........................................................................................................37 5.11.2 Tile level data storage ..................................................................................................................37 5.11.3 Station beam storage ...................................................................................................................37





5.11.4 Dedicated storage nodes .............................................................................................................38

6 Risk mitigation ..............................................................................................................................................38

7 Cost estimation .............................................................................................................................................39

7.1 Relative costing between Baseline and Proposed Designs .....................................................................42 7.2 Cost discussion .....................................................................................................................................42 7.3 Consortia interfaces and cost allocations ..............................................................................................43

7.3.1 Buildings ..........................................................................................................................................43 7.3.2 Power distribution ...........................................................................................................................43 7.3.3 Clock distribution .............................................................................................................................44 7.3.4 Telescope manager and TM data transport ......................................................................................44 7.3.5 Data transport to the correlator .......................................................................................................44 7.3.6 Data for correlation..........................................................................................................................44

8 Scaling to SKA Phase 2 ...................................................................................................................................45

8.1 Bunker design issues ............................................................................................................................47 8.1.1 Receiver and tile processor cards .....................................................................................................47 8.1.2 Tile signal Processing........................................................................................................................48 8.1.3 Station beamforming and system data rates.....................................................................................48

8.2 Configuration .......................................................................................................................................49 8.3 Extending the frequency range for SKA2 ...............................................................................................50

Appendix A. Possible station configuration for SKA1 Core ................................................................................52

Appendix B. Tile processing and communication requirements ........................................................................52

Appendix C. Bunker power requirement estimation ........................................................................................53

List of tables Table 1: Outline specifications for SKA-low for the Baseline Design and the Proposed Design ................................18 Table 2: Outline comparison of technologies that could be used for LFAA subsystems, for investigation and down-

select during Stage 1 Preconstruction. The example approach adopted in this proposal is shown shaded.......22 Table 3: Costing for Baseline and Proposed Design for LFAA (part 1) ......................................................................39 Table 4: Costing for Baseline and Proposed design for LFAA (part 2 .......................................................................41 Table 5: Outline predicted specification of LFAA, as a simple extension of Phase 1, in SKA2 compared to proposed

SKA1 .............................................................................................................................................................46 Table 6: Approximate tile processing requirements ...............................................................................................53 Table 7: Estimated bunker power requirements ....................................................................................................54

List of figures Figure 1: Computer-generated visualisation of SKA-low core showing the log periodic antenna prototype. Credit:

Swinburne Astronomy Productions/ICRAR/U. Cambridge/ASTRON ................................................................10 Figure 2 SKA-low layout showing a single processing bunker fed with signals from every individual antenna .........11 Figure 3: The sensitivity of SKA-low over frequency. This is as expected for the Baseline Design, limited to 350MHz

and the sensitivity at higher frequencies is also shown for the Proposed Design, limited to ~650MHz. Note that there is still good sensitivity even to a scan angle of 60°. ........................................................................14





Figure 4: Work structure and signal flow for SKA-low ............................................................................................17 Figure 5: SKA-low test array on the SKA site ..........................................................................................................24 Figure 6: SKA-low processing for the Baseline design .............................................................................................28 Figure 7: Processing for the SKA-low Proposed Design ...........................................................................................29 Figure 8: Outline layout of Receiver and Tile Processor. The industry standard SPF module houses the analogue

components for pairs of channels ..................................................................................................................30 Figure 9: Station beamforming accumulation approach (See Figure 11 for illustration of Aisle switch) ...................33 Figure 10: Outline LFAA rack, handling a total of 1024 antennas, using 8 shelves or 8 tile processors linked via two

36-port data switches. The rack shown has the second level, aisle switch ......................................................34 Figure 11: Outline of overall interconnect. Essential features are wide bandwidth into the correlator and the

Monitoring and Control system .....................................................................................................................35 Figure 12: Station beams in a tile beam. Stepped beamforming for off-centre beams on the right. ........................48 Figure 13: Enhanced configuration architecture for SKA2 ......................................................................................50





Definition and Acronyms Acronym Explanation

AADC Aperture Array Design and Construction

AD-n nth document in the list of Applicable Documents

AIV Assembly Integration and Verification

CE Conformité Européenne

CIDL Configuration Item and Document List

DRB Delivery Review Board

EIDP End Item Data Package

ENG Engineering

HW Hardware

ICD Interface Control Document

LFAA Low Frequency Aperture Array

MOM Minutes of Meeting

PA Product Assurance

QA Quality Assurance

RD-n nth document in the list of Reference Documents

RFP Request for Proposal

SKA Square Kilometre Array (www.skatelescope.org)

SKA-low The low frequency system for the SKA. Includes, LFAA, associated correlator and post processing

SKAO Square Kilometre Array Office

SOW Statement of Work

SW Software

TBC To be continued

TBD To be determined

TBS To be supplied

TRB Test Review Board

WBS Work Breakdown Structure

WP Work Package




http://www.skatelescope.org/


1 Introduction

1.1 Scope

This is the Technical Description for the Low Frequency Aperture Array, a major part of SKA-low, for implementation as Phase 1 of the SKA. This covers the Baseline Design as described in the RfP documentation and a Proposed Design with significantly enhanced performance. It is in response to the Request for Proposal from the SKA Office dated 11 March 2012.

This description should be read in conjunction with the rest of the AADC consortium response to the LFAA Element design.

1.2 Document Structure

• Section 1 introduces the purpose and scope of this document. • Section 2 lists the applicable and reference documents. • Section 3 describes the SKA-low concept. • Section 4 presents the SKA-low science with particular reference to the Proposed Design. • Section 5 provides a detailed description of the Low frequency Aperture Array by sub-system. • Section 6 discusses the principal risks, mitigations and consequences. • Section 7 discusses LFAA costing. • Section 8 considers the requirements of scalability to SKA Phase 2.





2 Applicable and Reference Documents

2.1 Applicable Documents

Id Title Code Issue

AD-1 SKA Request for Proposals SKA-TEL.OFF.RFP-SKO-RFP-001 1

AD-2 Statement of Work for the Study, Prototyping and Design of an SKA Element

SKA-TEL.OFF.SOW-SKO-SOW-001 1

AD-3 Statement of Work for the Study, Prototyping and Preliminary Design of an SKA Advanced Instrumentation Programme Technology

SKA-TEL.OFF.AIP-SKO-SOW-001 1

AD-4 SKA Pre construction Top Level WBS SKA-TEL.OFF.WBS-SKO-WBS-001 1

AD-5 SKA-1 System Baseline design SKA-TEL-SKO-DD-001

AD-6 The Square Kilometre Array Design Reference Mission: SKA Phase 1

SCI-020.010.020-DRM-002 3

AD-7 SKA System Engineering Management Plan SKA.TEL.SE-SKAO-MP-001 1

AD-8 The Square Kilometre Array Intellectual Property Policy

SKA IP Policy 1.3 Draft

AD-9 Draft Consortium Agreement PD/SKA.26-4 Draft

AD-10 Document Requirements Description SKA-TEL.SE-SKO-DRD-001 1

AD-11 SKA Document Management Plan SKA-TEL.OFF.MGT-SKO-MP-001 1

AD-12 SKA Product Assurance and Safety Plan SKA-OFF.PAQA-SKO-QP-001 1

AD-13 Change Management Procedure SKA-TEL.SE.CONF-SKO-PR-001 1

AD-14 SKA Interface Management Plan SKA-TEL.SE.INTERF-SKO-MP-001 1

2.2 Reference Documents

Id Title Code Issue

RD-1 LFAA Delivered Items and Document List AADC-TEL.LFAA.SE.MGT-AADC-PL-002 1.0

RD-2 LFAA Management Plan AADC-TEL.LFAA.SE.MGT-AADC-PL-0021 1.0

RD-3

RD-4





3 LFAA description The low frequency aperture array, LFAA, covers the lowest frequency band for the SKA, from 50MHz up to 350MHz in the Baseline Design or optionally up to >650MHz at a small additional cost of ~2%. It is an aperture array consisting of 262,144 wide bandwidth antennas of a single design. The configuration is very close packed with 75% of the antennas within a 2km diameter core and the remaining collecting area situated on three spiral arms, extending out to a radius of 50km and enabling higher spatial resolution observations.

The overall system is organised as logical stations constituting correlatable entities. The Baseline Design specifies 911 stations of 35m diameter; our "Proposed Design" offers selectable station sizes, as appropriate to the science with a commensurately varying number of stations.





Figure 1: Computer-generated visualisation of SKA-low core showing the log periodic antenna prototype.

Credit: Swinburne Astronomy Productions/ICRAR/U. Cambridge/ASTRON

The principal science specified for SKA-low is the Epoch of Re-ionisation experiment and the Baseline Design has been optimised for the requirements of this experiment. However, there is considerable additional science to be addressed with SKA-low; an overview is given in Section 4. To include this science and provide improved performance for the EoR experiment, the AADC consortium proposes an extended design that could reduce the overall cost of SKA Phase 1 (aperture arrays plus dishes).

3.1 LFAA Concept

While alternative implementations of the LFAA can be designed, the rest of this description focuses on a representative implementation to justify our assessment of the viability and costs of the LFAA. Alternative sub-system implementations are discussed in Section 5.1, some of which will be investigated during Stage 1 before the final design is tested in AAVS1.





The LFAA will be built using log-periodic antennas, which work well over a very large frequency range, providing a good impedance match to the low noise amplifier. An artist’s impression of part of the core of the LFAA is shown in Figure 1. Here, the antenna is a log-periodic optimised for the close antenna spacing required, dual polarisation performance and low cost. Each antenna houses analogue electronics: low noise amplifier, gain with filtering and communications driver. The signal from each polarisation of each element is transported to a signal processing site ("bunker") where it is digitised and processed into beams by combining with other antennas constituting a logical station. Each antenna requires a small amount of power that can be supplied by local solar power for individual antennas or groups of antennas, or by a power network using copper wires. This is further discussed in Section 5.3.

Our representative, practical implementation for the LFAA uses these antennas with a combination of solar power and RFoF. Figure 2 shows an overall LFAA layout which highlights that the system only requires a single processing facility for all the stations, making a conceptually simple arrangement that is highly flexible, upgradeable and easily maintained. An important consideration for SKA implementation is the opportunity to locate the SKA-low correlator in the same processing bunker as the LFAA beamforming hardware. The architecture shown in Figure 2 is a low-cost evolution of the Baseline Design, as it redefines the digital signal transport system as digital links across a single bunker, avoiding the need to use long distance links from individual stations.

Figure 2 SKA-low layout showing a single processing bunker fed with signals from every individual antenna

Local solar power (including cost-effective energy storage), maybe even as one power source per antenna, provides considerable benefit for the system, not least of which is potentially lower whole-of-life operating costs, since there will be no ongoing requirement to externally supply power to the antennas, which require about 1 watt each for electronics and optical drive (250kW total). The local solar power supply can have very low RFI and an integrated design, which can be assembled prior to deployment, would give a very consistent EM performance for the antenna. The combination of RFoF





and local solar power means that there is no galvanic connection throughout the array, potentially important across an all-electronic, highly distributed telescope. In addition, the infrastructure installation costs may be considerably reduced, with only optical fibres to install and maintain.

Within the processing bunker the analogue signals are digitised, channelised in frequency and calibrated into “tiles” of 16 antennas; tile beams are formed using the available bandwidth. The tile beams are combined into “station” beams of the selected station size and finally passed for correlation. With all the antenna signals coming to a single bunker it is straightforward to form stations of programmable size by combining beams from the required number of unit tiles. The stations may overlap for better beam performance or even support more than one station size concurrently.

4 LFAA science considerations The Baseline Design for the LFAA recognized the importance of Epoch of Reionization (EoR) science for SKA1 and proposed a design tailored to this application. The AADC Consortium proposal likewise recognizes the need to optimise EoR science and offers an extended design, the Proposed Design, identical to the Baseline Design in terms of sensitivity and angular resolution, with enhanced EoR performance and significantly wider scientific application. In particular, the AADC Consortium design gives:

a) greater flexibility in EoR observing techniques and calibration methods, leading to an enhanced likelihood of a robust EoR detection and a consequentially a large SKA1 scientific impact;

b) greater scientific utility of the SKA-low via:

i. enhanced frequency coverage,

ii. simultaneous, parallel observing programs enabled by collecting area re-use (multiple station beams giving widely-separated fields-of-view), and

iii. better matching of the instrument performance to specific experiments through selectable station size.

c) considerably enhanced SKA1 pulsar observation capability for both search and timing of a large number of objects, to reveal the detailed physics of matter in extreme conditions;

d) access to other leading-edge time-domain science, such as cosmic transients, via highly flexible signal aggregation and processing architectures, including transients buffers, opening the door to observations of the most energetic processes in the Universe; and

e) effective demonstration of new calibration and imaging techniques via SKA1 continuum and spectral line science, enabling much greater use of cost-effective sparse aperture array technology and representing an important step in demonstrating scalability for SKA Phase2.

While proposing the enhanced LFAA frequency coverage, we keep our focus for all science - EoR and other - below 350 MHz. Much of the extensive pathfinder science cases, including the comprehensive cases outlined for LOFAR [4-1] and MWA [4-2], are directly applicable to the LFAA and we note the exciting experiments being pursued by these telescopes in areas as diverse as very low-frequency pulsar studies, coronal mass ejections and space weather, advanced ionospheric modelling, and tracking of space junk.





4.1 EoR Science

The need for flexibility in EoR science arises from the currently incomplete state of pathfinding programs in instruments such as LOFAR, MWA and PAPER. Statistical estimates of the EoR rely on the reduction of noise to detect the faint (milli-kelvin) cosmological signal. Broadly, noise falls into three categories: radiometric noise due to sensitivity limitations; spatial sample noise due to sky-coverage limitations; and cosmic variance remaining after the whole sky has been observed. Most SKA pathfinder EoR instruments are thermal noise dominated on most spatial scales, and spatial sample variance is a secondary concern. For the LFAA, exceptional sensitivity will give low radiometric noise, and statistical measurements are expected to be dominated by spatial sample variance. Spatial sample variance is reduced by independent observations of different regions of the sky (incoherent averaging of power), while thermal noise is reduced most effectively by sampling identical sky (coherent averaging of visibilities). The balance between coherent and incoherent addition of information determines the balance of thermal and sample variance noise to the overall error budget; this balance is still being explored.

Small stations (such as in the Baseline Design) yield large angular FoV per beam, providing the sky area necessary for reducing spatial sample variance. Equivalently, they provide a compact beam in Fourier space, reducing the coherence of measurements made from differing baselines. This effect is countered by the increase in number of baselines from a large-N array (for constant total array collecting area). Conversely, larger stations sample a smaller sky area, with less ability to reduce spatial sample variance. Large station Fourier beams are less compact, yielding greater coherence from similar baselines, but fewer independent bins for incoherent addition. The information lost from averaging the sky signal over a larger station can be recovered effectively using two schemes:

a) drift scans to sample a larger fraction of the sky, reducing spatial coherence of measurements and improving sample variance reduction - this option used alone comes with the need for extra observing time per EoR field, the information coming from a single, small, station beam generated by the larger station; and

b) multiple station beams to observe simultaneously multiple fields across the same frequency range, recovering and exceeding the equivalent beam of the smaller station - this gives a substantial reduction of spatial sample variance from the addition of incoherent modes across all station beams.

In a recent analysis [4-3], which extended earlier work [4-4] examining observing strategy on EoR detection, the calibratability of the LFAA Baseline Design and designs using 50m and 100m diameter stations was examined. Smaller stations perform less favourably when radiometric noise is dominant but in the presence of noise dominated by background sources, all stations produce comparable measurement precision when one beam is formed. By extension, if larger stations with widely-separable, independent FoVs (many station beams) are used to give additional area sampling, calibration precision will not suffer but the EoR signal estimate will be improved via the reduction of spatial sample variance flowing from simultaneous independent measurements.

The LFAA can have a large processed bandwidth and duplicating bandwidth for an EoR experiment across multiple FoVs and station beams as in the AADC Consortium Proposed Design gives an instrument which is robust in the face of current uncertainty concerning thermal noise and spatial sample variance in practical experiments. The Proposed Design also offers almost the ultimate risk mitigation in that the





central LFAA collecting area can be beamformed into stations of selectable sizes. Assuming an acceptable intra-core antenna configuration can be found - highly likely given the density of antennas in the core - it will be possible to conduct experiments with a range of station sizes and FoV options.

As a related point, and since the Baseline Design document mentioned mosaicking, we note that combining multiple station beams (and more widely-separated FoVs) from an aperture array differs in principle from the classical, dish-based operation; the LFAA beams will be formed simultaneously via the same receptors and signal paths, potentially significantly improving the wide-field imaging capability for a range of SKA-low science, including EoR experiments.

4.2 High Band science

Figure 3: The sensitivity of SKA-low over frequency. This is as expected for the Baseline Design, limited to 350MHz and the sensitivity at higher frequencies is also shown for the Proposed Design, limited to ~650MHz. Note that there is still good sensitivity even to a scan angle of 60°.

While the array packing considerations in the AADC Consortium Proposed Design are the same as those in the Baseline Design (i.e., EoR driven), the Proposed Design offers a high-band (350-650 MHz) capability available for the substantial periods when EoR (or other low-band) observations are not feasible or not required. The LFAA will be sparse in the high-band but sensitivities across this band, as shown in Figure 3, are an improvement over the SKA-mid dish array at the bottom of the extended band and comparable at the top of the band. This, combined with multi-beaming capability, gives considerable additional SKA1 science potential. The principal driver for the high-band extension is time-domain science, including cosmic radio transients and pulsars.

In the field of transients trail-blazing discoveries [4-5]–[4-7] offer tantalizing prospects of a new view of the Universe and promise unparalled insight into hitherto hidden baryonic matter; new event-rate formalisms [4-8] link telescope architectures and resulting windows on the Universe; and recent





rigorous analyses [4-9] show that the LFAA is highly efficient as a transients detector, especially above 350 MHz. These factors, and the extremely low impulsive RFI of the LFAA site, are important motivations for the extended frequency range.

4.2.1 Pulsars

Pulsars are identified as an important part of the SKA1 science mission [AD-6] although the LFAA pulsar observing capabilities have not previously been considered in the DRM extensively. However, the extended frequency range and multi-beaming capabilities of the Proposed Design are particularly attractive for pulsar observations. Pulsars are particularly steep-spectrum sources (flux proportional to wavelength to power 1.8), but precision measurements at low-frequency are adversely affected by interstellar scattering (which scales as wavelength to the fourth power). Nonetheless, the LFAA’s large instantaneous sensitivity and FoV are very well suited to both pulsar surveys and the regular monitoring of the largest possible sample of known radio pulsars. Compared with SKA-mid pulsar surveys of the Galactic plane, where observing frequencies around 1.2GHz are optimal, the LFAA at ~500-600MHz is better matched for surveys at Galactic latitudes above five degrees, where the effects of scattering and sky background are significantly less severe and there is considerably more sky area to cover.

The LFAA can provide an efficient all-sky pulsar survey, including multiple passes to search for intermittent sources. Recent results [4-10]–[4-13] have shown that a significant fraction of the radio-emitting population of neutron stars are only sporadically observable – making a strong case for multi-pass surveys and large total on-sky time (i.e., longer dwell time can sometimes trump instantaneous sensitivity). Large total on-sky time is also critical for probing transient events with low event rates, similar to those of core collapse supernovae and gamma-ray bursts [4-14]–[4-16].

Regular monitoring of the largest possible sample of known radio pulsars is critical for studying their physics and for performing basic timing of SKA pulsar discoveries. With thousands of expected SKA pulsar discoveries, follow-up timing will be an observational challenge. Nonetheless, it is a challenge that has to be met if we want to reap the scientific harvest of these discoveries. High-cadence monitoring of known sources is very likely to reveal new surprises about the physics of pulsar magnetospheres. The last years have underlined the richness of the observable phenomena [4-17]–[4-19] and how understanding these is potentially critical for making the best use of pulsars as precision astronomical clocks. Lastly, high-cadence pulsar monitoring with SKA-Low will provide an unparalleled interstellar medium weather report, which can be used, for example, to map the Galactic magnetic field and to correct dispersion measure variations in the high-precision pulsar timing data collected with SKA-Mid at frequencies above 1 GHz.

4.2.2 High band imaging

Results in aperture array calibration and imaging emerging from LOFAR and MWA lead us to expect that the LFAA will be a formidable imaging telescope, even at its highest frequencies. The SKA Design Reference Mission and Baseline Design documents identify a number of science applications in addition to the EoR, including HI-line absorption against continuum sources. The AADC Consortium LFAA design will verify the capacity of the instrument to access a wide range of this science, illustrating unambiguously the imaging performance of sparse aperture arrays and potentially pointing the way to the use of these cost-effective receptors in a an SKA2 frequency domain hitherto assumed to require dishes or dense arrays.





4.2.3 Observing flexibility

As noted in the next Section, there are a vast number of trade-offs possible in assigning beams, bandwidths and number of stations within experiments. Data rates across the LFAA and total SKA-low processing system will be the subject of investigation during the design process but, from the astronomer's viewpoint, even simple architectures yield great flexibility. For example, the same LFAA architecture supporting a 911 station, single FoV, EoR experiment with 30 MHz of bandwidth could comfortably support a 200 station, 32 FoV version of the same experiment. Similarly, for transient experiments, the LFAA processing can handle the entire high band, with several FoVs. The LFAA processing is of course only part of the story, with final spectral and time resolutions set by the SKA1-low central signal processing, and software and data processing capability. Nevertheless, the flexibility of the LFAA architecture augurs well for the science potential of SKA1.

4.3 Section references

[4-1] van Haarlem, M. et al., "LOFAR: the LOw Frequency ARray", Astron. Astrophys (submitted), http://arxiv.org/abs/1305.3550v2, May 2013.

[4-2] Tingay, S. et al., "The Murchison Widefield Array: the Square Kilometre Array precursor at low radio frequencies, Publications of the Astron. Soc. Australia, vol. 30, e007, Jan. 2013.

[4-3] Trott, C. "Impact of array design on calibration of SKA-low", Astrophysical Journal (in preparation), 2013.

[4-4] Wijnholds S. J. and van der Veen, A., "Multisource self-calibration for sensor arrays", IEEE Transactions on Signal Processing, vol. 57, issue 9, pp. 3512-3522, 2009

[4-5] D. R. Lorimer et al., “A bright millisecond radio burst of extragalactic origin,” Science, vol. 318, no. 5851, pp. 777–780, 2007.

[4-6] E. F. Keane et al., “On the origin of a highly dispersed coherent radio burst,” Monthly Notices of the RAS, vol. 425, pp. L71–L75, Sept. 2012.

[4-7] Thornton, D. et al., “Discovery of a fast radio burst population at cosmological distances”, submitted for publication

[4-8] J.-P. Macquart, “Detection rates for surveys for fast transients with next generation radio arrays,” Astrophysical Journal, vol. 734, p. 20, June 2011.

[4-9] T. M. Colegate and N. Clarke, “Searching for Fast Radio Transients with SKA Phase 1,” Publications of the Astron. Soc. of Australia, vol. 28, pp. 299–316, Nov. 2011.

[4-10] M. A. McLaughlin et al., “Transient radio bursts from rotating neutron stars,” Nature, vol. 439, pp. 817–820, 2006.

[4-11] M. Kramer, A. G. Lyne, J. T. O’Brien, C. A. Jordan, and D. R. Lorimer, “A Periodically Active Pulsar Giving Insight into Magnetospheric Physics,” Science, vol. 312, pp. 549–551, Apr. 2006.

[4-12] J. S. Deneva et al., “Arecibo pulsar survey using ALFA: Probing radio pulsar intermittency and transents,” Astrophysical Journal, vol. 703, pp. 2259– 2274, Oct. 2009.

[4-13] E. F. Keane et al., “Rotating Radio Transients: new discoveries, timing solutions and musings,” Monthly Notices of the RAS, vol. 415, pp. 3065–3080, Aug. 2011. Reference : AADC-TEL.LFAA.SE.MGT-AADC-PL-002

of 56 Issue – Revision : 1 - 0 Date : 2013-06-08

RESTRICTED © 2013 AADC Consortium

This document shall only be reproduced for the agreed purpose for which it has been supplied.


[4-14] J. van Leeuwen and B. W. Stappers, “Finding pulsars with LOFAR,” Astronomy and Astrophysics, vol. 509, Jan. 2010.

[4-15] A. P. V. Siemion et al., “The Allen Telescope Array fly’s eye survey for fast radio transients,” Astrophysical Journal, vol. 744, p. 109, Jan. 2012.

[4-16] P. J. Hall et al., “Towards SKA studies of the radio transient universe,” in Resolving the Sky - Radio Interferometry: Past, Present and Future PoS(RTS2012)042, (Manchester, UK), Apr. 2012.

[4-17] M. Kramer, A. G. Lyne, J. T. O’Brien, C. A. Jordan, and D. R. Lorimer, “A Periodically Active Pulsar Giving Insight into Magnetospheric Physics,” Science, vol. 312, pp. 549–551, Apr. 2006.

[4-18] A. Lyne, G. Hobbs, M. Kramer, I. Stairs, and B. Stappers, “Switched Magnetospheric Regulation of Pulsar Spin-Down,” Science, vol. 329, pp. 408, July 2010.

[4-19] W. Hermsen et al., “Synchronous X-ray and Radio Mode Switches: A Rapid Global Transformation of the Pulsar Magnetosphere,” Science, vol. 339, pp. 436, Jan. 2013.

5 Detailed technical description The overall work flow of the AADC Consortium is shown in Figure 4; this is closely mapped to the required blocks of the SKA-low system and illustrates the signal flow. The sections below describe the sub-systems in detail.

Figure 4: Work structure and signal flow for SKA-low

To set the basis of the description, the specifications for both the Baseline Design and the Proposed Design are summarized in more detail in Table 1. The main difference between the Baseline Design and the Proposed Design is a significantly higher top frequency, two bands of operation, capability of higher station data rates (for multiple beams) and the ability to have programmable station sizes.





Table 1: Outline specifications for SKA-low for the Baseline Design and the Proposed Design

Parameter Baseline Design Proposed Design Comments

Number of antenna

262,144 262,144 250,000 antennas is for the core only. 911 stations of 289 antenna = 263,279. Rounded to 262,144 (218)

Types of antenna

1 1 The full frequency range will be covered by a single element type, as with the Baseline Design. This is for lowest cost.

Frequency – low

50MHz 50MHz Push the low frequency as low as reasonable without losing performance at 50MHz for future science capability.

Frequency – high

350 MHz 650 MHz The increased frequency limit is accommodated by the element design. The LFAA operates well at these frequencies and avoids the dish system having a large, expensive LF feed and a very large sub-reflector, while maintaining contiguous frequency coverage.

Element separation

1.35m (λ/2 at 111MHz)

1.35m (λ/2 at 111MHz)

The spacing of 1.35m is taken from the Baseline Design. This may not be optimal for the science and needs checking early in Stage 1.

Station diameter

35m 20-100m The “station” diameter, particularly in core, can be varied to suit the experiment. This is managed by the station processing. The variable station diameter should be able to mitigate central processing requirements for many of the experiments.

Polarisations 2 – linear 2 – linear Essential to have a dual polarisation system

Number of bands

1 2 The full available bandwidth is divided into 2, which handle mutually exclusive science cases. This is similar to switching feeds on a dish.

Max instantaneous Bandwidth

250-300MHz 335MHz & 300MHz

Because the full frequency range is covered in two bands, the output bandwidth is essentially the same as the Baseline Design but is reused through using a switched-in first alias on the digitisation.

Data rate into Correlator/ Beamformer

10Gb/s per 35m Station

10Tb/s total

≥10Tb/s total The performance of the SKA-low depends on the total data rate to the central processing. This can be increased as required.

Data flexibility

1 beam/station Flexibly assigned to beams within

a band

A great benefit of an AA is that data output can be assigned to arbitrary beamlets in arbitrary directions up to the total designed data rate. Then each experiment can be optimised, or concurrent experiments run.





Sample resolution

8-bit 4 or 8-bit capability

Many experiments will operate effectively with 4-bit data, hence doubling the total bandwidth for the same data rate. This needs to be agreed with the correlator and post processing groups.

5.1 Implementation options

The technologies used to implement the LFAA are subject to choice: generally with pros and cons for each approach. In effect the Baseline Design document has already made some decisions, which are supported by the AADC e.g. single antenna type, configuration of the array etc. Early down-select of implementation options as soon as a decision becomes clear is an important mechanism to both focus development on the final solution and save design resources.

5.1.1 LFAA design choices already made

The AADC has also made further design decisions as part of this proposal being put forward. In summary the following decisions have been made:

5.1.1.1 Single element: log-periodic antenna

The LFAA for Phase 1 of the SKA will only use a single element type to cover the full frequency range. The element type chosen is a log-periodic antenna that was suggested in the Baseline Design document. There will be one element since:

i. the log-periodic will readily cover the specified frequency range, and

ii. a large proportion of the cost of the LFAA is in the antenna, LNA and associated infrastructure making incremental cost for using two antenna types substantial.

Consequently, a single antenna type is a lower cost solution.

5.1.1.2 All beamforming in the digital domain

In principle, beamforming can be performed using analogue or digital electronics or a combination of both. The choice has been made to entirely use digital beamforming. The reasons are:

i. The beam performance requirements of the LFAA will need the precision associated with digital beamforming;

ii. Analogue beamforming for such large antenna spacing as the LFAA will require large and expensive switchable analogue delay lines. Digital beamforming in the timescale of SKA Phase 1 is likely to be more cost effective; and

iii. Digital beamforming is much more flexible in the forming of beams on arbitrary positions on the sky of different bandwidths, leading to a better scientific return.

5.1.1.3 Digitisation in the bunker

The location for digitisation of the input signals can either be near or at the antenna or in the bunker. Signal transport from the antenna to the bunker is then either digital or analogue. While digitisation at the antenna is ostensibly a good solution, there are some significant downsides:





i. Digitisation near the antenna requires a clock signal that is phase stable enough to not impact the beamforming precision. This implies transporting a clock signal to every antenna, which is difficult, expensive and may cause RFI;

ii. Digital electronics generate RFI that needs to be shielded. This is a risk, as this approach would involve many RFI tight enclosures throughout the LFAA. This is both expensive and potentially creates considerable self-generated interference;

iii. Digitisers draw significant power, which adds to the power distribution issues at the antenna; and

iv. Due to the wide distribution of components, it is very difficult to maintain the digitisation systems and upgrade to higher-performance components e.g. for wider bandwidth or higher resolution.

Since long-range analogue signal transport is available, and indeed is planned for ASKAP, these factors can all be mitigated by keeping the electronics near the antenna as simple as possible and transporting a wideband analogue signal to a bunker for digitisation and processing.

5.1.2 Ongoing design options for Stage 1 decisions

The detailed design of the LFAA has further alternatives for the different subsystems, which are not so clear-cut. These need to be considered and reviewed during Stage 1 of Preconstruction. It is likely that some of these can be down-selected early in the development. By SRR/PDR there is expected to be a single design for the LFAA, which will be further developed during Stage 2 and physically tested wherever possible on AAVS1. The list of significant ongoing technical options are listed in Table 2, it can be seen that there are pros and cons for each decision and some are obviously linked e.g. a single bunker is only possible with low-cost, long range analogue communications.

Further comments on options from Table 2:

5.1.2.1 Station processing

The Baseline Design requires relatively low data rates per station, which while specified as one beam, can be split up into multiple beams of narrower bandwidth. The implementation in this representative design uses a “daisy chain” approach whereby the tile beams are progressively accumulated by the tiles themselves, using data steering through switch network. This is described in more detail in section 5.7. This is elegant, cheap and uses conventional COTS components for the data steering and unless it does not operate efficiently or if there is a lower cost, easily realisable solution then this will probably be the chosen approach.

Alternative station processor implementations will be considered in stage 1 to find if there are lower cost solutions that meet the agreed specification. For example, if much larger station data rates are required which significantly exceed the capacity of a reasonable switch network then the “daisy-chain” approach may not be efficient. The principle alternative is to provide a second level of hierarchical beamforming in a station processor. This takes the output of all the tiles for a station and accumulates them to form station beams, many from each tile beam. If the tile processor data output rate is to be less than the station data output rate then hierarchical beamforming needs to be used as described in the upgrades for SKA2 in 8.1.3.





5.1.3 Existing experience

The knowledge that is being gained from a large AA installation in Europe, LOFAR, and the smaller instrument based on the SKA site, MWA, is invaluable. Also, ASKAP will illustrate the use of RFoF. Several AADC members are key proponents or leaders of these programmes and the pathfinder instruments are allowing detailed evaluations of technology choices, science applications and a plethora of real-world considerations; lessons learned will be applied to the AADC developments.





Table 2: Outline comparison of technologies that could be used for LFAA subsystems, for investigation and down-select during Stage 1 Preconstruction. The example approach adopted in this proposal is shown shaded.

Sub-system Approach Pros Cons

Analogue signal transport

Coaxial copper links Well proven on multiple RA systems. Can also be used to transport power

Short range – 100’s metres. Frequency dependent transmission. Material is expensive and attractive to thieves. Requires multiple processing bunkers.

Analogue RF over Fibre, RFoF

In use on existing phased arrays. Long range – 10’s km. Individual optical strands are cheap. Wide, flat frequency response.

Requires low cost, low power laser. Antenna power must be supplied separately.

Antenna signal transmission

Each polarisation on a separate channel

Guaranteed signal separation. Simple integration. No risk of RFI from a local oscillator, LO.

Requires two cables/fibres, per antenna. Maybe higher cost. Requires two analogue channels & ADCs in the bunker.

Multiplex both pols into one channel electronically (see Section 5.4.2)

Halves the number of analogue links. Can use a single, fast ADC per antenna at reduced system cost.

Risk of crosstalk between polarisations. Requires LO at antenna, so may get RFI issues. DSP needs to correct upper channel frequencies.

Optically multiplex both channels onto one fibre

Halves the number of analogue links. Guaranteed separation. No risk of RFI from LO.

More expensive lasers and combiners. Requires two analogue channels & ADCs in the bunker.

Antenna Power

Line power delivered via wire network

Low risk. Easily implemented with coaxial links.

Risk of RFI, ESD and lightning issues with expensive protection systems. Substantial additional infrastructure. Operational cost for antenna power.

Local Solar power to single or multiple antennas

Reduced global RFI, ESD and lightning issues. Simplified infrastructure. Potential operational cost savings Assemble w/antenna before deployment Green power usage.

Unit cost (offset by infrastructure/ antenna savings) Potential technical issues e.g. battery life, local power supply RFI. Efficiency degradation with time e.g. solar cell lifetime, debris accumulation.

Processing Facilities

Single bunker Lower cost. Flexibility in LFAA configuration. Ease of maintenance. Ease of system upgrade. Could accommodate correlator as well.

Requires long range, low cost analogue signal transport. Potential single-point of failure.





Sub-system Approach Pros Cons

Multiple bunkers Less cost in analogue signal transport. Can use copper analogue signal transport. Fewer LFAA single-points of failure.

Higher total cost for bunkers Higher risk of RFI. More power, clock and communication networks required. External digital backhaul required.

Tile processing

ASIC based Very low power for processing. No maintenance. Low part costs.

High development costs. Long lead time. Inflexible and hard to upgrade. Requires other technologies to perform complex tasks.

FPGA based Low power. Programmable, can be upgraded. Low NRE.

Expensive piece parts. Hard to programme.

Processor based Easily programmable. Very flexible. Low development costs and NRE. Ability to follow COTS developments.

Higher power. Possible insufficient communications bandwidth.

Station processing

“Daisy chain” tile processing using switches for data steering

All complex development on tile processor board. Use COTS for data steering. Very flexible. Low cost.

Station data output rate limited to tile processor output data rate. Cannot use hierarchical beamforming.

Dedicated station processor

Can supports high station output data rates. Less processing on tile processors. Can use hierarchical beamforming.

Higher cost to develop and deploy. Requires specific hardware development. Less flexibility on data routing for beam formation e.g. overlapping stations.

5.1.4 LFAA design presented in this document

In this document a Representative Implementation is described and costed as it is not practical to cost all variants at this point; much more detailed costing work will be undertaken in Stage 1 of Pre-construction. While the shaded approaches in Table 2 presents a reasonable set of choices, for final implementation, the architecture and options for LFAA will developed during Stage 1, with a discussion on the selection made for Stage 2 to be presented at SRR/PDR.

5.2 Antenna and LNA

The antenna sub-system consists of the antenna, analogue gain and filtering components feeding the communications link; all, ideally, will be solar powered.

The current Consortium prototype antenna is a log-periodic element, shown in Figure 5, as part of a test array at the MRO site. This antenna design provides a very wide frequency range, determined only by Reference : AADC-TEL.LFAA.SE.MGT-AADC-PL-002





the range of sizes of the dipole arms, with a good match to the LNA. The antenna is constructed using bent wire, which has been shown to give excellent performance in our testing to date. The manufacturing technique leads to a minimal use of material and is inherently low cost. Prototypes are easily manufactured on CNC wirebenders, with a clear route to SKA volume manufacturing. Although the choice of materials will be finalised in the design phase, stainless steel has survived very well in the Australian site conditions and represents a low risk, but at slightly higher cost than mild steel. A groundplane may not be required as it supports only the lowest frequency of operation, where the system in any case operates in a very high sky noise regime. At all the other frequencies the log-periodic provides its own “groundplane” via the lower frequency dipole arms. However, an integrated groundplane can be considered to give system predictability in different environmental conditions.

Figure 5: SKA-low test array on the SKA site

The electronics are housed at the apex of the antenna and will contain the LNA, analogue components, RFoF driver and connector. This is protected from the weather conditions with a cover and the printed circuit boards themselves are protected either with conformal coatings or potted, to be confirmed during Pre-Construction Stage 1 development.

The combination of antenna element and LNA define the noise performance of the system. Many tests on current prototype of all types have been conducted; the results verify that design simulations are





accurate and that the noise performance over the entire band of the antenna is very good. We are therefore confident in developing mechanical improvements to the antenna design and slightly extending its frequency range for the Proposed Design.

More quantitatively, the prototype antenna element has been shown to work well to 600MHz, and a single extra dipole arm element and redistribution of dipole arm sizes will extend operation to >650MHz. There are no additional signal processing costs since the system would use a switched analogue filters in the bunker to select baseband or the digitisation first Nyquist band. There is further discussion in Section 5.5.1.

5.3 Power for the LNA and antenna receiver

There are two alternatives for powering the antenna electronics: copper-based distribution of power from a central source or solar power. While copper wire is clearly able to be implemented, it suffers from a number of issues:

• The material and installation are expensive;

• It is susceptible to interference from its associated electronics, unless there is careful design of the antenna and bunker systems;

• The electromagnetic properties of the antenna can be affected; and

• In the event of a lightning strike, or an electro-static discharge, copper links can distribute damaging voltages across various systems.

Following an examination of projections for the cost of various power solutions in Australia, the AADC plan is to develop a solar power solution, which can be installed for each antenna or for small blocks of antennas. The putative solar solution will be compared for cost, performance and maintainability with a ”normal” copper power network across the LFAA system, allowing an informed decision to be made at SRR/PDR. Assuming that solar power proves viable and cost-effective then the ideal solution is to integrate the design and installation of the solar power unit with the antenna structure. This guarantees that performance conforms to the simulation and test results, and enables the power unit to be raised off the ground to avoid issues with flash flooding.

There are implementation issues to solve, including:

• Ensuring that the lifetime of the solar unit, particularly the battery to support operation overnight, meets with the maintenance cost and resources budgeted;

• Suppressing RFI from the solar unit to acceptable levels (e.g. use linear or low frequency regulation and charging for the battery); and

• Verification of efficient operation in the presence of long-term dust or other material.

A major benefit is that the "fuel" component of the operational cost of powering the antennas is zero, so increasing the power used by the LNAs and amplifiers at the antenna only results, to first order, in more up-front capital expense for the solar power unit.





5.4 Receiver

The receiver development is defined, as shown in Figure 4, to cover the analogue signal from the LNA at the antenna and delivery to the ADC in the processing bunker. This consists of three principal components:

• Gain and filtering at the antenna; • Signal transport using RFoF components; and • Analogue gain, filtering and pre-whitening in the processing bunker pre-ADC.

The actual installation of the fibre cables will be specified by the LFAA Local Infrastructure task and ultimately undertaken as part of SKA infrastructure site works.

5.4.1 Antenna front-end

This development will be in close collaboration with the Antenna and LNA work, indeed the implementation is likely to be on single circuit boards one for each polarisation channel (as they are with the prototype antenna).

It is unlikely that the implementation will use custom analogue chips since the moderate volume and long development timeline would not justify the development cost for the LFAA. The only justification for customization may be for power reduction purposes, and then only on the basis of a trade-off between the cost of solar power versus that of an ASIC. The requirement is for sufficient gain at the antenna to ensure that the system temperature is not increased due to any losses in the signal transport, as well as preserving good dynamic range headroom. Filtering is required, particularly a low frequency cut off to remove large RFI (such as over the horizon short wave radio) signals below 50MHz.

There are no dedicated control signals for the antenna and associated electronics; this would require a communications link from the bunker to the antenna which would be expensive and is not considered necessary. The health and signal condition of the antenna sub-system can be gauged from the signal strength and spectral analysis of the data signal received at the tile processor.

5.4.2 Signal Transport

The planned signal transport is analogue RF over optical fibre, RFoF. This is widely used commercially, e.g. duplicating cellular phone base-stations using specialist lasers developed for analogue communications; and is also planned for ASKAP. These are low cost and can transmit the signal up to 10km with the lowest cost laser and >50km using a higher performance device.

In our LFAA design solution, most lasers are low cost and will be integrated with the remainder of the gain chain. For simplicity and lowest risk we have chosen to use separate lasers and fibres for each polarisation from every element, however; it is conceivable to electronically multiplex both polarisations onto one fibre by heterodyning one polarisation up to a band of frequencies higher than the top frequency used. This can be done with a reasonably stable, but free running local oscillator, LO, at each antenna. The two polarisations can be recovered digitally and errors in the LO corrected following the spectral filter.

Phase stability of a fibre solution, particularly for the longer baselines, may become an issue. This can be due to temperature and environmental effects. Instability may be severe enough, even at these low frequencies, for the calibration schemes not to be able to get within one RF cycle. The proposed solution





for the few stations that may be affected is to use the phase and time transfer system (being developed by the SADT consortium) on one fibre strand in a bundle of 200-500 fibres. This effectively “measures” the optical length of a fibre and enables calibration to be successful. The costs are not substantial on just the long baselines.

The fibre bundles will be armoured and specified for simply laying on the surface of the site or in conduit. It is known that trenching costs are very high on the MRO site. Each cable of many fibres needs to be split near the antennas such that two fibre strands are delivered to individual antennas. This is an important development and discussions with the fibre cable manufacturers/suppliers to pre-construct this system prior to deployment.

The optical receiver is a simple, cheap PIN diode integrated with the receiver analogue board in the processing bunker.

Since the communications are all specifically point-to-point, there is no requirement for a bunker patch panel system, with the fibre bundles being taken directly within the bunker to the racks housing the receiver systems.

5.4.3 Analogue processing in the bunker

The full bandwidth analogue signal is presented at the processing bunker, and then bandpass filtered, pre-whitened and fed to the ADC. The analogue receiver board and digital processing boards will be organised to process a complete “tile” of antennas – probably 16 x 2-polarisation channels.

For the Baseline Design only one 50-350MHz filter would be used (Figure 6). For the Proposed Design, two switchable filters will be available: 50-375MHz and 375-650MHz (Figure 7), using the baseband and first alias of the ADC to provide separate, non-commensal low and high band operation.

Any control signals for the analogue processing system will be supplied by the tile processor e.g. selection of band and any required gain control signals.

5.5 Digital Signal Processing

As can be seen in Figure 4, Figure 6 & Figure 7, the signal processing requirement consists of two distinct parts: a tile processing system and the station beamformer. The tile system is fed from the analogue receiver on a per-tile basis. This is described below:

5.5.1 Tile processing

The tile processing cards can handle 32 incoming channels, consisting of 16 dual polarisation antennas. The role of the tile processors is to:

• Digitise the analogue signal using 8-bit high speed digitisers, which are available and able to accommodate the likely and predicted RFI levels.

• Channelise the bandwidth into relatively narrow 1MHz channels for a phase shift controlled delay approximation.

• Calibrate the signal as a function of frequency to compensate for bandpass errors, gain and phase errors etc.

• Apply beamformer weights. • Aggregate all 16 antenna signals into a tile beam or beams.





These functions are described in the following subsections.

A potential practical implementation of the tile processor card plus the bunker receiver card is illustrated in Figure 8. The system is mounted in a 4U high rack mounted shelf with a simple midplane.

5.5.1.1 Digitisers

The analogue to digital converters (ADCs) operate up to ~800MS/s, and feed the processing system.

For the Baseline Design (Figure 6), each signal covers the full bandwidth from 50-350MHz.

In the Proposed Design (Figure 7), the input band is switched between 50-375MHz and 375-650MHz. The ADC converts either at baseband for the lower band or using the first alias for the higher band. The ADC analogue input must be able to effectively sample and hold to the highest signal frequencies. To ensure that there is no discontinuity in LFAA frequency coverage, while also ensuring that there is no aliasing from the other band, the ADC sampling frequency is also switched: 800MS/s for the low band and 700MS/s for the high band.

Figure 6: SKA-low processing for the Baseline design

5.5.1.2 Channelisation and calibration

The digitised signals are passed to a processing device, probably FPGA-based, which splits the bandwidth into relatively narrow channels of ~1MHz. This gives a complex sample stream for each channel that can be corrected for amplitude and phase errors. The beamformer delay and phase weights are applied to each channel for each antenna ready for beamforming.

5.5.1.3 Tile beamforming

The frequency channels required for the station beams are selected, creating the bandwidth required for the science experiment. Also, channels with gross RFI, for example, might be excluded. The selected Reference : AADC-TEL.LFAA.SE.MGT-AADC-PL-002





channels are all summed individually for every antenna and passed to the output data stream. Each tile will use a distributed beamformer approach and the output data rate from each tile is identical to the full station output data rate, assuming no overlapping stations. With a large number of stations each with a relatively low data rate in the Baseline Design, this is the best implementation, since it can provide the best possible quality for the beams.

Figure 7: Processing for the SKA-low Proposed Design

5.5.2 Station beamforming

The data output of the tile processors need to be accumulated for station beamforming. The function of the station beamformer is now very simple: it sums corresponding time & frequency samples from all the tiles, as the tile beamformer has provided all the alignment and gain adjustment required on the data streams within the packets.

This function represents the aggregation of 17 tiles in the Baseline Design, or a variable number of tiles for selectable station sizes as put forward in the Proposed Design (e.g., for 250 stations there would be ~64 tiles being summed).

5.6 Implementation of bunker receiver and tile processor

There is considerable work to be done on the detailed design of the receiver and tile processing. However, it is clear that the number of elements handled per tile processor directly affects the scale of the installation in the Bunker. On our initial analysis, it is reasonable to assume that each tile processor will handle 16 antennas i.e., 32 signal paths for both polarisations. This assessment is from road mapping the processing and communication ability of an FPGA likely to be available in the 2018 timeframe. The expectation is that the most capable FPGAs at that time will have the processing ability to handle all 32 channels on one chip. The actual implementation may use two devices for lower cost and additional I/O capability, which contrasts with the 16 devices, needed using the present UNIBOARD system with 2010-generation components. Reference : AADC-TEL.LFAA.SE.MGT-AADC-PL-002





Figure 8: Outline layout of Receiver and Tile Processor. The industry standard SPF module houses the analogue

components for pairs of channels

Notwithstanding this advance, the construction of the receiver and tile is expected to be similar to the proven UNIBOARD layout, the primary difference being to move the ADC components onto the processing board to ease the complexity of the high speed digital interconnect. An outline design is shown in Figure 8. Particular care will need to be taken to minimise digital noise on the analogue signal, hence all the analogue inputs are housed in an industry standard SPF form factor carrier which integrates the laser receiver with the gain and filter components on a small internal board. Normally SPF modules, picture inset in Figure 8, house Tx and Rx channels in this case there are two Rx channels installed. Note that the modules are on both sides of the receiver carrier board to increase packing density. The main features of the design are:

• The overall construction is a double sided rack with a central midplane board or connector system.

• All the analogue fibres connect to a “receiver board” which then: o Converts the optical signal to an electrical signal; o Provides analogue gain and any necessary equalisation to compensate for any

frequency dependence in the fibre link; o Provides Nyquist filtering and low end cut off prior to the ADC. The signal should be

conditioned appropriately for the ADC; and o Transmits the analogue signal to the processor board through a back-to-back connector.

• The central mid-plane provides common services and the necessary physical sockets to accommodate the receiver board and the tile processor. The principal features are:

o Physical mounting for the interconnect for the receiver and processor cards;





o Support straight through back-to-back connectors to carry 32 differential analogue signals and control signals for each receiver and tile processor pair;

o Power distribution with separate circuits for the analogue system and the digital processing;

o Any control signals required for the receiver board will be supplied by the tile processor, which is controlled through the Monitoring and Control network via the communications link;

o Clock distribution if this is carried as an electrical signal. This may be an optical signal connected directly to the tile processor;

o Tile processor identification. This is to provide positional identification such that every tile processor can identify its position within the system; and

o For the Proposed Design: a signal from the tile processor identifying the low or high band filters.

• The tile processor connects to the midplane, and performs the digitisation and beamforming functions for the tile. The main characteristics are:

o Plug into the midplane board to pick up the 32 analogue signal channels, power, position identification and, possibly, the synchronisation clock;

o The analogue signals are taken to an array of 32 ADC channels which are directly linked into the FPGA(s) for processing;

o On the Proposed Design, the tile processor receives configuration information over the monitoring and control network and can configure the system for low or high-band operation by switching the sample clock signal rate for the ADCs and provides the signal for its associated receiver board for selecting the appropriate Nyquist filters; and

o The beamformed signal is presented at a high-speed optical link on the outside edge of the card for connecting to the station processing switch.

5.6.1 Tile Processing

The signal processing on the tile is a beamforming function that has been well proven on multiple installations e.g. LOFAR and APERTIF.

The tile processor performs the tile beam formation and is part of the Station processing chain, see 5.7 below.

Essentially, a simplified list of the processing tasks on the tile is:

1. Take each signal channel and put it through a polyphase filter for spectral separation into approximately 1024 frequency blocks;

2. Apply amplitude and phase calibration coefficients to each active frequency block; 3. Perform any polarisation corrections to pairs of signals from each antenna; 4. Select the frequency band(s) required for the observation; 5. Apply beamformer weight(s) required to each frequency block; 6. Sum the blocks for each polarisation from all the antennas on the tile into tile beams; 7. Sum incoming partial station beams with the tile beams; and 8. Transmit the new, partial station beam via a switch to the next tile; or if completed, station

beams to the correlator.





5.6.2 Tile output data rate

The tile data output link will be 40Gb/s or greater Ethernet or 56Gb/s or greater Infiniband. This is significantly more than the minimum 10Gb/s data rate required for the Baseline Design. These standards are available today and are cost effective at the volumes required for LFAA; it may well be advantageous to use the next speed increment in the standards. Importantly it means that:

• The packets of data can be time stamped to ensure synchronisation; • Better usage can be made of the antennas in the core, by overlapping and each tile contributing

to multiple beams to provide station apodisation; • The management and control signals can use the same data network; • With SKA-low configured (using the Proposed Design) with fewer, larger stations for specific

experiments then there is capacity to maintain the same aggregate data rate into the correlator as the Baseline Design (~9Tb/s), thus maintaining the survey speed specification; and

• There is little risk of packet collision on the output data.

5.7 Station processing

The beamforming at the station level is performed as a distributed beamformer, whereby the time/phase delays required for the station beam are applied to each individual antenna by the tile processor and the rest of the processing system simply sums all of the antennas in a hierarchical fashion. The advantages of this system are that every station beam is precisely calculated and has minimum errors and artefacts; also, it is relatively simple to implement. The disadvantage is that the data rate throughout the system at each partial beamforming stage is the same, or greater, than the station output data rate. Hence, relatively fast switches are required for controlling the configuration, illustrated in Figure 9.

The station processing is performed cumulatively by the tile processors, or if it is available, a modified data switch, which can route input datastreams to a single summing point which then outputs a completed station beam.





Figure 9: Station beamforming accumulation approach (See Figure 11 for illustration of Aisle switch)

The data routing for the station is shown in Figure 11. Here it is assumed that the racks are organised as 16 aisles of 16 racks each. The station processing requirements are to enable flexible linking of adjacent tiles, so each half rack of tile processors is connected via 36-port switch, all the half rack switches are linked to an aisle switch, which enables routing throughout the aisle. The aisle switches are “daisy chained to link across to the nearest aisles. By organising the antennas by location almost any configuration of stations should be feasible.

5.8 Processing Rack - construction

An individual processing rack consists of “shelves” of receivers + associated tile processors back-to-back with a mid-plane, mounted in a 19” rack. Each shelf will be 4U high and can accommodate 8 tile processing systems. These are reasonably spaced (~50mm) to provide good airflow. If testing shows they can be closer packed, then there will be fewer racks in the bunker. Also, the rack can accommodate the necessary data switches for data steering. An illustration of a rack is shown in Figure 10. The makeup of the rack is:

• 8 x Rx + tile processor of 4U each 32U • 2 or 3 x data switch of 1U each 3U • Power supplies estimate 4U 4U • Space for additional cooling / spare 3U

The total of 42U is a standard, low cost 19 inch rack format.

Each shelf can handle 128 antennas. For 256 antennas per station (slight modification to the Baseline Design) then 2 shelves can handle the entire tile processing for an individual station. Of course, using a switch to link the tiles that form a station means that any reasonable configuration of station can be implemented. Each rack can handle 1,024 antennas.





There are always two 36-port data switches to link all the tile processors into stations. Some racks also accommodate a 3rd switch which acts as a second tier linking up to 32 switches together. Consequently there are 16 racks fitted with the 2nd tier switch.

The result is that for the Baseline Design of 262,144 antennas that there will be 256 racks.

Figure 10: Outline LFAA rack, handling a total of 1024 antennas, using 8 shelves or 8 tile processors linked via two

36-port data switches. The rack shown has the second level, aisle switch





Figure 11: Outline of overall interconnect. Essential features are wide bandwidth into the correlator and the

Monitoring and Control system

The precise topology of this system is the subject of study in Stage 1.

5.9 Monitoring and Control

As well as the main data path through the tile processors to the correlator, the whole of the LFAA needs to be controlled and monitored. This requires an overlay data network to all the controllable sub-systems. By using a 40Gb Ethernet or 56Gb Infiniband data network, there is spare capacity to use the same network as the data path system. This approach reduces cost and makes installation more straightforward. Referring to Figure 11 it can be seen that the monitoring, control and calibration data is routed to and from the tile processor via the rack switches, the aisle switches and concatenated in a final switch linking all the aisles.

The LFAA monitoring and control is centred on a number of dedicated servers that are part of the network, they provide:

• Configuration control to set up the requirements of the observation; • Calculation of all the relevant coefficients for the beamforming on all the tiles;





• Regular updates of the coefficients in order to steer the beams on a fixed point on the sky; • Reception of calibration information for the hardware in the arrays and distribution of

appropriate calibration coefficients; • Narrow band correlations of the elements to calibrate astronomically or with an artificial source; • Checking and reporting of the health of the system by determining which tiles/antennas are

faulty; reporting and accommodating the missing sub-systems in the beamforming system; and • Communication with the overall Telescope Management system.

5.10 Bunker system and SKA implementation

The whole of the processing for LFAA is planned to be in a single, large bunker. This provides all the power, cooling and RFI shielding for the system plus connection to the rest of the SKA interfaces: data output to the correlator, telescope manager and the synchronisation clock for the digitisers.

In this implementation, illustrated in Figure 11, there are 256 racks, dependant on the precise number of antennas used. These are expected to be organised as 16 racks to an aisle and 16 aisles. Each rack has its own power supply, from a distribution of power around the bunker. Cooling is likely to be provided by a closed-cycle water-cooled door although the power requirements per rack are relatively modest at around 7kW. Clock reference signals will originate from the SKA system clock and be distributed to individual racks; this is likely to be via an optical network, at least to the rack. Studies in Stage 1 will determine the most effective distribution system, which may be in collaboration with the SADT consortium.

The analogue optical fibres are located along the aisles, racks and tile processors as a point-to-point system from the antennas. The rack and aisle switches are distributed among the main processing racks.

An additional rack houses the monitor, control and calibration servers, plus the master clock reception and distribution systems.

The total required processing load is the same, regardless of whether it is done near the stations or brought into one very large bunker. However, centralised processing is cost effective as it offers:

• Ease of maintenance; • Lower cost for the facility; • Ease of interconnect; • Single power and cooling systems; • Simple clock distribution; • Streamlined integration with the LFAA correlator; and • The opportunity to house the master maser clock.

5.10.1 Integration with LFAA correlator and post processing

With all the processing for the LFAA in a single facility, close to the core of the SKA, it would seem a sensible, cost-effective extension to the concept to integrate the LFAA correlator in the same facility. This would restrict digital communication to within the building, removing the need for a wide area network for this part of the system.

A software correlator for SKA-low using even 2016-era devices can be housed in <30 racks, which is a small additional space over the processing for LFAA. This should be readily able to be accommodated.





Also, the central maser time standard for the SKA-low can be housed in this central building, making the distribution of the clock for all the LFAA stations very efficient.

There is more investigation needed around the scale and power requirements of the post processing system but the practicality of integrating the post processing system and correlator should be investigated since it avoids transmission of large quantities of data over a long distance and gives a major opportunity for integrating the whole processing chain and try new, novel processing approaches in readiness for SKA2.

5.11 Transient buffering

A potential feature that is only alluded to in the Baseline Design and is not a Baseline requirement is the ability to buffer the incoming signal data and use a trigger to freeze the buffer, and then look back at a previous astronomical event. This is useful, for example, in transient detection and verification. All of these buffers would be expected to be circular buffers, whereby only a fixed period prior to the present time is available. While this is not costed at this stage it is worth noting some implementation options to be explored during Stage 1 pre-construction.

5.11.1 Element level data storage

This is the most flexible, but also the most expensive implementation. This option implies considerable memory storage at each of the tile processors. The tile processor streams a selected set of element data to a high capacity memory system on the tile processor. The amount of data (or length of time the buffer can look back) can be reduced by selecting only relatively narrow bandwidths, reducing the digitisation depth or summing the polarisations.

The advantage of storage at this level is that beams can be reconstituted anywhere within the scan angle of the array, > ±45°.

The disadvantage is that it is expensive on memory and processing and may be memory bandwidth limited to store 16 elements worth of data.

5.11.2 Tile level data storage

Here the tile beam(s) are stored, as above, on the tile processor. This is a much reduced data set and can therefore be much cheaper to store.

This would enable a station beam to be formed within the tile beam by post processing, perhaps to more accurately identify the position of a transient.

This is cheaper in memory and could be used as an observation-time-selected alternative to the element level storage.

5.11.3 Station beam storage

Storing station beams is again much less expensive in memory terms than the foregoing approaches. This storage could be at the tile processor or at the input to the correlator.

The correlator storage approach is flexible, particularly for a software correlator where there is a normal CPU or multicore processor to process the data; a lot of storage could be made available on these machines. The reprocessing could vary between beamforming and correlation.





5.11.4 Dedicated storage nodes

Using data routing switches, which can duplicate data on multiple ports if required, data could be routed to dedicated transient buffer boards or processors for storage/analysis.

6 Risk mitigation This design of the LFAA makes a few predictions on the availability, performance and cost of some subsystems for the LFAA. There are risk mitigation strategies to cover the possibility of not meeting these predictions, albeit at some combination of reduced performance, increased cost or more difficult maintenance.

There follows the principal risks for the LFAA, these do not include obvious “higher component cost”, “longer to develop”, “insufficient resources” etc.:

Risk Likelihood/ impact

Mitigation Consequences

1. The analogue fibre transport including laser is not low cost enough/low performance/too much power

Low/ high

Use copper based analogue signal transport.

Due to the relatively short range of the copper links, the system architecture will need to be modified to use local, small bunkers near to the stations with the correlator in a central facility.

2. The analogue fibre range is not as far as predicted.

Medium/ low

Use some local digitisation/processing bunkers near the long baseline stations

The result is fairly minimal, provided that the bulk of the core and immediate stations can be accommodated. The design is not as flexible and will require some wide area digital communications.

3. The analogue receiver cannot be made as small as predicted

Low/ medium

Use larger receivers. The system will require more racks of equipment leading to a larger, more expensive facility

4. The processing or communications performance of the 2018 devices is not up to the requirement of the tile processors

Low/ medium

Use the available processing devices There will likely be increased cost and size of the system

5. The ADCs are more expensive/higher power than predicted

Medium/ low

Use the ADCs that are available The cost/power of the LFAA will be higher than predicted, so may need to be reduced in scale

6. Solar power does not give the power required/short life/battery technology inadequate

Medium/ medium

Use a network of copper cables powered from the central power supplies and upgrade lightning/RFI measures.

The use of copper cables is likely to result in a higher cost and susceptibility to lightening and RFI. Potentially higher RFI emissions

7. The data communication switches have lower performance/unpredicted delays

Low/ low

Use different switches or compensate in the processing

The switches are already considerably over-specified, so unlikely to suffer low performance. The consequences though would be a longer development time and maybe more resources for the tile processor

8. Power consumption too high Medium/ medium

Further development or more likely to accept the available components

Running costs are higher until SKA Phase 2 is commissioned.





7 Cost estimation Table 3: Costing for Baseline and Proposed Design for LFAA (part 1)





1 System & sub-system #/system #/SKA %age Commentsof

Baseline Proposed Baseline Proposed Baseline Proposed Total€ € € € €k €k

Totals: 135,312 137,819

Sub total SKA-low 135,312 137,819 100.0%SKA.TEL.LFAA.AL

Antenna & LNA 262,144Antenna arms 4 9.00 9.50 36 38Antenna other mechanical 1 20.00 20.00 20 20Housing for electronics 1 3.00 3.00 3 3LNA & electronic comp'ts 2 9.00 9.00 18 18Connectors to Arms 4 1.00 1.00 4 4Conformal coating 2 0.50 0.50 1 1

Subtotal 82 84 21,496 22,020 16.0%

SKA.TEL.LFAA.REReceiver

Antenna end: 262,144Ant. Circuit boards 2 3.00 3.00 6 6Amplifier & filters 2 5.00 5.00 10 10Power supplies 2 1.00 1.00 2 2Shielding 2 0.50 0.50 1 1Laser control 2 1.00 1.00 2 2Optical lasers 2 8.00 8.00 16 16Optical connectors 2 2.00 2.00 4 4

Subtotal 41 41 10,748 10,748 7.8%

Bunker end (32 channels): 16,384Bunker circuit board 1 20.00 20.00 20 20Optical connector 32 1.00 1.00 32 32PIN receiver diode 32 2.00 2.00 64 64Amplifier 32 3.00 3.00 96 96Filters & switch 32 5.00 8.00 160 256Shielded boxes 16 1.00 1.00 16 16Box cct board 16 2.00 2.00 32 32Output connector 1 10.00 10.00 10 10"Glue" 1 20.00 20.00 20 20

Subtotal 450 546 7,373 8,946 6.5%

SKA.TEL.LFAA.SPSignal processing 16,384

ADC & Beamforming (32 chans)Input connector 1 10.00 10.00 10 10Circuit board 1 50.00 50.00 50 50ADC per channel 32 40.00 40.00 1,280 1,280Processor FPGA 2 400.00 400.00 800 800Comms out 40GigE 1 30.00 30.00 30 30Clock distribution 1 50.00 75.00 50 75"Glue" 1 20.00 20.00 20 20

Subtotal 2,240 2,265 36,700 37,110 26.9%

Rack Switches (2/Tile rack) 512Input cables 36 10.00 10.00 360 36036 port switch 1 3,000.00 3,000.00 3,000 3,000

Subtotal 3,360 3,360 1,720 1,720 1.2%

Aisle Switches (1/16 racks) 16Input cables 36 30.00 30.00 1,080 1,08036 port switch 1 3,000.00 3,000.00 3,000 3,000

Subtotal 4,080 4,080 65 65 0.0%

CostEach Sub-system Sub-Totals/SKA





Table 4: Costing for Baseline and Proposed design for LFAA (part 2

2 System & sub-system #/system #/SKA %age Commentsof

Baseline Proposed Baseline Proposed Baseline Proposed Total€ € € € €k €k

SKA.TEL.LFAA.LINFRDesert infrastructure

Solar power units: 262,144Solar panel 1 20.00 20.00 20 20Battery 1 30.00 30.00 30 30Control electronics 1 5.00 5.00 5 5Housing 1 10.00 10.00 10 10

Subtotal 65 65 17,039 17,039 12.4%

Fibre bundles (192 strands): 2,731Fibre separation 1 40.00 40.00 40 40 109 109 0.1%Core bundles: 2,596

Av length (km) 1.6 per km 3,000.00 3,000.00 4,800 4,800 12,460 12,460 9.0%Station bundles: 135

Av length (km) 20 3,000.00 3,000.00 60,000 60,000 8,093 8,093 5.9%

Bunker:Infrastructure 1

Building 1 6,000,000 6,000,000 6,000,000 6,000,000Internal power dist 1 100,000 100,000 100,000 100,000

Subtotal 6,100,000 6,100,000 6,100 6,100 4.4%

Tile Racks: 256Rack+backplane 1 4,000 4,000 4,000 4,000Power supplies 1 3,000 3,000 3,000 3,000

Subtotal 7,000 7,000 1,792 1,792 1.3%

Monitoring and Control 1Rack 1 2,000 2,000 2,000 2,000Input cables 36 30 30 1,080 1,08036 port switch 1 3,000 3,000 3,000 3,000Servers 3 3,000 3,000 9,000 9,000

Subtotal 15,080 15,080 15 15 0.0%

InstallationAntenna installation 262,144

Assembly at site - mins 20 1.00 1.00 20 20Deployment - mins 10 1.00 1.00 10 10

Subtotal 30 30 7,864 7,864 5.7%

Fibre DeploymentCables 2,731

Per cable term'n - mins 1000 1.00 1.00 1,000 1,000 2,731 2,731 2.0% 5 min per fibreKilometres 6,851

Laying per km 30 1.00 1.00 30 30 206 206 0.1%

Bunker (Construction included above)Racks 1 Total # of racksBuild & load a rack - mins 840 1.00 1.00 840 840 1 1 0.0% 2 person day/rack

Commissioning 1Labour 400,000 1.00 1.00 400,000 400,000 400 400 0.3% 4 person years

Verification 1 4 person yearsLabour 400,000 1.00 1.00 400,000 400,000 400 400 0.3%

CostEach Sub-system Sub-Totals/SKA





The estimated LFAA costings for systems which meet the Baseline Design and Proposed Design requirements are shown in Table 3 and Table 4. These are largely broken down in the blocks as shown in Figure 4. This implementation is our current assessment of the best implementation for the LFAA in the context of the SKA. There are some implications on the apparent costs for some of the other element consortia which are discussed in Section 7.3 below.

The costs are clearly not final or definitive, but represent our current best estimate, with a probable error of approximately ±20% overall. Not included are:

• Profit margins for companies to construct and test the various sub-systems and components; the actual costs need to be carefully negotiated, at the time of procurement there are excellent opportunities for reducing costs. Typical margins would be 10-20% for most of these systems, leading to an anticipated uplift of ~€15m;

• Equipment transport costs; and

• Commissioning, test and operating equipment.

The cost of the array in this implementation is:

• Baseline Design: €148M

• Proposed Design: €151M

7.1 Relative costing between Baseline and Proposed Designs

The basic architecture required for implementation of the Baseline Design is identical to the Proposed Design. The Proposed Design makes two principal modifications over the Baseline Design:

1. Enhance the frequency range of the log-periodic antenna to accommodate higher frequencies. There is an additional small dipole arm element to extend the frequency range – this is the smallest dipole arm element and is very low cost. The analogue amplification chain and RFoF signal transport needs to handle the wider frequency range, which will require some additional design work, but this is unlikely to add any significant cost since, even in the Baseline Design, the components are typically used at well under their maximum frequency; and

2. Additional Nyquist bandpass filtering and switching for the two bands implemented. The costs for this can be clearly seen in Table 3.

The use of identical signal processing and signal delivery means that the cost increment is small at ~2%. There are potential major savings for the dish element, avoiding the very low frequency requirement on SKA-mid.

7.2 Cost discussion

The costs (without profit) in the Tables show that 24% are in the antenna plus electronics; 35% in bunker based analogue and digital signal processing; 27% is in power and signal transport infrastructure; 6% in Bunker infrastructure and 8% in installation and commissioning. This seems a reasonable apportionment of resources. The upgradeable bunker based processing systems are about a third of the total.





Much of the external antenna and infrastructure can be used in SKA Phase 2. By upgrading the processing additional performance and cost savings can be achieved.

7.3 Consortia interfaces and cost allocations

The LFAA will interface to:

• SKA.TEL.INFRA Infrastructure o Power distribution and reticulation o Buildings and roads

• SKA.TEL.SADT Signal and data transport o Clock and frequency distribution o Data transport to the correlator o Data transport to and from Telescope Manager

• SKA.TEL.CSP Central signal processor o Data for correlation

• SKA.TEL.MGR telescope manager o Control and monitoring network information

There is no fundamental difference in the architecture of the LFAA in the context of SKA1, however, there are savings being made over expectations for other consortia:

7.3.1 Buildings

The cost of all the buildings in the SKA is the responsibility of the Infrastructure consortium. We have proposed a single building and included likely costs. This is bound to be much cheaper than having multiple buildings as was expected. The buildings also include all the cooling systems necessary, which is now much cheaper as one large facility. These are also the responsibility of the Infrastructure consortium.

The value of these potential savings needs to be assessed by the Infrastructure Consortium, and the appropriate cost "credit" given to the AADC when assessing and comparing LFAA solutions.

7.3.2 Power distribution

It is assumed that power is delivered by SKA.TEL.INFRA to the major installations of the LFAA. If the AA station processing and bunker power requirements are distributed, as was anticipated in the Baseline Design document, then the costs of reticulation are inevitably higher than delivering power to only one very large bunker (no additional reticulation is necessary with the proposed local antenna solar power). If, however, solar power proves not to be cost effective, then power will need to be reticulated within the LFAA by the Infrastructure Consortium.

Further, if the same facility houses the LFAA correlator (and possibly post processing) then the overall SKA system costs are further reduced and SKA.TEL.INFRA in particular.

The value of these potential savings needs to be assessed by the Infrastructure Consortium, and the appropriate cost "credit" given to the AADC when assessing and comparing LFAA solutions. Reference : AADC-TEL.LFAA.SE.MGT-AADC-PL-002





7.3.3 Clock distribution

As with power distribution it was anticipated in the Baseline Design that clock distribution would be to each of the processing bunkers at every AA station, then received and distributed amongst the station processing. It is clearly cheaper to deliver a single clock signal to a central bunker; indeed the LFAA processing bunker could house the master clock maser so there is no wide area distribution...

The amount of these potential savings needs to be assessed by SADT consortium, and the appropriate credit given to the AADC.

7.3.4 Telescope manager and TM data transport

Again, the expectation in the Baseline Design was to deliver the data from central facility to every AA station bunker. There is a saving if there is only one central bunker. Further, the TM systems for the LFAA could be housed in the same central bunker. It is certainly the case that TM is not required to send control and monitoring information to multiple locations and one can expect the LFAA systems to distribute and collect the information locally.

The amount of these potential savings needs to be assessed by SADT and TM consortia, and the appropriate credit given to the AADC.

7.3.5 Data transport to the correlator

The digital data backhaul systems are similarly simplified using an architecture with a single bunker. At the very least, there is only one central bunker to transport the data from for a correlator facility if they are in separate buildings. However, there would appear to be an opportunity to house both the beamforming equipment for LFAA and its associated correlator in the same facility, reducing the backhaul costs radically.

The amount of these potential savings needs to be assessed by SADT consortium, and the appropriate credit given to the AADC.

7.3.6 Data for correlation

This interface is a data definition interface with little opportunity for cost benefit.





8 Scaling to SKA Phase 2 The fundamental architecture of the Proposed LFAA SKA1 makes scaling to SKA Phase 2 completely feasible and Phase 1 can readily be seen as a “testbed” for SKA Phase 2 full implementation.

Table 5 below lays out the broad specification anticipated for LFAA in Phase 2, derived from the full SKA DRM, and, for comparison, the Proposed Design specification as described in this document.





Table 5: Outline predicted specification of LFAA, as a simple extension of Phase 1, in SKA2 compared to proposed SKA1

Parameter SKA2 Predicted SKA1 Proposed Comments

Number of antennas

3-4 million ~256,000 This is a big change between SKA1 and SKA2. A dramatic increase in sensitivity.

Types of element

1 1 The full frequency range will be covered by a single element type. This will have been demonstrated in SKA1.

Frequency – low

<50MHz 50MHz It is conceivable that the minimum frequency will move down for SKA2

Frequency – high

TBD 650 MHz The performance of relatively sparse antennas at the high-end frequencies will determined with SKA1. The upper frequency of SKA2 will be based on the capabilities of all parts of SKA2, taking much reduced processing cost in Phase 2 of the SKA into account.

Element separation

1.35m (λ/2 at 111MHz)

1.35m

The spacing of 1.35m can be adjusted if required. The SKA2 design will be scientifically justified

Station diameter

20-200m 20-100m With this many antennas, small stations may be hard to process due to the very large number of stations formed – even just in the core. However, flexibility in station size will be retained

Polarisations 2 – linear 2 – linear Essential to have a dual polarisation system

Number of bands

1 2 SKA2 processing will be capable of digitising and processing the full available bandwidth, both for scientific benefit and reducing implementation cost

Max. inst. Bandwidth

600MHz 335MHz & 300MHz

See above

Data rate 2.5 Pb/s (1015) ≥10Tb/s total This is a very great change between SKA1 and SKA2 and leads to the extreme performance capability

Data flexibility

Completely flexibly assigned

Flexibly assigned within

a band

Data output can be assigned to arbitrary beamlets in arbitrary directions up to the total designed data rate. Then each experiment can be optimised, or concurrent experiments run.

Sample resolution

4 or 8-bit 4 or 8-bit capability

Many experiments will operate effectively with 4-bit data, hence doubling the total bandwidth for the same data rate.

The considerations for making the scaling up to SKA2 are:





8.1 Bunker design issues

The main question when dealing with ~4 million antennas rather than 250,000 is how large and complicated does the bunker become? In SKA1 the plan is to process 1024 antennas per rack. If, somewhat arbitrarily, the reasonable limit for the number of racks in the bunker is 1000 – 1500, then that determines the density of processing required. This means that for a total of 4 million antennas the target for each rack to support is 4096 antennas.

8.1.1 Receiver and tile processor cards

Given that the spacing and number of receiver and tile processing cards should remain fixed, then this requires each tile processor to support 64 antennas, compared to 16 proposed in SKA1. To achieve this density the following design improvements are targeted:

8.1.1.1 Multiplex (electronically) both polarisations from an antenna onto one fibre.

This reduces the number of fibres by 50%, which makes a big difference to the handling and density of the communications links. Hence, the density of the fibre on the rack front panels has only increased by a factor of 2 over the SKA1, this appears achievable.

By reducing the number of fibres, the number of analogue gain and filter stages has been reduced by 50%, saving considerable board area.

8.1.1.2 Custom analogue devices

In order to reach the packing density required for the analogue chain it may be viable and cost effective to have custom devices manufactured for both the antenna and bunker ends of the analogue link. The volumes required more readily justify the investment and there will be sufficient time to plan and test such devices.

8.1.1.3 Digitise both bands simultaneously

While the digitisation bandwidth for SKA1 is half the total analogue bandwidth for lower cost, it would be anticipated that the performance and cost of ADCs will have improved substantially by the time of deployment on SKA2 in ~10 years time. Consequently, it is not only a performance improvement to digitise the whole 600+ MHz available per polarisation, but also it is cheaper since both channels can be digitised simultaneously. There is only one fixed Nyquist filter to encompass both polarisation channels, again a size and cost saving.

8.1.1.4 Dense, fast ADCs

The requirement at the front end of the digital signal processing is multi-ADCs per chip. They will need to be 8-bit resolution with a sample rate of >3GS/s. To accommodate the very high data transfer rate a suitable device could be mounted on a module with a processing device capable of processing all 64 antennas, or 128 signal channels after splitting the multiplexed signals. See below for discussion on processing devices.

To keep inside the envelope of a standard 42U high rack, which will probably need to accommodate extra data switches over SKA1, it will be necessary to reduce the height of the Receiver+Tile Processor to





from 4U to 3.5U or even 3U. The limitation is likely to be the packing density of the analogue fibre connections, although these may be moulded into multi-way plugs.

8.1.2 Tile signal Processing

There is little doubt that the processing ability of devices will match or exceed the requirements of a 64 antenna tile processing card; there will be sufficient bandwidth on a module. Indeed the drive to Exascale processing for many of the large ICT manufacturers requires these levels of performance and, by the time SKA2 is being deployed, Exascale processors will exist and as will the drive will be to reduce power and costs. Apart from processing, a key aspect for Exascale processing is the existence of on-chip very high-speed local network connections. This will be essential to meet the total data rates into the correlator.

8.1.3 Station beamforming and system data rates

The required total data rate into the correlator (which determines array survey speed for a given sensitivity) is very high, earlier concepts considered 10Tb/s for each of 250 stations. For the purposes of this analysis if it assumed that 1024 stations are required then the data rate per station drops to 2.5Tb/s. While the on-chip beamforming for the tile is likely to be able to be performed at this data rate, it is difficult to envisage being able to switch this much data between tiles in precisely the same approach as in SKA1. The consequence is that a hierarchical beamformer will probably need to be developed. In this scheme the tile beams are combined in a separate station beamformer and multiple beams are produced within a tile beam.

For the first stage of hierarchical beamforming and all tiles observe identical areas of sky for subsequent station beamforming. The tile beams determine a number of areas of sky that the AA is observing for subsequent station processing. The bandwidth between the tile processors and the station processors will be a key determinant of the quality of the station beams. The station beamformer forms the required number of beams within each tile beam. The number of good quality beams that can be formed within a tile beam is to be determined, but if for example 10 station beams are formed inside each tile beam, then the data rate for the tile is about 250Gb/s. This is a feasible data rate on the project implementation timescale.

Figure 12: Station beams in a tile beam. Stepped beamforming for off-centre beams on the right.

Tile beam

Station beams

Central ÔperfectÕ beam





While the station design needs to have a hierarchical structure due to the communication overhead, as with most reduction techniques there will be a level of compromise involved. By processing antennas as tiles, limited numbers of tile beams are used to create a large numbers of station beams. This structure leads to errors in the beamforming which increase the further off-centre from a tile beam it is. This is illustrated in Figure 12. The “perfect” station beam requires linearly increasing delays with distance across the whole array which is illustrated in Figure 12; in the hierarchical system each tile has the same local delay slope, these are then put together by the station processor to form station beams. Of course, the station beam that has precisely the same delay slope as the tiles can produce a perfect beam; this will be the central station beam with a tile beam shown in Figure 12 left hand illustration. However, for station beams off-centre from the tile beam a series of steps in the element delays across the array is created shown in the right hand illustration. The consequence of these regular steps is to produce substantial sidelobes and reduced beam sensitivity.

The implementation of the hierarchical beamforming will need some consideration. It could restrict the flexibility in station size due to the amount of data switching involved. For fixed station size then a set number of tiles feed a single station beamformer which creates all the necessary beams for transmission to the SKA-low correlator.

8.2 Configuration

The current specifications in the DRM for SKA2 suggest that baselines from the core will be up to a maximum of ~200km; this may be extended to cover confusion or provide improved resolution. This is beyond the range of analogue transmission over fibre. So, with a strongly core concentrated layout, as would be expected, the vast majority of links can be linked directly to the processing bunker, however, the longest baselines will now need remote digitisation and transmission of beams directly to the correlator. This is illustrated in Figure 13.





Figure 13: Enhanced configuration architecture for SKA2

8.3 Extending the frequency range for SKA2

The fundamental concept and performance of LFAA for SKA Phase 2 makes it practical to consider extending the system to use two antenna types to give a dual-band array. The log-periodic antenna discussed in this document would continue to be used covering the frequency range specified. However, the processing infrastructure coupled with the ability to move analogue signals considerable distances implies that a second, higher frequency antenna operating from (say) <500MHz to >1000MHz , maybe 1.4GHz could use the same processing system.

Assuming the new upper frequency antenna element to be a derivative of the current element, many performance lessons will be learned from SKA1. For example, the upper band of SKA2 would also be a sparse array, losing sensitivity at the higher frequencies, but still having a very large survey speed capability. Conversely, the important lower part of its band will have a high effective area, giving the large survey speeds required at those frequencies.

The LFAA processing system can be dual-ported to cope with the dual-band array and provide sufficient channels for the upper frequencies that will likely require more antennas. Alternatively the system can be replicated. The higher sampling frequency described in Section 8.1.1.4 above can be selected for most benefit from this system.

Assuming that RFoF is cost effective then the two arrays can be separated appropriately.





Using the same processing system for both antenna types in the same bunker also implies that the correlator and post processing systems can also be reused.





Appendix A. Possible station configuration for SKA1 Core There is a discussion on improving beam performance in the SKA-low core. The following is an extract from a paper by Keith Grainge. The configuration of processing described above supports this structure, albeit requiring each tile to contribute to a maximum of three beams. This may limit the total bandwidth of the station beam, however, if it is found to be important to support the full bandwidth using this improvement then a higher bandwidth switch network is required – either faster switches (which will probably be available at construction time) or an overlay network of switches giving a total bandwidth >60Gb/s for each tile processor.

The Paper is appended to this description.

Appendix B. Tile processing and communication requirements The cost and power requirements of the tile processing board are strongly dependent upon the relative performance of the FPGAs available for construction and processing requirements for 32 signal channels. In section 5.5.1 the number of FPGAs required per tile processor was estimated from scaling from the proven UNIBOARD system using 2010 vintage FPGAs. Here, the requirements on the tile FPGAs are estimated and compared to the known roadmaps.

The requirements on the FPGAs are to:

1. Provide sufficient processing performance

2. Provide links to 32 ADC channels

3. Provide control signals for the receiver electronics (if required)

4. Provide high speed ≥40Gb/s Ethernet or ≥56Gb/s Infiniband links for the output and control data

An estimate of the processing required per tile processor is shown in Table 6. This is the processing before accounting for any inefficiency. As can be seen the processing is ~2 TMACs/s.





Table 6: Approximate tile processing requirements

The devices used on UNIBOARD are ~0.5TMAC/s and UNIBOARD has 8 FPGAs. These are can process 16 signal channels. This would indicate that there is some inefficiency in a real implementation that might be a factor of 2. So, sufficient FPGAs with a processing ability >4TMACs should handle 16 dual polarisation antennas, however, having more processing available would provide more headroom.

The projected capability of 28nm FPGAs in the 2015 timeframe is ~3.5TMACs/s. There is likely to be another generation available before construction of SKA1. Hence, two devices at a fairly well negotiated price of €400 in ~2018 is a reasonable price/performance point.

Communication ability is also vital to the performance. The requirement for linking to 32 ADCs each sampling at 800MS/s with 8-bits per sample is an aggregate approaching 250Gb/s. This is readily accommodated by projected 28nm FPGAs.

The design decisions on one or two FPGAs of medium or largest sizes at the time will be on a price/performance/ease of implementation balance. The devices will certainly have the required capability.

Appendix C. Bunker power requirement estimation A rough estimate of the bunker power requirements is shown in Table 7. The total power is ~2MW, assuming that ~0.5MW is supplied by solar power for the antennas.

Spectral Filter/signal channel

Sm Sampling rate: 800 MS/sn No. of spectralchannels 1024 250kHz channels

ntap No. FIR taps 5

PPF PPF filter processing 76,800 MACs 5n(log(n)+ntap)PRch Processing rate 60 GMACs/s PPF.Sm/n

Total spectral filter processingsch Signal channels 32

PTsf Total spectral proc. 1,920 GMAC/s PRch*sch

BeamformingBFch Channels to b/form 34 2 of 16 signals + accumulationScplx Complex sample rate 400 MS/s Sm/2

CM Complex multiply 4 MACs MACs per complex accumulationPTbf Total b/form proc 54.4 GMAC/s BFch*Scplx*CM

Total processing 1,920 GMAC/s PTsf+PTbf





Table 7: Estimated bunker power requirements





Distribution List Group Other

Michiel van Haarlem (ASTRON) Jan Geralt Bij de Vaate (ASTRON) Andre van Es (ASTRON) Marchel Gerbers (ASTRON) Andre Gunst (LFAA) Ilse van Bemmel (ASTRON) Andrew Faulkner (University of Cambridge) Eloy de Lera Acedo (University of Cambridge) Nima Razavi-Ghods (University of Cambridge) Peter Hall (ICRAR) Tom Booler (ICRAR) Jader Monari (INAF) Davide Fierro (INAF) Jonathan Hargreaves (JIVE) Arpad Szomoru (JIVE) Kris Zarb Adami (University of Oxford/University of Malta) Mike Jones (University of Oxford) David Zhang (University of Manchester) Wanging Wu (KLAASA) Rui Cao (KLAASA) Ping Chen (KLAASA) Mathias Hoeft (GLOW)




1 Possible station configuration for SKA1 Core

Keith Grainge

Within the core region the elements will be randomly scatteredsubject to the constraints of a minimum distance between elements,dmin, and that the desired array filling factor, ffill be achieved. Thestation centres can then be placed in a hexagonal close packed con-figuration as shown in Figure 1. The scale of hexagonal close packed

Figure 1: Hexagonal close packed configuration for station centres. The blackcircles have a radius, rfull, equal to half the distance between adjacent stationcentres and all the elements within this area receive their full weight in the beamformation process. The green filled circles have radius equal to

√3rfull and

indicated the full extent of the area over which elements will contribute to thisparticular station beam. Red arrows shows one of the baselines between adjacentstations which will be discarded before imaging. The shortest legitimate baselineoccurs between stations whose green circles are adjacent to each other and donot share any elements and the blue line shows as example of these. The blackbaselines are both examples of legitimate baselines — note that both of thebaselines between one station and a pair of adjacent stations are legitimate,despite the fact that the baseline between these two is discarded.

configuration is defined by the radius of the circles of which it is com-prised, called rfull for reasons that will become apparent shortly.

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Stations will be formed from more elements than just those withina radius of rfull of the station centre, in fact all those elements with√

3rfull will contribute. However, those within rfull will be addedwith full weight during beam formation while those beyond will beapodised with a Hanning window function (see Figure 2) i.e.

w = 1 r ≤ rfull (1)

w =1

2

(1 + cos

(π (r − rfull)(√

3− 1)rfull

))rfull ≤r ≤

√3rfull (2)

(3)

Figure 2: Hanning tapered radial station weighting function (see eqn 1).

Therefore the signals from individual elements will be includedin multiple different station beams (see Figure 3); on average theywill appear in 3

√3/2.

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Figure 3: Diagram showing the degreee of overlap between stations and thatindividual elements will therefore be included in multiple different station beams

The possibility therefore exists that visibilities could be formedbetween stations which incorporates elements in common whichwould give rise to autocorrelation type terms appearing in the cor-relations; this is undesirable. Therefore any visibilities formed fromstations where the baseline length is less than 2

√3rfull must be dis-

carded — examples of these baselines appear in red and only occurbetween adjacent stations in the hexagonal close packed configura-tion. Approximately 3N of the N(N − 1)/2 baselines between theN stations are discarded. The shortest legitimate baseline occursbetween stations whose green circles are adjacent to each other anddo not share any elements. However, both of the baselines betweenone station and a pair of adjacent stations are legitimate, despitethe fact that the baseline between these two is discarded.

1.1 Benefits

• The tapering function greatly improves the station sidelobes,which will therefore suppress bright sources far from the fieldcentre and so in turn improve imaging and calibration, see Fig-ure 4.

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• Signals from all elements in the core are used without havingto leave gaps between circles or having a tessalating stationshape (e.g. square or hexagonal which would give rise to strongsidelobes in some directons.

Figure 4: Plot which shows on the left the beam pattern for a (filled) stationusing the weighting function in eqn 1; and on the right a uniformly weightedcircular station.

1.2 Disadvantages

• The beamformer will be more complicated: element signals willbe required by more than one station beamformer and the totalnumber of data points to be summed will increase by 3

√3/2.

1.3 Noise considerations

Compared to a telescope with circular stations of radius rfull, theproposed configuration has much the same number of baselines (apartfrom those that are discarded) but a significantly larger collectingarea per station; so it is tempting to conclude that this implies animprovement in the sensitivity, but this is not the case. The rea-son for this is that the visibilities formed from baselines such as theblack ones those shown in Figure 1 are significantly correlated andso the noise level will not decrease as

√nbaselines; this is discussed

further in Section 1.4. There will in fact be a degradation in themaximally achievable sensitivity resulting from the fact that someelements receive more weight than others due to their position. Theextent to which this is a problem can be judged from the summed

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weights 1; see Figure 5.

Figure 5: Plot showing the sum weights for a hexagonally close packed configu-ration of stations using the individual station weights according to Eqn 1. Theregions of very high weight arise from regions where 3 stations contribute withjust short of full individual weight.

1.4 Visibility correlations

As stated above, visibilities from baselines such as the black onesthose shown in Figure 1 are significantly correlated. However, this isnot necessarily a problem. It is always the case that for two visibili-ties, where there is overlap between their uv positions convolved bythe aperture illumination function, will be correlated, because theyare sampling a common region of the underlying uv-plane. Thiscorrelation between the signals must be accounted for in the co-variance matrix when analysing the data in the uv-plane, but thethermal noise is usually assumed to be diagonal. However, for these“black” baselines, there will also be a correlated component to theirnoises, which must included in the covariance matrix. This idea is

1I am not entirely sure that this is the correct metric as opposed to, say, the sum of squaredweights

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further illustrated in Figure 6. The signal from each station can beconsidered to be the sum of the voltages from each of its comprisingelements. Therefore the visibility formed by the correlation of thetwo stations is the sum of all the possible products between the twostations. The visibility is therefore a weighted sum of the values inthe uv-plane within a patch defined by the convolution of the twostation areas (i.e. the aperture illumination function). The “black”baselines will therefore share some of these product components be-tween individual elements. The result will be that these componentswill be upweighted with respect to the rest. This is therefore equiv-alent in standard inteferometry to arbitrarily upweighting certainvisibilities in the uv-plane and will therefore lead to a sub-optimalsignal-to-noise but will not introduce biases.

Figure 6: The lefthand plot shows 2 stations and some of the individual baselinesthat could be formed from correlating elements within each. These baselines arethen also shown in the righthand plot in the uv-plane together with the extent ofthe aperture illumination function for the correlation of the two stations. Theredarrow shows the baseline between the centres of the two stations and theredpoint show the corresponding uv position in the aperture plane.

2 Perturbations on this original idea

2.1 Randomised station positions

Using a hexagonal close packed configuration gives rise to a largenumber of redundant baselines, which will be poor for filling of theuv-plane and hence for imaging. Also, since these redundant base-lines will have somewhat different beam patterns due to the randomdistribution of elements, the level of redundancy is unlikely to be

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useful for calibation. Therefore a certain amount of reduction infilling factor and a randomisation of the station centres may bevaluable.

2.2 Different apodisation functions

Figure 5 shows that some elements are significantly upweighted withrespect the mean which will degrade the overall sensitivity of thetelescope. Randomising the station centres may alleviate this prob-lem to a certain extent. However, adopting a different weightingfunction should also be considered; plots for a tapered gaussianweight function of the same extent are shown in Figure 7. The re-sults look attractive and certainly imply that an improved weightingfunction should be sought.

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Figure 7: Plots for a tapered gaussian station weighting function. From topleft: the radial weighting function itself; the associated beam response; the sumweights for a hexagonally close packed configuration of stations.

Another possible way forward would be to extend the extent ofthe tapered region for each station, for example having a Hanningwindow out to say 2rfull. The disadvantages of implementing thisare:

• Increased complexity for the beam former.

• Increased number of baselines that must be rejected since thecomponent stations share elements.

One final possibility that should be considered is whether oneneeds a “guard area” between adjacent stations whose signals are

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correlated, since there will certainly be cross-coupling between ad-jacent elements leading to auto-correlation terms appearing in thevisibilities (even if these are significantly downweighted).

2.3 Apodisation through thinning

Apodisation can be achieved either through downweighting of out-lying elements in an element or through reducing the density ofthe elements as a function of radius (i.e. thinning) and weightingthem all equally. Downweighting of outlying elements will producethe better beam pattern, but outside the core it will be wasteful ofelement collecting area, so thinning may be a good comprimise; itwill certainly give a better beam pattern than a uniformly weighted,uniformly dense, circular station.

3 Autocorrelation

Throughtout this document I have assumed that introducing auto-correlation type terms into the visibilities will be disasterous. WhileI believe this assertion to be true, the impact of autocorrelationsshould be quantified.

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