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System Design Aspects of Distributed Beamforming in SKA: Insights and Challenges

This document outlines the system design aspects of distributed beamforming for the SKA project, emphasizing the differences in RF architecture between distributed and central beamforming systems. It highlights the technical specifications of the APERTIF prototype, including assembly times, system temperatures, and noise performance. The importance of manufacturability, deployment ease, and operational efficiency is discussed alongside system optimization points such as antenna technology selection across varying frequency bands. Key challenges include achieving desired Tsys specifications at low costs.

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System Design Aspects of Distributed Beamforming in SKA: Insights and Challenges

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  1. SKA(DS) System Design Aspects:building a system Laurens Bakker

  2. System model Distributed beamforming N/16 * N/16 N N N Central beamforming N N N N N N N *Assumes 2 indep. Beams, 2 pol, FOV 250sq.degree at 1GHz (both beams)

  3. System model Distributed beamforming N/16 * N/16 N N N Central beamforming N N N N N N N Central beamforming closely resembles FPA architecture

  4. Central beamforming • rather different RF architecture for distributed and central • We can’t use the same front-end (cost) in both cases • Central resembles FPA to a certain extend • Some numbers of FPA at ASTRON (APERTIF)

  5. Central beamforming • rather different RF architecture for distributed and central • We can’t use the same front-end (cost) in both cases • Central resembles FPA to a certain extend • Some numbers of FPA at ASTRON (APERTIF)

  6. APERTIF prototype 8 x 7 x 2 elements Vivaldi array Dual polarisation 112 antenna elements 112 amplifiers 60 cables 60 receivers Frequency range 1.0 – 1.7 GHz Element separation: 10 cm (l/2 @ 1.5 GHz) 30 MHz bandwidth (backend) Data recording backend (6.7 s) APERTIF prototype

  7. Some APERTIF numbers • It took about 2 days to assemble the antenna • It took about 2 days to connect all amplifiers and power • It took about 2 hours to connect the cables on both sides • The front-end is ‘expensive’ • Voltage regulators ‘needed’ for performance (noise)

  8. Some APERTIF numbers • It took about 2 days to assemble the antenna • It took about 2 days to connect all amplifiers and power • It took about 2 hours to connect the cables on both sides • The front-end is ‘expensive’ • For AA: • Ease of deployment important • Manufacturability important • In general: getting a system operational takes a lot of time

  9. System temperature • LNA noise temperature vs. Tsys • Current APERTIF LNA is ~50K (with 15dB gain, 50 ohm) • Current installed APERTIF front-end is ~65K (40dB gain, 50 ohm) • Current measured Tsys ~115K

  10. System temperature • LNA noise temperature vs. Tsys • Current APERTIF LNA is ~50K (with 15dB gain, 50 ohm) • Current installed APERTIF front-end is ~65K (40dB gain, 50 ohm) • Current measured Tsys ~115K • So Tsys about 65 K higher than LNA • 15K second stage front-end • Feed loss and loss connectors ~20K (‘expensive’ RF material used) • Low cost high performance connectors needed (or no connectors at all) • Noise coupling/mismatch about 10K (LNA has low Rn value) • Sky noise 3K • (spillover about 15K, not relevant for AA?)

  11. System temperature • LNA noise temperature vs. Tsys • Current APERTIF LNA is ~50K (with 15dB gain, 50 ohm) • Current installed APERTIF front-end is ~67K (40dB gain, 50 ohm) • Current measured Tsys ~115K • So Tsys about 65 K higher than LNA • 15K second stage front-end • Feed loss and loss connectors ~20K (‘expensive’ RF material used) • Noise coupling/mismatch about 10K (LNA has low Rn value) • Sky noise 3K • (spillover about 15K, not relevant for AA) • Quite some challenges ahead achieving Tsys numbers of (or even below) 50K as specified at low cost

  12. Sky noise and survey speed • Sky noise rather dominant below 500 MHz Tinst=40K Efficiency=75% A/T=10000

  13. Sky noise and survey speed • Survey speed increases when scaled with 1/2 • A rather constant survey speed from 300-1GHz can be achieved with aperture array Tinst=40K Efficiency=75% A/T=10000

  14. Some SKA system optimization points • Should optimize SKA system (and cost) as a whole • What should be the switchover frequency of AA ->dishes • How many different antenna technologies are required to cover the whole band? • 100-500MHz requires probably 2 different antenna types • 500-800Mhz can easily be met with one antenna type • 300MHz-800MHz (or even 1000MHz) is also achievable

  15. Some SKA system optimization points • Should optimize SKA system (and cost) as a whole • What should be the switchover frequency of AA ->dishes • How many different antenna technologies are required to cover the whole band? • 100-500MHz requires probably 2 different antenna types • 500-800Mhz can easily be met with one antenna type • 300MHz-800MHz (or even 1000MHz) is also achievable • We should try to minimize the required number of different antenna types • Running cost (esp. power consumption) should be taken in account early on in the design process

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