Speaker
Description
SKA-Mid Signal Processing Overview
Jayashree Roy
Signal Processing Engineer
Square Kilometre Array Observatory (SKAO)
Email id: Jayashree.roy@skao.int
Abstract:
Introduction: The Square Kilometre Array (SKA) project is an international effort to build the world’s largest radio telescope with an unprecedented sensitivity because of its huge collecting area. SKA will be a distributed observatory with radio telescopes at two different radio-quiet sites, SKA-MID in South Africa and SKA-LOW in Australia. The first phase of SKA-MID telescope will consist of ~150-kilometre diameter array of 197 offset Gregorian antennas (dishes). It will be a mixed array of 133 15-m SKA (phase 1) dishes and 64 13.9-m diameter dishes from the MeerKAT telescope, with total ~33,000 m2 collecting area.
Figure 1: SKA science goals, observation categories, and electromagnetic spectrum coverage. SKA-Low frequency range is shown in Green and SKA-Mid in Red (at the top of the figure).
Figure 2: Full SKA-Mid array configuration (Left); The inner 2 x 2 km region of the array with blue dots are the MeerKAT dishes (Middle). Simulated view of the inner part of the array (Right).
SKA Mid signal chain overview
The SKA (phase1) - Mid telescope will be able to achieve high sensitivity over a wide frequency range starting from 350 MHz to 15.4 GHz, in 6 selectable bands. Figure 3 shows the SKA1-Mid signal chain sub-systems. This summary paper describes an overview and progress on design of the Single Pixel Feed Receiver (SPFRx), and Correlator & Beamformer (CBF) system of SKA-Mid telescope.
Figure 3: Major sub-systems of SKA-Mid: The black or red lines indicate signal or data flow; the dash-dot lines for timing and synchronisation signals; dashed green lines for control and monitoring flow
Single Pixel Feed Receiver (SPFRx):
The SPFRx system which receives the RF signal from feed, implements conditioning, and performs digitization and packetization of the input signal before forwarding to the Correlator & Beamformer system. The SPFRx is a key performance-determining sub-system, because of its impact on telescope sensitivity, through signal-to-noise ratio (efficiency) and linearity of analog and digital components.
Figure 4: SPFRx simplified block diagram (left), Summary of SPFRx digitization parameters (right), frequency offset index ‘k’ ranges from 1 – 2222
Synchronization: The entire SKA-Mid telescope is synchronized by distributing phase-locked reference signals to each antenna, it also implements the “Sample Clock Frequency Offset (SCFO)” scheme to supress digitiser-generated self-interference and out-of-band interference by employing different sample clock frequencies at each dish. Subsequently, these sampled sequences are digitally re-sampled to a common sample rate prior to correlation/beamforming.
SPFRx system consists of two EMI shielded enclosures at the dish indexer for Band123 and Band45, and one shielded unit in the pedestal of the dish. The Optical Digital Link (ODL) architecture used for the receiver offers the best compromise between RF performance and EMI suppression by distributing the system components between the feed indexer and the shielded compartment in the dish pedestal, few major components of this module are:
• High performance ADC – TI ADC12DJ5200RF.
• Optical transceiver for transmission of sampled data, and monitor & control signal
• High-speed SERDES transceiver to perform serial-to-parallel and parallel-to-serial data conversion.
• TALON-DX Board with Intel Stratix-10 SX280 FPGA + HPS used at the receiver pedestal unit for Digital processing and packetizing.
Figure 5: SPFRx123 analog signal chain (left), SPFRx123 digital signal chain at the indexer (right)
Correlator & Beamformer (CBF) system:
The Central Signal Processor (CSP) is responsible for accepting data from the receptors, generating visibilities, generating VLBI tied-array voltage beams, performing Pulsar Search and Pulsar Timing processing in real-time, and forwarding the resulting data to the Science Data Processor (SDP).
Figure 6: Processing flow of SKA Mid CSP signal chain
CSP Capabilities: When performing the correlation, CBF calculates full-polarisation cross-correlation spectra with a configurable number of channels per polarization (up to ~380K) for every pair of receptors in each sub-array, each sub-array may be observing in a different Band, and spectral zoom could be employed to provide finer spectral resolution over a range of narrower bandwidths.
The Pulsar Search Engine (PSS) accepts (up to) 1500 Pulsar Search beams from CBF and searches each individually for pulsars and transient sources over a range of dispersion measures (DM), accelerations, and periods, at ~40 to ~800 MHz/beam, channelized into 107,520 Hz channels.
The Pulsar Timing Engine (PST) times up to 16 known pulsars, one in each of the up to 16 coherent beams produced by the CBF beamformer. The PST performs high-fidelity, high precision timing observations of known pulsars. The primary task is phase-coherent dispersion removal, a computationally intensive algorithm that requires performing many large Fast Fourier Transform operations in real time.
The CBF system within any sub-array, can perform correlation and beamforming independently and simultaneously. CBF adopts the “Frequency slice architecture” approach, for better parallel development and integration, and lesser hardware required for full capability - which Splits functionality into two parts:
- Band-dependant processing: generates “VCC Products” – Frequency Slices and Search Window Channels and distributes VCC Products to FSPs for further processing
- Band-invariant processing: - made up of several independent Frequency Slice Processors (FSP), having Four “Function Modes” – (1) Correlation, (2) Pulsar Search Beamforming, (3) Pulsar Timing Beamforming, (4) Very Long Baseline Interferometry
Figure 7: Simplified functional breakdown of Mid.CBF into VCC and FSP processing (Left), An illustration of RF band is split into frequency slices, where each slice is processed independently of the others (Right).
CBF Technology: Some of the key components of CBF hardware:
• Bitware IA-860m FPGA PCIe Card
• Intel Agilex M-Series AGM039 FPGA, 3 x QSFP-DD cages for 3 x 400GbE links (Hard MACs), x16 Gen5 PCIe interface (Hard IP), Board management controller (via USB)
• Bittware Terabox 1501B FGPA Server
• 1U host server with support for up to 4 x IA-860M cards, AMD EPYC 9354 CPU (3.2 GHz, 32C/64T), 512GB (VCC) or 128GB (FSP) DDR5-4800 (8 banks), 2 x 1TB M.2 SSD, Redundant Power Supplies, 16-lane PCIe 5.0 bus for each FPGA (512 Gbps), Mellanox ConnectX-7 200GbE NIC
• Edgecore network switch
• Cisco Nexus 9332D-GX2B network switch, 32 x 400 GbE QSFP-DD ports, 25.6 Tbps / 4.17B packets per second.
CBF’s reliability and robustness is increased using measures such - Selection of high reliability components, Design to have the fewest possible variations of equipment types and configurations, advanced diagnostics, use of components having plug and play modularity for rapid part replacement, etc.
Figure 8: Annotated Correlation (for imaging) end-to-end signal flow diagram
Conclusion: A brief overview of the SKA Mid signal chain architecture of the Square Kilometre Array radio-telescope has been provided. The latest technology developments on the design and prototyping of the Single Pixel Feed Receiver (SPFRx) and Correlator & Beamformer (CBF) system have been discussed. Prototype design and extensive qualification tests are being carried out at present. The consolidated effort from all the teams involved demonstrates that the SKA project is certainly on the path to success.
We would like to acknowledge National Research Council Canada (NRC), MDA Space (Canada), Qamcom Research and Technology (QRT), University of Western Australia (UWA), South African Radio Astronomy Observatory (SARAO) for their invaluable contribution and collaborative efforts to these works.
