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The MWAX correlator replaces the existing Voltage Capture System (VCS), FX correlator and on-site archive of the Murchison Widefield Array (MWA). All of the fielded instrument hardware (tiles, beamformers, receivers) remains the same, as described in The Murchison Widefield Array: The SKA Low Frequency Precursor by Tingay et al. (2013), and the Phase II description paper: The Phase II Murchison Widefield Array: Design Overview by Wayth et al (2018). The diagram below shows a high level overview of the complete signal chain, including the main MWAX components: Media conversion and Correlator.
Top-Level Architecture
MWAX is located on site at the MRO and comprises 24 new servers (+ 2 spares) and repurposes 10 existing on site servers. Together the equipment occupies three racks. Output visibilities are transferred to Curtin, and ultimately Pawsey, via existing fibre-optic links.
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The MWAX correlator design employs a multicast architecture, allowing all receivers to each send their streams of high time resolution data to any number of multicast consumers with no additional load on the sender. Initially there will be 24 multicast receivers (one for each coarse channel for correlation and voltage capture), however, the multicast architecture allows for additional multicast receivers to utilize the same high time resolution data for other purposes. For example: RFI monitoring, transient detection and external instruments such as Breakthrough: Listen could commensally consume some or all of the high time resolution data without impacting the operation of the telescope.
The MWAX correlator employs the “FX” correlation architecture where the input time samples for each signal path (antenna and polarization) are fine-channelized prior to cross-correlation, reducing the correlation process to complex multiplications in the frequency domain. Whereas the legacy correlator utilized a polyphase filterbank for the F-engine, the MWAX correlator employs the FFT. For the X-engine, development time/cost was minimized by utilizing the existing open-source GPU correlator library “xGPU” (the same library at the heart of the legacy correlator). The standard xGPU library is used essentially unchanged for MWAX, with the exception of a small but crucial change to improve speed and reduce bus/memory traffic, found to be essential to making the design scalable to 256T and beyond (see MWAX xGPU on github).
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Each of the MWA's existing sixteen 16 receivers in the field send eight 8 tiles worth of twenty four 24 coarse channels over forty-eight 48 fibre optic cables using the Xilinx RocketIO protocol. The fibre optic cables terminate in the MRO Control Building, where two bundles of three fibres connect to a media conversion (medconv) server via custom Xilininx FPGA cards. Six independent processes on each medconv server convert the RocketIO data into Ethernet UDP packets which are sent out to our Cisco Nexus 9504 switch as multicast data where each coarse channel is assigned a multicast address. This provides the "corner-turn" where each of the six processes on each mediaconv server is sending one third of the coarse channels for one eighth of the tiles in the array.
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