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Introduction

The MWAX correlator replaces the previous fine PFB, Voltage Capture System (VCS) (and media converter), 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.

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Top-Level Architecture

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The 24 coarse channels are processed by 24 GPU-accelerated compute nodes referred to as “MWAX Servers”, with two hot spares.  Each MWAX Server implements the functions shown in the figure above.  The real-time data flows on the MWAX Servers are managed through the use of input and output ring buffers that decouple its computational workflow from the input source and output destinations.  These ring buffers are established and accessed using the open-source ring buffer library “PSRDADA”.

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The output visibility modes of MWAX for 128 tiles that run real-time on the as-built MWAX hardware configuration are listed at this page: MWAX Correlator Modes (128T)

Signal Path/Data Flow

In this section we describe the flow of signals from the tiles and receivers to the media converter (Medconv) servers, MWAX Correlator and then into Long Term Storage at the MWA Archive at the Pawsey Supercomputing Centre.


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MWAX Media Conversion

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This section describes the data flow within the MWAX correlator servers. The function and data flow between components is shown in the below diagram:

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MWAX UDP Capture + Voltage Capture To Disk

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The figure below shows the processing stages and data flows within the MWAX correlator FX Engine process.

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The FX Engine treats individual 8 second sub-observations as independent work units. Most of its mode settings are able to change on-the-fly from one sub-observation to the next.  Each 8 second sub-observation file contains 160 blocks of 50 ms of input data each.   An additional block of metadata (of the same size as a 50 ms data block) is prepended to the data blocks, making a total of 161 blocks per sub-observation file.  At the start of processing each new sub-observation file, the metadata block is parsed to configure the operating parameters for the following 160 data blocks.

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The data capture process, running on each MWAX Server, reads visibility data off the output PSRDADA ring buffer and writes the data into FITS format. The data capture process breaks up large visibility sets into files of up to approximately 10 GB each, in order to optimize data transfer speeds while keeping the individual visibility file sizes manageable. The FITS files are written onto a separate partition on the MWAX Server disk storage. 

Transfer to Curtin Data Centre

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Cache Storage

Each MWAX server has enough disk storage for around 30 TB of visibilities plus 30 TB of voltage data, effectively replacing the need for a separate "Online Archive" cluster of servers as the legacy MWA had. In normal operating modes and schedule, this means the MWA can continue to observe for a week or two even if the link to Perth is offline- data will continue to be stored on disk until the link is online again, and will then begin transmission to Perth.

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