ASKAP Digital Systems
The ASKAP digital signal processing system (DSP) comprises a suite of special-purpose filterbanks, beamformers and a correlator which manipulate the data from the receiver (analog) systems at each antenna and combine the signals into a format which can be processed into radio images and spectral cubes - this being the astronomer's "result" from ASKAP.
ASKAP DSP Elements
Assembled Digital Receiver card in test jig (March 2009)
Each ASKAP antenna will have a digital beamformer which will take in ~100 dual polarisation baseband signals each of 300MHz bandwidth. These will be digitised and processed by a first stage polyphase filterbank. The data generated is then transported on a separate Signal and Data transport system to the beamformer. The beamformer will process this data to generate ~30 beams with the data for each beam channelised to a resolution suitable for spectroscopic observing. The beamformed data is then transported to a correlator which forms the cross power spectrum for the signals for each pair of antennas on each of the beams. The output from the correlator goes to the Computing system which will also provide control signals and monitor the operation of the correlator. The compute load in the filterbanks, beamformers and correlator is about 2 Peta arithmetic operations per second. This is computed on a data stream that totals 70 Tb/s.
The first filterbank reduces the fractional bandwidth of the filterbank outputs to less than 1%. Thus, across each band the beamforming weights can be approximated by a single complex number. The filterbank operation also allows a divide and conquer approach to be used in the beamformer: each processing element in the beamformer has data for all inputs over a subset of the bandwidth. This requires a cross connect operation. At the antenna the filterbank data from four inputs is driven onto 16 high speed serial lines which are then encoded onto four 10Gb/s optical links. These optical links connect the antenna to the Murchison Radio-astronomy Observatory (MRO) central site which houses the beamformers and correlator. At the beamformer the 16 serial signals are decoded and distributed across 16 processing boards which process 19MHz of bandwidth each. Further cross connections occur on the processing board.
On the beamformer boards all ~30 beams are calculated using a simple weight and sum procedure. At the same time a subset of the correlations between the input signals is calculated for calibration purposes. Also on the beamformer board is a second stage polyphase filterbank which decimates the data to the frequency resolution needed for spectral line observing. The resolution is limited by memory but with the use of cheap DRAM the full bandwidth can be decimated to the required resolution 18.5kHz. Coarser frequency resolution will be obtained by channel averaging in the initial correlator deployment; a single internal frequency resolution will save considerable time in programming and configuring the correlator. Eventually the system will be upgraded to work at a frequency resolution of 0.58kHz. The DRAM in the beamform will additionally allow a transient buffer to be implemented.
The data from each beamformer is then transported to the correlator. Again a cross connection is needed between the 36 beamformers and 256 correlator boards. This is partly implemented in the cabling between the systems and partly with the chassis that hold groups of 16 correlator boards. The similarity in data flow in a correlator and beamformer chassis allow the same board design to be used for both, further reducing the design effort. Each correlator board processes 64 of the 18.5kHz channels for all beams. At the correlator board data is available from all antennas so an additional function implemented is beamforming data from the separate antennas. This generates a tied array beam to be used for VLBI and high time resolution observations of targeted sources.
The team implementing the system have prototyped much of the design in existing systems that they have been involved in. This includes the MOPRA wide band spectrometer at Coonabarabran, the CABB 2GHz correlator at Narrabri, the SKAMP 384 input correlator at Captains Flat, the MWA correlator on the same site as ASKAP, and the Parkes Testbed, where a 48 (upgradable to 192) input beamformer has been implemented.
Opportunities for Industry
A pdf file describing ASKAP Digital Signal Processing Systems and giving an overview of industry opportunities can be found here.
Further information on opportunities for industry can be found on the industry page.