1. Aperture Array Simulations
The OSKAR Project
All digital, aperture array simulation for an SKA station.
Very general approach to system design for digital beamforming.
The People
S. Salvini, M. Jones, B. Mort, F. Dulwich, K. Zarb-Adami, A. Trefethen, S. Rawlings
Oxford e-Research Centre and Astrophysics.
Collaboration with other SKADS groups from the UK.
On the Web
Wiki:
In this talk I’m going to give an extremely brief overview of Aperture Array simulation being carried out at Oxford. --
The simulation project is called OSKAR, and has the goal of simulating beamforming for an all-digital SKA station … and in the next couple of slides I will give a very general overview of the work we are doing.
The project is principally a collaboration between the OeRC and Oxford Astrophysics, combining interdisciplinary expertise in HPC and with radio Astronomy.
We also strongly value links with the other UK SKADS people working with aperture array beamforming.
As this 10min talk will only provide a brief introduction, the address of our wiki site is on the slide where loads more information on the project can be found.
(poster)

2. The OSKAR Station Simulator
Scope and Goals
Narrowband (pre-channelised), all digital system.
Scalable to large numbers of antennas (~105).
Choice of number of beams and channels (limited by available processing power).
Simulation of noise, other time variable corruptions, corrections.
Design
Parallelised and scalable.
Hierarchical.
Separate data paths for signal and beamforming weights.
Simplified beamforming math.
Modular framework with data checkpoints.
This slide details a bit more about the simulator we are developing. --
While the OSKAR simulator is will be a very flexible framework for investigating aperture array beamforming, at present we are aiming the development effort at
simulating a pre-channelised (narrowband), all digital system.
As we will be modelling an entire SKA station, the simulation needs to scale to an extremely large numbers of antennas … around 105 antennas for an SKA station.
One of the main reasons for adopting an all-digital system is flexibility, and the simulator will, reflect this by allowing a complete choice of beams and channels (only limited only by the available processing power).
As the value of simulating an aperture array is to gain a better understanding of aperture array beamforming the simulator will be taking into account of noise effects, other corruptions on the beams and looking at the approach to how these might be corrected.
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The design therefore is both influenced by these goals as well as consideration of how digital processing in an SKA station might be realised.
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The design is therefore highly parallel, (achieved by) splitting the signal data between processors, and by implementing a hierarchical beamformer
As shown in the diagram, the simulator therefore performs beamforming hierarchically, with a number of levels producing sub-station or partial beams which are further subdivided until we get to station beams.
With this scheme we can choose the number of beamforming levels in the system and the number of processors in each level. So far tested with 200.
This reduces the computational load and data bandwidth on any of the individual components in the processing chain.
Another important feature of the simulator is the separation of signal path, and a beamforming weights generation. This serves a number of advantages as it allows simple repetitive beamformer processing module to be used with more complicated weights generation as well making it very easy to have the beamforming weights generated at a different rate to the sampled signal stream.
Using this approach the beamformer processing units are reduced to extremely simple math. Just a number of complex multiplies and accumulates between the signal and beamforming weights data streams.
The design is modular. This allows the data stream to be cut and started and stopped as at any point. As we choose to extend the complexity of the simulator, the modular design also allows new algorithms and effects to be integrated extremely easily.
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While the design for the simulator has been established, it should be made clear that implementing this is ongoing work.
We have the majority of the basic communication and processing framework complete and have produced our very first simple beams about two weeks ago, but are not at a stage where we can look at the more interesting problems the simulator is designed to investigate yet.

3. Using the OSKAR simulator
A tool to investigate…
System design.
Beamformer performance.
Targets
Processing structure design.
Processing hierarchy.
Beam patterns.
Array configuration, antenna response.
Algorithmic aspects.
Beam corrections.
Noise performance.
Extensible parallel framework with flexible communication structure.
This slide therefore outlines what we would like to investigate when the simulator is completed.
The aim is to provide a tool to for investigating both how an aperture array beamformer system might be implemented, and also the quality of beams we might expect from the system.
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On the way to investigating these aspects the simulator will be run to produce a number of target investigations.
To investigate system design, we will compare the various processing structures that the simulator can be configured for taking into account of for example the number of levels of beamforming.
We will be producing beam patterns which is large part of the point of simulating the aperture array.
By varying the array configuration and antenna responses we plan to see how this might effect an aperture array beamformer both from a system design perspective (limits the number of possible levels and quality of the beams?).
Also crucial to the investigation of station beamforming is the way beamforming weights are generated and we will look at various algorithms that might perform better (both processing performance and beam quality)
We will look at how the system might allow the possibility for a mechanism for making beam corrections at the station level (which can be incorporated into the beamforming weights).
Important to any beamformer system, we will also look at the noise performance of various beamforming regimes.
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The diagrams on the right are mainly there as pretty pictures, but they give some examples of the ouptut from a simple matlab test code we produced in the design of the simulator.
Finally, and importantly it should be noted that if any additional beamforming ideas come along, due to the way the simulator is designed these should be easy to integrate without having to completely rewrite code.

4. Emerging technologies
SKA station beamforming is a BIG problem…
~1017 flops per 1000 beams per 1024 channels!
~102 IBM Blue Genes, 104 clusters per station!
Cutting OSKAR data stream allows for experimenting on alternative architectures
GPU (SIMD, stream architecture)
Gate arrays / FPGAs
Massively multicore.
ASIC.
At present the simulator is targeted to run on a general purpose cluster.
While this is convenient and flexible for the purposes of simulation (and the computing platform we have availiable), this kind of processing technology will almost certainly not be used for an actual station beamformer as SKA station beamforming is a BIG computing challenge.
Considering the number of operations for beamforming with the SKA reference design for the mid-frequency aperture array.
To form 1000 beams for 1000 frequency channels the station beamformer alone would require of order 1017 flops.
To put this in context, this is three orders of magnitude what is available today in HPC architecture.
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As this problem is core to the design, the simulator should take this into account. The approach we plan to take is to exploit the modularity of the simulator to cut the output stream to provide input data to test processing elements run on different architectures.
I wont go into any detail on this as its outside the immediate scope of the project but we are very aware of the implications when designing code to allow the possibility of testing on on other hardware which might better match the requirements for an SKA station beamforming system.
Examples might be….
GPU….

5. Summary
OSKAR: simulation of all-digital, aperture array beamforming for an SKA station.
Modular, flexible, parallel simulator framework to investigate beamforming and system design.
OSKAR to be delivered Spring 2009
Project wiki:
Therefore to summarise,
We are working towards simulation called OSKAR, of all digital aperture array beamforming for an SKA station.
The code we are developing will provide a modular, flexible and parallelised framework for studying the aperture array beamforming.
The code will be open source and if you are interested it should be available for use in spring of next year.
And finally here is the link to our wiki site again.