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Station Processing

Station Processing. Philippe Picard Observatoire de Paris Meudon, 11th October 2007. Core station processing. Array processing. Beamforming Tied array Correlation Subarraying RFI mitigation. Outer station processing. Outer station processing. Main station processing tasks:

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Station Processing

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  1. Station Processing Philippe Picard Observatoire de Paris Meudon, 11th October 2007

  2. Core station processing Array processing Beamforming Tied array Correlation Subarraying RFI mitigation Outer station processing Outer station processing

  3. Main station processing tasks: • Beamforming • beam steering • beam shaping • Cross / auto correlaton matrix • compute and apply station calibration parameters • First level of ″F″ processing for a FX array correlator   • RFI mitigation at station level • beam nulling • RFI detection • blanking of corrupted channels • Station Monitoring and Control

  4. Sum of delays beamforming RF beam A Ant. 1, beam A1 delay RF beam A Ant. 2, beam A1 delay  Station beam A1 RF beam A Ant. 3, beam A1 delay RF beam A Ant. N, beam A1 delay For k station beams (A1 to Ak): k-fold duplicate of the sum of delays For m RF beams / antenna: k.m-fold duplicate Fine delay resolution required => approximate delays with phase shifts Phase shifts beamforming on narrow frequency bands => subbanding

  5. Beamforming weights Data type: complex Beam steering / shaping computation Amplitude and phase calibration Digital beamforming Amplitude control Phase shift Tile 1 RF beam A ADC + Subband filter (data type complex) Tile 2 RF beam A Tile 3  RF beam A LOFAR, EMBRACE: ADC 200 Ms/s, 512 subbands Tile N Station digital beams RF beam A amplitude and phase shifts : multiply by a complex value (weight)

  6. Calculate Initial vector for Beam forming Correlations averaged power Beams averaged power Separate Subbands Select / blank Subbands Form Beams Output Beams Station processing from array control Interferers coordinates Source coordinates To / from array control Subband frequency Calculate Nulling matrices Array geometry Nulling of Interferers Detect Interferer Apply calibration results Calculate calibration parameters Sync. cycle rate processing Store Store Output mode Time stamp Subband to be Processed / blanked Sample rate processing Antennas data 2N x Fs Ms/s To array processing

  7. Sample rate processing: • One of highest station processing load is subbanding • Use of polyphase filter banks (precise shaping of filter response) • Critical sampling filter: Nyquist sampling of subbands (LOFAR, EMBRACE) • Oversampling filter: oversampled (1.x to 2.0) subbands by overlapping two polyphases process • Oversampling filter reduces aliasing in the subband transitions zone • at the cost of duplicate polyphase processing and higher subbands data rate. • Technology for sample rate processing: • Silicon processing: ASICs, FPGAs, masked FPGAs • Software processing: Cell engines, GPUs

  8. Beamformer architectures Fully parallel processing X Subbands Ant. 1 Two processing board types Input bandwidth of the adder stage can be very high Ant. 2 Station beam  Ant. 3 X Subbands Ant. N Parallel / serial processing One processing board type Constraint: interconnection of all adders for all processing cells (FPGA), grows with station beams number Trade off between beams bandwidth and number of beams Station beam + + + + 0 X X X X Subbands Subbands Subbands Subbands LOFAR, EMBRACE topologie Ant. 1 Ant. 2 Ant. 3 Ant.N

  9. EMBRACE (LOFAR like) stations processing Westerbork Embrace array: 300 tiles in 75 cells of 4 tiles, 2 RF beams Nançay Embrace array:96 tiles in 24 cells of 4 tiles, 2 RF beams Sampling: 2 x 75 ADC, 200Ms/s, 12b. (2 x 24 ADC @ Nançay array) Subbanding: 512 subbands (195.3125 KHz) Hierachical 2 rings topologie (parallel / serial / serial processing) One processing board for 8 inputs (4 ant. 2 RF beams. or 8 ant. 1 RF beam.) 20 processing boards for the whole array (6 boards @ Nançay array) Processing for one « antenna, 2 pol. » fits in an « affordable » FPGA (90 nm) One more smaller FPGA / 4 antennas to manage monitoring, control, data output Trade off between beams bandwidth and number of beams. Station output: Successive beamformed data sets for selected subbands and steerings Data flow: 2.7 Gb/s on 4 Gb Ethernet links

  10. EMBRACE stations output beams Digital beams inside RF beam A Digital beams inside RF beam B Constraints : 2 steerings for one spectral window (number separate spectral windows).(window width) ≤ 42.1875 MHz

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