Philippe Picard 2 nd SKADS Workshop October 2007 Station Processing Philippe Picard Observatoire de Paris Meudon, 11th October 2007

Philippe Picard 2 nd SKADS Workshop October 2007 Beamforming Tied array Correlation Subarraying RFI mitigation Array processing Core station processing Outer station processing Outer station processing

Philippe Picard 2 nd SKADS Workshop October 2007 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

Philippe Picard 2 nd SKADS Workshop October 2007  Station beam A1 RF beam A Ant. 1, beam A1 delay Ant. 2, beam A1 delay Ant. 3, beam A1 delay 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 Sum of delays beamforming

Philippe Picard 2 nd SKADS Workshop October 2007 ADC + Subband filter (data type complex) Beam steering / shaping computation Beamforming weights Amplitude control Phase shift  Station digital beams Tile 1 RF beam A Data type: complex amplitude and phase shifts : multiply by a complex value (weight) RF beam A Tile 2 Tile 3 Tile N Amplitude and phase calibration LOFAR, EMBRACE: ADC 200 Ms/s, 512 subbands Digital beamforming

Philippe Picard 2 nd SKADS Workshop October 2007 Separate Subbands Form Beams Output Beams Correlations averaged power Beams averaged power Apply calibration results Nulling of Interferers Calculate Initial vector for Beam forming Calculate calibration parameters Detect Interferer Calculate Nulling matrices Subband frequency Array geometry Source coordinates Interferers coordinates Subband to be Processed / blanked Output mode Time stamp Antennas data 2N x Fs Ms/s Store To array processing Select / blank Subbands Sample rate processing Sync. cycle rate processing from array control To / from array control Station processing

Philippe Picard 2 nd SKADS Workshop October 2007 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

Philippe Picard 2 nd SKADS Workshop October 2007 Beamformer architectures  ++++ XXXX X X Subbands Station beam Station beam Parallel / serial processing Fully parallel processing Two processing board types Input bandwidth of the adder stage can be very high 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 0 Subbands Ant. 1 Ant. 2 Ant. 3 Ant. N Ant. 1Ant. 2Ant. 3 Ant.N LOFAR, EMBRACE topologie

Philippe Picard 2 nd SKADS Workshop October 2007 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 Nançay array) Subbanding: 512 subbands ( 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 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 EMBRACE (LOFAR like) stations processing

Philippe Picard 2 nd SKADS Workshop October 2007 Separate beams Separate spectral windows Window width (MHz) Separate beams Separate spectral windows Window width (MHz) Digital beams inside RF beam ADigital beams inside RF beam B Constraints : 2 steerings for one spectral window  (number separate spectral windows).(window width) ≤ MHz EMBRACE stations output beams