Philippe Picard2nd SKADS Workshop 10-11 October 2007

Station ProcessingStation Processing

Philippe Picard

Observatoire de Paris

Meudon, 11th October 2007

Philippe Picard2nd SKADS Workshop 10-11 October 2007

BeamformingTied arrayCorrelationSubarrayingRFI mitigation

Array processing

Core stationprocessing

Outerstation

processing

Outerstation

processing

Philippe Picard2nd SKADS Workshop 10-11 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 Picard2nd SKADS Workshop 10-11 October 2007

Station beam A1

RF beam A

RF beam A

RF beam A

RF beam A

Ant. 1, beam A1delay

Ant. 2, beam A1delay

Ant. 3, beam A1delay

Ant. N, beam A1delay

For k station beams (A1 to Ak): k-fold duplicate of the sum of delaysFor m RF beams / antenna: k.m-fold duplicate

Fine delay resolution required => approximate delays with phase shiftsPhase shifts beamforming on narrow frequency bands => subbanding

Sum of delays beamforming

Philippe Picard2nd SKADS Workshop 10-11 October 2007

ADC +

Subband filter (data type complex)

Beam steering / shaping computation

Beamformingweights

Amplitudecontrol

Phase shift

Stationdigital beams

Tile 1

RF beam A

RF beam A

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 Picard2nd SKADS Workshop 10-11 October 2007

SeparateSubbands

FormBeams

OutputBeams

Correlationsaveraged

power

Beamsaveraged

power

Applycalibration

results

Nulling ofInterferers

CalculateInitial vector forBeam forming

Calculatecalibrationparameters

DetectInterferer

CalculateNulling matrices

Subbandfrequency

Array geometry

Source coordinates Interferers coordinates

Subband to beProcessed / blanked

Output modeTime stamp

Antennas data2N x Fs Ms/s

Store Store

To arrayprocessing

Select / blankSubbands

Sample rateprocessing

Sync. cycle rateprocessing

from array control

To / from array control

Station processing

Philippe Picard2nd SKADS Workshop 10-11 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 Picard2nd SKADS Workshop 10-11 October 2007

Beamformer architectures

+ + + +X X X X

X

X

Subbands

Subbands

Stationbeam

Stationbeam

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

Sub

band

s

Sub

band

s

Sub

band

s

Sub

band

s

Ant. 1Ant. 2Ant. 3

Ant. N

Ant

. 1

Ant

. 2

Ant

. 3

Ant

.N LOFAR, EMBRACE topologie

Philippe Picard2nd SKADS Workshop 10-11 October 2007

Westerbork Embrace array: 300 tiles in 75 cells of 4 tiles, 2 RF beamsNançay 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 steeringsData flow: 2.7 Gb/s on 4 Gb Ethernet links

EMBRACE (LOFAR like) stations processing

Philippe Picard2nd SKADS Workshop 10-11 October 2007

Separatebeams

Separatespectralwindows

Windowwidth(MHz)

2 1 42

4 2 21

24

12

2110.5

8 4 10.5

. . .

48 24 1.75

. . .

272

136

7.031250.9765625

. . .

432 216 0.1953125

Separatebeams

Separatespectralwindows

Windowwidth(MHz)

2 1 42

4 2 21

24

12

2110.5

8 4 10.5

. . .

48 24 1.75

. . .

272

136

7.031250.9765625

. . .

432 216 0.1953125

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

EMBRACE stations output beams