Station Processing

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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

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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 gt approximate
delays with phase shifts Phase shifts beamforming
on narrow frequency bands gt subbanding
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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)

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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
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• 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

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Beamformer architectures
Fully parallel processing
X
Subbands
Ant. 1
Two processing board types Input bandwidth of
the adder stage can be very high
Station beam
Ant. 2
?
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
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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 _at_ 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 _at_ 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
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EMBRACE stations output beams
Digital beams inside RF beam A
Digital beams inside RF beam B
Separate beams Separate spectral windows Window width (MHz)
2 1 42
4 2 21
2 4 1 2 21 10.5
8 4 10.5
. . .
48 24 1.75
. . .
2 72 1 36 7.03125 0.9765625
. . .
432 216 0.1953125
Separate beams Separate spectral windows Window width (MHz)
2 1 42
4 2 21
2 4 1 2 21 10.5
8 4 10.5
. . .
48 24 1.75
. . .
2 72 1 36 7.03125 0.9765625
. . .
432 216 0.1953125
Constraints 2 steerings for one spectral
window ?(number separate
spectral windows).(window width) 42.1875 MHz