Philippe Picard1EMBRACE station processing SKADS Conference, Limelette, 4-6 November 2009 EMBRACE...

Philippe Picard 1EMBRACE station processingSKADS Conference, Limelette, 4-6 November 2009

EMBRACE station processing

P. Picard Station de Radioastronomie de Nançay

P. Renaud Station de Radioastronomie de NançayC. Taffoureau Station de Radioastronomie de NançayD. Benoist Station de Radioastronomie de Nançay

V. Macaire SKADS fundedL. Mercier SKADS fundedW. Paule SKADS funded


One RF beam signal is the phased sum of RF signals from 72 one pol. Vivaldi elements (analog RF beamforming)

Depending upon site configuration, for station processing one digitizer input is fed by a base element being: one IF beam coming from one tile through an RF downconverter one IF beam coming from a tileset of 4 combined tiles through an RF downconverterThe IF beams are in the 100 to 200 MHz frequency range after downconverting

Digitizer bandwidth: 100 MHz (200 Ms/s, 12 bits) sending a 2400 Mb/s data flow to each back-end digital input

Westerbork site (144 tiles): 144 digital inputs for beam A (tiles) => 345.6 Gb/s

36 digital inputs for beam B (tilesets) => 86.4 Gb/s

Nançay site (80 tiles): 20 digital inputs for beam A (tilesets) => 48 Gb/s20 digital inputs for beam B (tilesets) => 48 Gb/s

Station processing inputsStation processing inputs

RFbeam A

RFbeam B

EMBRACE tile72 two pol. Vivaldi elements

500 to 1500 MHzAnalog RF beamformingfor 2 beams (one pol.)

Station processing input data rate for EMBRACE


EMBRACE digital beamformingEMBRACE digital beamforming

Synthesis of a digital beam is done by a phased sum of all the digitized IF beams, each one being phase shifted by the proper value in order for the digital beam to point a sky direction.Using phase shift rather than true delays of IF beams signals is easier to implement, but works only in small bandwidth where a phase shift is equivalent to a true delay, requiring the use of bandpath filters before phase shifters

Digitized IF beam AAntenna 1

Digitized IF beam AAntenna 2

Digitized IF beam AAntenna n

Bandpath filter F, Δf Phase shifter Ф1

Phase shifter Ф2

Phase shifter Фn

+Bandpath filter F, Δf

Bandpath filter F, Δf

Digital beam F, Δf

{AZ, El}

.real data complex data

This generic system gives us a digital beam in a Δf subband.Duplicating this architecture with the proper bandpath filters and phase shifts allow access to many digital beams of Δf bandwidth (Δf = 195 KHz in EMBRACE)


To reduce development duration, LOFAR station Back-End is used as EMBRACE hardware platform for station processing.Base processing element is the Antenna Processor (AP).

One AP computes the phased sums of two antennas, same RF beam, for 248 data objects {[|AZ0, El0|,|AZ1,El1|], one subband} called beamlets (2 beams in one subband)

Xin

Yin

Xre,Yre Xre,Yre

Xim,Yim Xim,Yim

Xre,XimYre,Yim

Xre,XimYre,Yim

248 x[Sre,Sim](Az0,El0,s)[Sre,Sim](Az1,El1,s)

Subband filter Subband select Beamformer

Antenna nbeam A

Antenna n+1beam A

512 subbands Up to 248 subbands 2 antennas

All the required APs outputs are then summed to deliver the station digital beamlets

One station beamlet is the phased sum of all station antennas, same RF beam, for two sky directions and one subband. A station delivers 248 beamlets for each RF beam.

Antenna ProcessorAntenna Processor

e.g. of digital beams configurations for RF beam A:

2 sky directions in 248 subbands (48.43 MHz if consecutive subbands)or ……or 8 sky directions in 62 subbands (12.11 MHz if consecutive subbands)or …..or 496 sky directions in 1 subband (0.1953125 MHz)


For two antennas X, Y and one AP, beamforming weights to phase sum these two antennas in one subband and for two sky directions {Az1,El1} and {Az2,El2} are set in a 4 x 4 matrix and the beamforming process for one subband becomes a simple matrix multiply

APs input parametersAPs input parameters

Subband filter: this polyphase filter delivers 512 subbands (~195 KHz wide) using a fixed set of 16 K coefficients to define filter shape.

Subband select map: 248 subbands must be selected, with no constraints on these subbands (no required ascending nor descending order, multiple use of same subband…).All the APs must use the same subband select map at the same time.

Beamforming weights: these weights are the digital phase shifts and amplitude shifts required for each antenna to synthezise a beam in a sky direction.

g1cosφ1 -g1sinφ1 g2cosφ2 -g2sinφ2

g1sinφ1 g1cosφ1 g2sinφ2 g2cosφ2

g3cosφ3 -g3sinφ3 g4cosφ4 -g4sinφ4

g3sinφ3 g3cosφ3 g4sinφ4 g4cosφ4

Sum(X,Y,{Az1,El1})real

Sum(X,Y,{Az1,El1})im

Sum(X,Y,{Az2,El2})real

Sum(X,Y,{Az2,El2})im

= Xreal

Xim

Yreal

Yim

*s s s

Complex samples of antennas X and Y

signals in subband s

Phased sum of antennas X and Y,

subband s, towards sky direction

{Az1,El1}

Phase and gain shifts to apply to antennas X and Y,

subband s, to point sky direction

{Az1,El1}

beamlet


Subband width: 195.3125 Khz (input data sampled at 200 Ms/s) φ is one sky direction (Az,El)

SeparateSubbands

SelectSubbands

2 data flowsX and Y

(12b. real)2 x 2400 Mb/s

512 X subbands512 Y subbands

2 polyphase filterbanks

16 K coef.

Subband Select map248 X subbands248 Y subbands

Weights248 X weights248 Y weights

512 x[Xre + jXim]b

[Yre + jYim]b

b = 0 to 511(18b complex)

248 x[Xre + jXim]s

[Yre + jYim]s

s in [0, 511](18b complex)

248 x[Sre + jSim]s,φ0

[Sre + jSim]s,φ1


1024 samples time frame =>

248 x[FSre + jFSim]s,φ0

[FSre + jFSim]s,φ1


248 x[PSre + jPSim]s,φ0

[PSre + jPSim]s,φ1


FormBeams X

FormBeams +

Data rate => 7.2 109 b/s 3.4875 109 b/s 3.4875 109 b/s 3.1 109 b/s

Max[nb.s]= 248

Max[nb.s x nb.φ]= 496

Max[nb.s x nb.φ]= 496Constraints =>

two tiles or cellsof combined tiles

Stationoutput

From LCU (1s time frame)

From previous AP in the

chain

To next AP in the chain

Antenna Processor datapathAntenna Processor datapath


AP0

AP1

AP2

AP3

BP

CONF

PHY

PHY

SERDES

data

11 x

800

Mb

/s

2 x 800 Mb/scontrol

X0 input200 Ms/s 12b.

Y0 input


Y1 input


Y2 input


Y3 input

RSPboard

100 Mb/s raw

EthernetMonitorin

g and control

1 Gb/s raw EthernetStation data (partial)

From previous board4 x 2.5 to 3.125 Gb/s

(Infiniband)

To next board4 x 2.5 to 3.125 Gb/s

(Infiniband)

4b.

MII

8b. G

MII

Station processing boardStation processing board

APs : FPGA using 90 nm process


SelectSubbands

FormBeams

OutputBeams

ComputeSubbandsStatistics

ComputeCross

Correlations

ComputeBeams

Statistics

Correct forCalibration

Nulling ofInterferers

CalculateInitial vector forBeam forming

CalculateCalibration

DetectInterferer

CalculateProjection

Matrices for nulling

Subbandfrequency

Array geometry

Subbandfrequency

Array geometry

Source coordinates Interferers coordinates

Station Control Unit

Subbandsto be processed

Output modeTime stamp

LO1 beam A

LO1 beam B

LO2

Datarecording

Postprocessing

LocalControl

Unit

Antenna dataFrom RCU

N Tiles2 RF beams

Store Store Store

Tile arraysettings

CalculateTile arraysettings

Source coordinates

ExternalCorrelatorInterface

SeparateSubbands

Sw

itch

Monitoring andControl software

2N x200 Ms/s 12b.


EMBRACE station data outputEMBRACE station data output

1s averaged power of all 512 subbands for all antennas: subbands statistics

1s averaged power of all 248 station beamlets: beamlet statistics

1s averaged cross correlations of all antennas, for one subband

output of up to all 248 station beamlets on up to 4 x 1 Gb ethernet links

Locally stored 1s averaged data, for each RF beam:

External Correlator Interface with two analog outputs

output bandwidth: up to 20 MHz

starting frequency: 0 to 40 MHz

Analog output for external analyser systems:

High temporal resolution data (5.12 µs), for each RF beam:


External Correlator Interface: outputs two beams in 0-20 MHz analog signals

20 MHz bandwidth requires 103 consecutive subbands (20.14 MHz).

Input of ECI: 103 beamlets of consecutive subbands and 2 sky positions (the same for each beamlet)

Output of ECI: 2 x 1 data flow for the same 20 MHz bandwidth and 2 sky positions

Spectral domain of output beam A (digital)

Spectral domain of beam A (digital)F F+ 20.14 MHz

0 20.14 MHz

Spectral domain of output beam A (analog)0 20 MHz

Digital to Analogconverter + filter

Digital processing

Synthesis filter

IF inputs of WSRT correlator(fringe stopping in IF processor)

0 20 180 MHz40

Analog output for external analyserAnalog output for external analyser


Halfback-end

RF beam A144 inputs

Halfback-end

RF beam B36 inputs

ECI Digital beam A(Az0,El0) in one 20 MHz bandwidth, to analog output 1Digital beam A(Az1,El1) in the same 20 MHz bandwidth, to analog output 2

Data storage of up to124 beamlets from RF beam A and 248 beamlets from RF beam B

mode 0

mode 1

Note: Same configuration available with RF beam B to ECI and RF beam A to data storage

Da

tast

ora

ge

Gb switch

Halfback-end

RF beam A144 inputs

Halfback-end

RF beam B36 inputs

ECI No analog output 1No analog output 2

Data storage of up to 248 beamlets from RF beam AData storage of up to 248 beamlets from RF beam B

Westerbork array station processing configurations18 RSP boards for beam A and 5 RSP boards for beam B

mode 0

mode 0

Da

tast

ora

ge

Gb switch

tiles

tilesets

tiles

tilesets

Data output configurationsData output configurations


Data recording, access to post processingData recording, access to post processingP. Renaud Station de Radioastronomie de NançayJ. Borsenberger Observatoire de ParisF. Viallefond Observatoire de ParisHenrik Olofsson Observatoire de ParisS. Torchinsky Station de Radioastronomie de Nançay

S. Pomarede SKADS funded

Ongoing work on specific computer hardware configuration using COTS components to allow real time recording of at least 124 beamlets (2 x 1 Gb ethernet link) and recording software

Storage capacity to record N beamlets ( GBytes or Tbytes)

beamlets data flow 10 min 1 hour 5 hours 10 hours 62 93 MB/s 54.2 325.5 1.6 3.2 124 185 MB/s 108.5 651 3.2 6.4 186 278 MB/s 162.8 976.4 4.8 9.6 248 370 MB/s 217 1.3 6.4 12.8

Real time data recording


Tools for acquisition software and post processing:

Work on an Embrace data model in order to generate API in Python / C++ to deliver procedures to be used in test and observation software (F. Viallefond)

Work to define an Embrace Measurement Set to be used in data acquisition and post processing tools.

Data toolsData tools

Philippe Picard1EMBRACE station processing SKADS Conference, Limelette, 4-6 November 2009 EMBRACE...

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