Space-Time Digital Filtering of Radio Astronomy Signals

using 3D Cone Filters

Najith Liyanage1, Len Bruton2 and Pan Agathoklis1

1Department of Electrical and Computer Engineering

University of Victoria, BC, Canada

2Department of Electrical and Computer Engineering

University of Calgary, Alberta, Canada

RFI 2010 , March 29-31, 2010, Groningen, The Netherlands

MDSP Group, University of Calgary

•Dr Len Bruton, Leader MDSP GroupLiaison Engineer, SKA

• T. K. Gunaratne, PhD student• A. Madanayake, Pdf, moved to U of Akron

DSP Group, University of Victoria

• Dr Pan Agathoklis• Najith Liyanage, M.A.Sc

2

Outline1. Motivation

Pre-processing of signals in radio telescopes

2. Space-Time (ST) Signal Models and their Spectra SOIs on Focal Plane Arrays (FPAs) RFI Signals, and SOIs in Aperture Arrays (AAs) Mutually Coupled (MC) Signals

3. 3D ST Filter Design4. Mitigation of RFI Signals5. 3D Space-Time Filtering: an Illustration using 3D

simulation figures6. Filtering of SOIs in the Presence of RFI and MC

Signals on FPAs

3

The Region of Support (ROS) of the SOI and inteference signals are analysed in the 3D frequency domain.

For FPA signals the ROS in the 3D frequency domain is a cone (frustal) which will be the pass band of the 3D filter.

Significant parts of the 3D spectra of the electromagnetic RFI signals and of the mutually-coupled LNA noise signals lie outside of the cone-shaped pass band of the 3D Filter.

The 3D ST Cone Filter operates on the digitized spatial array of antenna signals to significantly attenuate RFI and MC signals during preprocessing.

sysT

1. Motivation

4

Motivation

3D Cone Pre-FX FPA Filtering

( , , )x y ctW

3D EM celestial

3D EM RFI

3D MC EM noise

w/e & LNA

ADC

3D LNA electronic noise

3D Cone Pre-FX Filter

( , , )x y tw n n n ( , , )x y tv n n n

( , , )x y ctV

3D FT

3D FT

( , , )( , , )

( , , )x y ct

x y ctx y ct

VH

W

We design

1. Motivation

5

SOIs on FPAs

FPA

F

D

)0,0,0(

y

x

z

eT

),( aF

Focal Plane

spill

),,( iFPA fyxf

1GHz

4GHz

2. Signal Models

6

Focal Field Distribution

[UC-FPFC]

[7]

Radio Frequency Interferences (RFIs) RFI: Interfering ST-PWs which do not fall in to the DOAs of SOIs

y

x

z maxSOIAA __

),,,(4__ ctzyxw DRFICangle

for possible BB RFI

Possible FOV angle (Span of all sky beams)

AA 0z

1_ SOIAA2_ SOIAA

3_ SOIAARFIAA_

(0,0,0)

spill

FPA

yx

z

),,,(4__ ctzyxw DRFIC

Paraboloidal Reflector

Focal Plane

anglefor possible RFI

(0,0,0)

max__ SOIFPA

SOIs

angle for possible RFI

RFIFPA _

0z

Aperture Arrays Focal Plane Arrays

2. Signal Models

7

)(),,,(4__ ctzdydxdwctzyxw zyxPWDRFIC

]cos,sinsin,cos[sin],,[ zyx ddd

SOIs

RFIs

SOIs

RFIs

2. Signal Models

Synthesised BB RFI/SOI on AAs

3D Space-Time

3D ST Frequency Domain

8

90,0

0,85 45,80

PW-1

PW-2PW-3

Mutually Coupled(MC) signals

MC Signal Types Caused by RFI/SOI

The ST Propagation model Circularly symmetric propagations with exponential damping in

spatial dimensions

2. Signal Models

9 a= 6.29 (2dB)

Synthesised BB MC

10

2. Signal Models

Inter-element attenuation of 2dB

Inter-element attenuation of 5dB Inter-element attenuation of 20dB

3D ST Frequency Domain3D Space-Time Domain

Circularly symmetric cross-section Linear phase Analytic filter which is capable of easily defining the

required passband angle

y

ct

x

(0,0,0)

),()( 21,

yxct jjNkC

jNk eeHeH

),()( 211,1

yxct jjNkC

jNk eeHeH

45

Spectral light cone boundary

)tan(. L

krk

thk

thk )1(

thk

thk )1(

11

3. Cone Filter Design

L

k

jjNkC

jNk

jjjDCFIR

yxctctyx eeHeHeeeH1

,3__ ),()(),,( 21

Cone filter approximation with L bands, each band consisting of a 1D linear phase FIR filter and a 2D zero phase circularly symmetric FIR filter.

RFI signals: PW-1 and PW-2

3D cone filter angle 30 35 40 43

Energy of the input 2 2 2 2

Energy of the output 0.0039 0.0069 0.0252 0.37

Supp. efficiency (%) 99.81 99.65 98.74 81.19

12

4. Mitigation of RFI signals

(PW-1) (θ1,1)=(85,80)(PW-2) (θ2, 2)=(90,10) GHz]4....1.1,05.1,1[f

13

5. 3D Space-Time filtering: an illustration using 3D simulations figures z

x

y

SOIw

RFIw

Aperture Array

MCwMutual Coupling (MC)

Radio Frequency Interference (RFI)

Signals of Interest (SOI)

(0,0,0)

3D ST 3D Frequency

Output : 3D ST

3D ST Cone Filter

Output : 3D Frequency

AAs3D ST filtering illustration

14

6. Filtering of SOIs in the presence of RFI and MC signals on FPAs

k j ioutputinput

k j ioutputinput

kjiIFR

kjiSOI

ut SIRInput/Outp2

/

2

/

10

),,(

),,(

log10

NoiseLNAMCBBC photonicRFI + BB MBBIFR outputinput =/

GHz]4....1.1,05.1,1[f 8181)12()12( yx NN

mf

TTs

sctyx 0366.01

,,

SOI in the presence of RFI signals PW-1 and PW-2 and ST MC of 20dB

inter-antenna attenuation (a=62.94)

3D cone filter angle ( ) 40 41.5 43

Input SIR (dB) -4.8011 -4.8011 -4.8011

SIR of the filtered output (dB) 2.5113 2.2185 0.6025

SOI in the presence of RFI signals PW-1,PW-2 and ST MC (a=62.93).

Where the energy of ST MC is 20dB higher than rest of the signals.

3D cone filter angle ( ) 40 41.5 43

Input SIR (dB) -20.0944 -20.0944 -20.0944

SIR of the filtered output(dB) -17.2668 -17.1747 -17.2217

Equal RFI and MC signals Energy

Dominant MC signals energy

Conclusions

15

3D space-time cone filtering methods have been proposed and investigated for enhancing the SNR of the far-field SOIs by

attenuating the RFI signals that propagate on or close to the surface of phased arrays, such as FPAs and dense AAs.

attenuating the MC noise signals that propagate on of FPAs

Numerical simulations, based on simulated broadband FPA data, show significant attenuation of over-the-horizon RFI signals and moderate attenuation of typical ST MC signals.

Relevant Publications

1. N Liyanage, L.T, Bruton, and P. Agathoklis, “On the Attenuation of Interference and Mutual Coupling in Antenna Arrays Using 3D Space-Time Filters”, IEEE PACRIM 2009, Victoria, Canada, May 2009.

2. T. K. Gunaratne and L. T. Bruton, "Beamforming of broadband-bandpass plane waves using polyphase 2D FIR trapezoidal filters", in the IEEE Transactions on Circuits and Systems -I, Regular Papers, vol. 55, no. 3, April 2008, pp. 838 - 850.

3. A. Madanayake and L.T. Bruton, "A Systolic-array Architecture for First-order 3D IIR Frequency-planar Filters", IEEE Trans. on Circuits and Systems-I: Regular Papers, Vol. 55, No. 6, July 2008, pp. 1546-1559.

4. A. Madanayake and L.T. Bruton, "A Speed-optimized Systolic-array Processor Architecture for Spatio-temporal 2D IIR Broadband Beam Filters", IEEE Trans. on Circuits and Systems-I: Regular Papers, Vol. 55, No. 7, August 2008, pp. 1953-1966.