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 Group
Liaison Engineer, SKA
T. K. Gunaratne, PhD student
A. Madanayake, Pdf, moved to U of Akron
DSP Group, University of Victoria

•
•
2
Dr Pan Agathoklis
Najith Liyanage, M.A.Sc
Outline
Motivation
1.

Space-Time (ST) Signal Models and their Spectra
2.



3.
4.
5.
6.
3
Pre-processing of signals in radio telescopes
SOIs on Focal Plane Arrays (FPAs)
RFI Signals, and SOIs in Aperture Arrays (AAs)
Mutually Coupled (MC) Signals
3D ST Filter Design
Mitigation of RFI Signals
3D Space-Time Filtering: an Illustration using 3D
simulation figures
Filtering of SOIs in the Presence of RFI and MC
Signals on FPAs
1. Motivation
Motivation
The Region of Support (ROS) of the SOI and inteference
signals are analysed in the 3D frequency domain.

T sys

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.
4
1. Motivation
3D Cone Pre-FX FPA Filtering
3D EM
celestial
3D MC EM
noise
w/e &
LNA
3D Cone
Pre-FX Filter
ADC
w(nx , ny , nt )
3D EM
RFI
3D LNA electronic
noise
We design
5
v(nx , ny , nt )
3D
FT
W (x , y , ct )
H (x ,  y , ct ) 
3D
FT
V (x , y , ct )
V (x ,  y , ct )
W (x ,  y , ct )
2. Signal Models
SOIs on FPAs
Focal Field Distribution [7]
Te
[UC-FPFC]
F ( a,  )
f FPA ( x, y, f i )
 spill
y
FPA
z
1GHz
(0,0,0)
x
4GHz
D
Focal Plane
F
6
2. Signal Models
Radio Frequency Interferences (RFIs)

RFI: Interfering ST-PWs which do not fall in to the DOAs of SOIs
angle
for possible RFI
Aperture Arrays
Possible FOV angle
SOIs
(Span of all sky beams)
z
 AA _ SOI 2
RFIs
 AA _ SOI 3
 AA_ RFI
wC _ RFI _ 4 D ( x, y, z, ct )
 AA _ SOI _ max
 FPA _ RFI
 AA _ SOI 1
angle
for possible BB RFI
SOIs
Focal Plane Arrays
wC _ RFI _ 4 D ( x, y, z, ct )
SOIs
y
x
 spill
z
(0,0,0)
RFIs
y
(0,0,0)
 FPA _ SOI _ max
x
FPA
z0
Paraboloidal Reflector
AA z  0
Focal Plane
wC _ RFI _ 4 D ( x, y, z, ct )  wPW (d x x  d y y  d z z  ct )
[d x , d y , d z ]  [sin cos, sin  sin , cos ]
7
angle
for possible RFI
2. Signal Models
Synthesised BB RFI/SOI on AAs
3D Space-Time
3D ST Frequency Domain
8
PW-1   0,   90
PW-2   85,   0
PW-3   80 ,   45
2. Signal Models
Mutually Coupled(MC) signals
MC Signal Types


The ST Propagation model


9
Caused by RFI/SOI
Circularly symmetric propagations with exponential damping in spatial dimensions
a= 6.29 (2dB)
2. Signal Models
Synthesised BB MC
3D Space-Time Domain
Inter-element attenuation of 2dB
Inter-element attenuation of 5dB
10
3D ST Frequency Domain
Inter-element attenuation of 20dB
3. Cone Filter Design



Circularly symmetric cross-section
Linear phase
Analytic filter which is capable of easily defining the required passband
angle
 ct
H FIR _ C _ 3D (e jx , e


H kN11 (e jct
j
) H CN,2k 1 (e j x , e y
(k  1)
)
H kN1 (e jct ) H CN,2k (e jx , e
j y
Spectral light cone
boundary
th
y
k th
)

45



(0,0,0)
rk 

k th
(k  1) th

11
k .
tan( )
L
x
j y
L
, e jct )   HkN1 (e jct ) HCN,2k (e jx , e
k 1
j y
)
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.
4. Mitigation of RFI signals
(PW-1)
(PW-2)
(θ1,1)=(85,80)
(θ2, 2)=(90,10)
f  [1,1.05,1.1....4]GHz
RFI signals: PW-1 and PW-2
12
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
5. 3D Space-Time filtering: an illustration using 3D simulations figures
wSOI
Signals of Interest (SOI)
z
AAs
3D ST filtering illustration
y


x
(0,0,0)
wMC
Mutual Coupling (MC)
3D ST
wRFI
Radio Frequency
Interference (RFI)
Aperture Array
3D Frequency
3D ST Cone Filter
Output : 3D Frequency
13
Output : 3D ST
6. Filtering of SOIs in the presence of RFI and MC signals on FPAs
2

    SOIinput / output (i, j , k ) 
k j i

Input/Outp
ut SIR  10 log10 
2 

    IFRinput / output (i, j, k ) 
k j i

f  [1,1.05,1.1....4]GHz
(2 N x  1)  (2 N y  1)  81 81
Tx, y,ct  Ts 
1
 0.0366m
fs
IFRinput / output = BB RFI + BB MC photonic BB MC LNA Noise
Equal RFI and MC signals Energy
SOI in the presence of RFI signals PW-1 and PW-2 and ST MC of 20dB
inter-antenna attenuation (a=62.94)
Dominant MC signals energy
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.
14
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
Conclusions

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.
15
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.