Transcript Analysis of Adaptive Array Algorithm Performance for Satellite Interference Cancellation in Radio Astronomy Lisha Li, Brian D.

Analysis of Adaptive Array
Algorithm Performance for Satellite
Interference Cancellation in Radio
Astronomy
Lisha Li, Brian D. Jeffs, Andrew Poulsen, and Karl
Warnick
Brigham Young University
XXVII URSI General Assembly 2002
BYU
Summary
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GLONASS, Iridium, ground-based radars, etc. create
overwhelming interference in important bands.
Adaptive beamforming/array processing algorithms are
promising for interference cancellation, but are not
well characterized in radio astronomy environment.
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Low SNR, very sparse arrays, very high gain elements.
We study five algorithms for a small telescope array.
A real-time MSC, LMS filter is implemented.
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The Algorithms
LCMV: Linear constraint minimum variance MSNR: Max. signal to noise ratio
GSC:
Generalized sidelobe canceller
SPSN: Subspace projection spatial nulling
MSC:
Multiple sidelobe canceller
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Algorithm Applicability
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Adaptive beamforming: LCMV, GSC, MSC, MSNR:
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For more compact arrays or sub-arrays.
Single channel output – array performs as single high gain
telescope (like GBT).
Candidate for SKA sub-arrays.
Array nulling: SPSN, modified MSC:
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For large imaging arrays.
Output is full array, usable with synthesis correlator.
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Calibrated Array SINR
Improvement Comparisons
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SINR at feeds is -60 dB,
SNR = -14 dB.
Average interference
reduction of 45 dB.
Source is at zenith.
8 MHz processing bandwidth.
MSC is least affected by
grating lobes.
Grating lobes are a big
problem for small array.
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SINR Performance with
Calibration Errors
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Circular complex
Gaussian calibration
error, mean = 1,
variance = 0.01
Relative performance
among algorithms similar
to calibrated case.
Significant SINR
improvements still, but
larger variation.
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LCMV Null Placement and
Mainlobe Distortion
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LCMV for 3 element
VSA, tight 15 ft.
spacing.
Interferer is in 3m
dish mainlobe, and
array grating lobe.
Null placement and
cancellation are good.
Array mainlobe is
distorted.
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A Real-time MSC, LMS Filter
Adaptive Cancellation Experiment
Interference
Cancelled
Signal
Primary
Antenna
d[n]
+
+
-
x[n]
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Periodogram
PSD
Estimator
Ss(wk)
y[n]
h[n]
Reference
(Auxiliary)
antenna
e[n]
m
Periodogram
PSD
Estimator
Si(wk)
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VSA Antenna Array in Adaptive
Canceling Experiment
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Reference
antenna aimed
at interference /
photographer.
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Right dish is
primary
channel, aimed
at Cassiopeia.
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Real-time LMS Filter Parameters
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13 full complex adaptive FIR filter taps.
500 kHz processing bandwidth, I-Q
baseband.
Complex LMS filter update algorithm.
VSA 3m dish antennas used for signal and
reference.
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Primary antenna steered to Cassiopeia.
Reference antenna steered to roof mounted
interference source.
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Real-time LMS Filter Parameters
(cont.)
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Interferer is F.M. sweep modulation,
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100 kHz BW
Carrier centered at 1420.66 MHz
-62 dBm at dipole 95 ft. from receiver dish.
1024 bin (500 Hz per bin) periodogram spectral
estimate computed in real-time.
Integrate and download every 2s.
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Advantages of Real-time
Cancellation
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Operates on raw pre-correlator sampled data.
Can be inserted as a transparent front-end process in an
existing telescope system.
No long-duration, high data rate recording needed.
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Useful if time-sample outputs are desired, not integrations.
Post processing adaptive filtering requires huge data storage.
DSP hardware (programmable and FPGA) are now fast
enough to support desired bandwidths.
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VSA Test Platform Receiver
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Analog receiver (foreground):
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4 channels
16 MHz bandwidth
59 K total system noise
DSP array processor in
(background):
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4 channels, 65 MHz A/D.
Digital Receiver front-end.
4 TMS320C6201 floating-point
processors.
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DSP Detail
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4-200 MHz processors
with digital receiver
front-ends.
In real-time, performs:
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Complex baseband, band
select, decimate, filter.
Two channel 1024 point
periodogram and
accumulate.
13 complex tap FIR LMS
adaptive filter. (four
multiplies per tap).
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Hydrogen Line and Interference
Signals
FM interference signal seen by
reference antenna. Real-time PSD
Estimate
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Hydrogen signal from Cassiopeia seen
by primary antenna. 28 minute PSD
integration. Automatic tracking.
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Real-time Cancellation Results
Signal and Interference Seen by the
primary antenna.
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LMS MSC adaptive canceller output.
Real-time result. 30 min. integration PSD.
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Noise Floor Calibration Signal
(using RF absorber in feed)
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Conclusions
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Adaptive beamforming algorithms are promising for
compact arrays, sub-arrays, array feeds.
MSC is most robust,
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No grating lobe problems.
Useful for both beamforming and imaging arrays.
The real-time LMS MSC was very successful, should
work in many environments.
Next step: test real-time MSC with GBT.
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Hydrogen Line Signal, Cassiopeia
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FM Interference
at Reference Receiver
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Signal plus Interference
at Primary Receiver
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Adaptive Canceller Output
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