Analysis of Adaptive Array Algorithm Performance for Satellite Interference Cancellation in Radio Astronomy

1
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

2
Summary

• 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.
• 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 GSC Generalized sidelobe canceller MSC
Multiple sidelobe canceller
MSNR Max. signal to noise ratio SPSN Subspace
projection spatial nulling
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Algorithm Applicability

• Adaptive beamforming LCMV, GSC, MSC, MSNR
• 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
• For large imaging arrays.
• Output is full array, usable with synthesis
  correlator.

5
Calibrated Array SINR Improvement Comparisons

• 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

• 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

• 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
Periodogram PSD Estimator
dn
en

Ss(wk)

-
xn
yn
hn
Reference (Auxiliary) antenna
m
Periodogram PSD Estimator
Si(wk)
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VSA Antenna Array in Adaptive Canceling Experiment

• Reference antenna aimed at interference /
  photographer.
• Right dish is primary channel, aimed at
  Cassiopeia.

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Real-time LMS Filter Parameters

• 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.
• Primary antenna steered to Cassiopeia.
• Reference antenna steered to roof mounted
  interference source.

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Real-time LMS Filter Parameters (cont.)

• Interferer is F.M. sweep modulation,
• 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

• 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.
• 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

• Analog receiver (foreground)
• 4 channels
• 16 MHz bandwidth
• 59 K total system noise
• DSP array processor in (background)
• 4 channels, 65 MHz A/D.
• Digital Receiver front-end.
• 4 TMS320C6201 floating-point processors.

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DSP Detail

• 4-200 MHz processors with digital receiver
  front-ends.
• In real-time, performs
• 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
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.
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

• Adaptive beamforming algorithms are promising for
  compact arrays, sub-arrays, array feeds.
• MSC is most robust,
• 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 Interferenceat Reference Receiver
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Signal plus Interference at Primary Receiver
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Adaptive Canceller Output