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

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. 5 December 2018

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 5 December 2018

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 December 2018

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. - - - - - - - - 5 December 2018

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. - 5 December 2018

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. 5 December 2018

A Real-time MSC, LMS Filter Adaptive Cancellation Experiment Interference Cancelled Signal Primary Antenna Periodogram PSD Estimator d[n] e[n] + + Ss(wk) - x[n] y[n] h[n] Reference (Auxiliary) antenna m Periodogram PSD Estimator Si(wk) 5 December 2018

VSA Antenna Array in Adaptive Canceling Experiment Reference antenna aimed at interference / photographer. Right dish is primary channel, aimed at Cassiopeia. 5 December 2018

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. 5 December 2018

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. 5 December 2018

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. 5 December 2018

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. 5 December 2018

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). 5 December 2018

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. 5 December 2018

Real-time Cancellation Results Signal and Interference Seen by the primary antenna. LMS MSC adaptive canceller output. Real-time result. 30 min. integration PSD. 5 December 2018

Noise Floor Calibration Signal (using RF absorber in feed) 5 December 2018

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. 5 December 2018

Hydrogen Line Signal, Cassiopeia 5 December 2018

FM Interference at Reference Receiver 5 December 2018

Signal plus Interference at Primary Receiver 5 December 2018

Adaptive Canceller Output 5 December 2018