Wideband Radio Frequency Interference Cancellation for High-Sensitivity Phased Array Receivers With True Time Delays and Truncated Hadamard Projection

Radio frequency interference (RFI) is a significant challenge for high-sensitivity phased array instruments. RFI can be suppressed using digital signal processing, but to improve dynamic range for wideband RFI, it can be desirable to remove interference in the analog domain before sampling. In previous work, it has been shown that analog true time delay (TTD) stages with a truncated Hadamard transform can place a wide-band spatial null on RFI from a given direction of arrival. We show that TTD and Hadamard projection is mathematically equivalent to a bank of classical narrow-band subspace projection beamformers, but with a structure that allows efficient implementation in either analog circuitry or digital hardware. We analyze how loss in the TTD blocks and time delay errors affect beamformer performance and propose methods for calibrating time delays. Simulation results show that ideal TTD and Hadamard projection matches the bank of subspace projection beamformers and places deep nulls over wideband RFI signals while achieving SNR performance comparable to the maximum signal-to-interference and -noise ratio beamformer.

I. Introduction

Radio frequency interference (RFI) is a serious problem for passive spectrum users in remote sensing and radio astronomy. Time and frequency blanking, beam pattern nulling and spatial filtering, adaptive filters, and many other methods have been used for RFI mitigation [1], [2]. Traditional digital RFI mitigation schemes for phased arrays such as maximum signal-to-interference and noise beamforming and subspace projection [1] are inherently narrow band and are implemented for wideband signals using a fast Fourier transform or polyphase filterbank after sampling. The interferer must be identified and canceled separately in each subband, leading to significant computational overhead for real-time signal processing receivers. Analog phase shift beamforming does not require real time digital computation to cancel RFI, but is also limited to narrow-band signals and does easily allow computing many beams in parallel. It would be desirable if there were a compromise between the two architectures that could implement a wideband RFI cancellation beamformer in analog while preserving signals of interest for filtering by downstream digital beamformers.