Mitigation of non-narrowband radio-frequency interference

The rapid development and implementation of wireless communication standards put increasing pressure on spectrum allocation, and therefore threaten the efficacy of radio astronomy. For example, digital audio broadcasting (DAB) is a wide-bandwidth broadcast technology that is now being implemented and has spectrum allocated in the L band. We demonstrate that standard narrowband subspace subtraction methods may provide insufficient suppression of such signals. We therefore propose two algorithms that take into account the non-narrowband nature of these signals. The first proposed algorithm is based on a flat-frequency-response model. The second is an approximation of the first that makes use of two frequency-shifted monochromatic sources (Zatman's model). An experimental demonstration of both proposed algorithms yielded an increase of approximately a factor six in bandwidth per channel that can be processed when compared to conventional narrowband techniques (for the same attenuation of the RFI signal). The performance of the two methods was identical for LOFAR station configurations with bandwidths between 763 Hz and 195 kHz. However, the model based on Zatman's algorithm required fewer operations (a speed-up of 1.3 was achieved). An equation is derived that gives the direction of arrival for a far-field non-narrowband signal that causes the greatest distortion of the visibilities. Another equation is derived for the appropriate frequency channel bandwidth at which to implement the proposed non-narrowband RFI mitigation algorithms.