Broadband low-sidelobe beamforming in time domain

In modern active and passive sonar systems, broadband digital beamforming for acoustic arrays is widely used to suppress unwanted interference and to detect interested target signals. A broadband low-sidelobe beamforming procedure in time domain is proposed. The first step of this procedure was to delay the outputs of each element in the acoustic array by digital delay lines to accomplish the integer part of the time delay. Then, finite impulse response (FIR) digital filters were used to implement the fractional part of the time delay. The weighting matrix for all array elements at different frequencies for low-sidelobe beam was also implemented with the FIR digital filters. Finally, the outputs of the digital filters were summed up yielding the time domain beam output. The design of low-sidelobe beam patterns and that of the FIR digital filters are two crucial technical issues in this beamforming procedure. We used the improved adaptive approach for the design of FIR digital filters, and the design requirements of these filters were specified by the weights which formed the low-sidelobe beams of each sub-band over the broad frequency band. The low-sidelobe beams of each sub-band were designed using the optimized beam synthesis approach based on the principle of MVDR beamforming. Results of computer simulation and lake-experiment for a twelve-element arc array showed that the beamforming procedure was not only correct but also promising in practical acoustic environment.