Comparison of shifted sideband and conventional beamforming techniques

For many array configurations the exact time delays required for beamsteering impose prohibitingly large sample rate requirements on digital beamformer implementation. This necessitates the use of approximate time delays to achieve moderate sample rate requirements. For conventional beamformer implementations this degrades the sidelobe structure of the beampatterns. This degradation is also experienced by shifted sideband beamformers (SSB) which utilize frequency translated single sideband representations of the input signals [J. Acoust. Soc. Am. 64, S116(A) (1978)]. However, since beamsteering is performed on translated signals rather than on the original signals, degradation of the beampattern for the SSB is less sensitive to absolute time-delay errors than for the conventional approach. This result is supported analytically and by examples. The phase shift beamformer, a limiting case of both a frequency domain beamformer and the SSB, is considered. Beamsteering errors experienced by the phase shift beamformer at frequencies other than that for which it was designed are discussed. Analytical results and examples are presented which demonstrate that these errors can be eliminated with the SSB.