I. Introduction
D Ata-Independent digital beamforming is an array signal processing method that may require a significant computational load (CL). A particular example of this is the generation of 3-D acoustic images through active systems [1] using wideband pulses and deploying planar arrays, especially for sonar applications. To reduce the CL, several beamforming techniques operating in the frequency domain have been proposed [2], [3] and compared. Among them, the technique based on the chirp zeta transform (CZT) is considered the most computationally convenient implementation to perform beamforming in an exact way. In recent years, CZT beamforming [3] has been extended to planar arrays [4], [5] and near-field conditions [6] and in all cases has demonstrated a considerable computational advantage with respect to the direct method (DM) for frequency-domain beamforming [3], [7]. In [7], we proposed a comprehensive technique for wideband 3-D imaging based on CZT beamforming that is specifically tailored to investigate a portion of space encompassing the near-field and far-field regions. Even though underwater sonar imaging and ultrasound medical imaging represent the most popular applications, other 3-D imaging systems can take advantage of such a technique.