A split-aperture transmit beamforming technique with phase coherence grating lobe suppression

Abstract

A small element-to-element pitch (~.5λ) is conventionally required for phased array ultrasound transducers to avoid large grating lobes. This constraint can introduce many fabrication difficulties, particularly in the development of highfrequency phased arrays at operating frequencies greater than 30 MHz. In this paper, a new transmit beamforming technique along with sign coherence factor (SCF) receive beamforming is proposed to suppress grating lobes in large-pitch phased-array transducers. It is based on splitting the transmit aperture (N elements) into N/K transmit elements and receive beamforming on all N elements to reduce the temporal length of the transmit grating lobe signal. Therefore, the use of synthetic aperture beamforming, which can introduce relative phase distortions between the echoes received over many transmit events, can be avoided. After each transmit-receive event, the received signals are weighted by the calculated SCF to suppress the grating lobes. After pulsing all sub-apertures, the RF signals are added to generate one line of the image. Simulated 2-way radiation patterns for different K values show that grating lobes can be suppressed significantly at different steering angles. Grating lobes can be suppressed by approximately 20 dB with K = 2 at steering angles greater than 25° and an element pitch greater than 0.75λ. A technique for determining the optimal transmit sub-apertures has been developed.

Experimental verification of pulse-probing technique for improving phase coherence grating lobe suppression

“…Higher values of α result in a larger suppression of grating lobes. We have considered α = 2 in our studies [21]- [23]. In the focal region, where all the echoes are lined up after inserting the delays, the standard deviation of sign-bits of echoes is small, resulting in scF close to 1.…”

“…The simulated twoway radiation patterns have shown that this technique is theoretically quite effective for suppressing grating lobes in large-pitch phased arrays [21], [22]. The purpose of the present paper is to experimentally validate the pulseprobing technique in conjunction with phase coherence imaging.…”

“…This makes the PcI technique impractical for suppressing grating lobes in conventional transmit beamforming becasue the signals received from the grating lobe region will have a high instantaneous phase coherence over a large group of elements, making it indistinguishable from the focal region. This problem has been extensively discussed and investigated in our earlier work based on impulse-response simulations [21].…”

Angular Resolution Enhancement Provided by Nonuniformly-Spaced Linear Hydrophone Arrays in Ocean Acoustic Waveguide Remote Sensing

“…Another approach for suppressing grating lobes uses nonsinusoidal transmit signals that also remove the periodic pattern in the beamformer output [43]. Grating lobe suppression can also be accomplished via adaptive beamforming that applies a weighting factor either to the echoes received on each sensor element [44,45] or to the transmit aperture [46,47]. Previous work in optimizing array design or performance in ocean acoustic remote sensing applications are based on numerical simulation for a given signal with deterministic frequency or bandwidth [36][37][38][39][40][41][42].…”

Continental-shelf scale Passive Ocean Acoustic Waveguide Remote Sensing of marine mammals and other submerged objects including detection, localization, and classification

“…Grating lobe suppression can also be accomplished via adaptive beamforming that applies a weighting factor either to the echoes received on each sensor element [176,177] or to the transmit aperture [178,179]. Previous work in optimizing array design or performance in ocean acoustic remote sensing applications are based on numerical simulation for a given signal with deterministic frequency or bandwidth [168][169][170][171][172][173][174].…”