Transmit Beamforming

02-05-2021 admin

Transmit Beamforming
What Is Beamforming
Transmit Beamforming Video

Throw a stone into a pond. You'll see circular waves travelling outwards from the point of impact. Throw two stones, and you'll see an interference pattern forming. At locations where maxima from the first stone coincide with the minima from the second stone, you'll get extiction, and where maxima meet maxima, you'll get amplification. Beamforming is about controlling this interference pattern, and, in most cases, it is about forming a beam-like interference pattern where the amplification occurs predominantly in one distinct direction.

In medical ultrasound, the stones are replaced by several small piezo-electric elements at the tip of an transducer, and the pond is replaced by the human body. If you apply an alternating voltage signal to an piezo-electric element, it will start vibrating and emit sound. If you select the spacing between your elements and the delay in the elements' signals just right, you can create an interference pattern that's to your benefit, in particular one in which the majority of the signal energy all goes out in one angular direction. When using the transducer to receive sound, the principles are the same. Received sound vibrations at the elements will be converted to an electric signal. Adjust the amplitude and delays of the received signal on each element, and you'll be able to receive from a chosen angular direction. If you transmit and receive along narrow beams in several adjacent directions and combine the received echo data, you can create an ultrasound image.

The 802.11n standard defines a few methods of beamforming. One method, referred to as 'explicit' beamforming, requires the downstream channel to be measured at the receiver, or beamformee, and relayed back to the transmitter, or beamformer. The beamformer uses the measured channel information to derive the transmit beamforming parameters. Adaptive Transmit Beamforming for Simultaneous Transmit and Receive by Daniel L. Gerber B.S., Massachusetts Institute of Technology (2010) Submitted to the Department of Electrical Engineering and Computer Science in partial fulfillment of the requirements for the degree of Master of Engineering in Electrical Engineering.

Beamforming visualization

In a recent student project a wave field simulator based on the Huygens principle was developed by Jon Petter Åsen. The simulator uses the graphics card to simulate and visualize arbitrary wave fields in real-time.

Research activities

The research activity the last years have mainly been within parallel beamforming. Using parallel beamforming, several receive beams are formed for each transmit event to increase the frame rate. Without any compensation, this technique is known to produce artefacts in the images. The following articles addresses the problems of parallel beamforming:

Bjåstad, Tore; Aase, Svein Arne; Torp, Hans. Velocity Sensitivity Mapping in Tissue Doppler Imaging. Ultrasonics Symposium 2005; 18.09.2005 - 21.09.2005

Hergum, Torbjørn; Bjåstad, Tore; Kristoffersen, Kjell; Torp, Hans. Parallel Beamforming using Synthetic Transmit Beams. IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control 2007;54(2):271-280