Beamforming

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Beamforming is a signal processing technique used with arrays of transmitters or receivers that controls the directionality of, or sensitivity to, a radiation pattern. When receiving a signal, beamforming can increase the gain in the direction of wanted signals and decrease the gain in the direction of interference and noise. When transmitting a signal, beamforming can increase the gain in the direction the signal is to be sent. This is done by creating beams and nulls in the radiation pattern. Beamforming can also be thought of as spatial filtering.

Beamforming takes advantage of interference to change the directionality of the array. When transmitting, a beamformer controls the amplitude and phase of the signal at each transmitter, in order to create a pattern of constructive and destructive interference in the wavefront. When receiving, information from different sensors is combined in such a way that the expected pattern of radiation is preferentially observed.

For example, to send a sharp pulse of sound towards a ship in the distance, simply transmitting that sharp pulse from every speaker in an array simultaneously fails because the ship will first hear the pulse from the speaker that happens to be nearest the ship, then later pulses from speakers that happen to be the further from the ship. The beamforming technique involves sending the pulse from each speaker at slightly different times (the speaker closest to the ship last), so that every pulse hits the ship at exactly the same time, producing the effect of a single strong pulse from a single powerful speaker.

To detect the clank of someone dropping a wrench, the beamforming technique involves combining delayed signals from each antenna at slightly different times (the antenna closest to the wrench has the longest delay), so that every clank reaches the headphones at exactly the same time, making one loud clank, as if the signal came from a single, very sensitive antenna.

The time delay is sometimes called a "phase shift", so the array of antennas, each one delayed a slightly different amount, is called a phased array.

The signal from each antenna is amplified by a different "weight." When that weight is negative by just the right amount, the faint clank received by that antenna can exactly cancel out the faint clank received by some other antenna, causing a "null." This is useful to ignore noise or jammers in one particular direction, while listening for events in other directions.

For the full mathematics on directing beams using amplitude and phase shifts, see the math section in phased array.

Beamforming techniques can be broadly divided into two categories:

conventional (fixed) beamformers
adaptive beamformers

Conventional beamformers use a fixed set of weightings and time-delays to combine the signals from the sensors in the array, primarily using only information about the location of the sensors in space and the wave directions of interest. In contrast, adaptive beamforming techniques, generally combine this information with properties of the signals actually received by the array, typically to improve rejection of unwanted signals from other directions. As the name indicates, an adaptive beamformer is able to automatically adapt its response to different situations.

Beamforming is a term that has been frequently used in the discussion of sensor networks.

1 Sonar beamforming
2 Beamforming schemes
3 See also
4 External links

[edit] Sonar beamforming

Sonar implementation is comparatively easy compared to electromagnetic implementation because of the relatively slow speed of sound in relation to the speed of the controlling hardware. Sonar arrays are used both actively and passively in 1, 2, and 3 dimensional arrays.

1 dimensional arrays are usually towed behind ships.
2 dimensional arrays are common in side-scan sonar.
3 dimensional arrays are used in 'sonar domes' in the modern submarine.

Sonar also differs from radar in that all directions can be listened to and, in theory, broadcast to, simultaneously, and the phases can be manipulated entirely by signal processing software, as compared to present radar systems that use hardware to 'listen' in a single direction at a time. At a certain level the human brain does the same thing, using signal processing on its 2 dimensional array (ears) to figure out where a sound comes from.

[edit] Beamforming schemes

A conventional beamformer is a simple beamformer also known as delay-and-sum beamformer. All the weights of the antenna elements have equal magnitudes. The beamformer is steered to a specified direction only by selecting appropriate phases for each antenna. If the noise is uncorrelated and there are no directional interferences the SNR of a beamformer with $L$ antennas receiving a signal of power $P$ is $\frac{P\cdot L}{\sigma_n^2}$ .
Null-steering beamformer
Frequency domain beamformer