Beamforming is a
signal processingtechnique used in sensor arrays for directional signal transmission or reception. This spatial selectivity is achieved by using adaptive or fixed receive/transmit beampattern. The improvement compared with an omnidirectionalreception/transmission is known as the receive/transmit gain(or loss).
Beamforming can be used for both
radioor sound waves. It has found numerous applications in radar, sonar, seismology, wireless communications, radio astronomy, speech, and biomedicine. Adaptive beamforming is used to detect and estimate the signal-of-interest at the output of a sensor array by means of data-adaptive spatial filtering and interference rejection.
Beamforming takes advantage of
interferenceto change the directionality of the array. When transmitting, a beamformer controls the phase and relative amplitudeof 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 in
sonar, to send a sharp pulse of underwater sound towards a ship in the distance, simply transmitting that sharp pulse from every sonar projectorin 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 projector at slightly different times (the projector 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 projector. The same thing can be carried out in air using loudspeakers, or in radar/radio using antennas.
In passive sonar, and in reception in active sonar, the beamforming technique involves combining delayed signals from each
hydrophoneat slightly different times (the hydrophone closest to the target will be combined after the longest delay), so that every signal reaches the output at exactly the same time, making one loud signal, as if the signal came from a single, very sensitive hydrophone. Receive beamforming can also be used with microphones or radar antennas.
With narrow-band systems the time delay is equivalent to a "phase shift", so in this case the array of antennas, each one shifted a slightly different amount, is called a
phased array. A narrow band system, typical of radars, is one where the bandwidth is only a small fraction of the centre frequency. With wide band systems this approximation no longer holds, which is typical in sonars.
In the receive beamfomer the signal from each antenna may be amplified by a different "weight." Different weighting patterns (e.g.,
Dolph-Chebyshev) can be used to achieve the desired sensitivity patterns. A main lobe is produced together with nulls and sidelobes. As well as controlling the main lobe width (the beam) and the sidelobe levels, the position of a null can be controlled. This is useful to ignore noise or jammers in one particular direction, while listening for events in other directions. A similar result can be obtained on transmission.
For the full mathematics on directing beams using amplitude and phase shifts, see the mathematical section in
Beamforming techniques can be broadly divided into two categories:
* conventional (fixed) beamformers or
switched beam smart antennas
* adaptive beamformers or
adaptive array smart antennas
** Desired signal maximization mode
** Interference signal minimization or cancellation mode
Conventional beamformers use a fixed set of weightings and time-delays (or phasings) 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. This process may be carried out in the time or frequency domains.
As the name indicates, an
adaptive beamformeris able to adapt automatically its response to different situations. Some criterion has to be set up to allow the adaption to proceed such as minimising the total noise output. Because of the variation of noise with frequency, in wide band systems it may be desirable to carry out the process in the frequency domain.
Beamforming can be computationally intensive. Sonar phased array has a data rate slow enough that it can be processed in real-time in software, which is flexible enough to transmit and/or receive in several directions at once. In contrast, radar phased array has a data rate so fast that it usually requires dedicated
hardwareprocessing, which is hard-wired to transmit and/or receive in only one direction at a time. However, newer field programmable gate arrays are fast enough to handle radar data in real-time, and can be quickly re-programmed like software, blurring the hardware/software distinction.
onar beamforming requirements
Sonar itself has many applications, such as wide-area-search-and-ranging, underwater imaging sonars such as
side-scan sonarand acoustic cameras. Sonarbeamforming implementation is similar in general technique but varies significantly in detail compared to electromagnetic system beamforming implementation. Sonar applications vary from 1 Hz to as high as 2 MHz, and array elements may be few and large, or number in the hundreds yet very small. This will shift sonar beamforming design efforts significantly between demands of such system components as the "front end" (transducers, preamps and digitizers) and the actual beamformer computational hardware downstream. High frequency, focused beam, multi-element imaging-search sonars and acoustic cameras often implement fifth-order spatial processing that places strains equivalent to Aegis radar demands on the processors.
Many sonar systems, such as on torpedoes, are made up of arrays of up to 100 elements that must accomplish beamsteering over a 100 degree field of view and work in both active and passive modes.
Sonar arrays are used both actively and passively in 1, 2, and 3 dimensional arrays.
* 1 dimensional "line" arrays are usually in multi-element passive systems towed behind ships and in single or multi-element side scan sonar.
* 2 dimensional "planar" arrays are common in active/passive ship hull mounted sonars and some
* 3 dimensional spherical and cylindrical arrays are used in 'sonar domes' in the modern
Sonar differs from radar in that in some applications such as wide-area-search all directions often need to be listened to, and in some applications broadcast to, simultaneously. Thus a multibeam system is needed. In a narrowband sonar receiver the phases for each beam 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.
Sonar also uses beamforming to compensate for the significant problem of the slower propagation speed of sound as compared to that of electromagnetic radiation. In side-look-sonars, the speed of the towing system or vehicle carrying the sonar is moving at sufficient speed to move the sonar out of the field of the returning sound "ping". In addition to focusing algorithms intended to improve reception, many side scan sonars also employ beam steering to look forward and backward to "catch" incoming pulses that would have been missed by a single sidelooking beam.
* A conventional beamformer can be a simple beamformer also known as
delay-and-sum beamformer. All the weights of the antenna elements can 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 signal-to-noise ratioof a beamformer with antennas receiving a signal of power is , where is Noise variance or Noise power.
Frequency domain beamformer
Beamforming history in cellular standards
Beamforming techniques used in
cellular phonestandards have advanced through the generations to make use of more complex systems to achieve higher density cells, with higher throughput.
* Passive mode: (almost) non-standardized solutions
** Wideband Code Division Multiple Access (
WCDMA) supports direction of arrival(DOA) based beamforming
* Active mode: mandatory standardized solutions
2G— Transmit antenna selection as an elementary beamforming
3G— WCDMA: Transmit antenna array (TxAA) beamforming
** 3G evolution — LTE/UMB: Multiple-input multiple-output (MIMO) precoding based beamforming with partial Space-Division Multiple Access (SDMA)
** Beyond 3G (4G, 5G, ...) — More advanced beamforming solutions to support SDMA such as closed loop beamforming and multi-dimensional beamforming are expected.
inverse synthetic aperture radar(ISAR)
Phased arrayantennas, which uses beamforming to steer the beam
Sonar, side-scan sonar
synthetic aperture radar
synthetic aperture sonar
Thinned array curse
Space–time block code
Dirty paper coding (DPC)
* Space-division multiple access
* B. D. V. Veen and K. M. Buckley. Beamforming: A versatile approach to spatial filtering. IEEE ASSP Magazine, pages 4-24, Apr. 1988.
* H. L. Van Trees, Optimum Array Processing, Wiley, NY, 2002.
* [http://www.spectrumsignal.com/publications/beamform_primer.pdf "A Primer on Digital Beamforming"] by Toby Haynes, March 26, 1998
* [http://www.vissta.ncsu.edu/publications/ahk/spm1996.pdf "Two Decades of Array Signal Processing Research"] by Hamid Krim and Mats Viberg in "IEEE Signal Processing Magazine", July 1996
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