Beamforming & Beamsteering Antennas

Beamforming and beamsteering are antenna techniques that are used to form and steer a radiated beam of energy from a phased array antenna.


Phased array antennas includes:
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Antenna beam forming and antenna beam steering are technologies or techniques that are finding increasing use with systems like cellular or mobile telecommunications and in particular 5G as well as many other wireless communications.

With the ever growing need for faster data rates, higher densities of mobile equipment and the like, antenna technology is developing in line with the other technologies used.

Antenna beam forming allows an antenna system consisting of a number of individual antennas to have the direction of the overall beam to be changed by altering the phase and amplitude of the signals applied to the individual antenna elements in the array.

Techniques required to improve the performance can utilise antenna beam forming techniques to enable individual users to have an individual beam directed at them. In this way they receive the an improved signal, and other users with their own beams receive a lower level of interference.

Difference between beamforming & beamsteering

Two terms are mentioned when looking at this type of antenna technology. Although inextricably linked, there are two different aspects to the technology which are described by the two different terms:

  • Beam forming:   This term refers to the basic formation of a beam of energy from a set of phased arrays. Using phased antenna arrays it is possible to control the shape and direction of the signal beam from multiple antennas based on the antenna spacing and the phase of signal from each antenna element in the array.

    Accordingly, the creation of the beam using the technique of interfering and constructing patterns is called beamforming.

  • Beam steering:   Beamsteering takes the concept of beam forming a stage further. It is the way in which a beam pattern can be dynamically altered by changing the signal phase in real time without changing the antenna elements or other hardware.

    Beamsteering is used in many instances, from 5G to Wi-Fi to focus the radiation or reception beam on a particular station to give the maximum gain for that station, EUE, etc and reduce the interference to others.

Beamforming and beamsteering are two linked techniques, but both are incorporated into the types of antennas that are being utilised with many new communications technologies like 5G.

Antenna beam forming: the basics

Beamforming and beamsteering antennas use phase array antenna technology as the basis for their operation.

There are many types of phased array antennas, but typically a beamsteering antenna will use a number of small elements as the frequencies used tend to be relatively high, say int he 2 - 5 GHz region or more.

For many types of phased array antenna, the phase of the signal emanating from the individual elements is fixed, often by the lengths of feeder used to connect the elements to the signal source. This would give a signal that would be a right angles to the axis or plane of the antenna.

However, by controlling and varying the phase of the signals to the antenna, it is possible to provide different directive patterns. It is possible to change the directive pattern of the antenna. The directive pattern can be varied so that the main beam of radiation from the antenna is directed towards the receiver. In this way the radiated power can be used as effectively as possible.

It is normal for the various elements in the beamsteering antenna to be spaced equally apart from one another. If there is no phase difference between the different elements, then the signals will combine together and reinforce one another in a direction that is perpendicular to the plane of the elements.

However if a phase difference is applied so that each antenna element has an equal phase shift from the one adjacent to it, then the signals will constructively combine at an angle different to the perpendicular, creating a wavefront at an angle to the perpendicular.

To achieve this in a beamsteering antenna, each antenna element is fed separately with the signal to be transmitted. However each antenna feed is controlled so that the phase and amplitude to each element can be controlled. This creates the required pattern of constructive and destructive interference in the wavefront.

A beam forming antenna array can be created by using a number of closely spaced antenna elements. If they are equi-spaced a distance "d" apart, then we can see the performance as below.

Application of antenna beam-forming as used with mobile or cellular telecommunications
Application of antenna beam-forming as used with mobile or cellular telecommunications
Ψ = 2 π d λ ( sin θ )

Where
    Ψ = phase difference between two adjacent beams.

If all the elements in the array are isotropic, i.e. they radiate equally in all directions, they all have the same gain, and are driven with a signal at the same phase and power, the resultant beam will point straight out of the plane on which they are mounted.

it is also possible to alter the phasing progressively between the antenna elements of the array to form a beam at a different angle. The phase difference between the elements determines the angle of the beam.

As with any antenna, the law of reciprocity applies and the equivalent performance is obtained in the receive direction - it is just easier to visualise the power distribution in the radiated pattern from the beam forming antenna.

Naturally beamsteering antenas a far more complicated than traditional passive antennas, but they are able to provide much better performance within radio communications, mobile communications and general wireless systems to enable more users to access a base station, access point etc and obtain the optimum signal with minimum interference.

Many beamsteering antennas now incorporate the electronics to provide the required functionality, and even though they required a considerable level of development, some are manufactured as integrated modules and for remarkably low cost considering the performance and functionality. Those required for mobile communications base stations need to be able to accommodate a very large number of users and therefore they will be very complicated and require a very high level of performance.

Extra dimensions

It has been shown that the a beam can be steered in the required angle in one plane, typically the horizontal plane by using a linear antenna. This is very useful as it enables the overall azimuth to be controlled. This can be key to many radio communications or mobile communications systems, and radar as well.

However in some instances it may be necessary to control both the azimuth and elevation of the antenna. For example, for a mobile communications systems, the base station antenna may be high up, and this means that users close to the tower will need to have the radio beam directed downwards towards them. Users further away will need the beam directed at a more horizontal angle.

In just the same way that the azimuth can be controlled, it is also possible to control the elevation, or more usually the angle of declination as mobile communications users are more likely to be lower than the antenna tower.

This can be achieved by using an array of antenna elements rather than just a linear series of antenna elements.

Although the antenna as well as the drive for it, is more complicated, the same techniques are repeated but in a planar rather than a co-linear fashion.

Sidelobes

As with any directional antenna a number of sidelobes are formed. For the cases where the spacing is less than a wavelength, the side-lobes appear either side of the main lobe with decreasing levels.

However, if the array elements are spaced more widely, the strength of the side lobes increases until, when the separation distance "d" matches the signal wavelength λ, unwanted beams with the same power level as the main beam appear at ±90°.

Sidelobes are normally unwanted as they result in power being radiated in directions that do not align with the main beam. This means that the efficient of the antenna is reduced compared to what is desired.

Analogue and digital antenna beam forming

As with may areas of electronics and with digital techniques extending further into all areas, it is hardly surprising to see that there are two methods of implementing antenna beam-forming:

  • Analogue antenna beam forming:   The analogue method of beam forming is probably the most intuitive. Using the analogue approach, a single data stream is handled by a set of data converters and an RF transceiver. The RF output is split into as many paths as there are antenna elements, and each of these signal paths is passed through a phase shifter, it is then amplified and passed to the individual array element.

    Analogue antenna beam forming in the RF path is the last complicated and it also uses a minimal amount of hardware, making it the most cost-effective way to build a beam-forming array. The main drawback is that the system can only handle one data stream and generate one signal beam. This limits its effectiveness in terms of the requirements for applications such as 5G where multiple beams are required.

  • Digital antenna beam forming:   Using digital antenna beam forming, each antenna has its own transceiver and data converters. It can handle multiple data streams and generate multiple beams simultaneously from one array.

    Application of antenna beam-forming with two beams as used with mobile or cellular telecommunications
    Antenna beam-forming with two beams as used with mobile or cellular telecommunications

    Using digital antenna beam forming, it is possible to generate several sets of signals and superimpose them onto the antenna array elements. In this way it enables a single antenna array to serve multiple beams, and hence multiple users in a scenario like 5G. This normally occurs on the same frequency channel, thereby enabling the optimum spectrum efficiency.

    The approach using digital beam forming requires more hardware and puts a greater burden on the signal processing in the digital domain than the analog approach, but enables much greater flexibility and capability.

Antenna beam forming and antenna beamsteering are two powerful antenna techniques that, even though complicated to implement are providing significant benefits.

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