Antenna beam forming and antenna been steering are technologies or techniques that are finding increasing use with systems like cellular telecommunications and in particular 5G as well as many other wireless systems.
Antenna beam forming allows an antenna system consisting of a number of individual antennas to have the direction of the 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.
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
As already mentioned the beam-forming antenna system consists or a number of individual antennas set up as an array.
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 a pattern of constructive and destructive interference in the wavefront.
The individual feed signals are controlled so that the overall sum of the instantaneous amplitudes from the different antenna elements add or subtract in such a way that the required beam is created.
A beam forming antenna array can be created by using a number of closely spaced antenna elements. If they are equispaced a distance "d" apart, then we can see the performance as below.
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 element synth e array to form a beam at a different angle. The case 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.
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.
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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