Implementation of 5G mm-Wave technology requires a lot of significant changes in antenna design engineers’ work. Antennas’ specification becomes more and more complicated: wideband and dual-band support, high gain, and wide coverage, scanning capabilities, and small size.
What happens with antenna elements at mmWave frequencies?
At 30GHz frequency, the wavelength is around 10mm in the air, and even smaller in dielectrics, so the antenna element becomes really small. Usually, the wireless device is bigger than 10mm, so from an antenna point of view, it contains several or tens wavelengths, which makes array placement a new significant challenge. On figure 1 you can see an mm-Wave 5G USB dongle with optimal antenna array location for end-fire radiation.
Figure 1. 5G mm-Wave USB dongle
Moving to antenna arrays instead of single elements allows beamforming and beam steering. To be able to have one main beam without significant side lobes distance between array elements should be half-lambda, which means half of the wavelength. This new requirement brings limits in radiating element size – it must be relatively small to fit. If the distance between antennas in the array becomes more than a lambda, the beam starts falling apart, especially while scanning.
Why do you even need antenna beamforming?
At higher frequencies such as mmWave signal becomes really weak due to enormous losses. To compensate that antennas are joined into arrays, which offers high directive gain and beamforming capabilities. Narrow beams decrease interference by direct targeting of users. This way cellular networks get bigger capacity and efficiency. In figure 2, a 16-element array with a total of 25 scanning positions is shown. This 4×4 array is symmetrical so it can scan in both elevation and azimuth from -60 to 60 degrees. Each of the 25 beams shown in figure 2 is achieved by setting a specific phase distribution to each antenna element in the array. In mmWave devices, this process is done typically by beamforming IC.
Figure 2. 16-element array with beam steering.
What challenges you might face with beam scanning?
By steering a narrow beam, you can achieve good enough coverage to pass EIRP and CDF requirements applied to 5G devices. However, even if the array behaves good enough at 0 degrees, it can be totally destroyed at 60 degrees by increased coupling between elements of an array. For example, in figure 3, a 60 degrees beam position is shown. You can notice that it has a sidelobe as big as the main lobe, which makes its behavior unpredictable.
Figure 3. 60-degree beam turn.
To take control over that, monitoring active S-parameters might help. When you notice, that active S-parameters become non-sufficient (above -6 level) it should be a starting point for further array improvement. Usually, corner elements behave differently from internal ones. They might require some changes in topology to compensate for those differences, so straight forward copy-past of the single antenna to array works only to some limits.
Another way to fix significant side lobes is to apply suitable amplitude distributions. It comes with a price, of course, which is decreased gain and wider beam, but it is still very helpful not only in lessening side lobes but also in creating nulls in desired directions if such nulls needed.
Beamforming in mmWave wireless devices is a must-have feature, that compensates for some disadvantages of this frequency range. Like almost everything in the 5G antenna design it has its newness and challenge but offers a lot of opportunities.