Radio waves are polarized waves

Radio waves with angular momentum: Is the shortage of frequencies over?

In the last few weeks, a groundbreaking research result has been making the rounds: Rotated waves should make it possible to transmit several signals at the same time on one frequency. In theory, there can even be any number. Will the shortage of frequencies, which has so far caused the mobile network providers to buy billions of dollars in licenses, soon be a thing of the past?

Inventory: What is already happening on one frequency today

Polarization at a 45 degree angle
Graphic: In order to take away the mystical effect of the researchers led by Fabrizio Tamburini, let us briefly introduce what is already possible today in order to transmit several signals on one frequency. Essentially, these are polarization and MIMO ("multiple input, multiple output").

The polarization indicates in which direction the electrical waves of a radio or mobile radio wave vibrate. This can be up / down (vertical), left-right (horizontal) or any other angle. Even circular polarization is possible, in which the plane of polarization rotates continuously as a wave progresses.

However, electromagnetic waves, which incidentally include not only radio waves but also light, have some complex properties. With regard to polarization, these essentially mean: Even if any number of polarization states are possible - you just have to set the polarization angle precisely enough - it is not possible to distinguish more than two types of polarization in the receiver. A circular polarization, or also a polarization at a 45 ° angle, can be broken down into "50 percent horizontal" and "50 percent vertical", for example. As a result, only one pair of opposite polarizations can be used for data transmission, for example "horizontal and vertical", "+ 45 ° and -45 °" or "counterclockwise and clockwise circular".

Even if the above statement is initially discouraging: Polarization is a tried and tested means of doubling the transmission capacity on a frequency. The best-known application is probably the 3D cinema: There are usually two polarization filters in the 3D glasses, one for left-handed light, one for right-handed light. In this way, two different versions of the same film - one from the perspective of the left eye, one from the perspective of the right eye - are simultaneously transmitted from the projector via the screen to the viewer.

But polarization is also used in classic broadcast technology, especially in satellite broadcasting: in the lists with the channel occupancy, in addition to the frequency, it always states whether there is horizontal and vertical polarization. Even if the exact same frequency is usually not polarized horizontally and vertically: The individual bands are typically 36 MHz wide, so that, for example, 11464 MHz are horizontal (Pro Sieben and Sat.1 in HD on Astra) and 11479 vertical (some French SD channels on Astra) overlap and could not be separated without polarization.

Complex calculation: MIMO

vertical polarization
Graphic: Even more capacity increases are possible with MIMO technology: Here, both the transmitter and the receiver use several antennas at the same time and on the same frequency. Even if you would like to think that only a "cacophony" of superimposed signals arrives at the recipient: The modern signal processing chips in the mobile phone and base station can separate the multiple voices from each other again.

horizontally polarized wave
Graphic: However, there are also problems here: To quadruple the transmission capacity, for example, four times more antennas are required. If you were to simply use four times the bandwidth instead, one antenna would still be sufficient. In addition, the antennas should, if possible, be at least a whole wavelength apart, but in any case a not only insignificant part thereof. When it comes to cell phones, where smaller devices are usually more popular than large devices, you quickly reach their limits. Multipath propagation, if the signal is reflected on house walls, the ground or other surfaces, can make the use of MIMO difficult or even impossible.

On the following page we explain the newly developed process in detail and how you can visualize it.