Antenna diversity
Antennendiversität ( English antenna diversity ) describes in radio technology the use of several antennas per transmitter or receiver . With the diversity achieved , disruptive interference can be reduced. This is particularly useful for mobile radio applications.
A distinction is made between macrodiversity, in which the antennas are sometimes far more than a hundred meters and more than a few hundred wavelengths apart, and microdiversity, in which the antennas are arranged in the range of one or a few wavelengths.
Reception diversity
When radio waves propagate, reflections of the radio wave occur on building walls or the ground, similar to sound echoes in the mountains. It can then happen that the direct radio wave is canceled out with a reflected radio wave at a certain point.This is referred to as destructive interference , as there is a transit time difference between the two due to the different path length and thus a phase shift. If the phase shift is half a wavelength, cancellation occurs. If the phase shifts differ slightly from this, the signal is weakened.
If several receiving antennas are used, the probability is high that at least one of the antennas is located at a point that is not affected by the signal cancellation. Accordingly, a function is required in the receiver that recognizes which of the antennas is currently receiving the best signal and then uses that signal. Regardless of this, there are also general disturbances and fading .
Working principle
The picture shows a method with switching in the baseband , in which each antenna has its own receiver that measures the antenna signal . The measurement result then goes to an evaluator that controls an electronic switch and connects the antenna to the main receiver (for example a radio), which generates the highest level or the best signal-to-noise ratio (“selection diversity”). In addition to the receivers in the antenna branches, there is also a third receiver which then processes the signal, for example as a picture or sound.
This method has the advantage of being able to measure antennas independently of one another without having to change antennas. One of the disadvantages is that there must be a separate reception branch for each antenna. This is associated with considerable costs, which are avoided as far as possible, especially in mobile devices (for example in the automotive industry, in mobile communications, ...).
In the field of radio transmission of microphone signals , the construction with several receivers is a quality feature and is referred to as "true diversity". High-quality radio microphones or their receiving parts are usually built according to this principle.
The term “scanning diversity” describes a process that involves switching different antennas to a receiver. If the currently connected antenna falls below a specified value or if the antenna has an interference signal, the receiver is offered the next antenna that fulfills the criterion. Additional antennas can be easily integrated (another port on the switch in front of the receiver and the evaluation logic). There is only one receiver in the entire system, which, in addition to transmitting messages, also serves to evaluate signals for the diversity logic.
The procedures mentioned require HF signal processing; mixed forms are possible.
The term “digital diversity” is used for a process in which the high-frequency antenna signal is mixed down into the baseband and then digitized. Further processing in the recipient is completely digital, with all the advantages and disadvantages that arise. One advantage is, for example, the use of digital filters for precise channel separation in the event of adjacent channel interference, a disadvantage is the enormous expense involved in integrating additional antennas (see above, increasing number of receiver branches). With more complex processes, such as those used in the UMTS mobile radio standard, the signals from several antennas can be weighted and added in order to optimize the signal-to-noise ratio . For this purpose, base stations usually have several (often two) receiving antennas for one cell. On the cell phone side, it is far more difficult and expensive to include more than one antenna in a device. Infineon presented such an HF chip (called SMARTi ™ UE +) for the first time in 2008.
Broadcast diversity
Since the implementation of reception diversity is too complex in some radio devices (for example cell phones), several antennas can also be used on the transmitter side (here: cellular radio base station). Since the simultaneous transmission of a radio signal via several antennas would lead to (undesirable) directional effects, it is either necessary to switch quickly between the transmitting antennas or it is sent with a time delay, which corresponds to multipath propagation from the recipient's point of view. However, this only makes sense if the receiver is able to combine the signals from several propagation paths.
Antenna arrangements
A distinction is made between spatial diversity and polarization diversity.
In the case of spatial diversity, (identical) antennas are mounted at a certain distance from one another but with the same orientation. A minimum distance of ten wavelengths (= rule of thumb ) should be adhered to in order to fully exploit the gain in diversity.
With polarization diversity, two antennas are mounted with an angle difference of 90 ° to each other. Since interference usually only affects one polarization direction at a certain point in time and place, an increase in diversity can be achieved with crossed antennas. This is due to the fact that (with linear polarization) the plane of polarization can be rotated by diffraction or reflection on objects. With rotating polarization, the direction of rotation can be changed by reflection. Two antennas with different directions of rotation are used here.
In contrast to diversity, with special antenna pairs the interference is not reduced, but amplified and used for direction measurements.
Application examples
Modern cars, for example, often have antennas built into the front, rear or side windows and sometimes also into the bumpers.
Many WLAN - access points multiple antennas are now being used.
GSM and UMTS base stations mostly use reception diversity. Some manufacturers of network technology already offer broadcast diversity.
Because of their movement in the transmission area, wireless microphones rely on diversity in order to deliver an uninterrupted signal. Here the receivers have two separate receivers and switch inaudibly to the better signal.
Smart antennas
The principle of smart antennas goes one step further than diversity. An array of four or more antennas is used here. The signals from the individual elements are combined using adjustable phase shifters. This creates a directional effect that can be adjusted electrically. The principle of the phased array antenna is used for this .
Another application is diversity in the context of HSDPA data cards .
DVB-T in vehicles
Thanks to diversity receivers, DVB-T reception is also possible in a moving vehicle . Depending on the channel, it may be possible to receive an interference-free TV signal. This works in the lower channels up to about 160 km / h.
See also
literature
- Eberhard Spindlert: The great antenna book. 11th edition, Franzis-Verlag, Munich 1987, ISBN 3-7723-8761-6
- Jürgen Detlefsen, Uwe Siart: Basics of high frequency technology. 2nd edition, Oldenbourg Verlag, Munich / Vienna 2006, ISBN 3-486-57866-9
- Hans Lobensommer: Handbook of modern radio technology. 1st edition, Franzis Verlag, Poing, 1995, ISBN 3-7723-4262-0
Web links
Individual evidence
- ↑ Product description infineon.com
- ↑ Press release, February 11, 2008 infineon.com