Antenna diversity

Antenna diversity (English "antenna diversity" ) refers to the use of multiple antennas per transmitter or receiver in the radio transmission, for example, as a strategy for reducing the interference effects. This is especially useful for mobile radio applications. A distinction between macro-diversity, the antennas are located in the part more than a hundred meters apart, and micro- diversity, in which the antennas are disposed, for example with only one wavelength distance.

Receive diversity

The propagation of radio waves are reflections of the radio wave on buildings or on the ground, similar to the sound echoes in the mountains. It may happen that the direct radio wave with a reflected radio wave at a certain point cancels (interference ), since it comes between the two because of the different path length in a run- time difference and thus to a phase shift. Is, for example, the displacement of half a wavelength, it is to eradicate or at least to a weakening of the signal.

If we use now multiple receive antennas, the probability is high that at least one of the antennas is at a location that is not affected by the signal cancellation. In the receiver according to a function is required which determines which of the antennas is receiving the best signal and uses the signal. In addition there are common problems and level fluctuations.

Principle of operation

The picture shows a method in which each antenna has its own receiver that measures the antenna signal. The measurement result is provided to an evaluation unit, which controls an electronic switch and the master receiver (e.g., a radio), the switches on the antenna having the highest level, or the best signal to noise ratio is generated ( "selection diversity "). In addition to the receivers in the antenna branches so there is a third receiver, which then processes the signal, for example, as an image or sound.

This method has the advantage of antennas and independent to each other measure without an antenna must be changed. One of the disadvantages is that for each antenna, a separate receive path must be present. This is associated with significant costs, which is currently in mobile devices (for example in the automotive industry, mobile, ...) try to avoid it.

In the field of technical radio transmission of microphone signals, the structure is a quality feature with multiple recipients and is denoted by " True Diversity ". High quality wireless microphones or their reception parts are usually constructed on this principle.

The term ' scanning diversity "refers to a process which includes the switching of different antennas to a receiver. Falls below the currently bridged antenna a fixed value or the antenna has a noise signal, so the recipient the next, fulfilling the criterion aerial is available. Other antennas can be easily integrated (another port on the switch in front of the receiver and the evaluation logic ). Throughout the system, a receiver is only available which also serves the signal evaluation for Diversity- module in addition to the messaging.

The above methods require an RF signal processing, mixed forms are possible.

The term " digital diversity " is used for a process in which the high-frequency antenna signal down-converted to baseband and then digitized. The further processing in the receiver is entirely digital, with all the offered advantages and disadvantages. One advantage is, for example, in the application of digital filter for accurate channel separation for adjacent channel interference, a disadvantage is the enormous cost of adding new antennas ( see above, increasing number of receiver branches ). With complicated methods such as are used for example in the UMTS mobile phone standard, signals of multiple antennas can be weighted and added in order to optimize the signal-to- noise ratio. Have base stations for this purpose usually several (often two) receive antennas for a cell. On mobile phone side, it is much more difficult and expensive to integrate more than one antenna in a device. Infineon ( called SMARTi ™ UE ) presents such an RF chip in 2008 for the first time.

Transmit diversity

Since some radios ( eg mobile phones ), the realization of receive diversity is too expensive, can the transmitter side ( here: cell phone base station ) to work with multiple antennas. Since the simultaneous transmission of a radio signal would lead directional effects over multiple antennas to ( unwanted ) must either be switched quickly between the transmit antennas or it is sent a time delay, which corresponds to the view of the receiver of a multipath propagation. However, this is only useful if the receiver is able to combine the signals from multiple propagation paths.

Antenna arrays

A distinction is made between space diversity and polarization diversity.

In the space diversity (identical) antennas are mounted at a certain distance to each other but with the same orientation. A minimum distance of ten wavelengths ( = thumb ) should be adhered to in order to fully exploit the diversity gain.

In polarization diversity, two antennas are mounted with an angular difference of 90 ° to each other. Since interference usually affects a particular time and place only one polarization direction can be achieved with crossed antennas diversity gain. This is due to the fact that ( for linear polarization), the plane of polarization can be rotated by diffraction or reflection of objects. For rotating the polarization can be changed by reflecting the direction of rotation. Here then are two antennas with different rotation direction for use.

In contrast to the diversity of the interference can not be reduced in special antenna pairs, but used the amplified and direction measurements.

Application Examples

Modern cars have, for example, often the antennas installed in the front, rear or side windows and partly also in the bumpers.

In almost all wireless access points now two antennas are used.

GSM and UMTS base stations mostly use receive diversity. Some manufacturers of network technology already offer in addition to transmit diversity.

Smart Antennas

Diversity goes a step further than the principle of smart antennas. Here, an array of four or more antennas is used. The signals from the individual elements are combined via adjustable phase shifter. This results in a directivity, which can be adjusted electrically. The purpose of the principle of phased array antenna.

Another use diversity in the context of HSDPA data card.

DVB- T in vehicles

Thanks diversity receivers DVB -T reception is possible even in a moving vehicle. Depending on the channel can be received possibly an interference-free TV signal. This works in the lower channels to about 160 km / h