Diversity combining

Diversity combining is the technique applied to combine the multiple received signals of a diversity reception device into a single improved signal.

Various diversity combining techniques can be distinguished:

Sometimes more than one combining technique is used – for example, lucky imaging uses selection combining to choose (typically) the best 10% images, followed by equal-gain combining of the selected images.

Other signal combination techniques have been designed for noise reduction and have found applications in single molecule biophysics, chemometrics among other disciplines.

When the focus is on the wireless transmission of longer signal sequences, such as for example Ethernet packets, specific performance characteristics regarding diversity gain on a parallel redundant wireless transmission system can be observed. For "timing combining", data packets are redundantly and simultaneously sent over parallel paths. On the receiving side, out of the branches the first arriving packet is selected and immediately processed towards the end application. Further copies of the packet -if arriving- are discarded. This type of postdetection combiner is called a "timing combiner", since a significant performance improvement is gained through the immediate processing of the first arriving packet.

In land-mobile radio, where vehicle-mounted and hand-held radios communicate with a base station radio over a single frequency, space diversity is achieved by having several receivers at different sites. Diversity combining, or voting, in two-way radio systems is a method for improving talk-back range from walkie-talkie and vehicular mobile radios.

The receivers are connected to a device referred to as a voting comparator or voter.

The voting comparator performs an evaluation of all received signals and picks the most usable received signal. In repeater systems, the voted signal is retransmitted. In simplex systems, it goes to the console speaker at the base station. Audio from a receiver that is not voted is ignored. Voting comparators in analog FM systems can switch between receivers in tenths- or hundredths-of-a-second, (faster than one syllable). So long as an intelligible signal gets to a single receiver in the system, the repeated audio, or audio sent to the console speaker, would be intelligible.

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