Various Techniques
Various diversity combining techniques can be distinguished:
- Selection combining: Of the N received signals, the strongest signal is selected. When the N signals are independent and Rayleigh distributed, the expected diversity gain has been shown to be, expressed as a power ratio. Therefore, any additional gain diminishes rapidly with the increasing number of channels.
- Switched combining: The receiver switches to another signal when the currently selected signal drops below a predefined threshold. This is a less efficient technique than selection combining.
- Equal-gain combining: All the received signals are summed coherently.
- Maximal-ratio combining is often used in large phased-array systems: The received signals are weighted with respect to their SNR and then summed. The resulting SNR yields where is SNR of the received signal .
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.
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