Multiple antennas can be used in a variety of ways, mainly based on three fundamental principles, schematically illustrated in below Figure
- Diversity gain: Use of the space-diversity provided by the multiple antennas to improve the robustness of the transmission against multipath fading.
- Array gain: Concentration of energy in one or more given directions via precoding or beam forming. This also allows multiple users located in different directions to be served simultaneously (so-called multi-user MIMO).
- Spatial multiplexing gain: Transmission of multiple signal streams to a Single user on multiple spatial layers created by combinations of the available antennas.