The use of multiple antenna technology allows the exploitation of the spatial domain as another new dimension. This becomes essential in the quest for higher spectral efficiencies
With the use of multiple antennas the theoretically-achievable spectral efficiency scales linearly with the minimum of the number of transmit and receive antennas employed, at least in suitable radio propagation environments.
Multiple antenna technology opens the door to a large variety of features, but not all of them easily deliver their theoretical promises when it comes to implementation in practical systems.
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.
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