The availability of multiple antennas at the transmitter and/or the receiver can be utilized in different ways to achieve different aims:
- Multiple antennas at the transmitter and/or the receiver can be used to provide additional diversity against fading on the radio channel. In this case, the channels experienced by the different antennas should have low mutual correlation, implying the need for a sufficiently large inter-antenna distance (spatial diversity), or the use of different antenna polarization directions (polarization diversity).
- Multiple antennas at the transmitter and/or the receiver can be used to “shape” the overall antenna beam (transmit beam and receive beam respectively) in a certain way – for example, to maximize the overall antenna gain in the direction of the target receiver/transmitter or to suppress specific dominant interfering signals. Such beam-forming can be based either on high or low fading correlation between the antennas.
- The simultaneous availability of multiple antennas at the transmitter and the receiver can be used to create what can be seen as multiple parallel communication “channels” over the radio interface. This provides the possibility for very high bandwidth utilization without a corresponding reduction in power efficiency or, in other words, the possibility for very high data rates within a limited bandwidth without a disproportionately large degradation in terms of coverage. Herein we will refer to this as spatial multiplexing. It is often also referred to as MIMO (Multi-Input Multi- Output) antenna processing.