Multi Antenna Systems

Multi Antenna Systems – fast facts

Basic characteristic

• The relation of the distance between the antenna elements in a multi antenna system and the mutual correlation between the radio-channel fading that is experienced by the signals at the different antennas defines many of the properties and capabilities of the system.

Benefits of multi antenna techniques

• Multiple antennas at the receiver and/or the transmitter can be used to provide additional diversity against fading on the radio channel.
• Multiple antennas at the receiver and/or the transmitter can be used to shape and direct the antenna beam in a certain way.
• Spatial Multiplexing (MIMO): Multiple antennas at both the transmitter and the receiver can be used to create what can be seen as multiple, parallel channels.

Maximum Ratio Combining (MRC) weights:

• Phase rotation of the signals received at different antennas to make them phase aligned when adding them together (coherent combining).
• Weight the signals in proportion to their corresponding channel gains, that is apply higher weights for signals that are received stronger (= more reliable).

Interference Rejection Combining (IRC)

• In situations with a single or a small number of interferers, improved performance can be achieved if selecting the weights so that the interferer is suppressed maximally.

The antenna weights can also be selected to achieve Minimum Mean Square Error (MMSE) combining.

In case of OFDM, due to the narrow bandwidth of the sub-carriers, there is little or no frequency selectivity on the channel. This means that low-complexity combining schemes such as MRC and IRC can be used.

Transmit antenna diversity

• Multi-path propagation may actually be beneficial if it can be used for delay diversity (RAKE)
• Space time coding is a general concept used to describe multi antenna transmission schemes where the symbols are mapped both in time and the spatial domain that the multiple antennas can provide.

Transmitter side beam forming

• If some knowledge of the downlink channel (phase) from the different antennas exist at the transmitter, the antennas can be used to provide beam-forming in a certain direction.
• The overall transmission beam can be steered in a certain direction by applying different phase shifts to the signals to be transmitted by the antennas.
• Beam-forming can increase the received signal strength with a factor that is in relation to the number of antennas

Spatial multiplexing (MIMO)

• Multiple antennas at both sides (transmitter and receiver)
• Possible to utilize high signal to noise ratio for higher data rates. F ex near the base station and/or in very good channel conditions.
• Data rate can be increased by a factor relative to the number of simultaneous receiver and transmitter antennas (rank, layers)
• In low signal to noise conditions, spatial multiplexing is less efficient as compared to beam-forming and/or diversity.