Multiple-input and multiple-output (MIMO) is when a radio system has multiple antennas in both the transmitter and the receiver. This way is possible to achieve a greater performance. This is done by exploiting the multipath propagation behavior of telecommunication signals.

Figure: MIMO

The MIMO system in LTE presents the following features:

- Transmit Diversity (TxD): On Transmit Diversity mode, the transmitter will send copies of the same data stream by each antenna. This will introduce redundancy on the system. This redundancy makes possible to reduce fading and also have a better signal-nise ratio (SNT) at the receiver. Since all the antennas are transmitting the same information there is no increase in data speed.

- Spatial Multiplexing (SM): On Spatial Multiplexing mode, the transmitter will send different signal by each antenna. Exploiting the fact that each signal from each antenna will go through a different path to get to the receiver it is possible to exploit that in order to reconstruct the signal on the receiver. Since there are many signals being transmitted in parallel it becomes possible to get higher data rates, but no diversity gains will be obtained. It is important to reminded that it is possible only because each signal goes through a different path, but since in real world there will be path correlations, that will be a big constrain in order to operate in this mode.

- Beamforming: An antenna array with closely spaced elements is used to focus de energy in the direction of the terminal. This is achieved by adapting the amplitude and gain of each antenna element to form the beam.

- Cyclic Delay Diversity (CDD): It is the increase of a delay in each signal by adding antenna specific cyclic shifts. That results in an additional multipath behavior increasing frequency diversity what will reduces intersymbol interference and improve SNR.

It is possible to switch between the Transmit Diversity mode and the Spatial Multiplexing mode. This is controlled by the eNBs A MIMO communication system can be simplified by the following equation:

Y=HX + N

X: The matrix of dimension NxT, being N the number of transmitting antennas and T the number of symbol durations.
H: The matrix that represents the propagation effect between all the N transmitter antennas and the M receiving antennas.
N: The noise over the M receiving antennas, of dimension MxT
Y: The signal received, of dimension MxT