Over the past few years, it has been shown that using multiple antennas can significantly increase the capacity and robustness of communication systems in fading environments. Capacity grows with the number of antennas used. Approximately twice the amount of information can be communicated using two transmit antennas and two receive antennas, without spending any extra time, bandwidth, nor power.
The multiple input multiple output (MIMO) scheme deploys multiple antennas at the transmitter or/and at the receiver. The data stream from a single user is demultiplexed and fed into the respective transmitting antennas all of which radiate in the same frequency band. By sharing the same frequency band the spectral efficiency is improved. The receiver is assumed to have ideal channel estimates so it can separate and decode the symbols transmitted from each antenna. The ability to separate out the symbols is due to the fact that in a scattering environment, the signals received at each receiving antenna from each transmitting antenna appear to be uncorrelated.