Multiple-Input Multiple-Output (MIMO) is a multiple antenna technology, which can improve the throughput performance of wireless communication systems. Multiple antennas can be used for both reliability and capacity enhancement with respect to different configurations of multiple antennas.
By the reception of multiple antennas at the receiver, we can improve the performance of signal reception. If two reception antennas are sufficiently uncorrelated, we can select or combine signals independently received signal at each antenna. Even if signals from two antennas are correlated each other, we can improve quality of the received signal by the coherent combining of two received signals. The quality of signal in wireless communications is usually defined as signal-to-noise-ratio (SNR) traditionally. That is, SNR can be improved by combining of two received signals coherently. If received signals are all fully correlated, we can obtain the maximum gain as follows:
SNR_ant = SNR_org * N_rx
where N_rx is the number of received antennas.