Multi-antenna transmission is also called Multiple Input Multiple Output (MIMO). MIMO can increase the efficiency of data transmission and reception using multiple transmit antennas and multiple receive antennas. MIMO schemes include spatial multiplexing, transmit diversity, beamforming, etc. A MIMO channel matrix formed according to the number of receive antennas and the number of transmit antennas can be decomposed into a plurality of independent channels and each independent channel is called a layer or stream. The number of layers or streams or a spatial multiplexing rate is called a rank.
While the legacy 3GPP LTE system (e.g., 3GPP LTE release 8 or 9) supports downlink transmission through up to 4 transmit antennas, the 3GPP LTE-A standard discusses support of downlink transmission through up to 8 transmit antennas in 3GPP LTE-A system evolved from the 3GPP LTE system.