Short summary of the functions and features of MIMO

Multiple Input Multiple Output (MIMO) may in a most generalized way be viewed as the use of pre-coding with multiple antennas at both the transmitter and the receiver arranged to operate in one of the 2 following ways: 

• Single stream: both diversity and array (beamforming) gain can be accomplished for increasing the carrier-to-interference-ratio which is typically used in bad channel conditions or at the cell edge borders for increased coverage. The pre-coding weights are selected such that the data streams from the two antennas can be combined coherently into a single stream. A new and modified type of CQI is used that consists of 5 bits carrying the Pre Coding Information (PCI) and the CQI itself.
• Dual stream: Transmission in multiple layers or streams for increasing the maximum achievable data rate. This is also known as spatial multiplexing and requires high carrier-to-interference-ratio and is therefore typically only used near the nodeB and in good channel conditions. Pre-coding is used in this case to create two orthogonal data streams that can carry separate flows of information. By choosing the weights for the second stream as the orthogonal eigenvectors of the covariance matrix at the receiver, the two streams will not interfere with each other and the bit rate may be doubled in this way. The physical layer HARQ processing for each stream is identical to the single stream case meaning that one ack/nack is transmitted for each stream. The CQI in this case has been extended to 8 bits and contains separate information for each flow.

All devices supporting MIMO has to be capable of receiving 15 channelization codes. The system has been extended in later/coming releases for higher number of layers/ranks each requiring an additional pair of transmit/receive antennas and allowing for a doubling in the bitrate for each doubling in the layers/rank.