Multiple Input Multiple Output (MIMO)
The major shift in the technology in the LTE suggested by 3GPP in its Release 8 was the implementation of MIMO in the radio environment. The use of MIMO has been made mandatory as per Release 8 to all the devices except for the category 1 device.
Two simultaneous streams of different set of data are transmitted from each transmitting end and both are received simultaneously. Basic idea to separate data streams from one transmitting antenna to another at the re-ceiving end is to use the precoding technique and the reference symbols.
Unlike WCDMA where the pilot channels are used to estimate the channel quality, ref-erence symbols are used in the LTE for the channel estimation. Different set of refer-ence symbols are used for different transmitting antennas so that the receiving antenna can differentiate signals coming from different antennas. Also since the data stream is now divided, better Signal to Noise Ratio (SNR) for each channel needs to be ensured. The UE makes the channel estimation based on the reference symbol measurements, calculates the coefficient of the weight matrix and reports back to the serving eNodeB. eNodeB then adjusts the power level for different channels according to the weight ma-trix to maximize the capacity. The process is called closed loop spatial multiplexing and the weight matrix is called Precoding Matrix Indicator (PMI).
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