3GPP LTE standard TS36.211 specifies the down link MIMO (Multiple-Input Multiple-Output) in section 6.3. The specification defines the DL MIMO in two types: transmit diversity and spatial multiplexing.
The characteristics of DL MIMO are as follows:
1. Precoding for transmit diversity adopted SFBC (Space Frequency Block Coding) for two antenna ports and balanced SFBC-FTSD (Frequency Shift Transmit Diversity) for four antenna ports.
2. Special multiplexing is used for improving the peak data rates, cell capacity and throughput.
3. Maximum number of cordwords is limited to two, and 1-2 mapping is used for three layers case and 2-2 mapping is used for four layers case (rf. Table 6.3.3.2-1: Codeword-to-layer mapping for special multiplexing).
4. A set of precoding matrices known at the UE and BeNodeB is constructed, and a codebook method is adopted to reduce the overhead in channel state information feedback (rf. 6.3.3.2.3 Codebook for precoding).
5. The precoding for special multiplexing is defined by the precoding matrix [W], which is selected among the precoder elements in the codebook.
6. Large-delay CDD (Cyclic Delay Diversity), defined by matrix [D][U] (rf. Table 6.3.4.2.2-1: Large-delay cyclic delay diversity), can be used compositely with precoding: [W][D][U].
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