Figure shows how the two codewords are used for a single user in the downlink. It is also possible that the code words to be assigned to different users to create multi-user MIMO (MU-MIMO). Depending on the channel information available in the eNB, modulation and precoding layers may be different to match the performance.
Precoding choices are defined in the search, known as codebook table. Codebook is used to quantize the options available, and thus limits the amount of information fed back from the receiver to the sender. Some precoding choices are clear. For example, Codebook index (CI) 0 is a direct set of codewords to layers and CI 1 applies spatial expansion.
Table 1 shows the choices codebook for one and two layers. Note only the case in two layers employs spatial multiplexing. Precoding with a layer is limited to a displacement 0 °, 90 ° or 180 ° of phase.
In operation, the UE will send a message to the scheduler ENB with the index codebook that is closest to the channel, even if the system can be configured for multiple codebook values, one for each group of resource block. To use this information while it is still valid, the scheduler must respond quickly, in a few milliseconds, depending on the speed of variation of the channel. If the EU is to provide information on a more regular channels, the information will be more accurate, but the percentage of resources used to increase the signal and set more challenging on ENB.