CSI RS Codebook in a Nutshell

What is Codebook ?

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What is Codebook ? It would many different things in different situation, but the meaning of Codebook under the context of CSI-RS is a set of Precoders (a set of Precoding Matrix).

Putting it other way, Codebook is a kind of matrix (a matrix having complex value elements) that transform the data bit (PDSCH) to another set of data that maps to each antenna port.

Two types of Codebook : Type I and Type II

There are two types of Codebook defined in 5G.

• Type I is based on the same logic as LTE codebook. NR Type I codebook is just a little bit more complicated and support more diverse types of matrices. Basically, this type uses a bunch of predefined matrices which is selected by UE report and RRC Configuration. (Ref [33])
• NR Type II is not based on predefined table, it is based on a specifically designed mathemical formula with a lot of parameters. Those parameters in the formula is determined by RRC and UE report. If you look at the Type II codebook, you would not see such a many different tables like Type I codebook. You would see only one table in Type II codebook but you would notice the formula defining the precoding matrix is much more complicated. With those complicated formula and a lot of parameters, Type II can apply more sophisticated precoding matrix than Type I.  (Ref [33])
• Type II is based on more detailed CSI report and designed mainly for MU-MIMO (Multi User MIMO)
• Both Type I and Type II codebook are constructed from 2-D DFT based grid of beams and enable the CSI feedback of beam selection as well as PSK based co-phase combining between two polarization.  (Ref [34])
• Comparing to Type I (Type I reports defines only the phase of selected beam, not the amplitude), Type II codebook based CSI feedback reports the wideband and subband amplitude information of the selected beams. (Ref [34])
• The difference between Type I and Type II is well illustrated by Ref [38] as shown below. The point is that Type I select only one specific beam from a group of beams, whereas type II select a group of beam and linearly combine all the beams within the group.