What is the correlation between the beam and layer in 5G?
When transferring I or 2 layers, there is a one-to-one mapping between the number of layers and the number of polarisations. ln this case, a beam can be generated by each polarisation to support the transmission of the 2 layers. When transferring more than 2 layers, it becomes necessary to use more than a single beam per polarisation. In these cases, the Codebook Mode does not have any impact and both modes apply the same selection procedure
Figure 399 illustrates the general concept of using multiple beams to support higher order MIMO. This means that multiple layers are transmitted by each polarisation but those layers are separated spatially by using different beams. These beams are pointing in different directions so the UE has to rely upon reflections and scattering to ensure that all layers can be received. The number of beams required is given by: ROUNDUP (Reported Rank I 2)
Source of above statement: 5G-NR-in-Bullets, page 483