ucture is similar to that of LTE FD-MIMO code- books, except rank 3/4 codebooks for 16, 24, and 32 antenna ports at the gNB. Let us con- sider codebook subset restriction for less than 16 antenna ports at the gNB. In this case, the beamforming vector for PMIs of all ranks is represented by 2D DFT beam denoted as bi, which is represented as Kronecker product of two 1D DFT vectors bi=unq wherein [66] e2nj((N,-1)/N101)n e27jj((N2-1)/N2O2)1 In this case, in order to restrict transmission in specific direction, a bitmap with size N1N2O1O2 can be specified, where each bit ai corresponds to DFT beam bi. If at least one layer of the PMI consists of bi, the PMI is considered to be restricted and cannot be 642 Chapter 4 Figure 4.90 Illustration of precoding for multi-panel antenna array [66]. reported by the UE. Rank 3/4 PMIs for 16, 24, and 32 antenna ports have different struc- tures compared to Type-I single-panel PMI. Therefore, it may not be possible to use the same approach for all ranks. The multi-panel codebook is constructed by DFT-based beam- forming per each panel and co-phasing of polarization and panels, where the same DFT beam is applied for all panels and polarizations. An example is shown in Fig. 4.90, where precoder p for rank-1 multi-panel codebook and two-panel antenna can be computed as fol- lows, in which C1, C2, C3 coefficients are independently reported in accordance to mode 2 of multi-panel codebooks [66]. In the preceding example, it is assumed that the antenna port indexing is performed in such way that [b c1b] corresponds to beamforming vector of the first polarization and [b c3/c2b] to beamforming vector of the second polarization. The direction of the transmission in the above PMI structure is determined by the DFT beam denoted by vector b and co-phasing coefficients C2 and C3/C2. Therefore, codebook subset restriction for multi-panel codebook PMI restriction consider all possible combina- tions of DFT beams and co-phasing coefficients, which are determining the direction of the transmission. T