  The orthogonal bases of discrete Fourier transform (DFT) has been recognized
as the standard spatial-domain bases for Type I, Type II and enhanced Type II
codewords by the 3rd Generation Partnership Project (3GPP). For uniform planar
arrays, these spatial-domain bases are derived as the Kronecker product of
one-dimensional DFT bases. Theoretically, each spatial basis corresponds to a
beam directed towards a specific angle of departure and the set of bases
represent the orthogonal beams that cover the front hemisphere of an array.
While the Kronecker-product based precoding scheme facilitates the concise
indexing of a codeword in the codebooks through precoding matrix indicators
(PMIs) in channel state information feedback, it introduces redundant spatial
beams characterized by high spatial-frequency components. This paper
investigates the presence of codewords representing high spatial-frequency
components within the Kronecker-product based codebooks. Through theoretical
analysis and simulations, we confirm the redundancy of these codewords in MIMO
communications, advocating for their removal from the codebooks to enhance
system performance. Several topics relevant to the high spatial components are
also involved in the discussion. Practical suggestions regarding future
standard design are provided based on our theoretical analysis and simulation
results.
