According to the agreed way forward for Rel-10 feedback [R1-101683], a precoding matrix for a sub-band is composed of two matrices. One matrix denoted by W1 targets wideband and/or long-term channel properties, and the other matrix denoted by W2 targets frequency-selective and/or short-term channel properties. The multiplication product of these two matrices is taken as the precoding matrix for precoding the data.
In proposal R1-105011, a two-stage feedback approach is proposed, with W1 being a M×Nb matrix and W2 being a Nb×r matrix. In the W2 codebook design of this approach, there is a co-phasing factor trying to match the phase between antennas 1˜M/2 and antennas M/2˜M. Therefore, this proposal needs to carry out data feedback for such as the co-phasing factor for the uniform linear array (ULA) antennas. This results in a certain amount of overhead, and the performance of the proposal is not satisfying.