The exact phrase: The N-bit overhead of CSI feedback in FDD Mu-Massive MIMO
I know that FDD is used for predetermined grid of beams with user equipments reporting their preferred beam. So, user equipments return the code of predetermined grid, but this is only one code... (or it send the whole matrix of predetermined grid?)
May be this phrase incorrect?
If you transmit a grid of beams in the downlink, then the user can measure the resulting SNR for each one of them. A codebook is then used for the user to indicate which beam it prefers. It doesn't have to be one of the beams that was transmitted, but could be a linear combination of multiple beams (since the chance is small that the channel is perfectly aligned with just one of the predetermined beams).
If N-bit feedback is used, then a codebook with 2^N entries can be used to inform the base station of the preferred beam.
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