User Centric Feedback Bit Allocation in CF-mMIMO under LoS/NLoS Channels

We consider the uplink of a cell-free (CF) massive multiple-input multiple-output (mMIMO) system under prob-abilistic LoSINLoS channels with limited feedback capacity between the access points (APs) and the central processing unit (CPU). We note that due to the limited feedback capacity, the information shared by the APs needs to be quantized before transmission, and use this fact to derive the rates achievable by the different users for different numbers of bits being used by the different APs. We then discuss the max-min feedback bit allocation strategy to develop the analogous optimization prob-lem, and observe its computational in-feasibility. Following this, we develop a user centric feedback bit allocation strategy that exploits the significantly different channel qualities offered by LoS and NLoS channels. Finally, we validate the performance of the proposed bit allocation scheme via extensive simulation experiments, and find that it leads to a performance almost as good as the unquantized case.