Impact of Channel Non-Reciprocity in Cell-Free Massive MIMO

In cell-free (CF) massive MIMO, a large number of access points (APs) distributed over the coverage area jointly serve a set of users over the same time/frequency resources. In this paper, we study the impact of channel non-reciprocity (NRC) and imperfect channel state information in CF massive MIMO systems. We derive analytical expressions of capacity lower bounds, including a physically inspired non-reciprocal channel model where the NRC variables vary slowly in time. The conclusion is that under conjugate beamforming, the achievable downlink rate is only sensitive to AP side phase non-reciprocity, hence the calibration requirements can be less restrictive since only the phase reciprocity errors have to be corrected. These findings are new compared to the existing literature where the NRC variables are commonly assumed to be of fast-fading nature.