  This work investigates the impact of imperfect statistical information in the
uplink of massive MIMO systems. In particular, we first show why covariance
information is needed and then propose two schemes for covariance matrix
estimation. A lower bound on the spectral efficiency (SE) of any combining
scheme is derived, under imperfect covariance knowledge, and a closed-form
expression is computed for maximum-ratio combin- ing. We show that having
covariance information is not critical, but that it is relatively easy to
acquire it and to achieve SE close to the ideal case of having perfect
statistical information.
