  We propose a novel algorithm to estimate the channel covariance matrix of a
desired user in multiuser massive MIMO systems. The algorithm uses only
knowledge of the array response and rough knowledge of the angular support of
the incoming signals, which are assumed to be separated in a well-defined
sense. To derive the algorithm, we study interference patterns with realistic
models that treat signals as continuous functions in infinite dimensional
Hilbert spaces. By doing so, we can avoid common and unnatural simplifications
such as the presence of discrete signals, ideal isotropic antennas, and
infinitely large antenna arrays. An additional advantage of the proposed
algorithm is its computational simplicity: it only requires a single
matrix-vector multiplication. In some scenarios, simulations show that the
estimates obtained with the proposed algorithm are close to those obtained with
standard estimation techniques operating in interference-free and noiseless
systems.
