1. A method comprising: measuring radio frequency signals received at different elements of an antenna system over time, the received signals corresponding at least in part to transmissions received from a system user; accumulating the signal measurements into a first matrix; generating a second matrix representing the transmissions of the system user; cross-correlating the first and second matrices to form a third matrix; and multiplying the third matrix with a product of the second matrix to form a fourth matrix in which the elements of the fourth matrix characterize the radio channel traversed by transmissions of the system user.