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 first cross-correlation matrix; cross-correlating the first matrix with itself to form a second cross-correlation matrix; and multiplying the second cross-correlation matrix with a product of the first cross-correlation matrix to form a fifth matrix in which the elements of the fifth matrix characterize the radio channel traversed by transmissions of the system user.