A method for estimating transmitted signals transmitted from a plurality of transmitters, the method comprising: simultaneously receiving at each of a plurality of receivers a respective received signal resulting from the transmitted signals; determining for each receiver a set of equation coefficients; forming for each received signal a characterisation of that received signal composed of elements weighted in accordance with the equation coefficients determined for the receiver at which that received signal was received; and processing the characterisations to estimate the transmitted signals; wherein the equation coefficients are determined such that the characterisations are substantially linearly independent.