1. A method of estimating distortion in the sub-carrier channels of signals received by a respective receiver from a plurality of transmitters, each of said signals including a portion that represented a plurality of known values when said signal was transmitted, said method comprising: extracting said portion of said signal to obtain a frequency domain data vector; multiplying each element of said frequency domain data vector by a respective element of a known vector to obtain a channel response vector, each element in said known vector being a function of a respective one of said plurality of known values, each element in said channel response vector being indicative of a distortion value; selecting respective ones of said channel response vector to form a plurality of further channel response vectors, each of said further channel response vectors corresponding to a respective one of said plurality of transmitters, and inserting, within said each of said further channel response vectors, at least one zero value before or after each non-zero valued element therein such that said elements of said each of said further channel response vectors correspond to a transmission pattern of said corresponding one of said plurality of transmitters; interpolating said elements of said further channel response vectors to obtain a plurality of distortion values each corresponding to a respective zero value element of said further channel response vectors, and substituting said plurality of distortion values in place of said corresponding zero value element; performing an inverse fast Fourier transform (IFFT) on each of said further channel response vectors to obtain a plurality of time domain further channel response vectors; substituting a zero value in place of any element of said time domain further channel response vectors that has a value indicative of signal noise; and performing a fast Fourier transform (FFT) on said time domain further channel response vectors to obtain a plurality of frequency response vectors.