transmitter located in a first mobile phone, wherein the transmitter comprises two or more transmitter antennas, wherein each transmitter antenna is configured to transmit an output signal;

receiver located in a second mobile phone, wherein the receiver comprises two or more receiver antennas, wherein each receiver antenna is configured to receive an input signal which corresponds to the output signals transmitted from the two or more transmitter antennas;

a filtering module located in the second mobile phone, wherein the filtering module comprises a causal finite impulse response filter having a channel degree; and

a signal processing module electronically coupled to the receiver of the second mobile phone and configured to estimate the output signals by the generation of one or more Toeplitz matrices, wherein the generation one or more Toeplitz matrices comprises the estimation of a parameter matrix by a minimization of a cost function comprising the channel degree and one or more matrices derived from the input signal,

wherein the cost function further comprises a corrective term which enforces a zero-lag matrix coefficient to be lower triangular with diagonal entries that are equal to 1,

wherein the causal finite impulse response filter is configured with a tap coefficient for each of a plurality of delay lines, and wherein the signal processing module is configured to estimate the tap coefficients from the minimization of the cost function and apply the estimated tap coefficients to the causal finite impulse response filter.