A user installs a Peer-to-Peer Payment Application on their mobile device. The first device processes the signals to be transmitted from the second device to the first device according to a model H ei, where H is a matrix representing the second communication channel and ei is a vector with all components of ei, except an ith component, being equal to zero. The closed-loop class-D amplifier circuit also includes a pulse generator to generate a pulse-width-modulated signal to drive an output stage of the closed-loop class-D amplifier based on the control signal.