A method comprising: at a first transceiver, transmitting a plurality of signals to a second transceiver and receiving corresponding receive signals from the second transceiver, wherein each transmitted signal is sent using a channel perturbation; measuring a plurality of phase values, wherein each phase value is a phase difference between one of the plurality of transmitted signals and corresponding receive signal; masking a subsequent phase modulated signal employing phase rotation at the first transceiver using the plurality of phase values.