According to various illustrative embodiments, a motion-monitoring system comprises at least one antenna fixed within a barrier, the at least one antenna transmitting at least one first radio-frequency signal through the barrier to at least one volume outside the barrier, the at least one first radio-frequency signal reflecting from at least one object in the at least one volume to produce a plurality of second radio-frequency signals, which are received by the at least one antenna, at least one sensing circuit sensing the plurality of second radio-frequency signals and generating a plurality of time-domain output signals, and at least one processor receiving the plurality of time-domain output signals and comparing at least one successive time-domain output signal to at least one previous time-domain output signal to estimate at least one characteristic of the at least one object, including a motion of the at least one object.