The invention relates to a method for monitoring the respiration activity of a subject, comprising the acquisition of a sensor signal of at least one Doppler-Radar sensor representing the respiration activity of a subject, the transformation of the sensor signal into a transformation signal being a series according to Formula (I) where ak is a set of predetermined constant coefficients specific for one individual subject, and processing the transformation signal S(t). The transformation signal can be analyzed with basic signal processing techniques that are applied in the field of inductive plethysmography. The invention is further related to a corresponding monitoring system.