A method of operating a radar sensor system for determining a number of passengers in a vehicle passenger compartment, the radar sensor system including a radar transmitting unit having at least one radar transmitting antenna and being configured for transmitting radar waves towards the vehicle passenger compartment, a radar receiving unit having at least one radar receiving antenna and being configured for receiving radar waves that have been transmitted by the radar transmitter unit and have been reflected by passengers that are present in the vehicle passenger compartment, and an evaluation and control unit that is at least configured for evaluating Doppler information from the radar waves received by the radar receiving unit, the method comprising at least steps of - operate the radar transmitting unit for transmitting radar waves towards the vehicle passenger compartment, - operate the radar receiving unit for receiving radar waves that have been transmitted by the radar transmitting unit and that have been reflected by a passenger or by passengers being present in the vehicle passenger compartment, - operate the radar receiving unit for generating received radar signals from the received radar waves, - mathematically decompose the received radar signals into a plurality of received signal components, wherein each received signal component has a different value regarding at least one characteristic parameter, wherein the step of mathematically decomposing the received radar signals comprises to perform a discrete wavelet transform, and further comprises a step of calculating the Hilbert transform for different levels of the wavelets to determine the at least one characteristic parameter that is given by an instantaneous frequency of the different levels of the wavelets, - provide values of the received signal components regarding the at least one characteristic parameter to a classifier that has been trained by supervised learning using data representing a plurality of scenarios with different numbers of passengers in the vehicle passenger compartment, - based on the provided values of the received signal components regarding the at least one characteristic parameter, identify one of the trained scenarios, - generate an output signal that is indicative of the identified scenario. - operate the radar transmitting unit for transmitting radar waves towards the vehicle passenger compartment, - operate the radar receiving unit for receiving radar waves that have been transmitted by the radar transmitting unit and that have been reflected by a passenger or by passengers being present in the vehicle passenger compartment, - operate the radar receiving unit for generating received radar signals from the received radar waves, - mathematically decompose the received radar signals into a plurality of received signal components, wherein each received signal component has a different value regarding at least one characteristic parameter, wherein the step of mathematically decomposing the received radar signals comprises to perform a discrete wavelet transform, and further comprises a step of calculating the Hilbert transform for different levels of the wavelets to determine the at least one characteristic parameter that is given by an instantaneous frequency of the different levels of the wavelets, - provide values of the received signal components regarding the at least one characteristic parameter to a classifier that has been trained by supervised learning using data representing a plurality of scenarios with different numbers of passengers in the vehicle passenger compartment, - based on the provided values of the received signal components regarding the at least one characteristic parameter, identify one of the trained scenarios, - generate an output signal that is indicative of the identified scenario.