Method for determining the distance between an authentication device and a vehicle

A method for determining the distance between an authentication device intended to be carried by a user and a motor vehicle, said device and said vehicle each comprising a wireless communication module allowing at least one data frame to be exchanged between the communication module, denoted transmitter, of the device, and the communication module, denoted receiver, of the vehicle, said data frame comprising a series of bits with a value of 0 or 1 modulated by the transmitter by changing the phase of a reference signal comprising a component, called “in-phase” component, characterized by a maximum amplitude value, and a second component, called “quadrature” component, phase-shifted by 90° in relation to the in-phase component and characterized by a maximum amplitude value, said method comprising:

• a step of the receiver receiving a modulated reference signal, which signal is sent by the transmitter and the phase changes of which characterize the values of the bits of a data frame, and
• a step of demodulating the received reference signal in order to extract the in-phase component and the quadrature component therefrom, the method being characterized in that it further comprises, following the demodulating step,
• a step of computing the power value of the reference signal on the basis of the maximum amplitude value of the in-phase component and of the maximum amplitude value of the quadrature component and
• a step of determining the distance between the transmitter and the receiver on the basis of the computed power value.