The present invention relates to a motion detector device (1) comprising a receiver (2) arranged for reception of at least one electromagnetic signal (3b, 4b, 5b) constituted by a corresponding transmitted electromagnetic signal (3a, 4a, 5a) being transmitted by a corresponding source (6, 7, 8) and influenced by a corresponding channel (9; 9a, 9b, 9c). The motion detector device (1) comprises predetermined information regarding each transmitted electromagnetic signal (3a, 4a, 5a). The motion detector device (1) further comprises analyzing means (10) arranged to analyze all components of the received signal to determine how certain parameters of each transmitted electromagnetic signal (3a, 4a, 5b) are influenced by each corresponding channel (9; 9a, 9b, 9c) by means of the predetermined information. The analyzing means (10) is also arranged to analyze the temporal variation of said certain parameters during a certain time. It is determined if said temporal variation exceeds a predetermined threshold. The present invention also relates to a corresponding method.