A detector for a user input to a mobile device, the detector comprising: an ultrasound demodulator (200, 300) having an input for receiving an ultrasound signal reflected from a user and an output for outputting a demodulated ultrasound signal; a gesture processor comprising: a time-frequency processing module configured to generate a time-varying ultrasound-image spectrum from the demodulated ultrasound signal; an image-feature extraction module configured to extract micro-doppler features from the time-varying ultrasound image spectrum; a feature selection module configured to select and compress the extracted micro-doppler features; a gesture detection module configured to compare the selected micro-doppler features with a known feature set and to output a detected gesture based on the comparison; characterized by an activation controller (216, 314) arranged between the ultrasound demodulator (200, 300) and the gesture processor, wherein the activation controller (216, 314) is operable to identify the ultrasound signal as a candidate gesture in response to the amplitude of the demodulated ultrasound signal exceeding a predetermined threshold, and to enable the gesture processor in response to a candidate gesture being identified. an ultrasound demodulator (200, 300) having an input for receiving an ultrasound signal reflected from a user and an output for outputting a demodulated ultrasound signal; a gesture processor comprising: a time-frequency processing module configured to generate a time-varying ultrasound-image spectrum from the demodulated ultrasound signal; an image-feature extraction module configured to extract micro-doppler features from the time-varying ultrasound image spectrum; a feature selection module configured to select and compress the extracted micro-doppler features; a gesture detection module configured to compare the selected micro-doppler features with a known feature set and to output a detected gesture based on the comparison; characterized by an activation controller (216, 314) arranged between the ultrasound demodulator (200, 300) and the gesture processor, wherein the activation controller (216, 314) is operable to identify the ultrasound signal as a candidate gesture in response to the amplitude of the demodulated ultrasound signal exceeding a predetermined threshold, and to enable the gesture processor in response to a candidate gesture being identified. a time-frequency processing module configured to generate a time-varying ultrasound-image spectrum from the demodulated ultrasound signal; an image-feature extraction module configured to extract micro-doppler features from the time-varying ultrasound image spectrum; a feature selection module configured to select and compress the extracted micro-doppler features; a gesture detection module configured to compare the selected micro-doppler features with a known feature set and to output a detected gesture based on the comparison; characterized by an activation controller (216, 314) arranged between the ultrasound demodulator (200, 300) and the gesture processor, wherein the activation controller (216, 314) is operable to identify the ultrasound signal as a candidate gesture in response to the amplitude of the demodulated ultrasound signal exceeding a predetermined threshold, and to enable the gesture processor in response to a candidate gesture being identified.