1. A detector for a user input to a mobile device, the detector comprising: an ultrasound demodulator 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; and 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.