An electronic device incorporating a liquid crystal display (LCD) screen comprises at least one at least one radio frequency (RF) antenna mounted behind a LCD panel of the LCD screen, and a processor. The RF antenna includes a directional transmit RF antenna that transmits an RF signal through the LCD panel to impinge on a human user, and includes a receive antenna configured to receive a RF signal reflected from tissue of the human user. The processor processes the reflected RF signal to generate a processed signal indicative of presence or absence of the human user, and responsive to the processed signal, controls an operating mode of the electronic device and/or enables control of the device by the human user.