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In this paper, we design, implement and evaluate RF-Beep - a high-accuracy, one-way sensing, energy efficient and light-weight ranging scheme for smart devices. RF-Beep is based on the well known Time-Difference-of-Arrival (TDoA) scheme that utilizes the different propagation speeds of both the acoustic and the radio-frequency (RF) signals. Unlike the previous works, RF-Beep utilizes both the audio interface (i.e., microphone, speaker and sound driver) and the RF interface (i.e., WiFi) at the kernel-level of commercial-off-the-shelf smart devices. Implementing the scheme at lower levels enables us to understand and address the challenges related to the timing uncertainties in transmitting and receiving the acoustic signal. Moreover, RF-Beep does not require any special hardware or infrastructure support. In this paper, we describe the complete implementation of RF-Beep at the kernel space of Linux OS. We evaluate RF-Beep under different indoor and outdoor real scenarios. Results show that the error in the estimated range is less than 50cm for more than 93% of the time.