Towards an Acoustic Keyless Entry System: Leveraging Off-the-Shelf Wideband Ultrasound for Cost-Effective Approach Detection

Digital car keys have become a central feature of smart car entry systems. However, existing solutions face some limitations: Bluetooth RSSI-based systems are prone to insufficient localization accuracy, while ultra-wideband (UWB) solutions come with high hardware costs. In this paper, we present an acoustic-based keyless entry system that leverages off-the-shelf audio hardware already integrated into modern cars. The system enables interaction-free unlocking by employing high-frequency acoustic signals transmitted from a smartphone and captured by the microphone array mounted on the car. By fully exploiting the inherent wide bandwidth of this commercially available audio system, we introduce a novel multi-frequency-based virtual microphone technique that significantly enhances angle estimation accuracy. Additionally, to improve the acoustic signal-to-noise ratio, we design a novel coprime multi-frequency waveform that concentrates signal energy and boosts overall effectiveness. Using only three microphones, the proposed system achieves an angle estimation performance comparable to that of a 21-microphone array. Extensive real-world experiments validate that the proposed approach offers a robust, low-cost passive keyless entry solution capable of operating reliably across diverse environmental and interference conditions.