RSSI-Based Direction-of-Departure Estimation in Bluetooth Low Energy Using an Array of Frequency-Steered Leaky-Wave Antennas

This paper presents a novel advanced Bluetooth Low Energy (BLE) beacon, which is based on an array of frequency-steered leaky-wave antennas (LWAs), as a transmitter for a Direction-of-Departure (DoD) estimation system. The LWA array is completely passive, fabricated in a low-cost FR4 printed-circuit board and designed to multiplex to different angular directions in space each one of the three associated BLE advertising channels that are used for periodically transmitting the ID of the beacon. This way, the use of more expensive hardware associated to electronic phased-array steering/beam-switching is avoided. Four commercial BLE modules are connected to the four ports of the array, producing an advanced BLE beacon that synthesizes twelve directive beams (one per each port and advertising channel) distributed over a wide Field of View (FoV) of 120 degrees in the azimuthal plane. Then, any BLE enabled IoT device located within this FoV can scan the messages from the beacon and obtain the corresponding Received Signal Strength Indicator (RSSI) of these twelve beams to estimate the relative DoD by using amplitude-monopulse signal processing, thus dispensing from complex In-phase/Quadrature (IQ) data acquisition or high computational load. We propose an angular windowing technique to eliminate angular ambiguities and increase the angular resolution, reporting a root mean squared angular error of 3.7 degrees in a wide FoV of 120 degrees.