In version 5.1 of the Bluetooth Core Specification, Bluetooth added an optional direction finding capability. Using this new feature, a Bluetooth device can determine the direction of a signal being transmitted from another Bluetooth device. This seemingly basic capability has the potential to significantly enhance Bluetooth location services solutions.
Bluetooth location services are currently using RSSI to estimate the distance between two devices and, in the case of RTLS and IPS solutions, use those distance estimates and trilateration to determine the position of a device. Now, with direction finding, those same devices can also use information about the direction of another device and, for RTLS solutions, use triangulation to improve location accuracy down to centimeter-level, for IPS solutions, will be able to provide even greater navigating experiences or require the deployment of fewer locator beacons.
The Bluetooth direction finding feature supports two methods for determining the direction of a Bluetooth signal, both of which are based on the use of an antenna array; angle of arrival (AoA) and angle of departure (AoD).
Direction Finding Using Angle of Arrival Method
In the angle of arrival (AoA) method, the device to which direction is being determined, such as a tag in an RTLS solution, transmits a special direction finding signal using a single antenna. The receiving device, such as a locator in that same RTLS solution, has multiple antenna arranged in an array. As the transmitted signal crosses the array, the receiving device sees a signal phase difference due to the difference in distance from each of the antenna in its array to the transmitting antenna. The receiving device takes IQ samples of the signal while switching between the active antenna in the array. Based on the IQ sample data, the receiving device can calculate the relative signal direction. The AoA method of direction finding is intended for use with RTLS, item finding, and PoI information location services solutions.
Direction Finding Using Angle of Departure Method
In the angle of departure (AoD) method, the device to which direction is being determined, such as a locator beacon in an IPS solutions, transmits a special signal using multiple antenna arranged in an array. The receiving device, such as a mobile phone in that same IPS solution, has a single antenna. As the multiple signals from the transmitting device cross the antenna in the receiving device, the receiving device takes IQ samples. Based on the IQ sample data, the receiving device can calculate the relative signal direction. The AoD method of direction finding is intended for use in IPS solutions, such as those used for wayfinding.
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