Bluetooth® positioning and distance measurements

Bluetooth® Low Energy proximity and tracing services

Bluetooth® Low Energy (LE) proximity and tracing services utilize link status and estimated pathloss to identify nearby peers and determine the distance to a peer. This is done using a received signal strength indicator (RSSI), which reflects the signal strength at the receiver’s end to estimate the distance between devices.

However, an RSSI-based ranging method offers limited accuracy with high uncertainty, due to the dependencies on actual radio channel behavior. It has also been observed that RSSI-based ranging can be manipulated without great difficulty. This poses a potential risk, especially in security applications such as digital keys. Recognizing these limitations, the Bluetooth® Special Interest Group (SIG) is actively developing the technology to introduce high-accuracy distance measurements (HADM) with built-in security features.

Bluetooth® positioning and direction finding

A Bluetooth® LE direction finding (DF) feature was defined in Bluetooth® 5.1 to enhance the capabilities of Bluetooth® positioning services. This feature supports two methods for determining the direction of a Bluetooth® signal:

• Angle of arrival (AoA) estimation: A Bluetooth® transmitter with a single antenna sends a specialized direction-finding signal. The receiving peer device, equipped with an antenna array, measures the phase difference to estimate the relative signal direction. This method is commonly employed for asset tracking.
• Angle of departure (AoD) estimation: A stationary Bluetooth® transmitter uses a switched antenna array to transmit direction-finding-enabled packets. The receiving device, equipped with a single antenna, captures the signals and measures the phase differences to estimate the relative signal direction. This method is suitable for applications that require precise directional information in dynamic scenarios.

The precision of AoA/AoD estimation for direction finding relies heavily on:

• Number of antennas
• Stable transmit power
• Low frequency offset
• Drift of DF signal (LE 1M or LE 2M) transmitting device
• Capability of receiving device to accurately sample I/Q data from received signals

To ensure robust performance, the Bluetooth® SIG has specified various test cases with distinct setups. The R&S®CMW wireless communications test platform supports these test cases with an automated test solution.

HADM with Bluetooth® Channel Sounding

Bluetooth® Channel Sounding uses phase-based ranging (PBR) to enable HADM between two Bluetooth® LE devices; it uses the phase shift between the received and transmitted radio signals of different channels. By utilizing two or more signals with different frequencies, the measured phase difference (Δθ) between these signals serves as a precise indicator for estimating distance.

• One device acts as the initiator, transmitting a packet with a constant tone extension (CTE).
• The second device functions as the reflector, responding to the first device by sending a packet back with the same phase as the received signal.

This process ensures accurate and reliable distance estimation between the Bluetooth® devices. Typical HADM applications include keyless entry systems, indoor navigation and asset tracking, with new use cases on the horizon.

The Bluetooth® SIG is actively specifying test cases focusing on precise measurement and stable frequency and phase generation of channel sounding signals. When antenna areas are utilized, the test cases also address the antenna switching integrity. Given the substantial impact of device design on phase difference and time delay measurements, it is best to conduct over-the-air (OTA) measurements when testing AoA, AoD and Bluetooth® Channel Sounding.