Bluetooth Low Energy 5.1 - Angle of Arrival and Angle of Departure Test Considerations

Eve Danel - Litepoint

Jul 15, 2021

Embedded in most mobile phones and countless consumer electronics devices, Bluetooth is primarily used for audio streaming and data transfer. The wireless technology is also widely deployed for proximity sensing, which uses distance estimation derived from the receive signal strength indicator (RSSI) measurement.

The RSSI method, however, provides only a crude estimation of distance, as the accuracy is limited by numerous factors like signal attenuation from environmental obstacles, transmitter power level and receiver calibration. Furthermore, a device’s direction is unknown since the receiver cannot tell the direction of the incoming signal; it can only detect that the tracked device is located in a circular zone around the receiver. The RSSI approach has shown good market adoption but due to the technology’s limitations its usage is mostly limited to proximity detection.

Bluetooth 5.1 Brings New Location Capabilities

There are many use cases that require high accuracy for indoor positioning, such as real-time locating systems for asset tracking or indoor positioning systems (IPS). For this reason, Bluetooth Special Interest Group (SIG) introduced direction finding enhancements as part of the Bluetooth 5.1 core specification.

The Bluetooth 5.1 capabilities greatly increase positioning accuracy over earlier generations, with new direction-finding enhancements where angle of arrival (AoA) or angle of departure (AoD) information can be determined. Depending on the details of the implementation and environment, the Bluetooth 5.1 direction finding method could provide sub one-meter accuracy. This level of accuracy in positioning opens the doors to countless new use cases for asset or personnel tracking in medical or industrial environments, wayfinding in public or private venues, item finding for smart retail or personal item finding.

Let’s take a closer look at how the direction finding enhancements work.

Angle of Arrival (AoA)

Angle of Arrival (AoA) is designed to be used in applications like asset tracking, where the moving transmitter sends Bluetooth 5.1 direction finding signals using a single antenna. Using this method, a fixed receiver, equipped through an antenna array with a minimum of two antennas, determines the direction of the transmitter using the angle of the received signal. The angle determination is based on the phase differences of the received signal as detected by the receiver’s antenna array.

Figure 1. Angle of Arrival

Angle of Departure (AoD)

Angle of Departure (AoD) is designed to be used in applications like wayfinding indoor navigation, for example, a person in a shopping mall that wants to locate a store. With AoD, the moving receiver receives Bluetooth 5.1 direction finding signals using a single antenna. The signal is transmitted by a fixed transmitter switching through an antenna array with a minimum of two antennas. The receiver determines the direction of the transmitter using the phase differences from the incoming signal.

Figure 2. Angle of Departure

Constant Tone Extension (CTE)

To enable AoA and AoD direction finding in Bluetooth 5.1, the signal phase on the receive signal must be measured. To do this, Bluetooth 5.1 includes a constant tone extension (CTE) field that is a bit sequence included at the end of the packets, with a variable duration from 16 microseconds to 160 microseconds. CTE is only supported for Bluetooth LE operating at rates of 1 Mbps, which is mandatory, or, optionally, 2 Mbps. The CTE field contains a series of modulated 1s, which must be transmitted at one frequency with a constant wavelength in order to measure the phase of the received signal. Thus, the signal is not subject to whitening, a process that scrambles signals to ensure there will be no long strings of 1s or 0s.  

Figure 3. Bluetooth LE Signal with 48μs CTE

Testing Bluetooth 5.1 CTE

Bluetooth SIG has updated the Bluetooth PHY test specifications to include validation of these new direction-finding capabilities. New test cases were added for transmitter and receiver tests for AoA and AoD methods, and the various combinations of supported Bluetooth data rates ( 1 Mbps and 2 Mbps), as well as the 1 microsecond and 2 microseconds switching and sampling times that are supported in the standard. 

Overall, there are 23 new test cases that were added to the PHY tests to cover AoA and AoD. These tests are not hopping and are performed on a single frequency/channel. The test cases and corresponding sections in the test suite document are summarized in the table below:

Figure 4. Table of Test Cases

CTE is a new concept for Bluetooth and the test cases are designed to ensure that it can be properly generated by the transmitter. On the receiver side, tests validate that the IQ measurements on the received CTE can be used to accurately derive the phase of the signal.

BT 5.1 devices with direction finding capabilities still only need a single transceiver but AoA receivers and AoD transmitters require multiple antennas in order to measure phase differences. The antenna array composed 2 or more antennas is controlled by a switch.

Accuracy of the phase measurements relies in large part on the switching and sampling time and switching and sampling sequence on the antenna array. 

Tests are designed to ensure that I/Q samples from the Device Under Test (DUT) yield to accurate phase measurements and the tester performs both test signal generation but also extraction and analysis of the DUT’s I/Q samples. The tester verifies that the measured relative phase values derived from I/Q data samples are within specs limits. Additionally, a dynamic range test verifies the receiver’s dynamic range capabilities by varying the signal level transmitted on the different antennas. 

Conclusion

The AoA and AoD direction finding enhancements in Bluetooth 5.1 can greatly increase positioning accuracy for a wide variety of use cases such as asset tracking or wayfinding. Verification testing of the AoA transmitter and receiver function, and the AoD transmitter and receiver function will be critical in ensuring success.

 

Contributed by :