RF-star Introduces The Realization Theory of Bluetooth 5.1 AoA Location Service

After two years of market development, Bluetooth AoA achieves two different applications: indoor positioning, and keyless entry. Let's know about indoor positioning.

1. Indoor Positioning Technology Summary

There are four solutions for indoor positioning: UWB, Wi-Fi, BLE beacon/BLE AoA. For the accuracy range, UWB is 0.1m~1m; Wi-Fi and BLE beacon is 5m~20m with a larger error range. As the first commercial BLE AoA positioning service company, Quuppa can control the accuracy range within 0.1m~1m (with the TI CC254x and Nordic nRF5281x chips).

The following is the test data of Quuppa.

2. Analysis of Indoor Positioning Theory

No matter what kind of positioning method, the indoor positioning system needs to draw the architectural modeling of the actual scene, deploy the receiver router in the indoor area, and mark the corresponding location on the modeling map. The mobile phone or the beacon module broadcast wireless signals. The receiver router deployed in a fixed position can confirm the location area range of the beacon after received the wireless signals.

The arrival angle of the beacon signal can be measured by multiple antennas after confirmed the location area range. Combined with triangulation positioning, the accurate position of beacon equipment can be calculated.

• Triangulation

Triangulation is to measure the position of the beacon device through two sets of AoA data. As shown in the figure below, two sets of antennas can measure one AoA data, and two AoA data can measure the position of device A. Both d and D are known parameters, how we can get θ1 and θ2.

3. AoA Measurement Theory

Frequency: Bluetooth operates in the ISM (Industrial Scientific and Medical) band from 2.40 GHz to 2.41 GHz with a bandwidth of 2 MHz. Bluetooth is divided into three broadcast channels 37, 38, and 39. In the Bluetooth v5.x core specification, the extended broadcast of the Bluetooth LE can be broadcasted in any channel from 0 to 39, that is, the Bluetooth v5.1 beacon can work on any Bluetooth channel. (The frequency of Bluetooth is inconsistent when it is working, which will affect the Bluetooth wavelength λ.)

Phase: The wireless signal is continuous in the air. The RX receiver receives and demodulates the signal of the entire wave cycle of 0 ~ 2 π under the frequency range.

AoA calculation: Supposing that there is a fixed-frequency Bluetooth beacon signal broadcasting in an open area (ignoring obstacles and other 2.4 GHz signal interference in the air). If the two receivers are on the same radius and at the same TX end, at a certain time t, the phase difference received by the RX receiver should be 0. But if the receiver is in a position with an inconsistent radius, at a certain time t, there is a difference in the phase of the RX receiver.

If the above diagrams are not easy to understand, let's change to the following diagrams to show the schematic. In the process of signal transmission, due to different antenna positions, the phase of the signal sampled at a certain time t is different.

Remark: In principle, the phase difference can be calculated with two antennas, and a multi-antenna array can help improve the anti-interference performance.

When the two antennas' distance d and the frequency (wavelength) of the beacon signal are known, the phase difference θ between the beacon signal and the antenna 1, and antenna 2 can be calculated. After calculating the two sets of θ at different positions, the position of the beacon signal can be calculated.

• AoA Measurement Error

Taking a 2-dimensional plane as an example, there will be two angles of α. And in real three-dimensional coordinates, there will be a trajectory of a circle with a radius of α. The AoA receiver only knows that the beacon signal is on the trajectory of the circle, however, it cannot judge where they are on the circle. At this time, an orthogonal antenna array is needed to confirm the position coordinates of the beacon point.

Antenna arrays can be arranged in a variety of ways. One mission is positioning, and the other mission is to minimize the number of receiver deployments. The common antenna array deployments are shown below.

• Example of Antenna Array

TI EVM Antenna Array

Quuppa Receiver Antenna Array

4. Bluetooth 5.1 AoA Software Deployment

The SIG has made corresponding specifications for the AoA protocol in the logical link layer in the Bluetooth 5.1 core specification.

AoA/AoD can work under 1M or 2M regular PHY (coding PHY is not supported) and can support broadcast mode and connection mode. The data transmission format has the following requirements: in the PDU data packet, the AoA/AoD specification is standardized. CTE is the extended data of AoA/AoD data packet, and the duration is 16μs–160μs. The 250kHz signal modulation is on the carrier wave without whitening and CRC validation. The signal is used by the receiver RX to sample the signal I/Q value at time t and calculate the phase difference.

5. AoA Design Challenges

• Signal Reflection Interference

The above discussion is under ideal conditions with an open environment and no other signal interference. In the real scenes, the receiver RX will receive the reflection signal of itself or other AoA and beacon signals, and the reflection signal also carry CTE extended data packets, however, they are noise signals that need to be eliminated.

• Switching Time Compensation

An RF core controls multiple RF antennas. The switching time of the RF switch should be compensated except for the time of signal transmission in the air.

• Angle Value Error

There must be an error in the measurement, and the error range is usually 3% ~ 5%. It needs an algorithm to reduce the error.