The Bluetooth Core Specification v5.1 is a significant advancement in the development of Bluetooth technology, especially its direction finding function. This feature improves the accuracy of location services and is crucial for applications such as indoor navigation and asset tracking.

Bluetooth direction finding is a cutting-edge technology that enhances the positioning services of various devices. There are two methods that can be followed: angle of arrival (AoA) and angle of departure (AoD).

In retail applications, data models that provide insights into product flow, utilization, and behavior patterns are being vigorously developed to generate business-related KPIs, such as service duration, most commonly used routes, hotspots, and other consumer behavior indicators (Figure 2).

The versatility of Bluetooth direction finding methods, such as connectionless and connection oriented operating modes, enables them to adapt to a wider range of applications and is expected to open up new prospects in wireless communication and location services in the coming years.

How does AoA work and what are the basic design principles?

Both AoA and AoD methods utilize the same fundamental principles of RF signal measurement, but differ in signal processing and antenna configuration methods. By utilizing antenna arrays, devices can more accurately determine the direction of signals than ever before (Figure 3).

The direction finding system consists of the following elements:

Sender (AoA tag)

Receiver (AoA locator)

Angle and Position Processing Unit

The direction finding technology used in Bluetooth technology involves the transmitter sending constant audio expansion (CTE) signals. Then, a receiver equipped with multiple antennas uses this signal to determine the direction of the source (Figure 4).

The main principles of the AoA mechanism can be summarized into several steps (Figure 5):

1. The tag initiates communication by broadcasting extensions on the main channel, and then periodically broadcasts on the secondary channel.

2. The locator is designed to detect this extended broadcast, synchronize with the tag, and then capture periodic broadcasts containing CTE signals.

3. The locator samples the CTE signal and generates a set of data called IQ samples.

4. These IQ samples are processed by an angle calculator to determine the angle between the label and locator.

After knowing the angles between the label and multiple locators, the system can perform triangulation on the position of the label, thereby achieving accurate position tracking in connectionless AoA mode.

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Figure 5: Data Flow of AoA Direction Finding System

When the transmitter emits a signal, it propagates outward from the transmitter in three-dimensional space at the speed of light. The path depicts a constantly expanding sphere.

To simplify the calculation of arrival or departure angles, let us only consider two dimensions, that is, on a two-dimensional Cartesian plane. This situation is currently the most common in asset tracking applications, as the third coordinate corresponding to the height of the object is not important (Figure 6).

By measuring the phase difference (PSI 2- PSI 1) between two receiving antennas, assuming that their distance (d) and signal wavelength (λ) are known, the angle of the signal can be calculated using basic trigonometry.

I/Q demodulation is a crucial step in modern radio receivers. It involves extracting I and Q data components from the received raw CTE signal. That is to say, the RF input is multiplied by complex phasors (I and Q components), and then filtered and downsampled to generate the IQ data stream (Figure 7).

The following is a typical workflow (Figure 8):

Down conversion: I/Q data multiplied by the carrier frequency or the complex phasor (sine and cosine pairs) near the carrier frequency. This operation moves the signal down to a lower intermediate frequency (IF).

Low pass filtering: After down conversion, the low-pass filter removes unwanted high-frequency components, leaving only the baseband signal.

Extraction: Downsampling the filtered signal to reduce data rate while preserving basic information.

Design considerations and industry restrictions

In a more accurate positioning system that provides a third Cartesian coordinate at the output, it is necessary to calculate at least two angles to display the relationship between the transmitter and receiver in three-dimensional space (triangulation). To obtain the second angle, a second AoA locator is required. These two angles are called azimuth and elevation.

Another method that does not require measuring any angle is called trilateration. It usually uses Time of Flight (ToF) to measure distance, for which channel detection (Bluetooth 5.4) or its Ultra Wideband (UWB) technology is used.

Channel Detection (CS), also known as High Precision Distance Measurement (HADM) in Bluetooth terminology, released with the Bluetooth Core Specification 5.4, can be seen as a high-precision alternative to RSSI based distance measurement. With the help of CS, applications that have already used low-power Bluetooth can achieve higher accuracy without additional hardware costs.

Ansenmei's RSL15 AoA Solution

The RSL15 5.2 microcontroller developed by Ansenmei can provide reliable asset tracking through the AoA method. This project consists of two independent parts:

Listening: ble_Scanner_DF_ locates multiple applications running on the board.

Broadcaster: The ble_ Advertiser_DF application running on the label board.

The wireless core of the monitor and broadcaster is the RSL15 5.2 system chip from Anson Mei. Alternatively, a more integrated solution based on the same Anson Mei product is Murata's System in Package 2EG device (Figure 9).

The monitoring project is responsible for receiving CTE signals sent by broadcast tags and obtaining IQ samples from them.

These samples are sent to applications running on PC or cloud to calculate the angle between the listener and broadcaster. Finally, convert the calculated angle to Cartesian coordinates and map it to two or three dimensions.

The example code for both of the above applications can be obtained for free in the Ansenmei CMSIS package, which can be downloaded from the Ansenmei website (Figure 10).

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Figure 10: An example of Anson's code for connected and unconnected modes

Everyone can download and use the Anson power estimator tool to verify which communication parameters and schemes to use and maximize battery life. This provides valuable theoretical reference for expected system performance and limitations (Figure 11).

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Figure 11: Ansenmei Battery Life Estimation Device and Direction Finding Indicators

CoreHW designs and provides antenna array boards in various sizes, achieving centimeter level excellent positioning accuracy. The frequency range of these antenna boards that can be immediately put into production is 2400-2483 MHz, with a maximum of 16 single ended antenna ports (Figure 12).

The antenna includes CHW1010 SP16T Bluetooth AoA and AoD antenna switches for antenna selection, as well as connectors for RF and digital control signals for connecting to BLE SoC control boards (Figure 13).

Unikie's low-power Bluetooth positioning engine is designed specifically for real-time tracking of low-power Bluetooth tags. The generated data can be processed on edge servers or in the cloud, ensuring flexibility and cost-effectiveness (Figure 14). In addition, the API of this engine facilitates seamless integration with enterprise systems and supports complex data modeling. This enables people to have a deeper understanding of material flow, utilization, and behavioral patterns, marking significant progress in location-based services and asset management.

Conclusion - How can the RSL15 solution help address industry challenges? What makes this solution different?

The key prerequisite for successfully adopting Bluetooth direction finding technology is still the service life and cost of the tag device. At Anson Mei, we view Bluetooth SoC as a big step towards this goal, and we are always focused on providing the market with top-notch ultra-low power wireless technology at a reasonable cost.

We are committed to sharing knowledge about the implementation of such systems, making it easier for our clients to develop and helping to bridge the gap between people and this transformative technology, in order to fully utilize it in their daily lives.