Bluetooth 5.1’s “direction finding function” takes IoT to a new level

Translated from — microcontrollertip, Mark de Clercq, Dialog Semiconductor

Summary:

The latest version of the Bluetooth communication standard was released in January 2018. The most notable feature of Bluetooth 5.1 is a new method to effectively identify the direction of Bluetooth transmitters and receivers. The specification also introduces other advances designed to speed up communication and improve energy efficiency. Bluetooth integrated circuit manufacturers have released hardware that supports the new 5.0 specification features. The following introduces the changes brought by Bluetooth 5.1 and application cases.

A Bluetooth receiver uses signal strength to estimate the distance to a Bluetooth transmitter. Bluetooth 5.1 provides two more accurate direction finding methods using multiple transmit/receive antennas.[1] , the receiver must have at least two antennas (minimum spacing 6.2 cm), while the transmitter can have one. In AoA, the receiver uses multiple received signals to infer the angle of the source. Similarly, Angle of Departure (AoD) requires a transmitter with at least two antennas. The receiving device can have one antenna, receive multiple signals, and derive the angle of the source from them.

As specified in the Bluetooth 5.1 specification, the Bluetooth direction finding feature supports two methods of determining the direction of a Bluetooth signal, both of which are based on the use of the angle of arrival (AoA) and angle of departure (AoD) of the antenna array.

In the Angle of Arrival (AoA) method implemented by Bluetooth 5.1, Bluetooth 5.1 is a device that transmits signals in a real-time positioning system (RTLS). There must be a multi-antenna array design between two Bluetooth devices. Multiple antennas form a plane. The different data received by these antennas causes a phase difference, and then an angle is calculated to measure the direction of the signal. Of course, combined with the previous method of calculating the distance, the location information is finally calculated.

In the Angle of Departure (AoD) method, a device such as a Bluetooth location beacon transmits a signal through an antenna array. The receiving device, such as a smartphone, receives the signal using one antenna, decodes it and calculates the relative signal direction. This direction finding method is used in indoor positioning systems such as wayfinding systems.

One thing to note: The range of devices following the 5.1 specification is the same as earlier versions. However, the accuracy of locating a Bluetooth source using signal strength alone is severely degraded when obstacles are in line of sight. The ability to triangulate positioning could lead to features such as real-time positioning systems and indoor positioning systems.

The direction finding function uses in-phase and quadrature (IQ) sampling to measure the phase of the RF received by the antenna. In AoA, the sampling process is applied to each antenna in the array, one at a time and in the order in which the array is designed. The sampled data is passed up to the Bluetooth protocol stack via the host controller interface (HCI), where an algorithm calculates the direction of one device to another.

The direction finding function uses in-phase and quadrature (IQ) sampling to measure the phase of the RF received by the antenna. In AoA, the sampling process is applied to each antenna in the array, one at a time and in the order in which the array is designed. The sampled data is passed up to the Bluetooth protocol stack via the host controller interface (HCI), where an algorithm calculates the direction of one device to another.

Some parts of the Bluetooth protocol were modified to support IQ sampling and are used by higher layers in the stack. For example, a new field called Constant Tone Extension (CTE) was defined at the link layer, which provides a constant frequency and wavelength signal material that can be IQ sampled without worrying about interference issues.

The 5.1 specification also makes some enhancements to GATT (Generic Attribute Profile), which defines how two BLE devices transfer data back and forth using concepts called services and characteristics. These improvements are generally aimed at improving energy efficiency and faster "handshakes" between Bluetooth devices. But some of them are being prepared for enhancements in future specification versions.

Traditional Bluetooth location services are mostly implemented based on RSSI and three-point positioning: (The left picture shows 3 receivers + 1 transmitter, which tracks the location of the tag through 3-point positioning (real-time positioning); the right picture shows 3 transmitters + 1 receiver, which calculates the location of the receiver by analyzing the signals of 3 transmitters (indoor positioning). Traditional methods can be used to provide meter-level positioning solutions, but the accuracy is not very high and the deployment and installation are complicated.

The AoA and AoD features in Bluetooth 5.1 are designed to bring greater accuracy to real-time location systems (RTLS) and indoor positioning systems (IPS). A Bluetooth RTLS setup for asset tracking uses Bluetooth receivers, commonly called locators, in fixed locations throughout a facility. The locators connect to a centralized server called a location engine. Bluetooth transmitters, commonly called tags, are attached to the entities the system is tracking. The tags periodically send a signal to the location engine that estimates the location of each locator. In IPs, Bluetooth transmitters called location beacons reside in fixed locations. Visitors typically use an app on their smartphones to listen for location beacons. The app uses the received signal strength to calculate their real-time location.

Fundamentally, Bluetooth 5.1 defines how to let BLE clients skip service discovery on devices where nothing has changed since the last communication. The client can now infer whether the device it is connecting to is the same type as in the previous connection and has a property table that the client already has cached. If some details are the same, the client may decide to skip certain parts of the connection protocol because it already has the required data.

A classic example is a Bluetooth smart lock, where a smartphone unlocks the door as the owner approaches. This technology only needs to be activated the first time the user opens the smart lock. Users may only notice a delay when the door opens for the first time, but from then on, they will experience a near-instantaneous response.

Advanced architecture

The new features included in Bluetooth 5.1 add additional computational load to the Bluetooth controller. Certain architectural features can come in handy when handling the additional work. For example, the triangulation process can employ complex algorithms. Therefore, architectural features including digital signal processing and large memory are helpful.

Additionally, because triangulation involves signals from multiple antennas, Bluetooth controllers must include methods for handling multiple RF signals. Because the market for single-antenna Bluetooth is still large, controllers implementing the Bluetooth 5.1 standard are likely to handle the need for multiple RF paths by including architectural features for fast-operating external RF switches, rather than including switches on the Bluetooth controller IC itself.

At present, some manufacturers have developed Bluetooth chips that meet the Bluetooth 5.1 standard, and Dialog is one of them. They have released the DA1469x series of products that support Bluetooth 5.1, which can achieve precise positioning through AoA and AoD features. Mark De Clercq, business director of Dialog, said that with its world-class radio front-end performance and configurable protocol engine, DA1469x can open up new opportunities for devices that require precise indoor positioning, such as building access control and remote keyless unlocking systems. He said that in the realization of the direction-finding function, antenna design is crucial, and Dialog provides antenna reference design. This is Dialog demonstrating the Bluetooth 5.1 development kit to the media.

Another point to note about the DA1469x chip is that programmers can use the API[2] Use AoA and AoD functions. This eliminates the need for detailed knowledge of how to process the raw AoA/AoD data.

Some other features of the DA1469x ICs are not specifically related to AoA/AoD functionality, but are very useful for the needs of modern IoT. These features include haptic feedback drivers, stepper motor drivers (designed to handle analog clock motion in hybrid analog/digital clocks), LCD drivers, high-precision ADCs for reading sensors and other dedicated analog blocks. Finally, some versions of the ICs include built-in Li-Ion and Li-Polymer battery charging facilities. All in all, Bluetooth 5.1 introduces a number of features that allow for better tracking of people and objects while enhancing the user experience.

Since the official launch of Bluetooth Low Energy in 2010, developers have been able to use Bluetooth to create powerful, low-cost location service solutions for various applications in consumer, retail, healthcare, public places and manufacturing. The new direction-finding function will enable Bluetooth to better meet the changing needs of the location service industry, making deployment more flexible and scalable, and able to cope with future needs, further accelerating the popularity of Bluetooth in the current location-based service market, while opening up business opportunities for new applications and use cases.

[1]  AOA positioning determines the position by the intersection of two straight lines. It is impossible to have multiple intersection points, thus avoiding positioning ambiguity. However, in order to measure the incident angle of electromagnetic waves, the receiver must be equipped with a highly directional antenna array.