Introduction of Direction
Finding Feature Expected to
Inspire a Wave of Innovative
Applications

According to Kai Ren, Bluetooth^® technology has a wide range of applications from audio streaming and data transfer, to location services and device networks. Bluetooth has become a standard for all smartphones and is expanding to tablets, cars, audio entertainment devices, connected devices, smart building, smart home, smart industry, smart city, and more. He mentioned that Bluetooth devices annual shipments will grow from 4 billion in 2018 to 5.2 billion in 2022.

Currently, Bluetooth location solutions are typically divided into two categories: proximity and positioning. With the help of Bluetooth proximity solution, two devices can tell when they are close to each other and evaluate their approximate distance. This category includes item finding solutions, such as personal property tags, and point of interest (PoI) information solutions, such as proximity marketing beacon.

“Bluetooth^® will be used in a wide range of emerging areas in the future, and the direction finding feature released today is sure to inspire many new applications,” says Ren. “We expect that by 2022, annual Bluetooth location device shipments will reach 400 million.”

The Principle of Bluetooth Direction Finding Technology

Ren says the current Bluetooth proximity solutions and positioning systems are mainly based on RSSI (received signal strength indication) positioning. For instance, in an iBeacon positioning scenario, when a Beacon broadcasts Bluetooth^® Low Energy packets periodically, these packets include the beacon’s latitude and longitude. Smartphones that enabled Bluetooth can receive the packets by RSSI (radio signal strength indicator) and the latitude/longitude. Using trilateration, the smartphone can calculate the position. When a user enters, exits, or roves within an area, the beacons’ broadcast spreads, and the distance between the user and the beacons are calculated through RSSI. Positioning is possible as long as three beacons are available, but only in meter-level positioning accuracy.

Then, how does Bluetooth improve its positioning accuracy to centimeter level with the Core Specification v5.1?

Angle of Arrival

“In fact, direction finding is not a new technology,” says Ren. “It has been around for a long time.” The basic principle is to use the phase difference of radio signals to calculate and convert location information. Two positioning methods are used in the latest version of the Bluetooth^® core specification. The first one is by using the angle of arrival (AoA) measurement method. By measuring the direction of arrival between the transmitter and the receiver, and then using triangulation, the azimuth and distance between the transmitter and the object are obtained. It is mainly aimed at RTLS (real-time location service), item tracking, and PoI information.

Its working principle is that when the transmitter sends out a special data packet through a single antenna, the receiver which has multiple antenna arrays will find the received signal phase difference because the distances from each antenna to the transmitter are different. When switching between active antennas, the system can obtain IQ samples from the received signals and then achieve the relative azimuth of the signal by data calculation. 

Angle of Departure

The second positioning method is by using the angle of departure (AoD) to measure. It also uses the signal phase differences, mainly for indoor positioning system (IPS). It works by sending special data packets when the transmitter switches between its active antenna arrays, while the receiver receives the signal through a single antenna and then obtains the IQ sample from the received signal. With the understanding of the antenna arrangement in the transmitter, the relative direction and distance of the signal are calculated. 

Ren says that with the introduction of this new direction finding technology, new features related to object directions can be added to Bluetooth^® proximity resolutions. For example, an item tracking solution will not only tell the user that they are close to the tagged item but also allow the user to know the direction in which it is located and significantly improve the user experience.

By adding the new direction finding function, this type of positioning system can improve their position accuracy to the centimeter level.

Other Innovations Inspired by Direction Finding

Ren says the addition of the Bluetooth^® Direction Finding function is expected to inspire many innovative applications. For example, a car door can be opened automatically via a mobile phone. With a locator installed in the car to determine the driver’s angle, the door can be opened automatically when the driver arrives at the door. It can also be used in access control systems by measuring whether an eligible person is close to a gate and thus open and close the gate automatically. Museums and art galleries can also realize precision-tour guidance with this feature. In addition, he says, Bluetooth also offers a mesh network topology. For example, many hotels and buildings have deployed Bluetooth mesh lighting systems and switches. These nodes can naturally be AoD transmitters or AoD receivers with the direction finding function. This fusion can inspire a lot of innovation, and there will be a lot of integration in this area in the future.

There are concerns about the problem of interference in direction finding. In this regard, Ren claims that the Bluetooth SIG is adding a new field called the Constant Tone Extension (CTE). The purpose of the CTE field is to provide a constant frequency and wavelength signal material that can be sampled for IQ, so there is no need to worry about interference problems.