Get Accurate, Low-Power Indoor Real-Time Asset Tracking Using Bluetooth Direction Finding

By Bill Giovino

Contributed By Digi-Key's North American Editors

Factories, warehouses, and manufacturing facilities are increasingly using tags to perform real-time location tracking of assets. The data is then typically integrated into an appropriate cloud-based Industrial Internet of Things (IIoT) inventory control system to allow remote asset tracking. The problem is that, apart from NFC, most asset tracking solutions depend upon tags that run on batteries, which requires keeping power consumption as low as possible. Additionally, some of the solutions can be unreliable and inaccurate when used indoors.

For example, GPS tags are unreliable indoors, particularly in steel and concrete buildings. Classic Bluetooth location systems are based on received signal strength indicator (RSSI) information, which, while useful, often doesn’t meet designers’ accuracy requirements. What is needed is a reliable, cost-effective, accurate, battery-powered wireless asset tracking solution that can be used indoors yet enables long battery life.

To meet these challenges, this article will describe the Bluetooth 5.1 Direction Finding protocol and how it operates. The article will then introduce a cost-effective Bluetooth module from Silicon Labs that supports this protocol and shows how it can meet both the accuracy and low-power requirements of an IIoT inventory control system.

What is asset tracking and why is it needed for the IIoT?

Advanced IIoT inventory control systems require real-time tracking of assets from the cloud anywhere in the world. Large warehouses that store high-value products and equipment may require location asset tags for inventory control and anti-theft assistance. This allows human warehouse workers as well as automated picking equipment to quickly and efficiently locate an item and prepare it for shipping. For inventory management, the existence and location of assets can be easily determined and itemized for regular status reports. This is a more reliable method of providing inventory status than manually reviewing shipping manifests that track incoming and outgoing assets.

Besides IIoT inventory management systems, real-time location tracking of assets is used in anti-theft systems. If an item in a warehouse is not scheduled for shipment, the IIoT system can alert security if it is tracked near an exit. Real-time asset location can also accelerate service and delivery in an age where next-day delivery is rapidly evolving into expectations of same-day delivery.

For asset tracking in volume, the asset location tag must be cost-effective and have a long battery life. NFC tags do not use batteries but require the receiver to be within 20 centimeters (cm) of the tag, limiting their usefulness. GPS trackers are unreliable indoors as the satellite tracking signals can be blocked, particularly by steel and concrete structures.

A popular asset tracking solution relies upon Bluetooth’s beacon location feature. This tracks the location of a tag by comparing a reference signal strength encoded in the beacon message to the signal strength of the received signal. The beacon location is then triangulated using three or more receivers to get an approximation of the beacon’s location. However, this approach does not provide the precision required for inventory management systems. In addition, the location accuracy can be affected by changes in humidity, as well as by moving objects such as forklifts, workers, and doors.

Bluetooth direction finding

The solution is Bluetooth direction finding, a feature included in the Bluetooth 5.1 specification.

Bluetooth direction finding triangulates the location of a battery-powered asset tag based on the phase shift of the received signal at two or more antennas. As a result, it is accurate to less than one meter (m) and is a cost-effective location tracking solution that can be used reliably indoors, while enabling years of operation off a single coin cell battery.

In Bluetooth direction finding, a new signal called a continuous tone extension (CTE) is added to the standard Bluetooth advertising packet. The CTE is a continuous tone sent over a frequency calculated to be the Bluetooth frequency + 250 Hz. Because the CTE is independent of the regular Bluetooth message packets, it does not interfere or delay these packets. This allows the receiving antennas to obtain a continuous, uninterrupted fix in real time, solving the problem of real-time location tracking.

Angle of arrival and angle of departure

Bluetooth direction finding uses two types of phase shift, antenna-based location detection mechanisms, referred to as angle of arrival (AoA) and angle of departure (AoD) (Figure 1). AoA is used when external systems must keep track of individual tags. An asset tag containing a compatible Bluetooth 5.1 or later module broadcasts a CTE. A Bluetooth receiver in the base station with two antennas receives the arriving signal. The receiver uses the phase difference between the two sampled signals received by the antennas to calculate via triangulation the distance to the asset tag.

Diagram of AoA and AoD methods of direction finding (click to enlarge)Figure 1: In the AoA method of direction finding (left), an asset tag broadcasts its signal to a Bluetooth AoA base station locator which measures the signal's arrival angle at two or more antennas to determine the tag’s location. With the AoD method (right), Bluetooth base stations transmit beacons to the asset tags which calculate their own position. (Image source: Silicon Labs)

To prevent sampling errors due to aliasing, the distance between the two receiving antennas must correspond to the wavelength of the Nyquist frequency of the received signal, which is the wavelength of the received signal divided by two. A Bluetooth signal of approximately 2.4 gigahertz (GHz) corresponds to a wavelength of 12.5 cm, so the distance between the two antennas must be 6.25 cm or less. Using the phase difference between the signals at the two antennas, the known fixed distance between the two antennas, and the known configuration of the two antennas, the distance to the asset tag can be calculated.

If an additional antenna receiving unit is used with two antennas of the same configuration as the first unit, the exact location of the asset tag in 3D space can be determined.

The AoD method is used when the asset tag must keep track of its own location. In the AoD method, the tag is the Bluetooth receiver and the base station with multiple antennas is the Bluetooth transmitter. The base station transmits a CTE from each antenna. The receiver firmware knows the number of antennas, the known fixed distance between each antenna, the known configuration of the multiple antennas, and uses the phase differences between the received signals to calculate its own location.

For an IIoT inventory control system in a warehouse, the battery-powered asset tags attached to boxes or containers would use AoA, while forklifts or automated pick and pack equipment would use AoD. Forklifts and other automated pick and pack equipment are heavy duty and not battery conscious, so they can transmit their location via Wi-Fi to the main IIoT hub. All of this can be tracked real-time in an IIoT cloud interface.

Low-power Bluetooth direction finding modules

For low-power Bluetooth 5.2 direction finding applications, Silicon Labs has introduced the BGM220 Bluetooth module family, which is specified to provide a 10-year battery life on a single long-life coin cell. The BGM220PC22HNA2 version is a Bluetooth 5.2 transceiver module  with a footprint of 12.9 x 15.0 millimeters (mm) and a profile of 2.2 mm (Figure 2). It requires a 1.8 to 3.8 volt power supply, making it appropriate for applications that can run off long-life 3.0 volt lithium coin cells, as well as larger rechargeable 3.6 volt lithium-ion (Li-ion) cells for consumer mobile devices. It can operate over -40°C to +105°C, making it particularly suited to harsh environments such as factories and industrial warehouses.

Image of Silicon Labs BGM220PC22HNA2 is a compact Bluetooth 5.2 moduleFigure 2: The BGM220PC22HNA2 is a compact Bluetooth 5.2 module that supports Bluetooth direction finding for up to 10 years on a single long-life coin cell battery. (Image source: Silicon Labs)

The BGM220PC22HNA2’s radio operates in the 2.4 GHz band and outputs 8 decibel referenced to 1 milliwatt (mW) (dBm). The module includes all necessary decoupling capacitors and inductors, as well as 38.4 megahertz (MHz) and 32.768 kilohertz (kHz) oscillators and an integrated ceramic chip antenna (Figure 3). The module is based on an Arm® Cortex®-M33 core supported by 512 kilobytes (Kbytes) of flash and 32 Kbytes of RAM.

Diagram of Silicon Labs BGM220PC22HNA2 Bluetooth module (click to enlarge)Figure 3: The BGM220PC22HNA2 Bluetooth module has everything needed to support a self-contained Bluetooth direction finding asset tag, including a 2.4 GHz radio, memory, an Arm Cortex-M33 processor, and an ADC. (Image source: Silicon Labs)

Peripherals available for firmware customization include a 76.9 kilosamples per second (kSPS) 16-bit analog-to-digital converter (ADC) that can also be configured to operate as a 12-bit 1,000 kSPS ADC. Up to 24 I/O pins are available for firmware customization. Four 16-bit timers and one 32-bit timer are available for timing firmware events. Two I2C interfaces can access external peripherals. The BGM220P also includes two multifunction USARTs that can be independently configured as a UART, SPI, smartcard interface, IrDA, or I2S. This allows flexibility in the selection of serial interfaces while lowering the pin count.

When using the BGM220PC22HNA2 in a Bluetooth direction finding asset tag, the application should use only the necessary peripherals, and turn off power to the unused ones to extend battery life. A minimal asset tag configuration would only contain the BGM220PC22HNA2 with a 3.0 volt battery in a non-metal housing that does not interfere with the transmission of Bluetooth signals. External switches could be connected to I/O pins for boot customization such as setting the identification of individual tags. One or more external LEDs could be attached, but designers need to be careful here as each LED is an additional drain on the battery. Ideally the LEDs would only be used during configuration.

Developing Bluetooth direction finding applications

For application development with Bluetooth direction finding, Silicon Labs provides the SLWSTK6103A BGM220P Wireless Gecko Bluetooth Module Starter Kit (Figure 4). It includes a plug-in radio board that is a carrier board for a BGM220P module. In the center of the board is a 128 x 128 LCD display, shown displaying the Silicon Labs logo with additional text.

Below the LCD display are two firmware-programmable pushbuttons. The LCD can be used during development to display status information, and the pushbuttons to control firmware flow. Debugging is supported over the USB connector. Additional connectors are available to support Silicon Labs’ energy monitoring software, allowing the application to be fine-tuned to draw only the minimum power needed.

Image of Silicon Labs SLWSTK6103A BGM220P starter kitFigure 4: The SLWSTK6103A BGM220P starter kit contains everything needed to develop firmware for a BGM220P module to support Bluetooth direction finding. (Image source: Silicon Labs)

The SLWSTK6103A also has a temperature and humidity sensor. For a Bluetooth direction finding asset tag, environmental sensors may be attached to an I2C interface to monitor the conditions surrounding the asset tag and transmit an alert over Bluetooth if conditions exceed pre-programmed thresholds. Additional I/O and peripheral pins are brought out to header connectors. The starter kit can be powered by an external USB connection or a coin cell battery.


Real-time asset tracking in IIoT inventory management systems requires an accurate, reliable, cost-effective solution that is small and low power. As shown, the direction-finding feature in the Bluetooth 5.1 specification can be quickly integrated into an asset tag using off-the-shelf modules to provide the required degree of real-time location tracking capability and performance.

Further reading

  1. Use Bluetooth 5.1-Enabled Platforms for Precise Asset Tracking and Indoor Positioning - Part 1
  2. Use an Advanced Bluetooth 5.2 SoC to Build Secure Low Power IoT Devices

Disclaimer: The opinions, beliefs, and viewpoints expressed by the various authors and/or forum participants on this website do not necessarily reflect the opinions, beliefs, and viewpoints of Digi-Key Electronics or official policies of Digi-Key Electronics.

About this author

Bill Giovino

Bill Giovino is an Electronics Engineer with a BSEE from Syracuse University, and is one of the few people to successfully jump from design engineer, to field applications engineer, to technology marketing.

For over 25 years Bill has enjoyed promoting new technologies in front of technical and non-technical audiences alike for many companies including STMicroelectronics, Intel, and Maxim Integrated. While at STMicroelectronics, Bill helped spearhead the company’s early successes in the microcontroller industry. At Infineon Bill orchestrated the company’s first microcontroller design wins in U.S. automotive. As a marketing consultant for his company CPU Technologies, Bill has helped many companies turn underperforming products into success stories.

Bill was an early adopter of the Internet of Things, including putting the first full TCP/IP stack on a microcontroller. Bill is devoted to the message of “Sales Through Education” and the increasing importance of clear, well written communications in promoting products online. He is moderator of the popular LinkedIn Semiconductor Sales & Marketing Group and speaks B2E fluently.

About this publisher

Digi-Key's North American Editors