How to use Bluetooth 5.1 to implement indoor positioning system

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How to use Bluetooth 5.1 to implement indoor positioning system [Copy link]

The new version of the Bluetooth core specification (version 5.1) makes it easier for developers to implement asset tracking and indoor positioning systems (IPS). Specifically, the specification adds a fixed frequency extension signal (CTE) to the Bluetooth data packet to enable the receiver to extract "IQ" data (in-phase and quadrature phase information required to calculate the transceiver's position) from the RF signal without interfering with the modulation. In addition, developers can now easily configure the protocol to perform IQ sampling by simply configuring the sampling controller using the host control interface (HCI).

However, extracting IQ data is still complex and requires the use of a properly designed antenna array in conjunction with a wireless microprocessor. Even if the IQ data is successfully extracted, it must still be processed to account for multipath reception, signal polarization and propagation delays, noise and jitter before it can be used to calculate the transmitter's location.

This article explains what to look out for in a practical solution and introduces development platforms and modules from Dialog Semiconductor, Silicon Labs, and Nordic Semiconductor that are suitable for building Bluetooth 5.1 direction finding applications. It also explains how to use these platforms to get started with prototyping, testing, and validating your design.

Bluetooth 5.1 Data Packet Structure

Bluetooth 5.1 packets contain a CTE consisting of a string of digital "1s" to ensure that the antenna receives this portion of the signal at a constant frequency (rather than the modulation frequency used to transmit Bluetooth data). In addition, the data string is not whitened (i.e., decorrelated). When a properly configured low energy (LE) Bluetooth radio receives a packet containing a CTE signal, it will perform IQ sampling during the CTE signal. A single IQ sample consists of the signal amplitude and phase angle, expressed in Cartesian coordinates (Figure 1).

Figure 1: The first step in direction finding is when a low-power Bluetooth receiver acquires phase angle and amplitude IQ samples during the CTE portion of a Bluetooth packet for each antenna in the array. These samples are expressed in Cartesian coordinates (I,Q). (Image source: Bluetooth SIG)

Bluetooth Core Specification v5.1 details changes to Bluetooth Low Energy controllers that enable AoA and AoD technologies to use either connected (“pairing”) or connectionless communications. However, AoA is often used for connected applications such as asset tracking, while AoD will be used for connectionless applications such as IPS.

Connected direction finding uses standard Bluetooth 5.1 packets with CTE appended to the end. In contrast, connectionless direction finding adds CTE to the end of Bluetooth periodic advertising packets (Figure 2).

Figure 2: The Bluetooth 5.1 packet structure shows the location and duration of the CTE. Connected applications add the CTE at the end of a standard packet, while connectionless applications use an advertising packet. (Image source: Bluetooth SIG)

For both connected and connectionless applications, developers must perform some setup and configuration steps to enable CTE at the transmitter and IQ sampling at the receiver, depending on whether the application is AoA or AoD based.

Building a Direction Finding Solution In applications such as asset tracking where AoA is appropriate, the transmitter is a movable object like a low-cost simple tag, and the receiver (or locator) is a fixed reference point. The advantage of an AoA implementation is that the tag only needs to transmit Bluetooth 5.1 protocol packets using a single antenna (rather than an array) and does not need to run computationally intensive algorithms to ultimately determine the transmitter’s location (see Part 1).

While the design of tags for asset tracking systems follows relatively simple radio frequency (RF) design principles, the tags need to be equipped with a Bluetooth 5.1 transceiver to configure the data packets to include CTE. When selecting a transceiver, it is important to note that CTE cannot be sent using the Bluetooth low energy coded PHY (the long-range radio used to implement Bluetooth 5 technology) but must be sent using an uncoded PHY.

There are commercial Bluetooth 5.1 products available, such as Dialog Semiconductor’s DA14691 Bluetooth 5 low-power SoC for location services applications. The chip uses an Arm Cortex-M33 microprocessor and includes 512 KB of random access memory (RAM). Dialog provides a Bluetooth 5.1 stack for the DA14691. Silicon Labs has also released a Bluetooth 5.1 stack for the EFR32BG13 Bluetooth low-power SoC; the chip uses an Arm Cortex-M4 microprocessor and provides 64 KB of RAM and 512 KB of flash memory. In addition, Nordic Semiconductor has taken a step further and released a new “direction finding” hardware and software solution, the nRF52811. This Bluetooth low-power SoC is compatible with Bluetooth 5.1, integrates an Arm Cortex -M4 microprocessor, and combines it with a multiprotocol radio from Nordic’s high-end device, the nRF52840. The chip offers 192 KB of flash memory and 24 KB of RAM.

These devices are suitable for use as transmitters and receivers in Bluetooth direction finding applications. Each device supports CTE transmission and can obtain IQ samples with the help of profile information that specifies the transmitter antenna layout. In theory, these devices can also perform complex calculations to calculate the angle of incidence of the incoming radio signal and the position of the transceiver. However, although the Arm Cortex-M33 and M4 processors used in these SoCs are relatively powerful, running complex direction finding algorithms while also monitoring the wireless protocol can result in poor application performance.

Depending on the performance and latency requirements of the application, developers may consider using a coprocessor that provides additional RAM and flash memory, especially for application software. For example, Nordic's nRF52811 is designed to connect to a coprocessor through an inter-integrated circuit (I2C) interface and a serial peripheral interface (SPI).

Another design challenge is that, to keep costs down, Bluetooth low energy SoCs typically do not have multiple antenna ports or the ability to systematically switch between antennas in the array. Therefore, an RF switch is required between the single antenna port of the Bluetooth low energy SoC and the multi-antenna array to switch between the antennas to collect IQ data from each antenna (Figure 3).

Figure 3: In an AoA direction-finding asset tracking system, a tag uses a single antenna and a traditional Bluetooth low energy SoC to send Bluetooth 5.1 packets that include CTE. The main computation is performed on the multi-antenna locator side of the system, where the signal data collected by the locator is sent to the location engine that runs the direction-finding algorithm. (Image source: Bluetooth SIG)

The receiver (or locator) needs to use the IQ data of the antenna array to detect the phase difference of the signal, which is caused by the difference in distance between each antenna in the array and the single signal transmitting antenna. Whether the application uses AoA or AoD depends on the phase angle difference of each antenna.

Antenna designs are generally classified into three types: uniform linear array (ULA), uniform rectangular array (URA), and uniform circular array (UCA). Designing antenna arrays requires a lot of experience, so it is often more efficient for developers to have third-party experts configure the optimal array and provide a bill of materials (BoM) for volume build, as described in Part 1.

The application's requirements for antenna arrays, coprocessors, additional memory, and antenna management not only increase the complexity of the locator side of the asset tracking solution, but also increase cost and power consumption. Fortunately, locators are usually installed in a fixed location and can therefore be powered by mains electricity. For most solutions, the number of devices required is much smaller than the number of tags.

The AoD implementation is slightly more complex. In this case, the transmitter contains an antenna array. The receiver performs IQ sampling, makes measurements on each antenna, and traces back to the specific antenna being measured based on the design details of the remote transmitter antenna array.

In an AoD implementation, a fixed locator beacon requires a Bluetooth 5.1 transceiver, RF switch, and multiple antennas to transmit the beacon signal, but unlike an AoA implementation, no additional processor and memory are required because no signal analysis is performed on this side of the link. However, a mobile receiver, while requiring only a single antenna, does require the appropriate hardware and software to perform the direction finding calculations (Figure 4). For example, in an IPS application, the receiver is typically a Bluetooth 5.1-compatible smartphone, so the processor and memory resources are sufficient for the task.

Figure 4: In an AoD direction finding IPS system, a fixed beacon uses an antenna array to send a Bluetooth 5.1 packet containing CTE. The main calculations are performed by a mobile device such as a consumer's smartphone. (Image source: Bluetooth SIG)

Prototyping with Bluetooth 5.1

Current solutions from Dialog Semiconductor, Silicon Labs, and Nordic Semiconductor focus on CTE transmission, packet reception, and IQ sampling execution in AoA and AoD applications. As a result, developers need to determine the resources (i.e., hardware and positioning engine firmware) required to perform the actual direction finding calculations. However, this may change soon as vendors rush to release enhanced direction finding solutions.

For example, for AoA asset tracking applications, various companies have launched corresponding development tools to support tag prototyping. The development process usually follows the development process of traditional low-power wireless devices. For example, the development kit contains a fully functional transceiver based on the Bluetooth 5.1 target device and peripherals provided by the factory, connects it to a PC or Mac to build a suitable integrated development environment (IDE), and uses the chip vendor's software tools to enable application development.

Dialog recommends using the DA14695-00HQDEVKT-P-ND development kit to develop Bluetooth 5.1-based applications. The kit includes a motherboard, a daughterboard based on the DA14695 Bluetooth 5.1 SoC, and a cable for connecting to a PC. In addition, the development kit also supports Arduino and MikroElektronika mikroBUS shields and has power measurement capabilities.

Silicon Labs has launched the SLWSTK6006A Wireless Gecko Starter Kit. The development kit comes with more than six daughter boards based on the EFR32BG21 Bluetooth 5.1 SoC, enabling the prototyping of asset tracking systems with multiple tags. The development kit can be used with the company's Simplicity Studio, which supports the Flex SDK application and configuration software development tool.

Nordic has launched the nRF52840-DK evaluation board based on the company’s nRF52840 SoC, which is fully compatible with the nRF52811 Bluetooth 5.1 SoC. The company’s nRF5 SDK is a software development tool supported by many popular IDEs and can be used to perform application development and system configuration.

Because Bluetooth 5.1 does not send packets containing CTE or perform IQ sampling by default, developers must configure the system to add these features through the vendor's development tools. These tools provide access to the Host Controller Interface (HCI), which the host can then use to configure the controller to generate CTE and perform IQ sampling.

For connectionless applications (the type of applications that AoD is suitable for), the host will perform the following controller initialization steps (Figure 5):
Configure extended advertising
Configure periodic advertising
Configure CTE transmission
Enable CTE advertising
Enable periodic advertising
Enable extended advertising
Set advertising data

Figure 5: Controller initialization steps performed by the host in a connectionless scenario (the type of application for which AoD is appropriate). (Image source: Bluetooth SIG)

A scanning device is designed to receive CTE data and obtain the IQ samples sent by the broadcast, so it must be configured as follows:

Configure extended scan
Start extended scan Synchronize
with received periodic broadcast sync packets Enable
connectionless IQ sampling

For connected scenarios (the type of application for which AoA is appropriate), a master or slave device requests the other device to send a packet containing a CTE. The request is performed by sending a Link Layer (LL) CTE Request packet, which contains a number of parameters that can be used to configure the CTE creation. If the remote device does not support CTE, the local device is notified. The local device then does not send further CTE requests using the current connection.

The specific process is as follows, the requesting device will:
configure CTE receive parameters in the controller
enable CTE requests in the controller
receive and process IQ reports
disable CTE request sending when no longer needed

The responding device will:
configure CTE transmit parameters in the controller
enable CTE responses in the controller
receive and respond to LL CTE requests from other devices

In the Bluetooth 5.1 specification, the HCI has a new command "low energy read antenna information" that allows the host to obtain information about the antennas supported by its controller. The procedure for obtaining antenna array information in a remote device has not yet been defined.

When performing IQ sampling using an antenna array, each sample taken must be traced back to a specific antenna and the sampling must be done in a systematic manner. Using the pattern specified in the HCI configuration command and strictly following the timing rules can help with systematic sampling. The application of these rules and the specific rules for a specific device depend on whether the application is based on AoA or AoD and whether the device is a transmitter or receiver. For example, a single antenna transmitter can continuously send packets with CTE. However, IQ sampling is always performed by the receiving device, regardless of the number of antennas used by the device.

The CTE processing time can be divided into a 4 s initial protection period, an 8 s reference period, and then a sequence consisting of switching slots, sampling slots, or alternating switching slots and sampling slots (Figure 6).

Figure 6: This example illustrates 1 s and 2 s switching and sampling timing in an AoA application. The single-antenna transmitting device continuously transmits packets containing CTE, while the receiving device performs IQ sampling according to the switching and sampling sequence. (Image source: Bluetooth SIG)

During the reference period, eight IQ samples are acquired at a time without switching antennas. The host can use the reference samples to estimate the signal frequency and infer the wavelength, thereby improving the accuracy of the phase angle calculation. [1]

Summarize

Enhancements to the Bluetooth Core Specification version 5.1 require the generation of the raw data required for direction finding using CTE and IQ sampling. The specification uses proven engineering techniques to determine signal direction and standardizes interfaces, configurations, and interactions. Another benefit is that today, all chip vendors offer Bluetooth solutions that enable high-precision direction finding.

Chip manufacturers are rushing to offer a variety of hardware solutions, software, development kits, and software development kits to get developers up and running quickly on how to configure systems that use Bluetooth direction finding. Commercial asset tracking and IPS applications still require a high level of development expertise, especially in the design of antenna arrays and location engine firmware. However, future Bluetooth direction finding configurations are expected to further simplify this challenge.