"The latest version of Bluetooth core specification (version 5.1) makes it easier for developers to implement asset tracking and indoor positioning system (IPS). Specifically, the specification adds a fixed frequency spread signal (CTE) to the Bluetooth packet so that the receiver can extract "IQ" data (in-phase and quadrature phase information required to calculate the transceiver 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, so it is necessary to use a reasonably designed antenna array with a wireless microprocessor. Even if the IQ data is successfully extracted, it must still be processed. It can be used to calculate the position of the transmitter only after full consideration of multipath reception, signal polarization and propagation delay, noise and jitter.
This paper illustrates the matters needing attention in the practical solution, and introduces the development platform and module for building Bluetooth 5.1 DF application launched by dialog semiconductor, silicon labs and Nordic semiconductor. In addition, this paper also illustrates how to use these platforms to start prototype development, test and verify design.
Bluetooth 5.1 packet structure
The Bluetooth 5.1 data packet contains a CTE composed of a string of numbers "" 1 "", so as to ensure that the antenna receives this part 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). After a properly configured low-power (LE) Bluetooth radio receives a packet containing a CTE signal, IQ sampling will be performed during the CTE signal. A single IQ sample consists of signal amplitude and phase angle and is represented in Cartesian coordinates (Fig. 1).
Bluetooth core specification V5.1 details the changes to the low-power Bluetooth controller to enable AOA and AOD technologies to communicate using connection ("pairing") or connectionless. However, AOA is often used for connected applications such as asset tracking, while AOD will be used for connectionless applications such as IPS.
The standard Bluetooth 5.1 data packet is used for connected DF, and CTE is attached at the end. In contrast, for connectionless DF, CTE is added at the end of Bluetooth periodic broadcast packet (Fig. 2).
Whether it is a connected application or a connectionless application, developers must perform some setting and configuration steps to start the CTE of the transmitter and IQ sampling of the receiver. The specific steps depend on whether the application is based on AOA or AOD.
Building DF solutions
In applications such as asset tracking applicable to AOA, the transmitter is a movable object such as a low-cost simple label, while the receiver (or locator) is a fixed reference point. The advantage of AOA implementation is that the tag only needs to use a single antenna (rather than an array) to transmit Bluetooth 5.1 protocol packets, and does not need to run a computing intensive algorithm to finally determine the location of the transmitter (see Part 1).
Although the tag design of the asset tracking system follows the relatively simple radio frequency (RF) design principle, the tag needs to be equipped with Bluetooth 5.1 transceiver to configure the data packet to include CTE. When selecting the transceiver, it should be noted that the CTE transmission cannot use the Bluetooth low-power coded PHY (remote radio for realizing Bluetooth 5 Technology), but must use the uncoded PHY.
Some commercial Bluetooth 5.1 products are available on the market. For example, dialog semiconductor's da14691 low-power Bluetooth 5 SOC is suitable for location service applications. The chip uses arm ® Cortex ®- M33 microprocessor, including 512 KB random access memory (RAM). Dialog provides a Bluetooth 5.1 stack for da14691. Silicon labs also released Bluetooth 5.1 stack for efr32bg13 Low Power Bluetooth SOC; The chip adopts arm cortex-m4 microprocessor and provides 64 kb ram and 512 KB flash memory. In addition, Nordic semiconductor went further and released a new "direction finding" hardware and software solution nrf52811. This low-power Bluetooth SOC is compatible with Bluetooth 5.1, integrates arm cortex-m4 microprocessor, and combines multi protocol radio from Nordic's high-end device nrf52840. The chip provides 192 KB of flash memory and 24 KB of ram.
These devices are suitable for transmitters and receivers in Bluetooth direction finding applications. Each device supports CTE transmission and can obtain IQ samples with the help of the configuration file information specifying the transmitter antenna layout. Theoretically, these devices can also perform complex calculations to calculate the incident angle of the incoming radio signal and the position of the transceiver. However, although the arm cortex-m33 and M4 processors used by these SOCS are relatively powerful, if the wireless protocol needs to be monitored while running complex DF algorithms, the application performance may be poor.
According to the performance and latency requirements of applications, developers can consider using coprocessors (which can provide additional ram and flash memory), especially for application software. For example, Nordic's nrf52811 design can be connected to the coprocessor through an internal integrated circuit (I2C) interface and a serial peripheral interface (SPI).
Another design challenge is that in order to reduce cost, low-power Bluetooth SOC usually does not have multiple antenna ports or the ability to systematically switch between antennas in the array. Therefore, a radio frequency switch needs to be connected between the single antenna port of the low-power Bluetooth SOC and the multi antenna array, so as to switch between each antenna to collect IQ data of each antenna (Fig. 3).
The receiver (or locator) needs to detect the phase difference of the signal through the IQ data of the antenna array, which is caused by the distance difference between each antenna in the array and a single signal transmitting antenna. Whether the application adopts AOA or AOD depends on the phase angle difference of each antenna.
Antenna design is generally divided 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 effective for developers to have third-party experts configure the best array and provide a bill of materials (BOM) for batch construction, as described in part 1.
The application requirements for antenna array, coprocessor, additional memory and antenna management not only increase the complexity of asset tracking solution locator, but also increase the cost and power consumption. Fortunately, the positioner is usually installed in a fixed position, so it can be powered by mains power. For most solutions, the number of devices required is much less 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, measures each antenna, and traces the measured specific antenna according to the design details of the remote transmitter antenna array.
In the AOD implementation, the fixed locator beacon needs to be equipped with Bluetooth 5.1 transceiver, RF switch and multiple antennas to transmit beacon signals. However, unlike the AOA implementation, because the signal analysis does not need to be performed at this end of the link, there is no need to add another processor and memory. However, although the mobile receiver only needs a single antenna, it needs to be equipped with corresponding hardware and software to perform DF calculation (Fig. 4). For example, in IPS applications, the receiver is usually a smartphone compatible with Bluetooth 5.1, so the processor and memory resources are sufficient to complete the task.
Prototype development using Bluetooth 5.1
At present, the solutions launched by dialog semiconductor, silicon labs and Nordic semiconductor mainly focus on CTE transmission, packet reception and IQ sampling execution in AOA and AOD applications. Therefore, the developer needs to determine the resources required to perform the actual DF calculation (i.e. hardware and positioning engine firmware). However, this situation may change soon as suppliers compete to release enhanced DF solutions.
For example, for AOA asset tracking application, each company has launched corresponding development tools to support tag prototype development. The development process usually follows the development process of traditional low-power wireless devices. For example, the development kit includes a full-featured transceiver based on Bluetooth 5.1 target devices and peripherals provided by the factory, connects it to a PC or Mac, constructs a suitable integrated development environment (IDE), and uses the software tools of the chip supplier to realize application development.
Dialog recommends using the da14695-00hqdevkt-p-nd development kit to develop applications based on Bluetooth 5.1. The kit includes a motherboard, a daughterboard based on da14695 Bluetooth 5.1 SOC, and a cable for connecting a PC. In addition, the development kit also supports Arduino and mikroelktronika mikrobus shield boards and has power measurement function.
Silicon labs has launched the slwstk6006a wireless gecko starter kit. The development kit is equipped with more than 6 daughter boards based on efr32bg21 Bluetooth 5.1 SOC, which can realize the prototype development of asset tracking system with multiple tags. The development kit can be used with the company's simplicity studio, which supports flex SDK application and configuration software development tools.
Nordic 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 ide commonly used by many audiences, which can be used to perform application development and system configuration（ For details on low-power Bluetooth application development, see digi key article "low-power Bluetooth SOC and tools compatible with Bluetooth 4.1, 4.2 and 5 can meet the challenges of the Internet of things".)
Since Bluetooth 5.1 does not send packets containing CTE or perform IQ sampling by default, developers must configure the system through the supplier's development tools to add these functions. These tools can access the host controller interface (HCI), which can then be used by the host to configure the controller to generate CTE and perform IQ sampling.
For connectionless applications (application types applicable to AOD), the host will perform the following controller initialization steps (Figure 5):
Configure extended broadcast
Configure periodic broadcast
Configure CTE sending
Enable CTE broadcast
Enable periodic broadcast
Enable extended broadcast
Set broadcast data
The scanning equipment is designed to receive CTE data and obtain IQ samples sent by broadcasting, so it must adopt the following configuration:
Configure extended scan
Start extended scan
Synchronization with received periodic broadcast synchronization packets
Enable connectionless IQ sampling
For the connection scheme (the application type applicable to AOA), the master device or the slave device requests other devices to send a packet containing CTE. The request is executed by sending a link layer (LL) CTE request packet in which many parameters can be used to configure CTE creation. If the remote device does not support CTE, the local device is notified. Thus, the local device will not send further CTE requests using the current connection.
The specific process is as follows. The requesting device will:
Configure CTE receiving parameters in the controller
Enable CTE request in controller
Receive and process IQ reports
Disable CTE request sending when no longer needed
The response device will:
Configure CTE sending parameters in the controller
Enable CTE response in controller
Receive and respond to ll CTE requests from other devices
In Bluetooth 5.1 specification, HCI has a new command "read antenna information with low power consumption", which allows the host to obtain the information of the antenna supported by its controller. The program used to obtain antenna array information in the remote device has not been defined.
When IQ sampling is performed using an antenna array, each sample obtained must be traced back to a specific antenna and the sampling must be completed in a systematic manner. Using the mode specified in the HCI configuration command and strictly following the timing rules helps to systematize sampling. The application of these rules and the specific rules corresponding to a specific device depend on whether the application is based on AOA or AOD, and whether the device belongs to a transmitting device or a receiving device. For example, a single antenna transmitting device may continuously transmit packets containing CTEs. However, IQ sampling is always performed by the receiving device, regardless of the number of antennas used by the device.
CTE processing time can be divided into 4 µ s initial protection period and 8 µ s reference period, and the subsequent sequence is composed of switching time slot, sampling time slot, or alternating arrangement of switching time slot and sampling time slot (Fig. 6).
summary
The enhancement of Bluetooth core specification version 5.1 requires the generation of raw data required for direction finding using CTE and IQ sampling. The specification uses proven engineering techniques to determine signal direction and standardize interfaces, configurations and interactions. Another advantage is that the Bluetooth solutions provided by all chip suppliers can achieve high-precision direction finding.
Major chip manufacturers compete to provide various hardware solutions, software, development kits and software development kits, so that developers can be familiar with how to configure the system using Bluetooth DF function as soon as possible. Commercial asset tracking and IPS applications still require a high level of development expertise, especially in the design of antenna array and positioning engine firmware. However, the future Bluetooth DF configuration is expected to further simplify this problem“
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