NAVIGATION
promise an angular accuracy of ±2° and around half-a-metre positional precision.
Design dilemmas
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vendor Nordic Semiconductor, Bluetooth 5.1 introduces ‘Direction Finding’ technology that significantly improves the protocol’s usefulness for indoor navigation and asset tracking. Leonard explains that the revision brings “precise positioning of things in three-dimensional space [that will have] a similar impact for indoor situations as GPS did for outdoor positioning”. The key to the enhancement brought by Bluetooth 5.1 Direction Finding results from combining RSSI with the apparent direction a signal is coming from. In this way, instead of just placing the transceiver somewhere on the circumference of a circle, its position in space is determined to an accuracy of about 1 m. The details of the new technology are complex and require a longer article to describe, but in essence, there are two methods for determining direction: • Angle of arrival (AoA) of the received signal • Angle of departure (AoD) of the transmitted signal
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More specifically, AoA is determined by measuring the phase difference between signals from a specific source arriving at multiple antennas. If the antennas are perpendicular to the transmitter, there will be virtually zero phase difference; as the angle increases, the distance from transmitter to each antenna will subtly change, increasing the phase difference. The phase difference data can then be crunched by an algorithm to estimate the angle between transmitter and receiver. AoA enables the receiving device to estimate the location of the transmitter. When employing the alternative AoD technique, the receiver uses only one antenna, and the transmitter is fitted with multiple antennas which sequentially transmit. AoD enables the receiving device to calculate its own position in space using angles derived from multiple fixed receivers. Direction Finding operates in either two or three dimensions depending on the complexity of the antenna array. And with a well-designed antenna array and software, AoA and AoD
Bluetooth 5.1 Direction Finding is an elegant theoretical approach to position confirmation, and several manufacturers already offer commercial solutions. But developing a practical application is far from easy. Many Bluetooth developers will be familiar with a transceiver’s lone antenna but not with antenna arrays. And even with a decent array, factors such as polarisation, multipath interference, clock jitter and propagation delays make it very difficult to extract the pure phase information from the noise. Because calculating the angle of a radio signal has historical applications across medical, security and military applications, there are some proven algorithms that designers can use as the basis for Bluetooth Direction Finding. However, they do require a lot of fine-tuning to suit the likely scenarios in which the target application is likely to be employed. And the algorithms demand the services of Bluetooth systems-on-chip (SoCs) with powerful processors and lots of Flash and RAM. Adding Bluetooth 5.1 Direction Finding to the wireless technology offers the potential for many new applications, including indoor navigation, asset tracking and a new generation of more sophisticated beacons. The Bluetooth Special Interest Group (SIG) — custodian of all things Bluetooth — forecast some 400 million Bluetooth ‘location services’ products per annum by 2022. But getting there won’t be easy. Curious engineers are advised to turn to their trusted distributor for guidance and advice before embarking on their first project. *Steven Keeping gained a BEng (Hons) degree at Brighton University, UK, before working in the electronics divisions of Eurotherm and BOC for seven years. He then joined Electronic Production magazine and subsequently spent 13 years in senior editorial and publishing roles on electronics manufacturing, test and design titles across the UK and Australia. In 2006, Steven became a freelance journalist specialising in electronics. He is based in Sydney. Mouser Electronics au.mouser.com
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