Bluetooth® wireless’ constant evolution is impressive; originally designed as a method to transfer information between handsets without cables, the technology—particularly since the 2010 launch of the Low Energy (LE) version as a “hallmark element” of Bluetooth 4.0—has expanded dramatically. Bluetooth LE extended the technology’s reach to devices with modest battery resources, and at a stroke, opened up wireless connectivity to thousands of previously ‘dumb’ products.
In the early years, the growth of Bluetooth LE was primarily driven by the market for “appcessories”—wireless products such as wearables, toys, bike lights, and coffee machines—that could be controlled directly from a smartphone. Fortuitously, the key attributes of the technology also ideally suited the demands of the wireless sensors that form the foundation of the Internet of Things (IoT). More recently, the introduction of Bluetooth® 5—which added enhanced throughput, increased range, and improved coexistence—cemented Bluetooth technology’s position as a major technology driving the “smart” future.
Bluetooth 4.0 and 5 were launched with the degree of hype that marketing folk do so well, and to be fair, those revisions to the standard did bring significant technical enhancements. In contrast, the latest upgrade, Bluetooth 5.1, received a little less marketing pizzazz yet promises a solution to a problem that no other wireless technology has addressed.
Global Navigation Satellite Services such as Galileo, Global Positioning System (GPS), and Global Navigation Satellite Systems (GLONASS) form the backbone of many guidance and tracking applications. But without “line of sight” to the satellites, the systems fail. Engineers have implemented alternative solutions such as determining a location based on the known position of a Wi-Fi router, but such solutions are limited to an accuracy of around ten meters.
For its part, Bluetooth has employed a Received Signal Strength Indicator (RSSI) methodology for estimating the position of a Bluetooth transceiver (embedded, for example, in a consumer’s smartphone). As the name suggests, the technique works by estimating the distance from the transceiver to a known fixed point (for example, a beacon) based on the Bluetooth signal strength. Such a system is generally unable to determine the exact location of the target transceiver being limited to an estimate of position on the circumference of a circle of known radius around the beacon (for a Bluetooth transceiver restricted to a horizontal plane such as a floor). Precision is further undermined by the (typically unknown) attenuation of signal strength by walls and other obstacles.
Both Wi-Fi and Bluetooth technology’s proximity systems have found favor in retail applications whereby consumers can be fed contextual information based on their approximate location, but the lack of precision of each means neither is up to the task of indoor navigation or asset tracking.
According to John Leonard, a Senior Product Marketing Manager with Bluetooth chip vendor Nordic Semiconductor, Bluetooth 5.1 introduces a “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 one meter. 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:
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 promise an angular accuracy of ±2° and around half-a-meter positional precision.
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 polarization, 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, U.K., 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 including What’s New in Electronics and Australian Electronics Engineering for Trinity Mirror, CMP and RBI in the U.K. and Australia. In 2006, Steven became a freelance journalist specializing in electronics. He is based in Sydney.
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