The Pitfalls of Contact Tracing are Addressed in New Wireless Ranging Tech and a BLE ChipMay 19, 2020 by Robin Mitchell
Radio Signal Strength Indicators, a common method of contact tracing, often yield inaccurate positioning results. Dialog Semiconductor says they have a better solution.
Many governments are considering contact tracing apps in an attempt to alert those who may have been in close contact with a person testing positive for COVID-19. These apps utilize Bluetooth and Radio Signal Strength Indicators (RSSIs) to determine the distance between two devices.
RSSI functions by measuring the power of a received radio signal to determine distance and position. But RSSI isn't the most reliable way to measure distance because radio signals can be obstructed, weakened, and diverted on their travel back to a receiving device, therefore obscuring an accurate measurement.
To address this flaw in RSSI function—especially in terms of COVID-19 contact tracing—Dialog Semiconductor has announced the release of their latest Wireless Ranging (WiRa) SDK for use in Bluetooth ranging and position technology.
Contract tracing helps healthcare officials alert those who have been in contact with a sickened person. Image (modified) used courtesy of Dialog Semiconductor
How does this SDK and its associated Bluetooth low-energy chip differ from traditional methods, like RSSI, and how can it be used in contact tracing against the COVID-19 pandemic?
The Basics of Bluetooth Positioning
To understand Dialog Semiconductor's new WiRa SDK technology, we should first review the basics of Bluetooth positioning.
Bluetooth positioning is a technique whereby a device's position can be determined based on incoming Bluetooth signals. According to Amos Kingatua's article on Bluetooth indoor positioning and tracking solutions, there are three common methods to determine position: angle of arrival, angle of departure, and RSSI (Radio Signal Strength Indicator).
- The angle of arrival establishes a single fixed beacon. Using a phased array antenna, the device can determine the angle of an incoming Bluetooth signal.
- The angle of departure draws positioning data from multiple beacons that emit regular beacon signals to be detected by a receiving device. The device determines the angle of each received signal and, with the positions of the fixed beacons known, can determine its precise location relative to the beacons.
- RSSI determines position by assessing the absolute signal strength and uses the inverse square law. This method requires multiple beacons. The device must also detect the distance from each beacon (via RSSI) for positioning.
Angle of arrival (left) and angle of departure (right). Image used courtesy of Silicon Labs
RSSI is not a reliable technique for determining distance for several reasons.
The first is that radio signals can reflect, refract, and diffract; any nearby interference will affect the received strength of the radio wave. Another is that the transmission power from the beacon may not be consistent and any variation will affect results. Finally, a system using RSSI requires a beacon to transmit power so that a device may compare the received signal strength to the initial transmission power.
As a result, RSSI can be imprecise for positioning.
The Wireless Ranging SDK
This week, Dialog Semiconductor announced that its latest SDK leverages the DA1469x Bluetooth LE SoCs to provide Bluetooth devices a better ranging solution. The new SDK, described as a Wireless Ranging (WiRa) SDK, is said to provide highly-accurate distance measurements with a resolution up to 10 cm—more than suitable for rough positioning in applications like contact tracing.
The Dialog Semiconductor solution uses a "radar-like" technique called phase-based ranging.
While the execution of phase-based ranging is complex, the theory is rather simple. Devices using this technique all emit a regular tone pulse on a Bluetooth channel along with any data. Nearby devices detect these regular pulses and send the same pulse back to the device. The receiving device looks at the phase difference between signals being sent and signals being received.
Wireless Ranging (WiRa) SDK. Image used courtesy of Dialog Semiconductor
The frequency of the tone is then adjusted to provide multiple phase shift values and with that data, the distance can be determined using simultaneous equations. The combination of the SDK and the DA1469x wireless family is able to filter out noise, reflections, and interference to determine the shortest distance.
Dialog Semiconductor says this technology not only provides reliable and accurate distance measuring but it also works on pre-existing systems that support the SDK with only a single antenna. There is no need for multiple fixed beacons or reliance on signal strength, making it functional in busy public spaces with hundreds of devices in close proximity.
Thus, the SDK may be a useful option for many applications, including asset tracking, indoor positioning, smart locks, and authentication. The SDK may also be beneficial to contact tracing applications that need reliable distance measuring.
The Accompanying Bluetooth Low-Energy Chip
The DA1469x features a 2.4 GHz on-chip radio which combines BLE data packages with a constant tone of frequency exchanges. Using the IQ samplings yielded from the on-chip sampling of radio waves, the device can determine distance.
The press release explains how the device along with the SDK avoids the pitfalls of RSSI: "Data processing algorithms filter the data for noise, interference, and reflections, to produce the shortest over-the-air signal path length as an accurate distance output."
Block diagram of DA1469x. Image used courtesy of Dialog Semiconductor
The SmartBond DA1469x family of Bluetooth MCUs offers three new cores that promise improved processing power, range, and battery life. Dialog Semiconductor says that with new BLE technology, these MCUs determine positioning with the angle of arrival and angle of departure based on the Bluetooth 5.1 standards.
The family also includes a Power Management Unit, which only activates certain cores as needed to preserve power and negate the need for a separate PMIC.
Slowing the Spread of the Virus
Should a decentralized system for contact tracing be used, matured tracing technology may have a bright technology for preventing the spread of future infections. This is possible thanks to the advanced mathematics behind phase-difference distance calculations and silicon transistors.
Dialog Semiconductor's Sean McGrath (senior VP of connectivity and audio business group) comments, “By adding unique, accurate distance measurement capabilities to our DA1469x SoCs, we expect to accelerate more tracing type applications and products into deployment on a global scale over the next few months. My expectation and hope is that this may help to significantly slow or stop the spread of the virus.”