Using AI Machine Learning with Bluetooth Angle of Arrival (AoA)

There’s new research from Universities in Piraeus, Greece and Berlin, Germany, together with U-Blox AG in Switzerland who create Bluetooth Angle of Arrival prototyping boards on Deep Learning-Based Indoor Localization Using Multi-View BLE Signal.

Processing of Bluetooth Angle of Arrival usually requires radiogoniometry spectral analysis of radio in-phase and quadrature-phase (IQ) signals in order to then determine location by triangulation. Instead, this paper proposes machine learning of IQ and signal strength (RSSI) data from multiple anchor points to determine location. AoA processing also uses distributed processing across the anchors to improve performance.

The developed machine learning models were found to be robust against modifications of room furniture configurations and materials and it’s therefore expected that they have high re-usability (machine learning generalisation) potential. The system achieved a localization accuracy of 70cm.

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Bluetooth Myths and Facts

There’s a useful new webinar at the Bluetooth SIG on The Myths & Facts About Bluetooth® Technology as a Positioning Radio. Fabio Belloni from Quuppa explains the main Bluetooth myths and facts:

  • Performance – There are misconceptions about accuracy, latency and reliability brought over from older systems using only received signal strength (RSSI). Newer systems based on Bluetooth direction finding provide much improved performance.
  • Communication Range & Coverage Area – People incorrectly think Bluetooth is a short range 10m – 15m technology. This isn’t so. Long range beacons can transmit up to 1.5Km and can work up to 100m in location finding scenarios.
  • Multipath Propagation – It’s wrongly perceived that Bluetooth is poor in harsh environments. Bluetooth is, in fact, designed for factory floor and additionally newer AoA direction finding can use spectral analysis to reduce the affect of radio reflections.

Gabriel Desjardins from Broadcom mentions how location technologies have overcome the peak of inflated expectations caused by UWB and are now in the plateau of productivity provided by Bluetooth LE.

Andrew Zignani shows the results of a survey on RTLS from 213 C-Level decision makers across five main verticals. Only 13% of businesses have already deployed RTLS and there will be a increased uptake over the next 5 years. Technology fragmentation and operational/maintenance cost are incorrectly seen as the barriers to adoption. The new Bluetooth AoA direction finding standard is easing fragmentation. The maintenance cost is actually very low compared to the ROI in most scenarios. Most want beacon battery life to be 90+ days and cost to be $11-$20 that are easily achievable with today’s beacons.

Angle of Arrival Accuracy Improvement

There’s new research from Department of Electrical and Information Engineering, Bari, Italy on A Linear Technique for Artifacts Correction and Compensation in Phase Interferometric Angle of Arrival Estimation that can be used with Bluetooth AoA Direction Finding.

The paper first discusses the main causes of error in AoA systems. This includes signal path length mismatches as avoided by the CoreHW AoA Hardware printed circuit board (PCB) tracks and mutual RF coupling effects that act on the antenna array.

The researchers devised and proved a technique to process IQ data to reduce systematic errors and first-order (linear) coupling effects. After a calibration phase they manged to reduce average absolute errors by more than a half in one test case more than a quarter in a second case.

Research on techniques such as this will make Bluetooth direction finding even more accurate.

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CoreHW AoA Hardware

CoreHW in Finland is a new entrant in the Bluetooth direction finding ecosystem. Their main product is the CHW10x0 chip that supports switching of complex antenna arrays needed for Bluetooth direction finding. It allows designs with only one component where three to five are usually required. The switch has a fast settling time for RF signals and a good phase balance between antenna ports providing better position accuracy.

CoreHW also has reference antenna arrays and 2D software for angle and position estimation to shorten time-to-market for AoA locator and AoD beacon manufacturers. They have a demo kit including 4 locators, 2 tags, a CorePatch antenna array board with CHW1010 chip, a Bluetooth T5.1 chip for IQ sampling and a USB interface (Ethernet) to connect locators with a Windows PC.

The CoreHW reference boards have some intriguing Medusa-type printed circuit board (PCB) tracks, presumably to keep the track length the same to each antenna to normalise RF signal delays.

We look forward to seeing CoreHW components in their customers’ production devices.

2D vs 3D Bluetooth AoA Direction Finding

Current AoA locators only have antennas in one plane which means they can only provide angles in two (elevation and azimuth) dimensions. A locator therefore sees assets as being somewhere along an imaginary line or ray emanating from the locator.

If the height, perhaps of a worn lanyard, is known and tends to not change much then it’s possible to estimate the 3D location. Obviously, if the person climbs some steps for kneels down then the location becomes less accurate in all dimensions.

The other solution is to use multiple locators to triangulate two or more locator lines. This is more accurate because it doesn’t rely on a known average height and provides the opportunity to use more than two locators to increase accuracy still further.

3D provides the best accuracy. 2D location allows use of fewer locators with the trade off of less accuracy. For example, the four locators in the Minew AoA kit can be placed in different rooms or areas rather than covering an overlapping area. 2D location also has the implicit advantage of supporting more beacons because the locators and subsequent systems are doing less work.

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Bluetooth Direction Finding Antenna Arrays

Bluetooth direction finding Angle of Arrival (AoA) uses multiple antenna in one device that uses the phase difference of signals received at different antenna to determine the angle and hence location of a beacon.

We are seeing a variety of designs but most use printed circuit board patches for antennas for reasons of cost and compactness.

All these designs use a radio frequency switch that switches each antenna, in turn, to just one Bluetooth chip to save cost and complexity. You can see this in some of the designs as tracks leading from each antenna to one chip and then one track from that chip to the Bluetooth system on a chip (SoC). The switch is very fast, of the order of 1 microsecond, to capture the same origin signal across all antennas.

Take care to purchase production-ready hardware. While there are currently many antenna array designs, some are just prototype or reference boards not intended for production. The software accompanying prototype or reference boards is also tends to be non-existent and in cases where it does exist, it won’t scale to more than a few beacons.

In practice, a location engine employing AoA radiogoniometry is required to process the radio signals from the Bluetooth SoC. The radio signals are also wirelessly noisy and have to be processed to mitigate reflections, interference and signal spread delays. Additional processing is needed to triangulate the angles from multiple locators. All this isn’t trivial given that the algorithms are computationally expensive and have to be executed extremely quickly to support a large number of beacons.

Minew AoA Kit

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Minew MWL01 AoA Beacon Insights

The MWL01 Beacon is an AoA beacon which means it advertises a constant tone extension (CTS) for angle of arrival detection. An accelerometer detects the degree of movement and adjusts the advertising across three levels between 100ms (fast) -> 250ms (normal) -> 4s (slow) to save battery life. The battery level is provided in the advertising data. Double clicking the button changes the advertising, for example, for SOS notification.

Here are some observations that aren’t immediately obvious in the documentation:

  • While AoA is Bluetooth 5.x, It actually advertises Bluetooth 4.1 rather than 5.x. This makes it suitable for non-AoA applications where it can be detected by non AoA Bluetooth 4.1 receivers. It’s particularly suited for asset tracking because it increases the period between advertising, when not moving, to significantly save battery.
  • You might think the change to 4s advertising isn’t working. Be patient. It takes 5 minutes of normal advertising before it switches to slow advertising. Note that it also takes 5 seconds to go from fast advertising to normal advertising. While the advertising rates can be changed, the times to transition can’t be changed.
  • The beacon that is supplied with the AoA kit is different to that supplied separately. The kit’s MWL01 is fixed to 100ms advertising and can’t be changed. This is because it much easier to evaluate AoA and develop software when the advertising is consistent over time. The kit MWL01 can be upgraded, if necessary, to support the changing advertising period. However, we recommend you keep them as 100ms advertising beacons for ease of testing.
  • The ‘Location Finding’ flag in the AoA advertising is actually an indication of the double button press rather than anything to do with direction finding. This flag stays on for a minute and the blue led flashes during this time. Again, the double button press is only available on the non-kit beacon.
  • The battery level isn’t in the AoA data advertising. The beacon advertises a second connectable broadcast frame that includes the MAC address and battery level every 1s, 1s and 4s in the fast, normal, and slow modes respectively. These times can’t be changed.
  • Unfortunately, Minew are stipulating that the settings and firmware upgrade are only available to people who have signed an NDA with Minew or BeaconZone.

Inside the Minew LR1 Locator

The Minew AR1 locator has multiple antenna that receive special constant tone extension (CTE) advertising from beacons.

The Minew antenna consists of 12 PCB patches. If you imagine a radio signal hitting the antenna array from the left hand side, the antennas to the right will receive the signal slightly later. The phase difference can be use to determine the angle.

Martin Woolley’s excellent Bluetooth Direction Finding Technical Overview explains the theory. The main concept is based on simple trigonometry:

In practice, if you do this across just two antenna and with one sample the result has very poor stability. Instead, you need to consider all the antenna patches, over time, as well as perform analysis in multiple directions. This requires use of advanced radiogoniometry techniques.

Minew AoA Kit

BluetoothLocationEngine™

Minew AoA Direction Finding Kit in Stock

We now have Minew AoA Direction Finding Kit and beacons in stock. The AoA kit consists of a G2 gateway, 4 locators, 3 beacons and interconnecting cables. It covers an area of 400m². Multiple kits can be used to cover larger areas.

The gateway outputs antenna IQ data that’s sent to your server or BeaconZone’s BluetoothLocationEngine™ for generation of angles.