Category: DirectionFinding

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.

Silicon Labs
IOSEA
Crepak
CoreHW
Telink Dipole
Telink PCB
Minew AR1
ublox
Texas Instruments

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 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 if you purchased the beacons from us.

RSSI vs AoA Bluetooth Asset Tracking

In a previous post on iBeacon Microlocation Accuracy we explained how assets can be tracked using Bluetooth Received Signal Strength (RSSI) or Angle of Arrival (AoA). We advised working out what accuracy is needed prior to seeking out an appropriate solution. However, accuracy isn’t always the only consideration and here is a more complete list of factors.

Accuracy

RSSI asset tracking can achieve accuracies of about 1.5m within a shorter range confined space and 5m at the longer distances. RSSI zone-based systems where beacons are found to the nearest gateway, are accurate to the inter-gateway distance that can be of the order of cm. However having such as large gateway density is usually only practical for very small areas.

AoA asset tracking achieves sub-metre accuracy. The accuracy depends most on the distance between the locator and beacon but is also affected by the locator hardware quality, radio signal noise, surfaces causing radio reflections, the accuracy of locator placement and beacon orientation.

Maximum number of beacons

AoA-based asset tracking produces and requires much more data which means the locators and software systems have to deal with more data. The data throughput for both types of system depends on the required minimum latency that in-turn depends on how often the beacons advertise. RSSI-based systems support up to high tens of thousands of beacons while AoA supports thousands of beacons.

Beacon variety and IoT

RSSI-based systems can use any beacon and hence support a large range of sensor beacons that can detect movement (accelerometer), movement (started/stopped moving), button press, temperature, humidity, air pressure, light level, open/closed (magnetic hall effect), proximity (PIR), proximity (cm range), fall detection, smoke, natural gas and water leak.

AoA beacons are more specialised and currently only support limited IoT sensing such as movement (accelerometer) and button press.

Cost

AoA locators, gateways and beacons are more complex and are therefore more costly. AoA also needs more locators/gateways per sq area. Hence, AoA systems are x3 to x4 more expensive than RSSI systems.

Setup effort

The accuracy of AoA requires that locators be more carefully positioned than for RSSI, in particular the site and AoA locator positions need to be carefully measured.

Beaconzone supplies both RSSI and AoA systems. Contact us to determine the best type of system for your needs.

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

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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 LocationEngine™ for generation of angles.

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How Accurate is Bluetooth Direction Finding?

Bluetooth direction finding promises sub-meter accuracy. In practice, the accuracy varies depending on factors such as the locator hardware quality, radio signal noise, surfaces causing radio reflections, the accuracy of locator placement and beacon orientation. The sophistication of the location engine software in mitigating some of the aforementioned factors can improve the accuracy.

As a guideline, our Location Engine with the Minew G2/AR1 tends to find beacons with a maximum angular error range between 6° to 10°, depending on the above factors. The error in position due to an error in angle gets magnified with distance from the locator. Hence, the accuracy also depends on the distance between the locator and the beacon.

Here are graphs of error vs distance for 6° error and 10° error:

The above accuracies are for hardware such as the Minew G2/AR1 with PCB antennas 50mm apart. It’s expected that greater accuracies might be achieved with hardware having greater inter-antenna distances.

It can be seen that the sub-meter promise has caveats. We have some tips to help reduce angular errors. Averaging data, over time, also reduces angular error with the trade-off of increased latency of detecting location changes. As with all locating technologies, headline performance claims need to be carefully examined and are only achievable in particular situations.

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Practical Bluetooth AoA Direction Finding Tips

Bluetooth Angle of Arrival (AoA) uses the phase difference of the radio signal hitting multiple antenna to calculate the elevation and azimuth angles to the beacon.

The accuracy is affected by physical aspects and the way the signals are processed. It varies from the order of a few centimetres to about a metre.

Here are some tips to maximise AoA accuracy:

  • Avoid metal objects close to the locators. Try getting better accuracy by adding boxing above locators to move them away from items such as metal beams.
  • Try to arrange that you have locators on all sides of a beacon. You ideally need locators all around the beacon. Accuracy is poor when the angle between beacon and locator is very large. This includes outside the perimeter of the locators where the angles get progressively larger.
  • The best, of the order of centimetres, accuracy is obtained when the beacon is close to a locator. If accuracy is particularly important, consider dropping the locators down with tall ceilings. Don’t drop too far as remember, from the last tip, accuracy is poor when the angle between beacon and locator is very large.
  • For an x, y z location, the worst accuracy is on up-down z axis. This is because all the locators are usually placed at the same height.
  • Accuracy is best when there’s line of sight between the beacon and locator. This favours the placing of locators on the ceiling/roof and beacons on top of items such as pallet loads.
  • While the physical room usually can’t be changed, be aware when testing that an enclosed space such as an office has more reflections and hence less accuracy than, for example, a warehouse.
  • For most systems, adding more locators in the same area produces more location angles that can be used to calculate a more accurate beacon position. Also try to stagger the locators so they are not in line. More data also means systems can also average the data to mitigate radio noise. However, more data means the location engine supports fewer beacons.
  • Another way to average more data, without stressing the location engine, is to filter over time. However, this increases the latency when receiving location updates.
  • Accuracy varies depending on the beacon orientation. The orientation that gives the best accuracy for one direction, say x, isn’t necessarily the best for y. While there’s usually nothing you can do about this, in some controlled scenarios you can arrange fix the beacon orientation to improve accuracy in a particular direction.
  • Too many beacons, in the same area, advertising too regularly causes Bluetooth packet collisions and loss of radio signal reaching locators. Large beacon populations require a longer beacon Bluetooth periodic advertising period that also has the affect of allowing the system to support a larger maximum number of beacons.
  • Accurate site and anchor measurement is important. Inaccurate initial measurement is a common cause of poor system accuracy. Use a laser measure. If you finding it difficult to measure the x, y location of a locator high up, fix a plumb bob line and measure the location at floor level.

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Why We Created Our Own Bluetooth Engine

In the post on Bluetooth (BLE) vs Ultra-Wideband (UWB) for Locating we mentioned how Bluetooth 5.1 direction finding solutions have been slow to come to the market and how the products that so far appeared all had shortcomings such we weren’t able to recommend them to our customers.

We spent a considerable amount of time (and cost) evaluating AoA solutions and were disappointed with what we found. While the hardware is generally good, the accompanying location engine software was very poor. This was perhaps understandable because software is not necessarily within a hardware manufacturer’s competence.

A location engine converts radio signals from multi-antenna hardware into x, y z location. There are many ways to implement a location engine using different radiogoniometry algorithms of different accuracy and computational complexity. The location engine also needs to filter the incoming data to mitigate the affects of multi-path reception, polarization, signal spread delays, jitter, and noise. It needs to be flexible to using just one locator or multiple locators for more accuracy. It needs to be performant to support the maximum throughput and hence the maximum number of beacons.

In our evaluations, the location engine wasn’t included in some solutions and where it did exist, it didn’t meet the above requirements. We found evaluation kits to be expensive and in some cases were ‘toys’ that demonstrated the principles but weren’t suitable for production. Most systems had vague performance specifications and it wasn’t clear how well they would scale. Most solutions used dongles for copy protection and some had resource limits linked to payment. Documentation tended to be poor, incomplete and require NDA for something that wasn’t at all commercially sensitive. APIs to access the location engine data tended to be incomplete. Systems were inflexible to beacon input rates and assume everyone needs 100ms updates that kills beacon battery life and severely limits the maximum number of tracked assets. Some only supported 2D rather than 3D locating. Some systems relied on applications polling rather than more efficient streaming the output data. The provenance of the software was unknown and whether it was secure, reliable and free of malware.

The above limitations caused too many uncertainties such that there was no way we could specify, demonstrate and supply to clients. It’s for this reason we created our own location engine that’s performant, flexible, accurate, reliable, secure and documented.

Analysis of location angles for a 12 antenna locator
The peak shows the most likely location

The location engine can be used on its own or as part of PrecisionRTLS™ for plotting onto floorplans, alerts and access to historical data.