Category: DirectionFinding

Managing Bluetooth LE Advertising Congestion

Bluetooth LE advertising congestion happens when there are too many Bluetooth devices in an area. As we will show, this rarely happens but with new Bluetooth technologies this situation is becoming more likely. We provide some ways to mitigate congestion.

Bluetooth LE advertising transmits periodically the period of which is configurable from typically 100ms to about 10 seconds.

Bluetooth LE advertising (from Bluetooth SIG)

If two Bluetooth devices happen to transmit at the same time, it’s like two people shouting at the same time. The signal is corrupted, the receiver can’t make sense of the signal and it is lost. This usually doesn’t matter because it’s likely the signal is seen the next time it is sent. The random advDelay in the above diagram ensures that the two sends don’t clash again. It’s very unlikely advertisers clash in the first instance because the transmit duration is very small compared to the advertising period. The above diagram isn’t to scale. Here’s an oscilloscope trace showing some real timing:

The advertising duration is very small, of the order of 1 to 2 ms (milliseconds). Advertising is also sent three times, on three different radio frequencies, so that if one is blocked, the radio signal might be heard on one of the others. All this means that advertising collisions rarely occur.

However, there are some newer Bluetooth protocols that as they are starting to roll out, are making collisisons more likely:

  • Bluetooth 5 advertising extensions – This allows advertising of more data, that takes longer than the typical 1 to 2 ms and hence increases congestion.
  • Bluetooth longer range – This transmits further thus effectively increasing the number of beacons advertising in a given area.
  • Bluetooth Mesh – This works by having relay beacons listen and re-transmit advertising, usually several times, to improve reliability.
  • Bluetooth direction finding – This also has longer advertising to send a constant tone extension (CTE) that is received by AoA hardware. However, of more affect is advertising more frequently. While beacons on assets used to advertise typically every second or longer, direction finding tends to use faster advertising to improve latency.

You can check how many devices are advertising by using a scanning app on Android. We recommend Nordic Semiconductor’s nRF Connect because it can decode the latest Bluetooth protocols. Use Android for full visibility because Apple made the poor design decision to obfuscate iBeacon advertising to coerce developers to only use the Apple iBeacon-specific APIs. Apple also hides devices’ MAC addresses making them more difficult to physically identify.

If you have a problem with congestion you might be tempted to increase the transmission power or advertise more often to increase the chances of being seen. However, this is counter-productive because you will be increasing congestion, especially if your devices are the main contributor to the congestion.

Instead:

  • Lower the transmit power so that beacons cover a smaller area. You can fine tune this using nRF Connect to measure the distance you need rather than needlessly advertising further. This will also conserve battery life.
  • Increase the advertising period to make collisions less likely.
  • Increase the receiver scanning period to make detections more likely.
  • Seek out and remove unwanted devices advertising too frequently, such as fitness devices, smartphones, displays and even cars.

Need more help? Consider our consultancy services.

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.

Read about PrecisionRTLS™

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

Read about BluetoothLocationEngine™
Read about PrecisionRTLS™

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

Read about PrecisionRTLS™

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.

Read about PrecisionRTLS™

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.

Read about PrecisionRTLS™