One of the most useful tools when deploying beacons is the Faraday Bag. A Faraday bag allows you purposely block beacons you haven’t yet placed so that they don’t affect testing. During development, it also allows you to simulate beacons or scanning devices going out of range without you even moving.
Faraday bags work by having a very thin gauze layer that block radio signals. Not all types of Faraday bag are the same. For BeaconZone Faraday bags, we had the manufacturer use two layers of gauze to block even the strongest Bluetooth beacons.
For any Faraday bag, the radio signal can leak through any larger holes in the gauze. This includes the opening that should be folded over and attached onto itself using the hook and loop fastening.
The following graph shows the standard deviation of the RSSI @ 1m, for some of our beacons, measured over a 60 second time period:
Smaller bars are better and represent beacons whose RSSI varied the least over time.
We found that beacons belonged to one or two groups. Firstly those with very stable RSSI and secondly those with an RSSI that had a standard deviation between about 4 and 6 dBm.
Signal stability is more important when you are using the RSSI to infer distance, either directly from the RSSI itself or indirectly via, for example, the iOS immediate, near and far indicators. RSSI varying without a change of distance might cause more spurious triggering. However, you should keep in mind that environmental factors can often cause variation much larger than the 4 to 6 dBm found in this test. Moving obstacles, for example people, will cause significant variation in RSSI.
Bluetooth LE advertising moves pseudo-randomly between radio channels. The channels use different radio frequencies that, in turn, results in fading of the signal at different distances. We experienced and mitigated similar behaviour in our BluetoothLocationEngine™. Different radio frequencies experience different constructive and destructive interference at different physical locations. Beacons that move more between channels can cause more rapidly varying received signal strength (RSSI).
The Bluetooth SIG, who manage the Bluetooth specifications, have a new market update that provides information on the Bluetooth ecosystem. The most interesting and pertinent segment for our customers is location services:
Location service device shipments are expected to have 25% compound annual growth over the next five years according to ABI Research:
Commercial RTLS are being driven by recent inventory and logistic challenges:
‘Retail and Services’ at 62% is surprising as we would have expected ‘Warehouse and Logistics’ to be the larger segment.
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.
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.
One of the issues with using Bluetooth beacons is that it’s not easy to predict how long batteries are going to last. Battery life depends not just on the battery capacity but also the transmitted power, advertising interval, beacon processor chip type and whether the beacon has timed transmission. Also, beacons vary from model to model, sometimes even between revisions of the same model. In some scenarios it’s essential to know which beacon models are the most power efficient and how long batteries will last.
Over the years we have spent a considerable amount of time investigating actual battery use. It’s not as simple as you might think. You can’t use an ammeter because it can’t see the short pulses in peak power. The majority of the power is expended in very short, few millisecond (ms) transmit pulses, in between which the beacon goes into low power sleep.
Nordic Bluetooth Advertising Power Use
Testing needs to integrate the current used over multiple advertising periods. The test equipment needs to capture this data at sub 1ms precision in order to catch the pulses. The testing also needs to be flexible enough to work for advertising periods from 100ms to 10 sec.
We have custom in-house designed test equipment capable of real-time battery current testing. This enables us to compare different manufacturers’ beacons having the same configured settings and provide our consultancy clients with beacon battery use data based on their exact configuration settings.
As an example, an interesting test we did was was to compare the Sensoro AA transmitting just iBeacon vs iBeacon at the same time as the 3x Eddystone advertising packets. With only ibeacon @ 760ms, 0dBm advertising, 4 typical alkaline batteries would last 7.7 years. Transmitting all 4 iBeacon and Eddystone frames reduces the battery life to 2.9 years.
Another interesting observation has been that the beacons that have the strongest signals aren’t necessarily the ones using the most battery power. Design of aspects, such as the antenna, contribute to power efficiency.
We often get asked what’s the best iBeacon? Unfortunately, there is no one best beacon for all scenarios. It depends on your particular project and business requirements. Having said this we have some favourites based on specific characteristics:
Best for Price: FSC-BP103 – Inexpensive beacon that transmits up to 10 channels simultaneously:
Best for Features:M52-SA Plus – Large easy replaceable battery, long range, temperature, humidity, accelerometer:
Best for Battery Life:SmartBeacon-AA Pro – Allows use of 4x AA batteries. Use lithium AA batteries for 7+ year battery life (also depends on settings).
Best for Setup App:Minew range – Minew’s latest BeaconPlus range (those supporting both iBeacon and Eddystone) provides the best in class app.
If you need a more rigorous description take a look at the book IoT Projects with Bluetooth Low Energy. It covers the fundamental aspects of Bluetooth Low Energy scanning, services, and characteristics. It goes on to describe examples of how to monitor health data, perform indoor navigation and use the Raspberry Pi for Bluetooth solutions. The book’s code is also available on GitHub.