Battery Power Use When Advertising Multiple Bluetooth LE Channels

Most beacons can transmit more than one type of advertising , for example iBeacon, Eddystone and sensor data. In practice, no beacon can send more than one kind of data simultaneously. Instead, they send the different data sequentially, one transmission very shortly, milliseconds, after the other. Many manufacturers describe this as sending data in different channels which shouldn’t be confused with different Bluetooth LE frequency channels used to reduce the affects of wireless interference.

Some devices such as Minew and Sato can send 6 channels that can include iBeacon, Eddystone UID, Eddystone URL, Eddystone TLM, sensor, acceleration and device info:

Sato setup: Channel types are shown at the bottom

Transmitting one type of data takes of the order of 1 millisecond (ms) every configurable 100ms to 10secs period. It’s during the sending that the majority of the battery power is used with the beacon sleeping between transmissions. The following oscilloscope trace shows the battery power used, over time, with one channel:

Care should be taken to configure only those types of data that are required. If you configure more than one channel then there’s a corresponding, almost linear, increase in use of battery power for every extra channel.

Bluetooth LE Advertising Channels

Bluetooth LE works at 2.4GHz, the same frequency used for WiFi and microwave ovens! There are two main types of Bluetooth LE transmission: advertising where the device only sends out data and connection-orientated using Generic Attributes (GATT) .

Bluetooth LE uses frequency hopping for increased reliability when there might be noise on a particular channel. When advertising, it uses channels 37, 37 and 39. If subsequently connected, it uses other channels:

Frequency Channels from RF Wireless-World

When only advertising, the transmission is very short of the order of 1 or 2 milliseconds. In between advertising, there is no transmission which is one of the main reasons why Bluetooth LE has very low power use:

Power use over time

Read about Choosing an Advertising Interval

New AKMW-iB004N PLUS (ZeroConf™)

We have a new variant of the iB004N, the AKMW-iB004N PLUS (ZeroConf™) that has evolved through this beacon being using with BeaconRTLS™.

Asset tracking systems don’t need beacons advertising of the order of hundreds of millisecs or even a second. Better better use can be made of gateways, servers and beacon battery life if the advertising period is longer. For the AKMW-iB004N PLUS (ZeroConf™) it’s set to 30 secs which gives a 10+ year battery life. The calculated life is actually much longer, of the order of 30 years. However, the shelf life of the Panasonic battery is only 10 years so other (chemical?) things might happen to the battery over this time so 10 years is a conservative estimate.

Another feature of asset tracking is that you usually have to set up lots of beacons and determine their Bluetooth MAC addresses. This can take a lot of effort and, being a tedious activity, can be prone to error. The AKMW-iB004N PLUS (ZeroConf™) comes already set up and advertising. The MAC address is on a label on the outside of the beacon.

Read about Using Beacons, iBeacons for Real-time Locating Systems (RTLS)

Read about Asset Tracking for Manufacturers

Testing if a Beacon is Working

It’s often the case you need to know if a beacon is working and advertising the correct information. It’s also sometimes necessary to differentiate between beacons, based on their signal strength, so you know you are setting up the correct beacon. Other times, you might want to know a beacon’s MAC address.

The best scanning app is Nordic nRF Connect that’s written by the manufacturer of the System on a Chip (SoC) in most beacons. Nordic nRF Connect detects all beacons and indeed all Bluetooth LE devices, irrespective of the SoC manufacturer because it just looks for standard Bluetooth advertising. nRF Connect is intelligent in that it works out the kind of beacon and displays the appropriate type of information.

It’s important you use the Android version of nRF Connect. Due to over-zealous efforts by Apple to hide identities, it’s not possible for iOS scanning applications to see advertising iBeacon (UUID, major and minor) information nor the Bluetooth MAC address.

Here’s an example scan:

In the above screenshot you can see two devices. The top one is a SensorMesh™ beacon. Below you can see an iBeacon that has been tapped on to show extra information. All devices have the MAC address and a Received Signal Strength Indicator (RSSI). The MAC address uniquely identifies the device.

Devices that scan for beacons will experience a signal strength (RSSI) that varies depending on the distance to the beacon. It’s expressed in dBm and is always negative. A more negative number indicates the beacon is further away. A typical value of -10 to -30 dBm indicates the beacon is close. A typical value of -110 indicates the beacon is near the limit of detection. You can use this to determine which beacons are closest. You usually configure beacons when they are right next to the phone and have a higher, less negative, RSSI.

nRF Connect also shows the advertising period that’s based on how often the app sees the advertising as opposed to what has been set in the beacon. The value is rarely exactly what you have set because Bluetooth requires some randomisation of the advertising period to reduce the possiblity of collisions between devices, in the vicinity, that are set to the same period. Also, being wireless, not all advertising is seen which causes jumps in the shown advertising period. Read more about choosing the advertising period.

There’s also a ‘RSSI at 1m’ which is the beacon’s self-declared value, in the advertising data, of what the RSSI should be at 1m. This can be used by scanning devices, such as apps, as a form of calibration for determining distance. In most cases this value isn’t used and should be ignored. Read more about power and the measured power calibration value.

What Advertising Interval for Eddystone-URL and the Physical Web?

As mentioned in the article on Choosing an Advertising Interval, Physical Web beacons tend to be associated with static things that user stands by for a longer time and hence advertising intervals tend to be set in the range of 700ms to 1000ms.

There was discussion on the Physical Web group where Scott Jenson of Google said:

“We recommend 700ms for the simple reason that we’d like to show results in the first 2 seconds of any scan. 700ms allows you to miss one broadcast and still gives you 500ms for the PWS roundtrip”