7 Power Consumption of the Low Power Node (LPN)

One of the key features of Bluetooth mesh is the introduction of a low power node. This mesh node feature allows for a battery powered device to receive messages at configurable intervals while the device sleeps in between each wakeup period. As the specification defines, a low power node must be paired with a friend node. The role of the friend node is to receive messages for the low power node and cache them. When the receive window starts, the low power node will wake up and the friend node will transmit the cached data to its recipient. The receive window reflects the amount of time the LPN listens for a response from its friend. An LPN can reduce current consumption by spending less time listening for a response, i.e. choosing a friend with a smaller receive window.

Poll timeout is the maximum time the LPN may take before requesting cached data from its Friend. This is a critical factor for low power nodes as this time represents the maximum amount of time the node may sleep for before the next wakeup event. If poll timeout is set too small, the LPN will wakeup more often resulting in more current consumption. Setting the poll timeout too large may result in additional latency when receiving updates from the LPN, since it wakes up less frequently.

To test the power consumption of an LPN, one friend node and one LPN are configured with the features listed below. All LPNs are configured with a scan delay of 0 ms and the Secure Network Beacon feature is disabled. Setting the scan delay to 0 ms ensures that the LPN wakes up in sync with its friend node to send a poll message. This saves power because the LPN maximizes the receive window and is not in receive mode (RX) for longer than necessary. During each test, 20 measurements were taken with a sample rate of 2.56 µs and a buffer size of 128k. The measured power consumption shown is the average current of an LPN taken over a period of 1 minute in the configuration detailed below.