LoRaWAN Battery Life

Reducing Device Power Consumption

March 3, 2024 (1mo ago)

The LoRaWAN specification defines three devices types:

  • Class A
  • Class B
  • Class C

Only Class A and Class B devices are typically battery powered. As their described behavior allows for the radio to turned off.

Class C devices support a continuous receive window, allowing them to received downlink messages at almost any time. Constantly keep their radio on draws lots of power, as a result these devices are often mains powered.

Firmware Optimization

A significant factor for battery life in LoRaWAN devices and LoRa devices is how the firmware is implemented. Depending on the MCU, different power saving modes can be entered which improve battery life.

The STM32 SoC provides many power saving modes that can be highly categorized into the following:

  • Run mode
  • Sleep mode
  • Deep Sleep mode
  • Shutdown mode

Run Mode

By default, the microcontroller is in Run mode after a system reset or a power reset. This mode can be described as the ‘default’ setting and consumes the most power.

In this mode at least one of the CPUs (if multi-core) is executing code and peripheral are turned on.

In the context of LoRaWAN devices it means the LoRa radio is powered and is able to perform transmission and reception.

The power consumption ins Run mode can still be optimized. By slowing down the system clocks, and/or gating the clocks to the APB and AHB peripherals when they are not in use.

Sleep Mode

Low-power modes are available to save power when the CPU does not need to be kept running.

In the case of Class A LoRaWAN devices, the CPU does not need to be kept running until its next uplink transmission.

Sleep mode stops the processors clock, but leaves other system and peripheral clocks running, allowing a quick wake up.

Deep Sleep Mode

Deep sleep modes stops most of the STM32 system and peripheral clocks. At a implementation level this corresponds to the Stop and Standby modes defined in the STM32 reference manual.

Active peripherals with wake-up capability can be used to wakeup the device.

Shutdown Mode

In Shutdown mode core system and peripheral clocks are disabled. The Low-Speed External Oscillator (LSE) can be left on. At this point the microcontroller can only be woken up by the LSE or external events such as a rest.

Communication Standard

LoRa communication is designed for low energy consumption. Configurations can be made to some LoRa parameters that greatly affect the power consumption of the device.

Spreading Factors

LoRa is modeled on Chirp Spread Spectrum (CSS) technology, where chirps (symbols) are the carrier of data.

The spreading factor controls the chirp rate / speed of data transmission. LoRa modulation has a total of 6 spreading factors from SF7 to SF12. Spreading factors influence data rate, battery life, and receiver sensitivity.

Without getting into the specifics a higher Spreading Factor needs longer time-on-air, this is because the chirps are, as the name suggest, more spread out. This results in longer active times for radio transceivers and shortened battery life.

There are trade offs when choosing a smaller spreading factor if you’re looking to save battery. These include Receiver Sensitivity, Distance, and Data rate. A LoRaWAN device should taken into account all these factors for its intended application.

Wireless Device Batteries

When developing wireless devices for Internet of Things (IoT) application, choosing a battery is often a challenging step in the process that is overlooked.

When working with batteries for LoRaWAN devices the form factor, capacity, cost, etc are all characteristic which can influence a battery selection.

Here are some considerations provided by Semtech to identify minimum requirements for the application

  • Physical dimensions – Available space in the product
  • Electrical parameters – Supply voltage range, peak current consumption
  • Operating profile – Frequency and duration of transmissions, receive windows, measurements, and sleep durations
  • Life expectancy – Shelf life and service life
  • Environmental – Storage and operating temperatures, moisture, and humidity
  • Other considerations – Cost, rechargeability and disposability