Academy for LoRaWAN®

The LoRaWAN protocol is a low-power standard. The low-power transmissions use a low duty cycle, meaning that a transmission is followed by a long period of inactivity before the next transmission is sent. Between data communication cycles, the end device idles. To save power, it can be put into deep sleep mode.

Transmitting data is very power-intensive compared to other end device operations. If an end device transmits hundreds of times a day, its battery will drain in a few months, if not weeks.  To receive a packet, the device consumes about 10% of the energy that was used to transmit it.  Also, note that the RX window is open for a much shorter time than the TX window, because the likelihood of a downlink message is very small and the receiver only listens long enough to detect a preamble.

Fortunately, we can manage the current draw and increase the battery life significantly.

The key to low-power performance is to put a device into deep sleep mode for more than 99.9% of the time. This is the only way to make a small battery last for several years. The difference in the energy consumption of an end device in deep sleep mode, compared to the consumption of one that is sending data, can be up to four orders of magnitude. For instance, Microchip’s LoRa® transceiver modules (RN2483 and RN2903) consume 1.6μA while in deep sleep mode. When sending data, the power consumption increases to 38.9mA. With their LoRa-enabled chip, SAM R34/R35, Microchip has managed to bring the power consumption down to 790nA.

Let’s put this into perspective. Take, for example, a 250mAh battery and an end device with a current draw of 15mA. To calculate the battery life, divide the 250mAh by the 15mA current. For our example, this results in a battery life of about 16 hours. However, in idle mode, the same battery can sustain a current draw of 500µA for 500 hours before it runs out of battery. If the end device is in deep sleep mode, a current draw of approximately 15µA can be reached. With the 15µA current draw, the battery can last for more than 16,000 hours, which is about two years.

End-Device Wake-up Calls

Once an end device is in deep sleep mode, an interrupt can be used to wake the device. The interrupt can be triggered in many ways, often by a timer or a sensor. If a sensor reading needs to be conducted periodically, the end device can be programmed to wake up at a corresponding time interval. Some sensors can wake a device when they sense a change in the environment. Examples of this type of interrupt include sensing movement with an accelerometer, detecting whether a car is being parked using a magnetic sensor, or ringing a doorbell when someone presses a button.