In this exclusive article for IoT Insider, Martin Lesund, Technical Marketing Manager – Cellular IoT at Nordic Semiconductor investigates battery life in LPWAN devices
Supported by the latest innovations in low power wireless technology, cellular IoT products can achieve unprecedented efficiency.
Due to its global reach, robustness, low power consumption, and advanced security features, cellular IoT is becoming the leading low power wide area network (LPWAN) technology. From asset tracking and smart metering to smart city and smart agriculture, cellular IoT enables connected devices to communicate over kilometer-plus distances while using modest amounts of energy.
But just how much energy is determined by the efficiency of the cellular IoT device and its application? Poor design choices can detrimentally impact battery life leading to users having to charge or change batteries often, generating frustration and inconvenience. One solution is to use a larger battery, but that adds cost, volume and weight. A better way is for the developer to take a systematic approach to IoT device design to ensure that not a single joule of energy is wasted.
The journey starts with selection of the most efficient hardware and software and ends with connection to Cloud Services that allow the developer to do things like trade-off power consumption for location accuracy in asset tracking applications.
Minimising power consumption
While not the only factor impacting battery life, radio transmission is the most significant contributor to current draw. The faster an LTE-M/NB-IoT radio can be switched on, send its data and go back to sleep, or the faster a GNSS radio can fix on a group of satellites, determine location and go back to sleep, the more efficient the IoT device will be.
There will always be some radio activity necessary to ensure the cellular IoT device remains registered with and connected to the network such that data can be sent virtually instantaneously when required. However, there are several power saving techniques that can be used to minimise radio transmission time while ensuring a reliable network connection.
The first is extended Discontinuous Reception (eDRX). When using eDRX, the IoT device monitors downlink paging messages from the network less frequently, thus saving power by not having to turn on the radio so regularly. The IoT device remains attached and registered to the network but spends longer periods in a sleep mode.
There is a trade-off: when the cellular IoT device is not monitoring downlink paging, it is unreachable by the network – such unavailability introduces latency into cellular communications. The eDRX interval is typically programmable between minimum and maximum limits which allows the developer to strike a balance between power consumption and latency to suit the needs of the application.
The second energy conservation technique takes power saving to a greater level by putting the cellular IoT device into a deep sleep state. Power Saving Mode (PSM) shuts off the modem, while the device is still registered to the network. The device will be unreachable for a configured time but can wake up whenever it wants (for example, in response to an alarm).
However, the device must still maintain a connection to the network. It does this by waking up to send a regular Tracking Area Update (TAU) to the network at a frequency determined by the Periodic TAU timer. Alternatively, the application could wake up the device to send an uplink message before the Periodic TAU timer has expired – although this uses more power than waiting for the timer to do the job.
Making the most of every microwatt
Nordic Semiconductor has a long history of “making the most of every microwatt”. One example of Nordic’s power saving features is “reduced mobility”—which limits swapping between cells—to decrease modem activity for devices that are mostly stationary.
Another is “country-specific search optimisation” whereby network search parameters for 70 countries can be pre-loaded – saving the power consumed during the initial search for a network in a new location. A third is “abort network search early”; in poor radio conditions, the modem can be instructed to abort initial attempts to connect to a network and try later rather than expend energy on an extended search.
Longer battery life for asset tracking applications
Asset tracking is a major cellular IoT application. Nordic’s nRF Cloud Location Services supports three power-saving locationing techniques.
The first is Assisted- and Predicted-GPS (A-GPS and P-GPS). A-GPS and P-GPS provide GPS levels of positional accuracy but use less battery power than conventional GPS. They access satellite assistance data stored in a ground-based GPS database which is relayed to the IoT device via the LTE network; the IoT device can then find the satellites in seconds instead of minutes, conserving energy. The P-GPS technique builds on A-GPS by providing over two weeks of assistance data to the IoT device. The result is even greater power savings for devices that will sleep for long periods of time.
The second location technique, LTE location services, can be used to save battery power compared with P-GPS and A-GPS. It locates the asset tracking device by identifying in which telecoms cell the tracked device is situated and then referencing the cell identification against a database of known base station locations. It offers accuracy down to kilometer level while only modestly impacting battery life. Multi-cell location builds on the single-cell technique by referencing the position of several nearby base stations instead of just one to offer accuracy down to a few hundred meters while still keeping power consumption low.
nRF Cloud Services also supports Constrained Application Protocol (CoAP) which is a very efficient protocol compared to Message Queuing Telemetry Transport (MQTT), representational state transfer (REST) and Hypertext Transfer Protocol (HTTP). CoAP support helps to further reduce battery drain.
Highly efficient cellular IoT SiPs and nRF Cloud Services allow, for example, asset tracker manufacturers to build lightweight, compact devices with powerful processing capabilities that can run for years from a single battery charge – lowering maintenance requirements. And because battery production and disposal are dramatically reduced, the environment benefits too.
Author: Martin Lesund, Technical Marketing Manager – Cellular IoT at Nordic Semiconductor
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