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Many IoT devices are battery-powered and one of the most important operating parameters is how long those batteries last in normal service. Maximizing battery life improves the user experience, lowers maintenance costs, and reduces wastage.
An engineer could opt for a larger capacity battery to extend battery life, 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 during operation.
Cellular IoT devices use either LTE-M or NB-IoT protocols. Both are designed for low power operation but are only part of the story. The cellular IoT device’s overall energy consumption is determined by the efficiency of its key elements and the application for which it is being used.
- Read more: Cellular IoT dominates LPWANs
- Read more: Inside the next generation of efficient and compact power management
Cellular IoT power saving techniques
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.
Some radio activity is 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 minimize radio transmission time while ensuring a reliable network connection.
The first is called “extended Discontinuous Reception” (eDRX). When using eDRX, the IoT device connects to the network less frequently, thus saving power by not having to turn on the radio so often. There is a trade-off: when the cellular IoT device is asleep, it is unreachable by the network - such unavailability introduces latency. The developer must decide on a suitable eDRX interval to strike a balance between power consumption and latency to suit the needs of the application.
The second energy conservation technique puts the cellular IoT device into an even deeper sleep state. Called “Power Saving Mode” (PSM) the method shuts off the modem, while the device remains registered to the network. The device will be unreachable for a pre-set time but can wake up whenever needed (for example, in response to an alarm).
nRF9151 gains from Nordic’s own power saving techniques
eDRX and PSM are standard power saving technologies available to any cellular IoT device maker. Nordic’s renowned ultra-low power expertise has allowed it to take energy saving significantly further in its latest low-power cellular IoT solution, the nRF9151 SiP. The nRF9151 was designed from the ground up to minimize power consumption and includes full support for eDRX and PSM in addition to Nordic’s own power saving features.
One example of Nordic’s technology is “reduced mobility”—which limits swapping between cells—to decrease modem activity for devices that are mostly stationary. Another is “country-specific search optimization” 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.
Nordic’s nRF9151 modem also offers a “pre-evaluation of a connection” feature. Before connecting and transmitting data, the modem evaluates the estimated power consumption it would most likely use when sending data, and provides an estimate (“excellent”, “good”, “normal”, “poor” or “bad”) and information about raw parameters such as radio link quality, signal-to-noise ratio (SNR), TX power and path loss. By performing this analysis of the cell downlink radio environment, the application can decide whether to send data depending on its energy efficiency requirements.
Supplementing battery power
The design decisions made when developing the nRF9151’s included consideration for powering the SiP solely from harvested energy (such as photovoltaic (PV) power) for certain applications. No matter how intrinsically efficient the cellular IoT product might be, energy harvesting will significantly extend its battery life. And in the case of the nRF9151 SiP, energy harvesting doesn’t place any constraints on the chip’s connectivity or computational capabilities.
However, energy harvesting might restrict the application’s duty cycle. Fortunately, the flexibility built into Nordic cellular IoT solutions makes it easy to optimize the duty cycle to meet the predicted energy reserves of the cellular IoT product’s battery.
nRF9151 supports Class 5 output
Another enhancement to the nRF9151 that helps extend battery life in certain applications is support for Power Class 5 20 dBm output power - complementing the existing nRF91 Series Power Class 3 23 dBm support. The additional output power support allows developers to save transmit power—helping eke out battery energy—if the Class 5 output power matches the needs of the application. This enhancement offers developers greater flexibility, broadening the scope of use cases for which the SiP can be employed.
Highly efficient cellular IoT SiPs 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.
- Read more: How Wi-Fi locationing complements GNSS and cellular
- Read more: New cellular IoT SiPs power smaller, more efficient asset trackers
