IoT Hacking Series #8: How Does NB-IoT or LTE-M Actually Save Power?

Low Power Wide Area Network technologies (LPWAN) emerged in 2013. For years, analysts have said that NB-IoT or LTE-M will replace other cellular technologies for most IoT use cases because of lower power consumption.

Today, the LPWAN market is still in its early stage, but their global footprint is growing steadily despite the current limitations.

Let’s leave the rollout plans aside and look at how both technologies achieve lower power consumption.

GSMA has specified minimum key features for NB-IoT(Cat-NB1) or LTE-M (Cat-M1) deployments and suggestions for Mobile Network Operators (MNOs).

The GSMA’s documents include the required frequency bands, power-saving features, and global roaming.

• NB-IoT Deployment Guide
• LTE-M Deployment Guide

Power Saving Mode (PSM)

Power Saving Mode (PSM) is one of the most critical features of LPWAN technologies. The basic concept is simple and not that revolutionary on its own.

The cellular radio module is turned off when not in use, and it will stay off depending on the use-case for the maximum amount of time.

Device manufacturers have tried implementing the same features on other cellular technologies like 2G/3G/4G. Yet the limitation used to be that every time the cellular module is turned off, it needed to detach from the network.

Once you are ready to use the module again, you need to reattach to the network. Detaching and reattaching consumes power every time. PSM avoids this power consumption.

The unique power-saving feature that PSM introduces is that it enables the device to set sleep and active timers which are forwarded to the network. The network will keep the device registered in the network for the specified time, and if the device wakes up within that period, no reattach procedure is needed.

The maximum sleep time is about 413 days, and the maximum active timer is about 186 minutes.

It's recommended that the MNO will store device terminated packets or an SMS of 100 bytes which are sent to the device while it is sleeping. Once it's back awake, the network forwards those to the device.

Extended Discontinuous Reception (eDRX)

Discontinuous Reception (DRX) is a common feature used in today's LTE networks and smartphones. It helps the device save power by switching off the receiving section of the cellular module for a fraction of a second. During this time the device is not reachable, but as the period is under a second, the phone user will not recognise any problems or delays.

Extended Discontinuous Reception (eDRX) is doing as the hints –extending the time of regular DRX. For IoT/M2M devices, the delay of a few seconds or minutes in receiving incoming packages is not that critical.

During eDRX, the device remains always attached to the network, and at least 100 bytes of packages terminated to the device will be forwarded once it's receiving section is awake.

The eDRX time can be configured up to ~175 minutes for NB-IoT and ~45 minutes for LTE-M, but this will impact the data reception.

eDRX doesn't provide the same amount of power savings as PSM, but it offers better device reachability. You should decide based on the specific use-case if you want to use PSM, eDRX or both in tandem.

To improve battery life, even more, the SIM (UICC) or eSIM (eUICC) can be turned off during eDRX. This is allowed when the required Elementary Files (EF's) are set.

Wake Up Signals (WUS)

Another handy feature reducing the unnecessary power consumption is Wake Up Signals (WUS).

The devices regularly check for paging messages (update messages between the device and cell tower)—every check consumer power. Wake Up Signals allows the device to skip the paging and begin the procedure once WUS is received.

Power Class 5 & Power Class 6

Currently deployed LTE networks are based on the assumption that devices are using Power Class 3. Transmitting with a power level of 23dBm.

For NB-IoT and LTE-M use new Power Class 5 and Power Class 6 could be implemented by the MNOs. Both would reduce power output. Power Class 5 transmits with a power level of 20dBm and Power Class 6 with 14dBm. Not only will this save battery life, but it would lower the device cost and footprint size.

However, there are a few points to consider when MNOs implement these new power classes.

LTE networks were set up with 23dBm level of power transmission in mind. Reducing the power level can lead to a reduction in coverage. It means that devices that used to have good signal before may end up out of coverage. Lowering power transmission reduces the ability to transmit signals underground in parking garages or tunnels.

MNO's cell planning can overcome the problem. Some MNOs might set up classes 5 and 6 without problems, where others can't do it.

And there is more...

PSM, eDRX, WUS, Power Class 5, and Power Class 6 provide the most significant power savings. But smaller features will help reduce consumption even further. Look for keywords like CIoT optimisation and Battery Efficient Security for low Throughput(BEST).

Even though power-savings with LPWAN have great potential in IoT, the power-saving comes with drawbacks. For many of the use cases, these drawbacks can be irrelevant and implementing PSM, WUS, and eDRX is fitting.

All in all, it comes down to specific configurations. It would help if you were careful not to misconfigure, as it may cause connectivity problems and unreachable devices.

1oT is regularly updating the possibilities of NB-IoT and LTE-M deployments across the world.

At the moment we've partnered with KPN to offer LTE-M in over 20 countries across all the continents. We are determined to extend our coverage network-by-network, country-by-country. If you are interested, please contact sales@1oT.com for more information on availability and pricing.

For technical questions about LPWAN technologies, contact hacking[at]1oT.com.