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

Tags: IoT
A picture of Ken-Tristan  Peterson
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Ken-Tristan Peterson

There has been much buzz about Low Power Wide Area Network (LPWAN) technologies. Minimized battery consumption and lower overall costs are the main reasons.

1oT receives more and more inquiries about the technology. One of the main questions still is how the lower power consumption is achieved?


GSMA has specified minimum key features for NB-IoT (Cat-NB1) or LTE-M (Cat-M1) deployments and suggestions for Mobile Network Operators (MNOs) on what or how to implement in the networks.

The documents list the required frequency bands, power-saving features, and global roaming. In this blog post, we cover the key elements that help you to reduce power consumption.

Power Saving Mode (PSM)

Power Saving Mode (PSM) is one of the essential features of LPWAN technologies such as NB-IoT and LTE-M. The basic concept is simple and not that revolutionary on its own.

The cellular radio module is turned off when not in use. It stays off depending on the use-case for the maximum amount of time.

Device manufacturers have implemented this on other cellular technologies like 2G/3G/4G, but every time the cellular module is turned off, it needed to detach from the network. Once you are willing to use the module again, you need to reattach to the network. Detach and reattach procedure consumes power every time. PSM is trying to avoid that.

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 keeps the device registered in the system for the set 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 also recommended that the MNO stores 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 should forward 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 to 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 does not recognize any problems or delays.

For IoT/M2M devices, most often the delay of a few seconds or minutes in receiving incoming packages is not that critical. Extended Discontinuous Reception (eDRX) is doing precisely that by extending the time of regular DRX. During eDRX, the device remains always attached to the network and at least 100 bytes of packages terminated to the device is 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 has an impact on the data reception.

eDRX doesn't provide the same amount of power savings as PSM, but it offers better device reachability. Users should decide based on the device’s use-case to either 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 only allowed when the required Elementary Files (EF’s) are set.

Fig.1 eDRX Cycle


Wake Up Signals (WUS)

The device must regularly check for paging messages (update messages between the device and cell tower) but most often performing this check is unnecessary and consumes additional power.

The device consumes battery with every check and avoiding all unnecessary paging checks would save power. Wake Up Signals allows the device to skip the paging and only start the procedure once WUS is received.

Power Class 5 & Power Class 6

Currently deployed and implemented LTE networks are based on the assumption that devices are using Power Class 3, which can transmit with a power level of 23dBm.

MNOs could implement Power Class 5 or Power Class 6 for NB-IoT and LTE-M. Power Class 5 transmits with a power level of 20dBm and Power Class 6 with a level of 14dBm. Not only will this save battery life, but it would also lower the device cost and footprint size.

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

LTE networks were set up with 23dBm level of power transmission in mind. Reducing power transmission can lead to a reduction in coverage. Devices may be out of coverage in places that had good signal before. The ability to transmit signals underground in parking garages or tunnels is reduced significantly. Some MNOs might be able to set up classes 5 and 6 without problems, where others won’t.

And there is more...

PSM, eDRX, WUS, Power Class 5, and Power Class 6 provide the most significant power savings. However, there are also smaller features that help to reduce consumption even further. Have a look at the keywords CIoT optimization or Battery Efficient Security for low Throughput (BEST).

As you see, power-saving comes with some drawbacks. Still, for most use cases, these drawbacks can be irrelevant and implementing PSM, WUS, and eDRX is the way to go.

On the other hand, you have to be careful not to misconfigure these as it may cause connectivity problems and unreachable devices.

1oT is looking to offer NB-IoT and LTE-M solutions as soon as possible. Unfortunately, no global roaming agreements make it currently a hassle. However, with our eSIM (eUICC) solution, we can provide NB-IoT and LTE-M in some parts of the world already. We are determined to extend our coverage network-by-network, country-by-country. If you are interested, please contact sales[at]1oT.mobi for more information on availability and pricing.

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

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