LPWA (Low Power Wide Area) technologies for the IoT
How LTE-M technology conserves power
LTE-M standards allow for simpler and cheaper chipsets to be manufactured. These standards also give the IoT application developers and customer options on how best to conserve power. There are two ways of doing this, depending on what is best for your application:
- PSM (Power Saving Mode)
- eDRX (Extended Discontinuous Reception)
LTE-M Power Saving Mode (PSM)
LTE-M Extended Discontinuous Reception
PSM is not the only way to manage power consumption on an LTE-M connection. The normal ‘paging cycle’ of an LTE device is 1.28 seconds, this is where the device ‘checks in’ with the cell tower to make sure it is connected. This may be fine for mobile phones or IoT devices where there is a mains or vehicular power supply, but not if your device is relying on batteries. LTE DRX extends the paging cycle, allowing the device to ‘sleep’ for 10.24 seconds, these extended sleep cycles are called hyper frames (HFs). DRX is useful for some IoT applications but still not enough to achieve a battery life of many years. The eDRX (extended) feature on LTE-M has been developed to use hyper frames in a different way. eDRX allows your device to specify how many HFs it would like to sleep before checking back in. The maximum number of HFs you can have is set by the operator but should be at least 40 minutes, which is about 240 HFs. Running your application in eDRX with your device checking in ever 10 minutes (60 Hyper Frames), should give a battery life (2 x AA) of 4 to 5 years.
eDRX allows more flexibility than PSM if you do need to communicate with your device in an asynchronous way, either to grab some data following an event or deliver a firmware update.
More information about LTE-M power saving modes
For those of you designing your IoT solution, writing the application or simply wanting to know more about LPWA cellular technology, here is a link to a GSMA document that gives more detail on LTE-M power saving modes (pages 10-13) and pages 6 to 8 explain more of those IoT acronyms we love so much.