LTE-M and NB-IoT both support the power saving techniques Power Saving Mode (PSM) and extended Discontinuous Reception (eDRX) to minimize power consumption.
The reason for employing power saving techniques, instead of just turning off the modem, is that the UE stays connected to the network and doesn’t have to go through the power consuming attach procedure to re-establish the connection. Power saving techniques allow the UE to switch off the radio while still maintaining connectivity with the network, mitigating the need for the connection re-establishing procedures.
The Radio Resource Control (RRC) protocol is used for communication between the UE and the eNB. The technical details of the RRC protocol and the RRC connection is beyond the scope of this course.
We will only focus on the fact that the UE can be in one of two RRC modes: RRC Connected and RRC Idle.
RRC Connected mode
RRC Connected mode refers to the time when the UE’s radio is actively sending or receiving messages. During RRC Connected mode, the UE must maintain its synchronization with the network, which consumes a lot of power.
In RRC Connected mode, the radio is mostly used for application-specific activity:
Sending uplink (UL) messages from the UE to the eNB, i.e. reporting a sensor value
Receiving downlink (DL) from the eNB, i.e. configuration updates
Neighbor cell measurements
RRC Idle mode
RRC Idle mode refers to the state when the UE is switched on, but the radio is not executing high battery-consuming tasks. In RRC Idle mode, the UE does the bare minimum to stay connected to the network and to stay reachable, so the battery consumption is low compared to RRC Connected mode, but still high compared to being in power saving mode.
In RRC Idle mode, the radio performs keep-alive activities:
Periodically listening to paging messages to be informed when the eNB requires a status update or value reporting
Periodically performing tracking area updates (TAUs) to update the eNB about the UE’s location for reachability
Paging is the mechanism in which the network tells the UE it has messages for it. The UE will then decode the content and initiates the appropriate procedure.
The UE must monitor paging, which is the process of monitoring whether the network is sending paging messages. This happens while the UE is in RRC Idle mode.
Since continuous page monitoring consumes power, the UE sleeps between monitoring pages. This method is called Discontinuous Reception (DRX).
This is a short explanation of the function of timers in cellular connections. The three timers that will become relevant later in this section are:
Periodic TAU timer (T3412): Time that the UE will stay in power saving mode before waking up to send a Tracking Area Update (TAU) to the network.
Active timer (T3324): Time that the UE is monitoring paging in RRC Idle mode, before going into PSM.
RRC Inactivity timer: Time that the UE stays in RRC Connected mode after transferring data. After this time, the UE receives the event RRC Release from the network
Understanding the different timers, and the different connection statuses and modes is important for understanding and calculating the power consumption in your application, and for debugging your cellular application.
It is important to note that although some of these timer values can be requested from the network, the network ultimately decides which value to give and doesn’t have to follow a requested value.
Power saving mode (PSM)
Power Saving Mode (PSM) is a state where the UE reduces its power consumption to a bare minimum to stay connected, by going into sleep mode. In PSM, the UE is not monitoring paging and as a result, becomes unreachable. To avoid the re-connection procedure when waking up again, the UE is allowed to keep its connectivity status.
For this to happen the UE must wake up regularly and send a Tracking Area Update (TAU) to the network. This period is decided by the Periodic TAU timer. A UE in PSM will wake up for one of two reasons, either triggered by the Periodic TAU timer to send a TAU, or by the application to send UL messages.
Either way, the device wakes up, enters RRC Connected mode, sends and receives data packets, then stays in RRC Connected mode until the RRC Inactivity timer has run out. Then it will go to RRC Idle mode for the Active timer period to monitor paging, before going back to power saving mode.
The image below explains what this process looks like.
The difference between using PSM and simply turning off the modem is that in PSM the UE can still be configured to wake up to update the network about its status and stay reachable for a while. The advantage of PSM is that the UE goes into deep sleep and the radio only consumes a few microamps. The UE does not lose its connection information on the network, meaning that upon UE wakeup, the connection is established relatively quickly. This reduces the power consumption compared a powered-off modem waking up and having to re-establish a connection with the network from scratch.
As we mentioned earlier DRX (Discontinuous reception) is a method where the UE and the network negotiate phases in which data transfer occurs. The UE wakes up regularly to check with the network if it has any incoming messages (pages), and will otherwise go into sleep mode. However, the sleep time is only around 2.5 seconds, which can work well for cell phones but when it comes to IoT devices, DRX is not enough.
Extended Discontinuous Reception (eDRX) takes this a step further, by extending the amount of time the UE can sleep before having to wake up and check for pages. In addition, the UE can request the duration it will sleep, referred to as the eDRX Cycle, and the paging time window.
Note that the device is not reachable while it is sleeping, so the time it takes to reach the device depends on the duration of the eDRX Cycle.
The image below shows two different scenarios for eDRX connection states. On the left, the UE is waking up to monitor pages without having any. On the right, the UE wakes up to a page that requires a data transfer. Then is in RRC Connected mode for the duration of the RRC Inactivity timer, before monitoring paging and going back to sleep until the next eDRX Cycle.
PSM and eDRX
Although we presented PSM and eDRX as two separate power saving techniques, they can also coexist as they implement changes to different parts of the connection process.
The image below shows the connection states when using both PSM and eDRX.
When requesting values for the Active Timer and eDRX Cycle in this case, it is important to consider that the Active Timer must be larger than the eDRX Cycle by a few magnitudes. The eDRX Cycle indicates the time for one paging window and sleep interval, while the Active Timer (usually) indicates the time it takes for multiple intervals. Therefore if the eDRX Cycle requested is larger than the Active Timer, the value won’t be granted by the network.
Different use cases
To put it very simply, the difference between PSM and eDRX is how deep the device sleeps, and for how long it can sleep. PSM sleep times are much longer than eDRX, allowing the device to enter a deeper sleep which consumes less power. The downside being that the device will take longer to wake up and it will be active for a much longer period of time.
With “sleep”, we mean that in PSM, the radio turns off both RX and TX while it’s still registered to the network. In eDRX, TX is turned off and RX is intermittently turned on to listen for paging messages.
When the UE is sleeping, the application core can still be running, reading sensor data or other operations that don’t involve the modem.