How does handover works in eDRX mode?

[Yusuke]

I'm attempting to receive SMS in eDRX mode. It succeeds when nRF9160 device is left in a place without moving it.

However, when I move it to a far place like a few kilometers away, the device stops receiving SMS. I expect this is related to handover.

I really appreciate it if you help solving this issue. Thank you in advance!

[Yusuke]

I may figure it out.

I expand paging time window(PTW) to receive SMS during eDRX. 

When a cell station is switched to another, eDRX parameters seem to be handed over except PTW value.

Why does this occur?

I hope PTW value is configurable in prj.conf. I need to fix lte driver now to do that.

[Heidi-Irene Harvey Sollie]

Hi!

When a cell station is switched, the UE (your nRF91 DK) will request the eDRX configuration that the user wants (such as the PTW), but it is up to the base station if these are accepted. So the PTW value should be handed over, but it could be rejected by the network. 

To confirm that this is what happens you could take a modem trace, see this tutorial, and send it to me for analysis.

You could also use the eDRX command +CEDRXRDP to read out the dynamic eDRX parameters such as the PTW.

Best regards,

Heidi

[Yusuke]

Let's say eDRX interval is 11min.

Does a modem check if the cell station is switched every 11min?

[Heidi-Irene Harvey Sollie]

No. The time between tracking area updates (modem checking if the cell station is switched) is decided by the network you are connected to. You can read out this value (called periodic TA) from the modem by using the %XMONITOR command.

[Yusuke]

Thank you for explaining.

So the modem remembers the TAU interval and wakes up when TAU occurs. This interval is independent of eDRX interval.

Is this correct?