modem network drop to 3G from LTE, but can't back to LTE network

[Dan Williams]

On Fri, 2014-10-31 at 09:39 +0800, dailijin wrote:
> Hey,
> 
> 
> I am using MC7354 modem with auto technology preference mode and it can work via libqmi on LTE network at the beginning, after run long time, it drop to 3G network and not back to LTE. if I reconnect the modem, it will work on LTE mode

What firmware version are you using?

What's the output of:

--nas-get-system-selection-preference
--nas-get-technology-preference
--nas-get-system-info
--nas-get-serving-system
--nas-get-signal-info
(XXXX out anything you'd like to keep private)

My best guess would be a firmware issue, because when the auto
technology preference is set, the modem should be registering on the
best technology it can find.  I've seen this type of issue being
corrected in firmware updates in devices in the past.

> For to trace the issue why it can't back to LTE mode, I print all cells informations per 5 seconds to check RSSI,RSRP,RSRQ etc. Before modem don't get connected, I can see all key value for LTE,WCDMA,GSM, but after it connect LTE, I only can see the key value for LTE,  and when it drop to 3G, I only can see the key value for WCDMA and can't see the key value for LTE  or other RAT(radio access technology), so I want to know  is there method via libqmi to check the key value for all RAT regardless of the modem connect or disconnect?

I don't think this is anything to do with QMI/libqmi (though I could be
wrong), it seems like a firmware issue where the modem either doesn't
see any other RATs or doesn't try to hand-off up to LTE.  That's usually
a firmware issue.

> Based on my understand, modem should take charge of cell reselection and RAT reselection,  so I think the modem should back to LTE automatically. AM I right?

Yes, I believe that if you have auto technology preference set then the
modem should handle this and jump back to LTE when it can.

However, when you're "locked" to WCDMA, what happens if you run
--nas-network-scan?  Does the modem soon jump back to LTE now that it
has a full network list?

Dan

> About this issue, Could you can give me some suggestions?
> 
> 
> Thanks in advance.
> 
> 
> 
> 
> 
> --
> 
> Best Regards,
> Dai,Lijin
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