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

[Bjørn Mork]

Dan Williams <dcbw at redhat.com> writes:
> On Fri, 2014-10-31 at 09:39 +0800, dailijin wrote:
>
>> 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.

I believe the described behaviour is normal. Temporarily swichting the
radio to another band to do measurements steals capacity.  So you won't
normally get any RSSI,RSRP,RSRQ etc values for the other available RATs.
But the network will transmit information about the available RATs, and
can instruct the modem to start measurments in preparation for an
inter-RAT handover if necessary.

Google pointed me to this high level description of the process:
http://lyle.smu.edu/~skangude/Inter-IRAT%20Handover%20in%20LTE_v3.pdf


Bjørn