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

[dailijin]

Hi Mork,


Thanks your reply and share document. it is helpful for me to understand how to handover during different RATs.


Based on  my test result,  once the modem drop to  UMTS from LTE, it will never  back to LTE.  it seems that there is no handover for my modem. 
Now I get cells information via --nas-get-cell-location-info, then get the RSSI,RSRP etc,  I want to analyze this issue depend on them ,  but this is useless for me now, Because when the modem work on LTE, I only can get LTE cell information, no UMTS info. also can't get LTE cell information when it work on UMTS.  


so do you know is there other method to trace the handover process?


Thanks in advance.





--

Best Regards,
Dai,Lijin



At 2014-11-01 18:30:02, "Bjørn Mork" <bjorn at mork.no> wrote:
>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
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