When does the nrf9160 modem decide to perform an LTE network search?

[EDLT]

Hello,

I'd like some clarification on the nrf9160 modem behavior. What is the criteria for the modem to perform a network search?

I've used the default search pattern for ultra-low power profile and subscribe to modem events. When I put the device into a shielded enclosure and it disconnects, I don't see any subsequent search events. However, when I remove the device from the enclosure, I am connected again to the network quickly.

What is the modem doing when it is disconnected? I would expect to get a number of search events within the first few minutes of not being connected. 

Apologies in advance if I'm missing something clearly stated in the 3GPP spec that the describes what is actually going on there.

Thanks!

P.S. Using MF 1.3.1 with NRF Connect 1.9.1

[Michal]

Hello,

How are you checking for the connection events? Have you done a modem trace?

Best regards,

Michal

[EDLT]

I've spent some time with the power analyzer which was informative. I've come to the conclusion based on power usage that searches are being performed as expected.

However, we are only notified of the search's completion after the first search of its kind (light or full). This seems like a bug in the lte_lc or modem event reporting...not sure which. This is what was confusing me and caused the initial inquiry.

Do you guys see the same behavior of only the first modem search event is report to the LTE even handler?

[Achim Kraus]

In my experience, these "Search Events" are good hints, but not that complete . E.g. HPPLMN searchs are not covered, at least not with mfw 1.3.2 .  

[gjoasen]

I'm also seeing this, only get notified of the first search.  I'm on modem fw 1.3.4 and nrf connect v 2.2.0. 

Michal, is this a known bug and something that might be resolved in later releases?

[Michal]

I will ask the experts about it and get back to you.

Best regards,

Michal

[Michal]

It is an expected behavior. Could you share with us what is your exact use-case for that, so I can check with the experts about the possibilities?

Best regards,

Michal

[Achim Kraus]

There are two issues mentioned here:

The initial comment with the device in the shielded enclosure.

And a second one, that in some cases the search events are not reported.

> It is an expected behavior

Maybe you refer, which one of the two is the expected behavior.

[Michal]

In general the notification is only sent after the first search after startup and after waking up from PSM.
The purpose of this notification has been to let application know that the every band has now been scanned once and depending on the use case application can then decide whether to deactivate modem for now or let it do more searches periodically (according to %PERIODICSEARCHCONF).

Best regards,

Michal

[Achim Kraus]

Thanks for the explanation!

[gjoasen]

Hi

When setting the search pattern using PERIODICSEARCHCONF it is possible to set the band_optimization parameter to specify how often a full search should be carried out. However, I want to have the first searches of my search pattern to be light searches, but as the period between searches becomes longer I want every search to be a full search. I don't see how I can do that with the PERIODICSEARCHCONF command. 

However, if one could get the LIGHT_SEARCH_DONE event when a the light search has completed, I could set every search to be a full search, and then stop the first searches once the light search has completed, but let the full search complete once the interval between searches is longer. 

Looking at the lte_lc.h header file, it is commented under the lte_lc_modem_evt enum:
"Indicates that a light search has been performed. This event gives the application the chance to stop using more power when searching networks in possibly weaker radio conditions."

This does not give any indication that only the first light search will generate a light search event, and should probably be updated? 

[Michal]

Hello,

Would a following procedure be suitable?

1. Activate modem and let modem start searching.

2. When modem notifies that light searches has been completed, deactivate modem with +CFUN=4.

3. Wait a short period, e.g. 10s or what ever was the idea to be in idle between the first searches.

4. Repeat steps 1-3 as many times wanted.

5. Configure %PERIODICSEARCHCONF for longer idle periods (shorter periods were already passed above).

6. Activate modem and wait until modem finds a cell by searching according to the configuration in step 5.

This approach possibly (depending on use case) requires also configuring the %PERIODICSEARCHCONF again after suitable cell has been found and modem has successfully registered to network. This is because if later the cell is lost, the periodic searches are started again. If the configuration still is such that the idle periods are very long, it could take very long time before a suitable cell is found again. Therefore, it should be considered whether a regular periodic search configuration (that contains also the shorter periods) should be taken again into use after successful registration.

Best regards,

Michal