nrf9160: tracking time spent searching for LTE

[rjshade]

Hi,

I’m experimenting with the nrf9160 LTE modem in poor network conditions. I would like to be able to keep track of how long the modem spends searching for LTE networks. I had assumed that we could do this by observing the registration status events emitted by the modem and tracking time spent in the LTE_LC_NW_REG_SEARCHING state but that appears to be insufficient.

Here are some timestamped logs I captured during testing, showing the values of the lte_lc_nw_reg_status events emitted from the modem:

00:01:42.464 Sleep exit: LTE_LC_MODEM_SLEEP_FLIGHT_MODE(4)
00:02:01.638 Reg status: LTE_LC_NW_REG_UNKNOWN (4)
00:03:27.269 Reg status: LTE_LC_NW_REG_SEARCHING (2)
00:03:29.888 RRC status: LTE_LC_RRC_MODE_CONNECTED (1)
00:03:30.947 Reg status: LTE_LC_NW_REG_REGISTERED_HOME (1)

From power measurement I can see that the modem begins LTE searching after the LTE_LC_NW_REG_UNKNOWN event, and it continues to search until 03:29 when it connects.

What can be inferred about the modem’s search behavior when we receive the LTE_LC_NW_REG_UNKNOWN event?

Why is modem searching not strictly associated with the LTE_LC_NW_REG_SEARCHING state?

Is there a correct approach to keeping track of time spent searching, either using these events or by some other means?

Thanks,

Robbie

[dejans]

Hi,

LTE_LC_NW_REG_UNKNOWN specifies unknown reason, but it probably means that your device lost the LTE connectivity. It might be that the device is considered to be out of the LTE network coverage due to poor network conditions. Can you provide more information about the network conditions that you test in?

Can you provide more information about your application used for testing? What is modem expected to be doing in your case?

Which device and modem firmware version do you use?

Best regards,
Dejan

[rjshade]

Thanks for the reply!

I guess my main point of confusion is that the nrf9160 continues to perform searches even in the LTE_LC_NW_REG_UNKNOWN state. What causes a transition from REG_UNKNOWN to REG_SEARCHING and what do the two states mean in terms of search behavior?

The application used for testing is simply our production firmware image running on a nrf52. It sends the request to the nrf9160 to ask it to register, and I observer the power consumption as the modem attempts to search for the network. I am simulating poor network conditions by disconnecting the LTE antenna from the device.

We are using nrf9160 FW 1.3.5.

Thanks.

[dejans]

Hi,

Thank you for additional information.

Registration status 4 is sent when there is no PLMN available and means that modem has performed all searches and did not find a suitable cell to register to. After that modem will perform new searches periodically, until it is able to register, or the modem is powered off or put into offline mode. The time between searches depends on the periodic cell search configuration. It can be configured either using %XDATAPRFL or directly using AT%PERIODICSEARCHCONF. 

Best regards,
Dejan

[rjshade]

"After that modem will perform new searches periodically"

This is the part that confuses me. So to confirm: the modem will perform periodic searches in both REG_SEARCHING and REG_UNKNOWN states? 

Can we say for sure that whenever we get a REG_UNKNOWN event that the modem is entering the periodic search pattern configured via XDATAPRFL OR PERIODICSEARCHCONF? Or are there other scenarios by which it can enter REG_UNKNOWN and *not* be searching?

Thanks,

Robbie

[dejans]

Hi Robbie,

If modem is camped on the cell but in limited-service, modem reports +CEREG: 2 (not connected but trying to connect). The same CEREG value is reported when the search starts. Therefore, it does not seem possible to keep track of how long modem spends in searching for LTE networks. In case where modem is not camped on the cell and does periodic searches, it should be possible to track time between states +CEREG: 2 and +CEREG: 4 but this might be used only for testing. 

Best regards,
Dejan