Hi Nordic team,
I’m working with an nRF9160-based device and have observed inconsistent LTE behavior when the device operates in areas with poor or unstable signal strength.
In low-signal environments, the modem repeatedly attempts to connect to the network. The sequence of LTE events varies between connection cycles.
Sometimes LTE_LC_NW_REG_UNKNOWN and LTE_LC_NW_REG_NOT_REGISTERED both come when the device is in a bad connection area, and sometimes only the LTE_LC_NW_REG_UNKNOWN event comes. If I don’t call lte_lc_func_mode_set(LTE_LC_FUNC_MODE_DEACTIVATE_LTE), then the pattern shown in the attached screenshot is observed.
If I call this function lte_lc_func_mode_set(LTE_LC_FUNC_MODE_DEACTIVATE_LTE) on UNREGISTERED event, then it works well and I can turn LTE back on after some time. However, the issue is that at low voltages, around or below 3.4 V, this function sometimes blocks the thread and never returns.
Also, as seen in the power profiler pattern the lte is trying to connect but the events are not triggered most of the time. For example, during the first attempt I see the LTE_LC_NW_REG_UNKNOWN and LTE_LC_NW_REG_SEARCHING events, but for the next 15 minutes, as shown in the screenshot, I only see one LTE_LC_NW_REG_NOT_REGISTERED event.
If I start LTE in an area where it cannot get a connection, the RRC first goes to CONNECTED, then IDLE after about 8 seconds, then LTE_LC_NW_REG_SEARCHING after a few seconds and then both events (LTE_LC_NW_REG_UNKNOWN and LTE_LC_NW_REG_NOT_REGISTERED) appear.
This makes it difficult to implement consistent application-level logic for managing LTE state transitions and power optimization.
Questions:
1. What is the expected pattern or sequence of LTE events during:
* Normal attach/detach
* Network loss
* Reconnection attempts
2. Is there an official document or timing diagram describing the LTE event flow in the nRF9160 modem firmware under various signal conditions?
3. In poor signal conditions, what is the recommended way to detect and manage repeated network failures?
Device: nRF9160
Modem firmware version: 1.3.7
SDK / NCS version: 2.7.0
Zephyr OS: v3.6.99-ncs2
