Not connecting to US LTE-M network

[JThomson]

Hello,

We're experiencing a strange issue where a device we sent to a partner in the US wasn't able to connect over there after rigorously testing this before shipping here in the EU and tuning it for the US. However, when it was shipped back to the Netherlands it was able to connect to EU networks as soon as it arrived.  

What's even stranger is we replicated the issue with another device in the US using the same low-power software and it wasn't able to connect. Then when flashing it with the same software build (only with debug enabled) it was able to connect. 

Has this problem been seen before? Is it possible the nRF9160 has a different procedure/method for scanning through LTE-M bands while in low-power vs. active? 

[Martin Lesund]

Hi JThomson,
Are you using the Lte_lc library to configure the modem when booting up?
Do you select the systemode (lte-m) or any Bandlock when booting up, or do you configure the XDATAPRFL?

What do you define by "active" vs. "low power"? 

It would be helpful with more information of the configuration used, and some serial output logs. 

CONFIG_SERIAL=y
CONFIG_LOG=y
CONFIG_LTE_LINK_CONTROL_LOG_LEVEL_DBG=y

[JThomson]

Yes the LTE_LC library is used, the function used for connecting is lte_lc_init_and_connect(). Also no bandlock is applied but the power level is set using the command "AT%XDATAPRFL=0". 

By active I mean one where the systems are functionally identical but the UART socket isn't disabled like in the low-power build. 

Here's the configuration used:

CONFIG_NEWLIB_LIBC=y
CONFIG_NEWLIB_LIBC_NANO=n
CONFIG_BSD_LIBRARY=y
CONFIG_GPIO=y
CONFIG_STDOUT_CONSOLE=y
CONFIG_UART_INTERRUPT_DRIVEN=y
CONFIG_TEST_RANDOM_GENERATOR=y
CONFIG_NETWORKING=y
CONFIG_NET_SOCKETS_OFFLOAD=y
CONFIG_NET_SOCKETS=y
CONFIG_NET_SOCKETS_POSIX_NAMES=y
CONFIG_RING_BUFFER=y
CONFIG_TRUSTED_EXECUTION_NONSECURE=y
CONFIG_LOG_DEFAULT_LEVEL=4

# Trace not LP
#CONFIG_THREAD_NAME=y
#CONFIG_SEGGER_SYSTEMVIEW=y
#CONFIG_USE_SEGGER_RTT=y

# LTE link control
CONFIG_LTE_NETWORK_MODE_LTE_M_GPS=y
CONFIG_LTE_LINK_CONTROL=y
CONFIG_LTE_AUTO_INIT_AND_CONNECT=n

# This time out is 2 x 90s scans + 10s between
CONFIG_LTE_NETWORK_TIMEOUT=190
CONFIG_LTE_NETWORK_USE_FALLBACK=n

# Heap and stacks
#CONFIG_MPU_STACK_GUARD=y
CONFIG_MINIMAL_LIBC_MALLOC_ARENA_SIZE=16384
CONFIG_NEWLIB_LIBC_ALIGNED_HEAP_SIZE=16384
CONFIG_HEAP_MEM_POOL_SIZE=16384
CONFIG_MAIN_STACK_SIZE=2048
CONFIG_SYSTEM_WORKQUEUE_STACK_SIZE=2048
CONFIG_MAIN_THREAD_PRIORITY=7

# eDRX Cycle = 327.68s
CONFIG_LTE_EDRX_REQ_VALUE="1010"

# Watch Dog
#CONFIG_WDT_LOG_LEVEL_DBG=y
CONFIG_WATCHDOG=y

# Enable SUPL client support
CONFIG_NRF9160_GPS=y
CONFIG_NRF9160_GPS_LOG_LEVEL_DBG=y
CONFIG_SUPL_CLIENT_LIB=y

#ESP8285
CONFIG_UART_1_NRF_UARTE=y
CONFIG_BSD_LIBRARY_TRACE_ENABLED=n

#AT commands (Not LP)
CONFIG_AT_HOST_LIBRARY=y
CONFIG_LOG=y

# For LP
#CONFIG_AT_HOST_LIBRARY=n
#CONFIG_BOOT_BANNER=n
#CONFIG_PRINTK=n
#CONFIG_LOG=n

# ADC
CONFIG_ADC=y
CONFIG_ADC_0=y
CONFIG_ADC_NRFX_SAADC=y

# NVS
# FLASH MEMORY
CONFIG_FLASH=y
CONFIG_FLASH_PAGE_LAYOUT=y
CONFIG_MPU_ALLOW_FLASH_WRITE=y
CONFIG_NVS=y

[Martin Lesund]

Hi JThomson,

I forgot to ask, which modem firmware versions are you using?

I will try to replicate the issue.

[JThomson]

Thanks, it's v1.2.0

[Martin Lesund]

Is it possible to replicate this with the CONFIG_BSD_LIBRARY_TRACE_ENABLED=y?

If so please share the modem trace.

[JThomson]

We're based in the EU and our partner in the US (where the issue occurs) is not available for any further testing until next week. Once he's back we can try this and then I'll share the modem trace. 

One question though. Between the low power and debug build, the only difference is in the configuration. This section specifically:

#AT commands (Not LP)
CONFIG_AT_HOST_LIBRARY=y
CONFIG_LOG=y

# For LP
#CONFIG_AT_HOST_LIBRARY=n
#CONFIG_BOOT_BANNER=n
#CONFIG_PRINTK=n
#CONFIG_LOG=n

For the debug build AT commands, printk's, logs etc. are enabled and for the low power it's disabled. To the best of your knowledge, does this affect modem performance at all? Possibly in scanning through bands or choosing a provider to connect to? 

[Martin Lesund]

Hi JThomson,

I do not see why those configs should differentiate the ability to connect to the network at all.
As I've understood this, this is only happening on a specific network in US?

Please provide the modemtrace of the working and non-working version.
Note: mfw 1.2.1 is released: https://www.nordicsemi.com/Products/Low-power-cellular-IoT/nRF9160/Download#infotabs