nrf9160 : LTE connection issue

[Stratosphere]

modem fw.: 1.3.2

sdk: 1.6.0

SIM: ibasis

We recently switched to iBasis sims (previously using Bell) and i have been seeing some behavior on LTE connection attempt that I am trying to figure out.

LTE connection timeout is set to 120 seconds.

Trying to connect
LTE cell changed: Cell ID: -1, Tracking area: -1
LTE cell changed: Cell ID: 141267985, Tracking area: 55012
RRC mode: Connected
LTE cell changed: Cell ID: -1, Tracking area: -1
RRC mode: Idle

lte_lc_init_and_connect failed: -116

Questions:

1. What does RRC mode: connected mean ? Very often (even with Bell and Rogers) I see RRC mode: connected but then the connection attempt times out and lte_lc_init_and_connect returns -116 after 120 seconds.

2. On many occassions i see LTE cell changed: Cell ID message even in RRC Idle mode. What causes this behavior? Is the modem trying to look for a better signal or connection? our device when idle will not send messages for a few hours but in between it will change cell towers many times while on PSM. How can I manage or disable this to not drain the battery prematurely?

3. What can I do to make the connection attempt faster, stable and reliable? Right now on a 10 minute connection cycle test (where I disconnect and reconnect every 10 minutes) sometimes I can connect within 5 seconds, at other time it will take 30-60 seconds, while occassionly it fails to connect. All of this while the device is on a desk and LTE signal stregnth is excellent in the office.

4. Do newer SDKs (2.1) and the modem lib that comes with it improves the LTE connection? Will switching to newer SDK provide me any improvement on this aspect?

Thank you in advance for all the help and support

Noaman 

[Hakon]

Hello,

1. What does RRC mode: connected mean ? Very often (even with Bell and Rogers) I see RRC mode: connected but then the connection attempt times out and lte_lc_init_and_connect returns -116 after 120 seconds.

You can read about RRC states here. I'm not sure why the connection times out, but you can try to use the at client + link monitor to get more information, and also maybe a modem trace would be helpful.

2. On many occassions i see LTE cell changed: Cell ID message even in RRC Idle mode. What causes this behavior? Is the modem trying to look for a better signal or connection? our device when idle will not send messages for a few hours but in between it will change cell towers many times while on PSM. How can I manage or disable this to not drain the battery prematurely?

I don't think this is possible using LTE-M. For NB-IoT automatic handover is not possible, but it should not work with iBasis SIM, so you will need to use a different SIM in that case.

3. What can I do to make the connection attempt faster, stable and reliable? Right now on a 10 minute connection cycle test (where I disconnect and reconnect every 10 minutes) sometimes I can connect within 5 seconds, at other time it will take 30-60 seconds, while occassionly it fails to connect. All of this while the device is on a desk and LTE signal stregnth is excellent in the office.

The best way is probably with xbandlock. Also, the %xcountrydata cmd can be considered to decrease the time.

4. Do newer SDKs (2.1) and the modem lib that comes with it improves the LTE connection? Will switching to newer SDK provide me any improvement on this aspect?

Probably not. You would most likely need to upgrade the modem firmware to see any significant improvements to the LTE performance.