nRF9160: High signal cell tower is congested and uploads fail, how to force device to use lower signal cell tower where uploads are successful

[cole.morgan]

Our IoT device is connecting to 2 different cell towers on the same carrier. Cell tower 'A' has signal of -90dBm, tower 'B' has signal of -105dBm.

The device is mainly attempting tower A b/c of the good signal, but when it does, it sees failed registrations, receive timeouts, and socket open failures. Data uploads using lower signal tower B consistently succeed without issue.

We are using the nRF9160 with modem firmware 1.3.7, and the stock SLM for the app core. There doesn't appear to be any way to block the modem from using specific cells. Am I missing some other command we could use to force the modem onto the lower signal tower in these scenarios?

[Simonr]

Hi

Michal is away this week, so I am looking after this ticket in his absence.

Can you specify what carrier you're using here? If you can provide the Cell ID from the XMONITOR or CEREG notifications we can try to ping the carrier to possibly fix the cell from our side. 

There isn't a specific command to force the modem onto a signal tower. This wouldn't be a good solution either way, this could cause problems for NW load balancing.

Best regards,

Simon

[cole.morgan]

there's no command to block a specific cell either? I imagine other people face this problem, in situations where a certain cell tower is causing issues and needs to be blocked from the device. I'd rather have a general solution that we could apply to other situations in the future rather than chase down every bad cell tower with the carrier.

[Achim Kraus]

> I imagine other people face this problem

As you will find in this forum, this is pretty rare and in some of the cases, the root cause turns out to be different. (After analyzing the modem-trace, if possible.) 

> general solution

Doesn't this depend on what is considered as "solution" and what as "workaround"?

Assuming, that the situation on the base-station of that operator are not "static", blocking cells would also be not "perfect". And to maintain lists of "broken cells" is also not too easy nor smart. 

Anyway, if there are other operators in that area, a workaround, which is available today, is to select the other operator.

Using the "blacklist" of the SIM-card may be an option in the same direction (blocking the PLMN), but usually requires to implement some "low level SIM functions" with AT+CRSM. 

Or you may test, if both RATs (LTE-M and NB-IoT) are affected. If only one RAT is affected (maybe more an issue of older LTE base stations), chose the other one.

> nRF9160 with modem firmware 1.3.7

AFAIK, there are no plans for new features. So a "cell-block" may anyway only get available for the nRF9151, if at all.

[Simonr]

There's no command to block a specific cell tower, no. For the main US carriers at least, a bad cell tower is rare, and the carriers are generally keen to fix these if one is spotted.

There are some mechanisms to try prioritizing and limiting certain cell changes like %XCOUNTRYDATA, %REDMOB, but that probably won't resolve this for good.

Best regards,

Simon