Inter-frequency handover not successful

[oapilo]

Hi,

We are evaluating the effect of mobility for applications running on nRF9160 in the LTE-M mode. The evaluation is done in a private research and testing network. At the moment, intra-frequency handovers (from B3 cell to another B3 cell) are working fine but inter-frequency handovers (from B3 cell to B1 cell, or vice versa) never happen. We are using nRF9160DK with Serial LTE modem loaded and periodic (once per second) MQTT publishing to keep the UE in the RRC-Connected mode. The band locking is set such that only bands B1 and B3 are monitored to make the handover easier for the device. Inter-frequency handovers with conventional LTE phones are working normally with these cells.

Any ideas what is preventing inter-frequency handovers? Should nRF9160 be somehow configured to do inter-band measurements also for neighboring cells?

Our setup is:

nRF9160DK HW: 0.9.0 2019.20

modem FW: 1.3.1

Serial LTE modem: 1.9.0

Thanks for any help, Olli

[Didrik Rokhaug]

Hi,

Could you take a modem trace, so we can see what the modem does?

Also, you should note that modem FW v1.3.1 (and other 1.3 versions) are only tested and certfied with rev2 SiPs (marked B1), while your DK has a rev1 SiP (B0). The combination should still work for development, but I would highly recommend you getting a new DK.

Best regards,

Didrik

[oapilo]

Hi Didrik,

Unfortunately, I'm having issues getting any trace out of the modem. I followed the instructions from the link above but the trace files returned by the Trace collector tool are empty. Its logs state only that the trace file is opened but no errors or warnings. I re-checked that the version of Serial LTE modem is actually 1.9.1 instead of 1.9.0 and it has the compilation flag CONFIG_NRF_MODEM_LIB_TRACE_ENABLED on by default. I tested with three different baud rates 115200, 921600 and 1000000 set by the AT#XSLMUART command. I also tried to manually set AT%XMODEMTRACE=1,2 but still the trace files were empty. Any ideas?

Best regards,

Olli

[Didrik Rokhaug]

oapilo said:

I re-checked that the version of Serial LTE modem is actually 1.9.1 instead of 1.9.0 and it has the compilation flag CONFIG_NRF_MODEM_LIB_TRACE_ENABLED on by default.

Are you sure about that?

In my copy of the SDK, the option is in the prj.conf file, but commented out. You need to remove the '#' at the beginning of the line for the option to be included.

oapilo said:

I tested with three different baud rates 115200, 921600 and 1000000 set by the AT#XSLMUART command. I also tried to manually set AT%XMODEMTRACE=1,2 but still the trace files were empty. Any ideas?

Yes, you need to use AT%XMODEMTRACE=1,2 manually with the SLM.

Modem traces are sent with baudrate 1000000 over UART1. If you have changed the firmware running on the nRF52840, it might be that the UART lines are not routed to the interface chip, but to the external headers. You can find the original board controller FW here, in case you need it.

[oapilo]

Hi Didrik,

Sorry my bad about the compilation flag. It was indeed commented. I'll blame on moving to summer time tiredness for not noticing ;)

Please find the trace file of the inter-frequency handover test attached. UE stays in the B1 cell around 104 seconds after which the connection is lost for around 6 seconds. Then UE does the cell-reselection and attaches to the B3 cell. According to the EPC logging, there is no X2 or S1 handover (cells belong to different eNBs).

We also received some feedback from the support of the eNB manufacturer. They confirmed that the features related to LTE-M inter-frequency handover are correctly configured.

Best regards, Olli

trace-2022-03-29T06-33-08.359Z.bin

[Didrik Rokhaug]

Thank you. This trace is valid.

I've forwarded it to our modem team, so they can have a look.

They usually answer within a couple of days, and I will come back to you when I get a reply from them.

[Didrik Rokhaug]

RRC connected mode inter-frequency measurements may be enabled with the REL14FEAT AT command.

[oapilo]

Thanks for the tip. I enabled the last bit interf_cmm_intraf_rsrq in the AT command. However, I didn't see any notable difference in the performance. Please see the attached modem trace. Now the UE transmits data in the B1 cell approx. 83 seconds, then there is disconnection for about 5.3 seconds. Finally, UE attaches to the B3 cell. Again, there is no indication on any X2 or S1 handovers in the EPC logs.

Best regards,Olli

trace-2022-03-30T10-09-28.722Z.bin