nrf9160 radio access: LTE vs GNSS

[amessner]

Hello,

I'm working on an experimental application that tries to collect GNSS data as often as possible and upload the data to our server as frequent as possible.

My setup: MFW 1.3.5, nRF Connect SDK 2.4.0

After uploading the data I am using AS-RAI to transition from RRC connected to RRC idle as quickly as possible to maximize the time GNSS can access the radio.
The application is set to use NB-IoT only, as the local LTE-M network doesn't seem to support AS-RAI. I have PSM enabled, but realistically we won't even enter PSM in this specific application. 

I ran into two scenarios that I don't quite understand:

1. My assumption was that if the modem reports to be in RRC idle, then the GNSS can access the radio. However, I have seen scenarios where the GNSS flag "NRF_MODEM_GNSS_PVT_FLAG_DEADLINE_MISSED" is set even with the modem in RRC idle. What causes the modem to block GNSS even if it is in RRC idle?
2. I tried to see how frequent we can upload data and still track our GNSS position. Setting the upload rate to 6s sometimes works perfectly, and sometimes it does not: Usually when sending data I see the modem immediately transition from RRC idle to RRC connected, however in this case we sometimes see a delay of ~5s before the modem switches to RRC connected to send the data. Is this the cell tower not allowing us to switch to RRC connected or is the modem actively slowing down this transition?

Thanks

Alex

[Johnny Nguyen]

amessner said:

This is a theoretical question that you should be able to answer without a modem trace. Are there any possible scenarios where GNSS would report to be blocked by LTE if the modem is in RRC idle?

Our developers state there are some LTE operations which can block GNSS also when an RRC connection is not active. For example, during cell search LTE uses the RF and GNSS needs to be blocked.

[Øyvind]

Hello,

amessner said:

I'm planning to collect a modem trace later this week

Any update on this?

Kind regards,
Øyvind

[shamilton]

Hello, I am a colleague of amessner at the same company. Sorry for the wait, but we managed to capture a modem trace which highlights the delay in switching from RRC idle to RRC connected. Attached is the modem trace from a test, the application log associated with the same test, and a pdf document that annotates the application log to help explain what is happening.  In the test, we managed to capture examples of both normal functionality, and when there are delays in the transition to RRC connected.

We hope the modem trace will help you to understand what might be causing these delays

Please let us know if there is any other info we can provide.

Thanks,

Stuart

trace-2023-10-30T21-37-01.203Z.bin

[Øyvind]

Thanks Stuart! I have forwarded this to our modem team and hope to have an answer by tomorrow or Monday. 

Kind regards,
Øyvind

[Øyvind]

Stuart, my sincere apologies for the late reply. Our modem team had a look at the logs and provided the following response:

The reason why the RRC connection activation sometimes gets delayed is a 12 second preferred time window granted to GNSS. During a preferred time window, GNSS blocks LTE. When the GNSS preferred time window ends, the RRC connection activation starts immediately.

This is as per design and there’s currently no way around it. GNSS can only get a preferred time window right after the RRC connection release. Because of this the delay is not visible with the longer upload cycle, because the preferred time window has already ended before the next upload.

Kind regards,
Øyvind