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

[Øyvind]

Hi Alex, 

Could you provide more details on your issue? Do you have any application logs and/or modem traces that you can provide us with? 

Remember that operation is time multiplexed with LTE modem, and it is possible to use the GPS and QZSS receiver to obtain position either while the LTE is in RRC Idle mode or power saving mode (PSM), or when the LTE modem is completely deactivated.

How long does the GNSS run vs LTE modem? 

Thanks. 

Kind regards,
Øyvind

[amessner]

Hi Øyvind,

GNSS is running in continuous mode, LTE (NB-IoT) is enabled at all times. GNSS tracking should be active any time the radio is not used for the upload cycle.

Periodically we upload data, the upload rate is configurable. During an upload the modem transitions from RRC idle to RRC connected, uploads the data and transitions back to RRC idle. The transition from RRC connected back to RRC idle would usually take 5s (with our local network), but we use AS-RAI to immediately transition back to RRC idle, this reduced the time for the complete RRC connected session to a ~3 seconds.

Here is a specific scenario of a upload cycle of 15s:
- The first ~3 second the LTE link is used to upload data (RRC connected session)
- The remaining ~12 seconds the GNSS accesses the radio to track satellites

This works fine, however when we increase the upload cycle to run every 6s then it sometimes works and sometimes doesn't:
When running the upload cycle every15s upon opening a socket and sending some data the modem always transitions immediately from RRC idle to RRC connected, however with an upload cycle every 6s we sometimes see a delay of ~5s before the modem switches to RRC connected to send the data (with sometimes I mean hours at a time).
QUESTION #1: Is this the cell tower/network not switching us to RRC connected sooner and I am missing something in regards to the LTE protocol or is there some process in the modem that adds this delay of 5s?

QUESTION #2: The second question is more generic and not directly related to the use case described above. My assumption is that if the modem reports to be in RRC idle, then the GNSS can always access the radio and track satellites and is never blocked by LTE. 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. Is my assumption wrong? If yes, then what causes the modem to block GNSS even if the modem is in RRC idle?

I will try to collect modem traces to support question #1 and attach them here.

Thank you

Alex

[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

[shamilton]

Hi Øyvind,

Thanks for the response. Due to the 12 second preferred time window, we will probably have our minimum upload rate be 15 seconds. We appreciate the team looking into this for us.

Regards,

Stuart

[Øyvind]

Sounds good! Please test and let us know how that works! 

Kind regards,
Øyvind