Skylo NTN: Signal Quality Check feature

[JordanYates]

The NTN AT command guide documents a "Signal quality check" option for performing a pre-check before attempting to re-attach to the Skylo network:

https://docs.nordicsemi.com/r/bundle/ref_at_commands_nrf91x1_ntn/page/ref/at_commands/ntn/skylo_set.html

I can't find any information on how this feature works elsewhere in the docs:

• When the check runs
• Whether it can be manually triggered by the application (without the attach step if it passes)
• How the application is notified on failure or success

Being able to manually trigger the check and receive a result in the application (ideally with signal strength information) would be extremely advantageous for quickly evaluating connection strength without having the send dozens of packets to calculate success rates. In a production environment, getting a notification on failure is important for handling pending data.

Alternatively, does `AT%XMONITOR` serve the purpose of application-level signal strength evalulation for NTN?

[Syed Maysum Abbas Zaidi]

Hi,

Thanks a lot for waiting. After consulting with the modem team, here is our response to your questions:

The signal quality check is an automated modem process specified by 3GPP. If a lower layer failure occurs (example an RRC connection release, radio link failure, or random access channel timeout before receiving a network response), the modem detects this and performs operations such as incrementing failure counters, backing off, or entering a limited service state, depending on the EMM cause code. If lower layer failures have occurred, before retrying a connection (such as an Attach Request, Tracking Area Update (TAU), or Service Request), the modem evaluates the signal conditions; if they have not improved, no new connection attempt is made.

This blocking (back-off) is visible in the +CGEV notification and the requested procedure is restricted:

+CGEV: RESTR <cause>,<validity>

<cause>
4 – Signal quality check fails at connection establishment. See %FEACONF <feature_id> value 7.

<validity>
1 – Permanent restriction. Clearing requires a power-off, UICC change, or a configuration change.
2 – Temporary restriction. Clearing requires, for example, back-off timer expiry.

However It is not possible to manually trigger this evaluation. One option to evaluate signal and cell availability without enabling the power-hungry TX is to use RX-only mode, so you can activate LTE in RX-only mode (cell selection with no network signaling; only search, SI reception, and NW evaluation are performed):

1. Case where cell is available:
   

   => AT+CFUN=2
   <= +CEREG: 51,"0001","01A2D101",14,,,"11100000","11100000"

   
   After CEREG: 51, it is possible to evaluate the RX signal quality
   

   => AT%XMONITOR
   <= %XMONITOR: 1,"","","90198","3A9B",14,255,"0004CE7B",3,228821,20,27,"","00001000","00111000","11100000",20,3,8,4

2. Case where no suitable cell is available:
   Activate LTE in RX-only mode and observe the search status:
   

   => AT+CFUN=2
   <= %MDMEV: SEARCH STATUS 1 (light search complete)
   <= %MDMEV: SEARCH STATUS 2 (full search complete)

   
   Receiving search-status events without a +CEREG: 51 indicates that no suitable PLMN was found.

I hope it helps.