As an independent NTN communication module, how is nRF9151 connected to an external MCU on the circuit?

[Baoxiang]

Hello, I am currently using the nRF9151 LACA A1A SIP module for the development of NTN communication application terminals.

In my hardware design scheme, the nRF9151 module is used as a standalone NTN module. The module communicates with the external application MCU through a serial port, and the external MCU controls the working state, data transmission and reception, etc. of the nRF9151 module through AT commands.

Without secondary development, the nRF9151 module will be programmed with two Nordic-provided firmware: one for NTN communication and the other for serial modem functionality, enabling the nRF9151 module to possess comprehensive NTN communication and serial AT command capabilities.

The hardware design has referenced the reference design of the development board nRF9151_SMA_DK, but there are still some issues. Please help me answer them. Thank you!

The current problem encountered is:
1. The nRF9151 module has two serial ports for external communication, which are respectively used for NTN AT commands and Modem trace and debug. Could you please help me identify which serial port is used for AT commands and which one is used for trace and debug, and provide the corresponding pin numbers for the nRF9151 module? According to the reference design of the development board nRF9151_SMA_DK, my current understanding is that UART1's Tx and Rx are P0.27 and P0.26, while UART2's Tx and Rx are P0.29 and P0.28. Is this correct?

2. For the aforementioned UART1 and UART2 types, besides using Tx and Rx, are the corresponding RTS and CTS also required to be wired out and controlled? If they are not wired out, what would be the impact?

3. Can the NTN firmware of the nRF9151 module officially released operate stably for a long time? Can it be used for commercial applications?

4. In my application, which firmware should I program into the APP MCU firmware inside the nRF9151 module? I noticed that the official website provides many firmware options, but I'm not sure which one is suitable for my application. Could you please provide me with the name of the APP MCU firmware that should be programmed into the module.

[Achim Kraus]

Baoxiang said:

3. Can the NTN firmware of the nRF9151 module officially released operate stably for a long time? Can it be used for commercial applications?

In my expperience (last 8 months) the "stability" (of the communication) seems more to depend on the antenna ;-).

As also for other modes, e.g. LTE-M or NB-IoT, you will need to use a "reconnect strategy", when the network gets deregistered. In many case the modem is able to register to the network again automatically. in some cases, it may require to restart the modem with a cfun=0/cfun1 cycle, if the modem doesn't get reconnected within a timeout (e.g. 1h). And, unfortunately, in my experience it requires in rare cases to reboot the device, if it can't reconnect for longer (e.g. 6 h).

But with that, my experience is pretty well. 

Simon D-M said:

But still, I would recommend you to have some kind of FOTA/DFU capabilities in order to update the NTN modem firmware to a newer version as more feature and bug fix will probably be added in the future.

Let me recommend to consider also the costs for a FOTA over NTN ... I guess you will do this only if it's really, really required.

[Baoxiang]

Thank you very much for sharing your application experience and suggestions. I can sense that you have rich application experience. We hope to have the opportunity to consult you more in the future regarding issues that may arise during our development process.
The antenna is indeed a crucial component. Our prototype utilizes the development board antenna provided by Taoglas, and we may later engage a professional antenna team to design a built-in antenna (FPC antenna). I'm wondering if you have experience with built-in FPC antennas, and how their communication performance is when connected to Skylo's satellite network?
Thank you！

[Achim Kraus]

Baoxiang said:

I'm wondering if you have experience with built-in FPC antennas

Unfortunately, no. I just bought an external antenna to run the device in my office and put the antenna outside.

[Simon D-M]

Hi,

Achim Kraus said:

Let me recommend to consider also the costs for a FOTA over NTN ... I guess you will do this only if it's really, really required.

That's indeed a good thing to point out. I maybe need to explain myself a bit here.

This is correct that FOTA over NTN would be very expensive and shouldn't be done unless something major happens. My advice applies more to products that are using NTN as a fallback, or devices that have periodic terrestrial communication. In this case the FOTA probably shouldn't happen during an NTN connection, but instead be done when the device is connected to a "normal" terrestrial network. This way you have FOTA for a reasonable budget.

Baoxiang said:

Here is mentioned the " need to program the 'Serial Modem Add-On' application (link). This application is handling the link between the host MCU and the actual modem in the nRF9151. It also has some more features, like some custom AT commands, FOTA/DFU capabilities, ..."
How to understand this sentence? Does my application development require further programming on the nRF9151 module?

You do not need to do further development on the Serial Modem Add-On application. The source code for the application is available and extensive documentation has been made. So if you want to modify and/or compile the application you are very welcome to do it. For example, if you want to change the UART pins that the application uses it is quite easy to do it.

Or maybe, your concern was more on a wording issue with "need to program the 'Serial Modem Add-On' application". What I meant here was that you need to flash/burn this application to the nRF9151.

You can find all the different pre-compiled builds for the Serial Modem Add-On in the "Releases" tab of the GitHub (link).

Best regards,

Simon D-M