nRF9151 Proprietary PSM

[Vincent44]

Hello,
I am currently working a project, initially based on nRF9160, which is now equiped with nRF9151.
The device sends data to nRFCloud using CoAP, every 15 min. The device is stationnary.

The issue I am currently looking at is the lack of PSM in some countries/operators.
With the nRF9160, the device has to disconnect from network (to save battery), then reconnect 15min later. With nRFCloud, this implies a full handshake/authentification, which consumes a lot of power and SIM data. Overall, we are looking at battery life divided by 2 for device without PSM access.

This is the main reason why I now upgrade the design with nRF9151.
At first, I though that SO_KEEPOPEN would be my savior. But it appears that if the modem is switch off (because no PSM), the SO_KEEPOPEN doesn't provide any benefit: the handshake needs to be re-made (that what the Nordic AI told me).

Here are several questions on this topic:

1#
So, I turned to "proprietary PSM", on which I was unable to find any detail explanation.
Is there any details on this feature, other than nrfconnectdocs.nordicsemi.com/.../psm.html

2#
From my understanding, this feature simply consists in putting the modem in sleep mode, without informing the network. Is that correct ?
If so, is there a way to apply the same principle to the nRF9160, using some AT commands ?

3#
How should this be implemented ?
Is this the right way to do it:

- Connect to network
- Send data
- Check if PSM is available / Negociate with network ?
    -> Yes, PSM can be used: just let the modem go idle on its own
    -> No, PSM can't be used
        -> Enable Proprietary PSM, using %FEACONF
        -> Should AT or TAU be negociated again ?
        -> Should PSM modem be disabled with lte_lc_psm_req() ?


4#
Is there a way to test this feature ?
At my workplace, all operators accessible to my device provide "normal" PSM.

5#
If the modem goes idle, the expected power consumption between 2 messages should be identical to what I have when the modem is switched off ?

Thanks a lot for your help.

[Syed Maysum Abbas Zaidi]

Hi,

Thanks for reaching out and your understanding regarding unavailability of PSM and SO_KEEPOPEN looks correct. Here is our responses to your questions:

1. Yes, currently this is the explanation we have for this proprietary PSM that can be configured using AT%FEACONF as mentioned in nRF91x1 Feature configuration %FEACONF. If you want any specific information related to it, let us know and we can look into it.

2. Yes, that is essentially correct. Proprietary PSM operates by modem entering deep sleep (the same way it would under regular PSM) without negotiating informing the network operator. The modem still maintains its registration context and will automatically wake up to perform required signaling or when the application sends data. And this AT command is not for nrf9160.

3. You can configure PSM and Proprietary PSM together before attaching as proprietary PSM kicks in when the device has asked for PSM and the network refuses. And Proprietary PSM is entirely on the device side and the network grants nothing, so there's nothing to renegotiate. And you should not disable standard PSM with lte_lc_psm_req() as that would remove the condition proprietary PSM depends on.

4. The straight forward way would be to use the network that refuses PSM but as you are saying it may not be possible then I think you can use a cellular call box / Network simulator and configure the simulated cell to reject PSM requests when the device attaches. However if your SIM supports both LTE-M and NB-IoT, you can check whether the network grants PSM on the other technology too. PSM support can differ between them on some operators, but there's no guarantee. 

5.Actually during the actual deep sleep period between uploads, the device's floor current is essentially the same as when the modem is switched off, modem in PSM Sleep, the typical PSM floor current for the nRF9151 and nRF9160 is 2.7 µA . Whereas when the modem is turned off, the typical sleep current of the application processor ranges from 1.4 µA (System OFF mode, wake on GPIO/reset) up to 1.8 µA - 2.2 µA (System ON Idle mode with RTC on).

But PSM will yields a substantially lower total average current because it eliminates the high energy cost of re-attaching and re-authenticating every cycle.

You can check more about maximizing battery lifetime in cellular IoT though PSM in this blog.

Best Regards,
Syed Maysum