Reliable method for LTE-M signal verification

[Gmaciek]

Hi,

I am developing an application for Thingy:91 in which I want to send data from the device to MQTT server. I am using 1.1.1 modem firmware and 1.2.0 ncs tag.

The whole MQTT procedure is more or less the same as in mqtt_simple_sample, but since the whole application is supposed to continuously send GPS data, and therefore the device is on the move, I need to make sure that I am actually able to connect with MQTT client every time I try to send data. What I believe is the right approach is to check the output of CEREG and/or CESQ at commands, but I do have some doubts:

- is it correct to say that the network can be registered (CEREG will return 5 or 1), but the signal will be to weak and the client connection or data publish procedure will fail?

- if so, what is the required signal strength to perform this procedure?

- is it true that if modem gets the AT+CFUN=21 command once, it will automatically try to reconnect to the network after losing the signal (no need for sending AT+CFUN=21 again)

- since in the last modem firmware version (1.1.1) there were introduced AT+CFUN= 21, 31 and other parameters related to activating/deactivating LTE and GPS (or at least I wasn't aware of them in the 1.1.0 and previous versions of MFW), is it correct to say that the AT+CFUN=21 "activates LTE" in a sense that previously was done by AT+CFUN=1 together with AT+CEREG=5?

Regards,

Maciek

[Hakon]

Hey,

 

- is it correct to say that the network can be registered (CEREG will return 5 or 1), but the signal will be to weak and the client connection or data publish procedure will fail?

 If CEREG returns 5 or 1 it means that the device is connected and should be able to send and receive. Otherwise, if the signal is too weak it should return 4.

 

- if so, what is the required signal strength to perform this procedure?

 You can find the receiver sensitivity in the product specification under section 7.6. Then if the RSRP drops below that, the device could be disconnected from the network. The RSRP can be read with the AT+CESQ command.

 

- is it true that if modem gets the AT+CFUN=21 command once, it will automatically try to reconnect to the network after losing the signal (no need for sending AT+CFUN=21 again)

 Yes, it's true.

 

- since in the last modem firmware version (1.1.1) there were introduced AT+CFUN= 21, 31 and other parameters related to activating/deactivating LTE and GPS (or at least I wasn't aware of them in the 1.1.0 and previous versions of MFW), is it correct to say that the AT+CFUN=21 "activates LTE" in a sense that previously was done by AT+CFUN=1 together with AT+CEREG=5?

 AT+CFUN=21 just activates the LTE without changing whether the GNSS services are turned on or off. I'm not sure if that answers your question.

[Gmaciek]

Hi Hakon, thank you for answer.

Hakon said:

 If CEREG returns 5 or 1 it means that the device is connected and should be able to send and receive. Otherwise, if the signal is too weak it should return 4.

So that means that the CEREG output 5 or 1 should be the sufficient indicator, that the device is able to connect to MQTT client and send data?

I did some outdoor tests and my device recovered from no coverage state and succesfully reconnected few times, but sometimes it doesn't do that. The only thing I verify related to network status is checking the registration status with AT+CEREG output - if it's registered I proceed to connecting to MQTT client and sending data, if not - I wait and check again each few seconds. I'm trying to disqualify the network registration, signal strength or any other networking related problems as a possible reason of failure.

[Hakon]

So the device will not connect to the MQTT broker even when you are registered? Could it be that it loses coverage while you are trying to connect?