Questions about signal quality polling in nRF9160

[Fredrik]

Hi,

I am mapping NB-IoT and LTE-M1 indoors coverage using the nRF9160, mostly just to familiarize myself with the nRF91 DK. I have a basic application which toggles between NB and M1 mode and records the RSRP for each of them, and I have a few questions about the CEREG and CESQ commands.

Below is the log output when connecting to NB-IoT and polling CEREG and CESQ every 10 seconds. In the log, '--->' denotes commands sent to the modem, and '<---' denotes the modem response.

[00:00:01.866,394] ---> AT+CFUN=4
[00:00:01.887,420] ---> AT%XSYSTEMMODE=0,1,0,0
[00:00:01.911,285] ---> AT+CFUN=1

[00:00:01.962,219] ---> AT+CESQ
[00:00:01.976,470] <--- +CESQ: 99,99,255,255,255,255
[00:00:01.985,198] ---> AT+CEREG?
[00:00:01.999,664] <--- +CEREG: 0,4,"FFFE","FFFFFFFF",9

[00:00:12.015,777] ---> AT+CESQ
[00:00:12.030,120] <--- +CESQ: 99,99,255,255,255,255
[00:00:12.038,879] ---> AT+CEREG?
[00:00:12.053,253] <--- +CEREG: 0,4,"FFFE","FFFFFFFF",9

// some time passes

[00:01:02.250,152] ---> AT+CESQ
[00:01:02.264,495] <--- +CESQ: 99,99,255,255,255,255
[00:01:02.273,223] ---> AT+CEREG?
[00:01:02.287,628] <--- +CEREG: 0,4,"FFFE","FFFFFFFF",9

[00:01:12.297,027] ---> AT+CESQ
[00:01:12.311,370] <--- +CESQ: 99,99,255,255,19,46
[00:01:12.319,915] ---> AT+CEREG?
[00:01:12.334,320] <--- +CEREG: 0,2,"9FC5","0103A167",9

[00:01:22.343,902] ---> AT+CESQ
[00:01:22.364,318] <--- +CESQ: 99,99,255,255,19,46
[00:01:22.372,802] ---> AT+CEREG?
[00:01:22.393,463] <--- +CEREG: 0,2,"9FC5","0103A167",9

Initially, the modem status reported by the CEREG command is 4 (unknown), and then it flips to 2 (not registered, searching). At the same time I receive valid RSRP values from the CESQ command. The modem status never changes after this. Is this expected? Can I have valid RSRP, <tac> and <ci> on NB-IoT without having modem status equal to 1 or 5? Repeating the same procedure for LTE-M1 always gives initial modem status 4, then 5.

I'm on nRF Connect SDK v1.0.0, modem FW mfw_nrf9160_1.1.0. The SIM card is an iBasis SIM which was provided with the nRF9160 DK.

[Håkon Alseth]

Hi,

Initially, the modem status reported by the CEREG command is 4 (unknown), and then it flips to 2 (not registered, searching). At the same time I receive valid RSRP values from the CESQ command. The modem status never changes after this. Is this expected? Can I have valid RSRP, <tac> and <ci> on NB-IoT without having modem status equal to 1 or 5? Repeating the same procedure for LTE-M1 always gives initial modem status 4, then 5.

Yes, this is expected. Since you are polling the modem via AT commands, it has likely successfully attached somewhere between timestamp 01:02 and 01:12.

If you look at the timestamp, you're receiving the CESQ response before querying CEREG. This indicates that you were already connected when checking the RSRP.

 

Try running the at_client, and see how that behaves. It will print notifications from the modem as they come in.

PS: I would recommend that you update to ncs tag v1.1.0, as that is now released. It shouldn't be a problem to modem v1.1.0 on tag v1.0.0, but in case you want newer features; like GPS, check that out.

 

Kind regards,

Håkon

[Fredrik]

Hi Håkon, thanks for getting back to me.

I updated to ncs v1.1.0 and ran the same test again with notifications as you suggested, and I am still a bit confused here.

[00:00:00.464,904] ---> AT+CEREG=1
[00:00:00.492,370] ---> AT%CESQ=1
[00:00:00.537,414] ---> AT+CFUN=4
[00:00:00.564,697] ---> AT%XSYSTEMMODE=0,1,0,0
[00:00:00.594,848] ---> AT+CFUN=1

[00:01:05.232,055] <--- %CESQ: 76,3,26,3
[00:01:05.247,650] <--- +CEREG: 2
[00:01:06.794,067] <--- %CESQ: 255,0,255,0
[00:01:14.438,110] <--- %CESQ: 54,2,19,2
[00:04:56.711,944] <--- %CESQ: 255,0,255,0
[00:05:04.841,613] <--- %CESQ: 53,2,22,3
[00:08:47.977,386] <--- %CESQ: 255,0,255,0
[00:08:55.247,039] <--- %CESQ: 52,2,22,3
[00:12:38.802,825] <--- %CESQ: 255,0,255,0
[00:12:45.652,465] <--- %CESQ: 52,2,23,3

I get the first signal quality update at 01:05 which states that the RSRP is good. Immediately after the signal quality notification, through, I get a modem update with state = 2, which means "Not registered, but UE is currently trying to attach or searching an operator to register to" (from the nRF91 AT Commands Reference Guide). For the next 10 minutes or so, the modem switches between notifying valid and invalid signal quality notifications, but the modem state never changes. Shouldn't the modem state be either 1 or 5 when there is a valid connection?

[Håkon Alseth]

Here's my testing:

I see the same behavior. It seems to be picking up the RSRP in the process of attaching, and reporting that before the ATTACH is fully completed.

By adding timestamps in teraterm, it seems that the RSRP comes around 1 second before the ATTACH is fully complete:

[2019-11-22 10:31:38.293] %CESQ: 30,1,0,0
[2019-11-22 10:31:38.524] %CESQ: 35,1,6,0
[2019-11-22 10:31:39.499] +CEREG: 1

 

Kind regards,

Håkon

[Fredrik]

Håkon,

In your testing, did you connect to NB-IoT or LTE-M1? As I said, everything works fine in LTE-M1, and I get the same results as you. In NB-IoT mode, however, the modem state never changes. It notifies state == 2 once, then it never changes. I get valid RSRP notifications, though, which is why I'm confused.

Thanks for your help.

[Håkon Alseth]

Fredrik said:

In your testing, did you connect to NB-IoT or LTE-M1? As I said, everything works fine in LTE-M1, and I get the same results as you. In NB-IoT mode, however, the modem state never changes. It notifies state == 2 once, then it never changes. I get valid RSRP notifications, though, which is why I'm confused.

 I tested M1. I have to apologize, I have misunderstood the scenario. I thought the issue was that the RSRP was reported before the ATTACH cam, but it turns out I have not read your questions properly.

You're stating that the ATTACH newer comes. I just tested with iBasis, on NB1, and I see the same.

The other SIM I have is also NB1 capable, so that works just fine on both M1 and NB1, but iBasis does not seem to work on NB1 in this region, unfortunately.

Do you have a Telenor SIM to test with? That should work on NB1 and M1.

 

Kind regards,

Håkon

[Fredrik]

Hi,

I tried with a Telenor SIM, and I got the exact same results: valid RSRP with modem status 2 on NB, no issues on LTE-M1. I called Telenor to make sure both LTE-M1 and NB-IoT is included in the subscription, and that there is coverage on both in my area.

Could this be an issue with my DK? It's an early model PCA10090 v0.8.2.

[Håkon Alseth]

Fredrik said:

I tried with a Telenor SIM, and I got the exact same results: valid RSRP with modem status 2 on NB, no issues on LTE-M1. I called Telenor to make sure both LTE-M1 and NB-IoT is included in the subscription, and that there is coverage on both in my area.

Could this be an issue with my DK? It's an early model PCA10090 v0.8.2.

This shouldn't be a problem with the DK.

Could you try programming your kit with "at_client", input the following:

AT%XSYSTEMMODE=0,1,0,0

AT+CEREG=5

AT+CFUN=1

 

Then wait 10 minutes, and upload the output log here?

 

Kind regards,

Håkon