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]

This is similar to the behavior that I see on my older telenor SIMs (and the iBasis one for that matter), where the network doesn't allow NB1 on the specific SIM:

+CEREG: 2,"9DD1","0145B864",9,0,0,"11100000","11100000"
+CEREG: 2,"FFFE","FFFFFFFF",9

 

Have you contacted telenor, supplying them with the IMSI (the numbers physically printed on your SIM), and asked them to verify that NB1 is enabled for that specific SIM card? If your SIM is old, it might be that you need a new one.

 

Kind regards,

Håkon

[Fredrik]

I called telenor again, and the customer rep said he didn't know but he'd open an internal case and get back to me in a few days. I'll put this on hold until i hear back from them.

Thanks for your help.

[Fredrik]

It turns out regular consumer SIM cards from Telenor do not support the IoT bands. I ordered a dedicated IoT SIM card from them instead, and NB-IoT works fine with that.

Thanks for your help!

EDIT: NB-IoT does not seem to work with the new SIM either.

I ran the original test again with the new SIM (which I am sure supports NB-IoT) outside in an open area. This is the log:

[00:00:00.431,793] ---> AT+CPSMS=0
[00:00:00.454,589] ---> AT+CEREG=4
[00:00:00.475,494] ---> AT%CESQ=1
[00:00:00.496,215] ---> AT+CFUN=4
[00:00:00.516,937] ---> AT%XSYSTEMMODE=0,1,0,0
[00:00:00.540,435] ---> AT+CFUN=1
[00:00:27.303,375] <--- %CESQ: 94,4,18,2
[00:00:27.311,584] <--- +CEREG: 2,"9FC5","0114CF67",9,,,"11100000","11100000"
[00:00:32.353,790] <--- +CEREG: 4,"9FC5","0114CF67",9,,,"11100000","11100000"
[00:00:32.393,127] <--- %CESQ: 255,0,255,0
[00:00:32.401,489] <--- +CEREG: 4,"FFFE","FFFFFFFF",9,,,"11100000","11100000"
[00:00:42.296,203] <--- %CESQ: 97,4,23,3
[00:00:42.304,412] <--- +CEREG: 2,"0EDB","020D8112",9,,,"11100000","11100000"
[00:00:43.590,759] <--- +CEREG: 2,"0EDB","020D8112",9,,,"11100000","11100000"
[00:00:43.725,555] <--- %CESQ: 255,0,255,0
[00:00:43.733,947] <--- +CEREG: 2,"FFFE","FFFFFFFF",9,,,"11100000","11100000"
[00:00:51.500,823] <--- %CESQ: 94,4,20,2
[00:00:51.509,002] <--- +CEREG: 4,"9FC5","0103A167",9,,,"11100000","11100000"
[00:00:58.853,546] <--- %CESQ: 89,4,20,2
[00:01:03.973,602] <--- %CESQ: 89,4,15,2
[00:01:06.533,630] <--- %CESQ: 89,4,20,2
[00:01:14.213,745] <--- %CESQ: 89,4,10,1
[00:01:16.773,773] <--- %CESQ: 94,4,18,2
[00:01:21.893,829] <--- %CESQ: 88,4,18,2
[00:01:34.694,000] <--- %CESQ: 88,4,13,1
[00:01:39.814,056] <--- %CESQ: 88,4,18,2
[00:01:44.934,112] <--- %CESQ: 93,4,18,2
[00:01:50.054,138] <--- %CESQ: 93,4,12,1
[00:01:52.614,227] <--- %CESQ: 93,4,19,2
[00:01:55.174,224] <--- %CESQ: 88,4,12,1
[00:01:57.734,222] <--- %CESQ: 88,4,17,2
[00:02:00.294,250] <--- %CESQ: 88,4,23,3
[00:02:07.974,334] <--- %CESQ: 88,4,18,2

Around 51.500 seconds I receive modem status 4 ("unknown") with valid tac etc. However, I also receive very strong RSRP values. I can't make sense of this. As usual, LTE-M1 works fine.

[Fredrik]

Hi Håkon,

Any update? I edited my previous reply and rejected the "verified answer", but I'm worried you were not notified.

[Håkon Alseth]

Hi,

 

My apologies for the delay.

Could you take a modem trace so we can look at what happens?

Here's a howto on enabling modem traces: https://devzone.nordicsemi.com/nordic/nordic-blog/b/blog/posts/how-to-get-modem-trace-using-trace-collector-in-nrf-connect

 

Kind regards,

Håkon

[Fredrik]

Sure. Here is the modem trace:

trace-2019-12-12T12-47-42.604Z.bin

Pseudo-log of AT commands sent/received:

[00:00:06.754,486] ---> AT%XMODEMTRACE=1,2
[00:00:06.778,350] ---> AT+CEREG=5
[00:00:06.800,872] ---> AT%CESQ=1
[00:00:06.823,181] ---> AT+CFUN=4
[00:00:06.848,907] ---> AT%XSYSTEMMODE=0,1,0,0
[00:00:06.875,885] ---> AT+CFUN=1
[00:00:30.485,015] <--- %CESQ: 54,2,23,3
[00:00:30.500,701] <--- +CEREG: 2,"9FC5","0103A167",9,0,0,"11100000","11100000"
[00:00:38.578,460] <--- +CEREG: 4,"9FC5","0103A167",9,0,15,"11100000","11100000"
[00:00:38.631,622] <--- %CESQ: 255,0,255,0
[00:00:38.644,073] <--- +CEREG: 4,"FFFE","FFFFFFFF",9,0,15,"11100000","11100000"
[00:00:49.760,467] <--- %CESQ: 72,3,22,3
[00:00:49.805,938] <--- +CEREG: 2,"0EDB","020D8126",9,0,15,"11100000","11100000"
[00:00:51.073,577] <--- +CEREG: 2,"0EDB","020D8126",9,0,14,"11100000","11100000"
[00:00:51.231,048] <--- %CESQ: 255,0,255,0
[00:00:51.239,105] <--- +CEREG: 2,"FFFE","FFFFFFFF",9,0,14,"11100000","11100000"
[00:00:56.075,988] <--- %CESQ: 54,2,22,3
[00:00:56.083,831] <--- +CEREG: 4,"9FC5","0103A167",9,0,14,"11100000","11100000"
[00:01:00.846,466] <--- %CESQ: 54,2,27,3
[00:01:49.487,640] <--- %CESQ: 47,2,27,3
[00:01:54.607,757] <--- %CESQ: 54,2,27,3

[Håkon Alseth]

Hi,

 

Thank you for the trace logs.

I can see that the eNB (the cell tower) releases the connection very quickly. This is very similar behavior with one of my SIM cards that does not support NB1.

I will ask R&D to look at the modem trace as well, in case I have missed something.

 

Kind regards,

Håkon

[Håkon Alseth]

Hi,

 

Got confirmation from R&D that the eNB or SIM seems to be the issue.

There is an attach reject with cause value #15 (No suitable cells in tracking area) from the home network for this SIM. This cause value means basically that you could get service from this network, either on some different area or via different access technology.

There were no cells from other home networks tracking areas available so we also made an attempt to another vendor (the one marked with 0EDB) on the log file. That network rejects attach attempt with a cause value that includes the whole network (#14 EPS services not allowed in this PLMN).

 

Have you tried connecting to NB1 in a different area (at work vs. at home, for instance)?

 

Kind regards,

Håkon

[Fredrik]

I just got off the phone with Telenor who (finally) decided that the subscription I had did not support NB-IoT. My SIM does support NB, however, and we swapped my subscription to allow NB traffic. Telenor is now certain that everything looks good on their side. They also made sure that there is good coverage in my area.

My case is starting to look similar to Terje's case as Telenor asked me to make sure I was using the telenor.iot APN. Simon's answer here indicates that PDP context should be handled by the modem firmware, and I don't know if I should mess with the APN manually.

I'll try connecting at home over the week-end.

[Fredrik]

Tried at home, same results unfortunately.

trace-2019-12-14T11-44-48.241Z.bin

[00:00:06.709,075] ---> AT%XMODEMTRACE=1,2
[00:00:06.732,971] ---> AT+CEREG=5
[00:00:06.755,523] ---> AT%CESQ=1
[00:00:06.824,432] ---> AT+CFUN=4
[00:00:06.850,219] ---> AT%XSYSTEMMODE=0,1,0,0
[00:00:06.877,227] ---> AT+CFUN=1
[00:00:30.016,876] <--- %CESQ: 60,3,21,3
[00:00:30.032,623] <--- +CEREG: 2,"A0F1","013DD964",9,0,0,"11100000","11100000"
[00:00:35.076,416] <--- +CEREG: 4,"A0F1","013DD964",9,0,15,"11100000","11100000"
[00:00:35.142,730] <--- %CESQ: 255,0,255,0
[00:00:35.155,181] <--- +CEREG: 4,"FFFE","FFFFFFFF",9,0,15,"11100000","11100000"
[00:00:52.689,270] <--- %CESQ: 60,3,24,3
[00:00:52.734,252] <--- +CEREG: 2,"0EDD","02100113",9,0,15,"11100000","11100000"
[00:00:54.002,990] <--- +CEREG: 2,"0EDD","02100113",9,0,14,"11100000","11100000"
[00:00:54.162,994] <--- %CESQ: 255,0,255,0
[00:00:54.171,112] <--- +CEREG: 2,"FFFE","FFFFFFFF",9,0,14,"11100000","11100000"
[00:01:00.727,142] <--- %CESQ: 60,3,19,2
[00:01:00.735,046] <--- +CEREG: 4,"A0F1","013DD964",9,0,14,"11100000","11100000"
[00:01:10.647,857] <--- %CESQ: 60,3,25,3
[00:01:15.767,974] <--- %CESQ: 60,3,20,2
[00:01:20.888,122] <--- %CESQ: 60,3,26,3
[00:01:23.448,181] <--- %CESQ: 60,3,18,2

Telenor agreed to send me a new SIM, just in case the one I got is misconfigured somehow.

Thanks for all your help!