MC7455 stuck on 3G

We’re trying to upgrade our devices to use the mc7455 modules (Rev A). Our devices are running opensuse linux with qmi_wwan drivers for the modems. For the last few years we’ve been using the mc7304 modems without issue. I have done tests on the mc7455 with the latest generic firmware and the Vodafone firmware but ran into issues with both.

When two mc7455 modules (using the Vodafone firmware) are connected and both have Vodafone SIMs, one of them gets 4G service while the other gets no service at all. I’m not sure what determines which SIM gets the 4G service, since it seems to change randomly after each reboot, but I haven’t tested this too much because it’s not something I could replicate on the generic firmware.

However I’ve now encounter another issue that occurs on both firmwares; after changing SIMs and rebooting the device, occasionally the mc7455 will connect but only get 3G service. So far I’ve managed to get it stuck on 3G with both Vodafone and EE SIMs. The temporary “fix” I’ve found is that if I run the at!reset command it then reconnects with 4G service. Usually it doesn’t take me more than 2-3 reboots and SIM swaps for it to occur on Vodafone SIMs, sometimes even occuring on the first boot. I’m confident that this is not a signal issue, because I’ve replicated this in multiple areas with good 4G coverage. Power was another consideration, because the mc7455 modules do draw more than the mc7304 modules. However, this behaviour occurs when one mc7455 module is fitted, and our board is designed to support two. I have also tried using a generic third party modem holder, connected to a spare USB port on the motherboard, but again this gives the same results.

Vodafone SIM with 4G

at!gstatus?

!GSTATUS:

Current Time:  210              Temperature: 27
Reset Counter: 2                Mode:        ONLINE
System mode:   LTE              PS state:    Attached
LTE band:      B20              LTE bw:      10 MHz
LTE Rx chan:   6300             LTE Tx chan: 24300
LTE CA state:  NOT ASSIGNED
EMM state:     Registered       Normal Service
RRC state:     RRC Connected
IMS reg state: No Srv
 
PCC RxM RSSI:  -50              RSRP (dBm):  -81
PCC RxD RSSI:  -98              RSRP (dBm):  -139
Tx Power:      0                TAC:         6082 (24706)
RSRQ (dB):     -13.1            Cell ID:     000E0614 (919060)
SINR (dB):     16.8
 
OK
at+csq

+csq: 29,99

OK
at+cops?

+cops: 0,0,"vodafone UK",7

OK

Vodafone SIM stuck with 3G (until modem reset)

at!gstatus?

!GSTATUS:

Current Time:  633              Temperature: 26
Reset Counter: 1                Mode:        ONLINE
System mode:   WCDMA            PS state:    Attached
WCDMA band:    WCDMA 2100
WCDMA channel: 10687
GMM (PS) state:REGISTERED       NORMAL SERVICE
MM (CS) state: IDLE             NORMAL SERVICE

WCDMA L1 state:L1M_PCH_SLEEP    LAC:         004A (74)
RRC state:   DISCONNECTED       Cell ID:     02CC9281 (46961281)
RxM RSSI C0:   -53              RxD RSSI C0:  ---
RxM RSSI C1:    ---             RxD RSSI C1:  ---
 
OK
at+csq

+csq: 30,99

OK
at+cops?

+cops: 0,0,"vodafone UK",2

OK

Has anyone had similar issues? Or have any suggestions for things I can try?

Thanks,
Matt

Hi Matt,

It is a good practice to connect the DIV antenna.

Without seeing the log I don’t know why the modem is not registering to 4G. I assume both SIMs are 4G.
you can try to lock the modem to LTE:
at!band=?
Index, Name
00, All Bands
01, Europe 3G
02, North America 3G
03, Europe 2G
04, North America 2G
05, GSM ALL
06, Europe
07, North America
08, WCDMA ALL
09, LTE ALL

OK

at!band=9
OK

Hi Oedenburger,

What logs do you mean? Where can I find them?

Yes, all SIMs I have tested are 4G.

I have now tried locking the modem to LTE, as you suggested, and that does appear to fix it. After about two dozen reboots and SIM swaps (where I always got 4G Vodaphone service), I’m reasonably confident that it’s stable. This solution does have at least one major flaw though. In areas where 4G coverage doesn’t exist or is very poor, there is no ability for the modem to fail over to 3G.

Is there any information I could provide that might give further insight?

Thanks,
Matt

Hi Matt,

I was referring to the modem’s internal DM log but it is complicated to capture- we don’t need that.
This could be a NTWK issue.
Try to move your setup to a different area and enable “all bands” again. – also connect the DIV antenna too.
If you cannot repro the issue then it is a NTWK problem.
It could happen that the modem acquires 3G but in the SIBs the NTWK could instruct the modem which LTE EARFCN to lock to. Very rarely the NTWK is configured to the wrong channel.
There is no way you can find this out.

When you lock the modem to 4G what is your signal level in the problematic area?
At!gstatus?
RSRP should be better than ~-105dBm (better is closer to 0)
SINR should be a positive number

Thanks,
James

Hi James,

After doing further testing, it was initially stable but now I’ve had another issue that’s probably related. Now I have a Vodafone SIM that’s (while locked to LTE) trying to connected to EE 4G and getting no service. Once I stop restricting it to LTE it connects to Vodafone 3G. Then running at!reset makes it connect correctly to Vodafone 4G.

Vodafone SIM attempting to connect to EE 4G network (at!band=9)

Feb 1 15:00:56 - 0 [234-30] NO SERVICE [0] 21 Normal:35.00 CNormal:3350 mV (ADC: 36102)

at!gstatus?
!GSTATUS:
Current Time:  168              Temperature: 34
Reset Counter: 1                Mode:        ONLINE
System mode:   LTE              PS state:    Not attached
LTE band:      B20              LTE bw:      10 MHz
LTE Rx chan:   6400             LTE Tx chan: 4294967295
LTE CA state:  NOT ASSIGNED
EMM state:     Deregistered     Attach Needed
RRC state:     RRC Idle
IMS reg state: No Srv

PCC RxM RSSI:  -75              RSRP (dBm):  -104
PCC RxD RSSI:  -79              RSRP (dBm):  -105
Tx Power:      --               TAC:         40B0 (16560)
RSRQ (dB):     -9.3             Cell ID:     01ACAE82 (28094082)
SINR (dB):      8.8


OK
at+csq
+csq: 31,99

OK
at+cops?
+COPS: 0

OK

Vodafone SIM stuck on 3G service again (at!band=0)

Feb 1 15:30:10 - vodafone UK [null-null] H+ [7] 14 Normal:39.00 CNormal:3347 mV (ADC: 36084)

Vodafone SIM with 4G service (at!reset)

Feb 1 15:39:08 - vodafone UK [234-15] 4  [9] 16 Normal:37.00 CNormal:3352 mV (ADC: 36102)

Any ideas what’s going on here?

Thanks,
Matt