VoLTE with SIM7600E-H HAT#
Making VoLTE calls with SIM7600E-H modem over o2/Telefonica Germany carrier network
Note
This post is part of my Press 1 for Fun project and still a work-in-progress.
The current status documents establishing a call, however only via GSM, as there seems to be a problem with establishing an IMS session.
The Waveshare SIM7600E-H HAT comes with two SMA to U.FL cables, a SMA GPS antenna and a SMA rod antenna with an unspecified frequency range…#
So the Waveshare SIM7600E-H HAT supports VoLTE, but it’s very badly documented and the configuration is not part of the official SIM7600 AT command manual. Instead, it’s hidden in the Wiki FAQ section of the HAT.
I’m using minicom via serial USB (USB to UART port) to send AT commands to
the modem.
sudo minicom -D /dev/ttyUSB0 -b 115200
AT+VOLTESETTING=1
AT+CNV=/nv/item_files/modem/mmode/ue_usage_setting,1,01,1
Doing quick call test didn’t yield any results. Enforcing LTE mode throws an immediate error, and on automatic mode, I’m hopping around bands until the modem decides it doesn’t find a suitable carrier band.
AT+CMEE=2
AT+CNMP=38
AT+CPSI?
+CPSI: LTE,Online,262-03,0xB7FD,48893735,241,EUTRAN-BAND1,250,3,3,-120,-878,-556,13
OK
ATD+491234567890;
ERROR
AT+CMEE=2
AT+CNMP=2
AT+CPSI?
+CPSI: LTE,Online,262-03,0xB7FD,48893735,241,EUTRAN-BAND1,250,3,3,-124,-871,-565,12
OK
ATD+491234567890;
OK
AT+CPSI?
+CPSI: LTE,Online
OK
AT+CPSI?
+CPSI: LTE,Online
OK
AT+CPSI?
+CPSI: LTE,Online
OK
AT+CPSI?
+CPSI: LTE,Online
OK
NO CARRIER
The signal strength is -124dBm, which isn’t terrible. It’s horrible… GSM might be able to struggle along with this strength, but for VoLTE this is out of the question. So I’m seemingly having a major problem with reception. My phone is getting -96dBm, that’s what I should be aiming for with the SIM7600 as well…
Note
On stock Android, or Google Pixel devices, dial *#*#4636#*#* to enter
the Testing menu. Under Phine information you can retrieve network info,
including the signal strength in dBm.
Network information via Android Testing menu on GrapheneOS#
Signal Strength Test#
The included antenna is connected to the MAIN U.FL jack only.#
To further investigate reception issues, I did some more signal strength testing.
For all of the tests, the HAT was connected to a 5V@3A USB power supply, as well as the USB to UART connector.
The device is on automatic mode and allows quad‑band GSM, UMTS 850 and 2100, a very wide LTE band set and all (six) TD-SCMA bands [1].
AT+CNMP?
+CNMP: 2
OK
AT+CNBP?
+CNBP: 0x0002000004400180,0x480000000000000000000000000000000000000000000042000001A0000800D5,0x000000000000003F
OK
GSM without antenna#
AT+CNMP=13
OK
AT+CPSI?
+CPSI: GSM,Online,262-03,0xb7ac,61767,976 EGSM 900,-102,0,0-0
OK
Signal strength: -102dBm
GSM with antenna#
AT+CNMP=13
OK
AT+CPSI?
+CPSI: GSM,Online,262-03,0xb7ac,61767,976 EGSM 900,-99,0,7-7
OK
Signal strength: -99dBm
LTE without antenna#
AT+CNMP=38
OK
AT+CPSI?
+CPSI: LTE,Online,262-03,0xB7FD,48893723,436,EUTRAN-BAND3,1600,5,5,-117,-1051,-732,13
OK
Signal strength: -117dBm
LTE with antenna#
AT+CNMP=38
OK
AT+CPSI?
+CPSI: LTE,Online,262-03,0xB7FD,48893735,241,EUTRAN-BAND1,250,3,3,-109,-885,-587,11
OK
Signal strength: -109dBm
GSM, on the EGSM 900 band is the best of the worst with -102 dBm, that’s a very bad signal already. The antenna did a mediocre improvement. On LTE it’s even worse. The signal strength is so bad, that I’m constantly hopping bands and on EUTRAN‑BAND1, the best signal I was able to get was -109 dBM, which is definitely too weak for a stable LTE connection that I need for Voice‑over‑LTE (VoLTE).
The stock antenna gave me a few dB improvement, enough to prove the hardware was alive, but nowhere near the margin needed for reliable LTE. At -109 dBm, you’re whispering across a soccer stadium. LTE typically needs something closer to -95 dBm or better for a usable VoLTE call. Below that, the modem spends more time negotiating with the tower than actually transmitting data. Calls drop, registration fails, and the whole experience feels like trying to hold a conversation in a noisy bar.
So I guess the included antenna is only a suggestion… Maybe if you’re outside, right next to a cell tower, it might suffice, but not if your sitting in a heated office, somewhere in the hilly mountains of the Taunus in Germany…
I bought this antenna of eBay, which was dropshipped via Amazon… Strange new world…
The antenna (supposedly) has a frequency range of 698-3980 MHz, which is fine for the LTE bands supported by the SIM7600E modem.
The new antenna in comparison to the old antenna, which looks comically small…#
Warning
Be very careful when buying antennas. Make sure they clearly advertise if they are broadband antennas, what their frequency range is and also make sure that the cable is less than 2 meters.
LTE with dual antenna#
AT+CNMP=38
OK
AT+CPSI?
+CPSI: LTE,Online,262-03,0xB7FD,48893723,436,EUTRAN-BAND3,1600,5,5,-74,-849,-533,16
OK
Signal strength: -74dBm
Wow… Well, that is a very good signal! I guess my reception problem is solved!
Retesting#
First, I’m ensuring that I’m properly registered in all networks.
AT+CREG?
+CREG: 0,1
OK
AT+CGREG?
+CGREG: 0,1
OK
AT+CEREG?
+CEREG: 0,1
OK
I am properly registered to all networks (home networks).
AT+CNMP=2
OK
ATD+491234567890;
OK
AT+CPSI?
+CPSI:LTE,Online
OK
AT+CPSI?
+CPSI:LTE,Online
OK
AT+CPSI?
+CPSI:LTE,Online
OK
AT+CPSI?
+CPSI:LTE,Online
OK
AT+CPSI?
+CPSI:LTE,Online
OK
AT+CPSI?
+CPSI:LTE,Online
OK
AT+CPSI?
+CPSI:GSM,Online,262-03,0xb7ac,61767,976EGSM900,-97,0,0-0
OK
BUSY
My phone was finally ringing! However, when checking the system status, while it was ringing, I sadly had to see that the modem dropped to GSM, so the call wasn’t handled via VoLTE.
Trying again, but with mode set to LTE-only, the modem threw an error right away, just as it did when I thought the reception was the problem…
AT+CMEE=2
AT+CNMP=38
OK
ATD+491234567890;
ERROR
AT+CGDCONT?
+CGDCONT:1,"IP","internet","0.0.0.0",0,0,0,0
+CGDCONT:2,"IPV4V6","ims","0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0",0,0,0,0
+CGDCONT:3,"IPV4V6","","0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0",0,0,0,0
+CGDCONT:4,"IPV4V6","","0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0",0,0,0,0
+CGDCONT:5,"IPV4V6","","0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0",0,0,0,0
+CGDCONT:6,"IPV4V6","","0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0",0,0,0,0
AT+COPS?
+COPS:0,0,"o2-de",7
OK
AT+CGACT?
+CGACT:1,1
+CGACT:2,1
+CGACT:3,0
+CGACT:4,0
+CGACT:5,0
+CGACT:6,1
OK
AT+VOLTESETTING?
+VOLTESETTING:1
OK
Everything is seemingly fine: PDP context is correctly set and activated, I’m on the right network, and VoLTE is enabled on the modem.
It’s tricky to debug right now, because I am not receiving any error string or
code from the failing ATD command.
I’m not quite sure how the IMS registration process with o2/Telefonica works, it might be that they are blacklisting certain IMEIs? Maybe the modem doesn’t advertise its capabilities correctly?
IMS (IP Multimedia Subsystem)
IMS is the control layer that makes modern voice and messaging work over LTE and 5G. Without IMS, the modem can attach to the data network but won’t be allowed to set up VoLTE calls. Think of it as the “voice switchboard” inside the operator’s IP core.
The IMS PDN (packet data network) is the special data session the modem opens to talk to the IMS core. It usually uses the hidden APN ims. If this PDN isn’t active, VoLTE can’t register. The IMS PDN is listed as a separate PDP context alongside the normal internet APN.
A PDP context is the logical pipe between the modem and the operator’s
packet network. Each APN configured (like internet or ims)
corresponds to a PDP context. Activating one tells the network “I want a
data session here.” VoLTE depends on the IMS PDP context being alive and
accepted.
Hopefully though, it’s just a faulty PDP configuration and the PDN connection isn’t properly established.
Todo
I will first try and figure out if there is a newer firmware for the modem and HAT and flash it, should there be one. In the meantime, however I’ll work on the audio driver for HAT first, since I don’t care whether the calls are circuit- or packet-switched…
To summarize so far: VoLTE isn’t just about signal strength… It’s about convincing my operator’s IMS core that my modem belongs. The modem can shout “I’m LTE, I have an IMS APN!” all day, but unless the IMS registration succeeds, the network won’t hand the modem a voice channel. That’s why calls drop back to GSM, the IMS gate never opened.
So I now want to figure out if my hypothesis is correct and then figure out why exactly the IMS core rejects my device.
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