Question

I want to use the ME909u-521 with LTE radio access technology only.

I use the command AT^SYSCFGEX="03",40000000,2,4,40000000,"","" or similar, but the module won't lock to LTE only.

Why?

Solution

For modules with firmware 12.631.07.00.00 to 12.631.07.01.00



Since voice was included as a feature in this firmware, Huawei have removed the feature to lock to LTE only. The reason is that the majority of the current LTE networks doesn't support VoLTE (voice over LTE). The way the ME909u-521 solves this is by using a technology called Circuit Switched Fallback. This requires the use of the WCDMA or GSM network. Therefore the module won't allow blocking theses technologies.



For modules with firmware 12.636.11.01.00 and forward



The problem with voice still exists, so the module will need to do a Circuit Switched Fallback to handle this. With this firmware, Huawei introduced the command AT^CEUS, which can set the module in either data centric mode (1) or voice centric mode (0). If the module is in data centric mode, there will be no need to use the voice feature. Then the module can be locked to LTE. If you want to use the voice feature of the ME909u-521, you cannot lock to LTE at the moment.