I got an answer in Linkedin
LWA gives you supplemental capacity to offload non-priority data over WiFi. Split between LTE and WiFi happens at PDCP level in the APs. CN still sees a single S1 connection towards LTE eNB. Signaling and VoLTE calls will be carried over LTE. If you need VoWiFi capabilities with LWA, you’ll have to use non-3GPP access based mechanism defined in the specifications (ePDG etc.)
Now I have question:
For my understanding, so if I want to use VoWiFi, the WiFi APs is connected to ePDG in CN via IPSec Tunnel, that is straight forward process. However, in LWA scenario, I will have only 1 transport from eNB to ePDG, not via IPSec Tunnel, do I need a spliter (which I do not know how the CN will differentiate before ePDG) to split the LTE and WiFi traffic? Or VoWiFi is not possible, then LWA allows only offloading non-priority data, but not voice over Xw interface through an eNB to EPC (S1 Link)