Hello Mohammad,
The AS ABAP (Fiori Front End Server) cannot be configured to authenticated users directly with AD username/password. That's the reason to include AS Java (SAP SSO) in the scenario. The AD credentials are checked by the AS Java system and after successful authentication an SAML 2.0 assertion is issued. The SAML 2.0 assertion is used to authenticate to the Fiori Front End Server. For the end user this is transparent and he only needs to provide his AD credentials.
Using SAP Authenticator you can achieve also Mobile SSO for Android and iOS devices using the setup described above.
If you need further details or would like to see the scenario working we can have a web conference. Just contact me via email at dimitar.mihaylov<AT>sap.com.
Best regards,
Dimitar Mihaylov