Apparatus and methods are provided for on-demand SI request procedure. In one novel aspect, the RACH procedure is used for one-demand SI information updates through unicast. In one embodiment, the UE sends an embedded SI request message in Msg3 requesting one or more on-demand SI including a UE ID. The SI response and the information of the requested SI are included in the Msg4 or in a separate unicasted message. In another embodiment, one or more assistant information is included in Msg3 or are associated with the preamble in Msg1. In another novel aspect, the preamble of the Msg1 is associated with a group X of SIB. Upon detecting the failure of transmission of the SI request, the UE monitors and receives updated SI information of the whole group X through a broadcast channel.