A method for establishing a trust relationship in an ultra dense network is provided. The method comprises receiving, by a user equipment (UE), a reconfiguration request from a macrocell; deriving, by the UE, a user plane encryption key according to information in the reconfiguration request; transmitting, by the UE, a first user plane signaling message to a first microcell in a group of microcells when the UE is attached to the first microcell; and transmitting, by the UE, a second user plane signaling message to a second microcell in the group of microcells when the UE is attached to the second microcell, wherein the first user plane signaling message and the second user plane signaling message are both encrypted according to the user plane encryption key.