After 4G network becomes a global commercial success, the researchers are now looking at the future 5G technologies, both in standardization bodies and in research projects. Researchers find that the security architecture used in 4G can not satisfy the needs of 5G. Some researchers proposed a kind of unified network security architecture which we find it inflexible. In this paper, we will propose a more flexible hierarchical security architecture which can be adjusted to the needs of the application scenarios of 5G-eMBB, mMTC and URLLC. Under this architecture, we will introduce a cross-layer light weight authentication scheme and a novel physical layer security assisted encryption scheme which is based on both the key streams and the channel information in f-OFDM system.