Secrecy Performance of Two-Level Superposition Transmission in Untrusted Multiuser Wireless Downlink Systems

Abstract

In an untrusted multiuser downlink system (UTMS) where a desired user is opportunistically selected and all the other users are potential eavesdroppers (Eves), the system performance with security requirement will deteriorate as the number of users grows. In this letter, we have proposed two-level superposition transmission (SuT) in which secure signals are superimposed with ordinary signals that are not necessarily to be secure. The ordinary signals play as jamming signals but are made not decodable at Eves with appropriate power allocation at the sender. We have proved that SuT achieves the full diversity order in secure transmission in UTMS. We also have shown that average secrecy rate with SuT obeys growth rate loglog ${K}$ as the number of users ${K}$ grows in UTMS. Numerical investigation is used to verify the proof and derivations obtained in this letter.