Physical Layer Security Designs for 5G NOMA Systems With a Stronger Near-End Internal Eavesdropper

Serial interference cancellation (SIC) technology is the key to ensure a higher system capacity for NOMA. However, SIC may also introduce some critical security risks, since it requires near-end users (NU) demodulate far-end users' (FU) signals and make the cancellations. It is hard to obtain positive secrecy rate in the presence of a stronger NU eavesdropper. To this end, this paper studies the security designs for a 5G NOMA system with a stronger NU internal eavesdropper. We first consider the two-user case and then extend our results to a multi-user case. After adjusting the order of SIC and employing a cooperative jammer, an optimal power allocation problem is built subject to users' data rate and total power constraints. The global closed-form optimal solution is achieved for the two-user case, while a closed-form sub-optimal solution is obtained for the multi-user case. For better insight, the jamming gain is introduced to approximately measure the performance gain achieved. Simulation results show that positive secrecy rate is obtained while the users' data rate are guaranteed.