Secure transmission in downlink non-orthogonal multiple access based on polar codes

Non-orthogonal multiple access (NOMA) is deemed to have a superior spectral efficiency and polar codes have became the channel coding scheme for control channel of enhanced mobile broadband (eMBB) in the fifth generation (5G) communication systems. In this paper, NOMA combined with polar codes is used to achieve secure transmission. Both degraded wiretap channel and non-degraded wiretap channel are considered, where an eavesdropper intercepts the communication between base station (BS) and users. For the degraded wiretap channel scenario, a secure polar encoding scheme is proposed in NOMA systems with power allocation to achieve the maximum secrecy capacity. With regard to the non-degraded wiretap channel, a polar encoding scheme with multiple-input-single-output (MISO) system is proposed, where artificial noise is generated at BS to confuse the eavesdropper's channel via transmit beam-forming. The security and the secure rate are employed respectively in order to measure the secrecy performance. We prove that the proposed schemes for each scenario can achieve the secure rate and can transmit the signal securely and reliably. The simulation results show that the eavesdropper hardly decoding the secure signal when the legitimate receiver can decode the signal with very low block error rate (BLER).