Artificial Noise Aided Secure Communications for Cooperative NOMA Networks

Non-orthogonal multiple access (NOMA) has been envisioned as a promising multiple access technique to improve spectral efficiency and provide massive connectivity in future wireless networks. However, the inherited security issues with NOMA should be carefully addressed to further exploit its potential benefits in NOMA enabled wireless networks. As such, we consider a cooperative NOMA network, where a source uses the NOMA to simultaneously communicate with a multi-antenna near-user and a far-user. While directly communicating with the near-user, the source employs multiple full-duplex (FD) decode-and forward (DF) relays to establish communication with the far-user in the presence of a passive eavesdropper. To address the eavesdropping in this cooperative NOMA network, we propose a new two-phase FD-based artificial noise (AN) scheme with different relay selection techniques. In the first phase, the selected FD relay emits AN to confuse the eavesdropper while receiving the superimposed signal from the source. In the second phase, the selected relay performs exclusive OR (XOR) operation on both the message intended to the far-user and the AN before broadcasting the resulting mixed signal. By utilizing null-space beamforming, self-interference cancellation techniques and DF-XOR cooperative protocol, the AN in the proposed scheme can be efficiently eliminated at the near-user and far-user as well as at the selected relay. However, the AN cannot be suppressed at the eavesdropper which serves the purpose of AN through degrading the decoding capability of the eavesdropper. We evaluate the performance of the proposed scheme in terms of security-reliability trade-off (SRT). For the AN-aided scheme with max-min and partial relay selection techniques, we theoretically derive the exact and asymptotic closed-form expressions of the outage probability and intercept probability. Numerical results have been provided to validate the derivations. In addition, the results reveal that the SRT of the near-user and far-user can be improved by increasing the number of antennas at the near-user and the number of relays.