  The amalgamation of non-orthogonal multiple access (NOMA) and physical layer
security is a significant research interest for providing spectrally-efficient
secure fifth-generation networks. Observing the secrecy issue among multiplexed
NOMA users, which is stemmed from successive interference cancellation based
decoding at receivers, we focus on safeguarding untrusted NOMA. Considering the
problem of each user's privacy from each other, the appropriate secure decoding
order and power allocation (PA) for users are investigated. Specifically, a
decoding order strategy is proposed which is efficient in providing positive
secrecy at all NOMA users. An algorithm is also provided through which all the
feasible secure decoding orders in accordance with the proposed decoding order
strategy can be obtained. Further, in order to maximize the sum secrecy rate of
the system, the joint solution of decoding order and PA is obtained
numerically. Also, a sub-optimal decoding order solution is proposed. Lastly,
numerical results present useful insights on the impact of key system
parameters and demonstrate that average secrecy rate performance gain of about
27 dB is obtained by the jointly optimized solution over different relevant
schemes.
