  With non-orthogonal multiple access(NOMA), we tackle the maximization of the
secrecy rate for the strong user subject to a maximum allowable secrecy outage
probability, while guaranteeing a constraint on the transmission rate to the
weak user. For the first time, the dependence between the eavesdropper's
ability to conduct successive interference cancellation and her channel quality
is considered. We determine the optimal power allocation and the redundancy
rate, based on which the cost of security in terms of the reduction in the
strong user's secrecy rate is examined and the benefits of NOMA for secure
transmissions are explicitly revealed.
