  Signaling design for secure transmission in two-user multiple-input
multiple-output (MIMO) non-orthogonal multiple access (NOMA) networks is
investigated in this paper. The base station broadcasts multicast data to all
users and also integrates additional services, unicast data targeted to certain
users, and confidential data protected against eavesdroppers. We categorize the
above MIMO-NOMA with different security requirements into several communication
scenarios. The associated problem in each scenario is nonconvex. We propose a
unified approach, called the power splitting scheme, for optimizing the rate
equations corresponding to the scenarios. The proposed method decomposes the
optimization of the secure MIMO-NOMA channel into a set of simpler problems,
including multicast, point-to-point, and wiretap MIMO problems, corresponding
to the three basic messages: multicast, private/unicast, and confidential
messages. We then leverage existing solutions to design signaling for the above
problems such that the messages are transmitted with high security and
reliability. Numerical results illustrate the efficacy of the proposed
covariance matrix design in secure MIMO-NOMA transmission. The proposed method
also outperforms existing solutions, when applicable.
  In the case of no multicast messages, we also reformulate the nonconvex
problem into weighted sum rate (WSR) maximization problems by applying the
block successive maximization method and generalizing the zero duality gap. The
two methods have their advantages and limitations. Power splitting is a general
tool that can be applied to the MIMO-NOMA with any combination of the three
messages (multicast, private, and confidential) whereas WSR maximization shows
greater potential for secure MIMO-NOMA communication without multicasting. In
such cases, WSR maximization provides a slightly better rate than the power
splitting method.
