  This paper proposes a secure users oriented multiple-input and single-output
(MISO) non-orthogonal multiple access (NOMA) downlink transmission scheme,
where multiple legitimate users are categorized as quality of service
(QoS)-required users (QU) and the security-required users (SU) overheard by a
passive eavesdropper. The basic idea is to exploit zero-forcing beamforming
(ZFBF) to cancel interference among SUs, and then several QUs are efficiently
scheduled based on the obtained beamforming vectors to divide the legitimate
users into several user clusters, in such a way that the QUs could share the
concurrent transmissions and help to interfere with the eavesdropper to enhance
SU secrecy. The goal is to maximize the achievable minimum secrecy rate (MSR)
and sum secrecy rate (SSR) of all SUs, respectively, subject to the secrecy
outage probability (SOP) constraint of each SU and the QoS constraint of each
QU. To provide a comprehensive investigation we consider two extreme cases that
the eavesdropper has perfect multiuser detection ability (lower bound of
secrecy) or does not have multiuser detection ability (upper bound of secrecy).
In the lower bound case, the Dinkelbach algorithm and the monotonic
optimization (MO)-based outer polyblock approximation algorithm are proposed to
solve the max-min secrecy rate (MMSR) and max-sum secrecy rate (MSSR) problems,
respectively. As for the upper bound case, an alternative optimization
(AO)-based algorithm is proposed to solve the two non-convex problems. Finally,
the superiority of the proposed cases to the conventional orthogonal multiple
access (OMA) one is verified by numerical results.
