  We investigate joint power allocation and decoding order selection (PA-DOS)
aimed at enhancing the outage performance of non-orthogonal multiple access
(NOMA) systems. By considering the diverse target rates of users, new important
properties of NOMA are revealed: When users' target rates satisfy certain
conditions, the channel state information (CSI) is not required by PA-DOS to
minimize the system outage probability, and different users' outage
probabilities can be minimized simultaneously; When such conditions are not
satisfied, the opposite situation occurs. Following these properties, two
PA-DOS strategies are designed regarding distinct user priorities, which ensure
the minimization of the system outage probability and the user outage
probability of the high-priority user. Especially, these strategies do not
require CSI or only require one-bit CSI feedback depending on users' target
rates. Analytical and numerical results are provided to demonstrate that the
proposed strategies significantly outperform the existing strategies in terms
of both system and user outage performance. Furthermore, the results show that
although under some target rates the minimum user outage probabilities cannot
be simultaneously achieved, they can be closely approached at the same time in
the high-signal-to-noise-ratio regime by the proposed strategies.
