  In this paper, we aim to design an adaptive power allocation scheme to
minimize the average transmit power of a hybrid automatic repeat request with
chase combining (HARQ-CC) enabled non-orthogonal multiple access (NOMA) system
under strict outage constraints of users. Specifically, we assume the base
station only knows the statistical channel state information of the users. We
first focus on the two-user cases. To evaluate the performance of the two-user
HARQ-CC enabled NOMA systems, we first analyze the outage probability of each
user. Then, an average power minimization problem is formulated. However, the
attained expressions of the outage probabilities are nonconvex, and thus make
the problem difficult to solve. Thus, we first conservatively approximate it by
a tractable one and then use a successive convex approximation based algorithm
to handle the relaxed problem iteratively. For more practical applications, we
also investigate the HARQ-CC enabled transmissions in multi-user scenarios. The
user-paring and power allocation problem is considered. With the aid of
matching theory, a low complexity algorithm is presented to first handle the
user-paring problem. Then the power allocation problem is solved by the
proposed SCA-based algorithm. Simulation results show the efficiency of the
proposed transmission strategy and the near-optimality of the proposed
algorithms.
