  In this paper, we propose a cache-aided non-orthogonal multiple access (NOMA)
scheme for spectrally efficient downlink transmission. The proposed scheme not
only reaps the benefits associated with NOMA and caching, but also exploits the
data cached at the users for interference cancellation. As a consequence,
caching can help to reduce the residual interference power, making multiple
decoding orders at the users feasible. The resulting flexibility in decoding
can be exploited for improved NOMA detection. We characterize the achievable
rate region of cache-aided NOMA and derive the Pareto optimal rate tuples
forming the boundary of the rate region. Moreover, we optimize cache-aided NOMA
for minimization of the time required for completing file delivery. The optimal
decoding order and the optimal transmit power and rate allocation are derived
as functions of the cache status, the file sizes, and the channel conditions.
Simulation results confirm that, compared to several baseline schemes, the
proposed cache-aided NOMA scheme significantly expands the achievable rate
region and increases the sum rate for downlink transmission, which translates
into substantially reduced file delivery times.
