  Non-Orthogonal Multiple Access (NOMA) and caching are two proposed approaches
to increase the capacity of future 5G wireless systems. Typically in NOMA
systems, signals at the receiver are decoded using successive interference
cancellation in order to achieve capacity in multi-user systems. The leveraging
of caching in the physical layer to further improve on the benefits of NOMA is
investigated, which is termed cache-aided NOMA. Specific attention is given to
the caching cases where the users with weaker channel conditions possess a
cache of the information requested by a user with a stronger channel condition.
The probability that any of the users is in outage for any of the rates
required for this NOMA system, defined as the "union-outage," is derived for
the case of fixed-power allocation, and the power allocation strategy that
minimizes the union-outage probability is derived. Simulation results confirm
the analytical results, which demonstrate the benefits of cache-aided NOMA on
reducing the union-outages probability.
