  Two emerging technologies towards 5G wireless networks, namely non-orthogonal
multiple access (NOMA) and cognitive radio (CR), will provide more efficient
utilization of wireless spectrum in the future. In this article, we investigate
the integration of NOMA with CR into a holistic system, namely cognitive NOMA
network, for more intelligent spectrum sharing. Design principles of cognitive
NOMA networks are perfectly aligned to functionality requirements of 5G
wireless networks, such as high spectrum efficiency, massive connectivity, low
latency, and better fairness. Three different cognitive NOMA architectures are
presented, including underlay NOMA networks, overlay NOMA networks, and
CR-inspired NOMA networks. To address inter- and intra-network interference
which largely degrade the performance of cognitive NOMA networks, cooperative
relaying strategies are proposed. For each cognitive NOMA architecture, our
proposed cooperative relaying strategy shows its potential to significantly
lower outage probabilities. Furthermore, we discuss open challenges and future
research directions on implementation of cognitive NOMA networks.
