  Non-orthogonal multiple access (NOMA) is a promising radio access technique
for next-generation wireless networks. In this article, we investigate the
NOMA-based cooperative relay network. We begin with an introduction of the
existing relay-assisted NOMA systems by classifying them into three categories:
uplink, downlink, and composite architectures. Then, we discuss their
principles and key features, and provide a comprehensive comparison from the
perspective of spectral efficiency, energy efficiency, and total transmit
power. A novel strategy termed hybrid power allocation is further discussed for
the composite architecture, which can reduce the computational complexity and
signaling overhead at the expense of marginal sum rate degradation. Finally,
major challenges, opportunities, and future research trends for the design of
NOMA-based cooperative relay systems with other techniques are also highlighted
to provide insights for researchers in this field.
