  Due to massive connectivity and increasing demands of various services and
data-hungry applications, a full-scale implementation of the fifth generation
(5G) wireless systems requires more effective radio access techniques. In this
regard, non-orthogonal multiple access (NOMA) has recently gained ever-growing
attention from both academia and industry. Compared to orthogonal multiple
access (OMA) techniques, NOMA is superior in terms of spectral efficiency and
is thus appropriate for 5G and Beyond. In this article, we provide an overview
of NOMA principles and applications. Specifically, the article discusses the
fundamentals of power-domain NOMA with single and multiple antennas in both
uplink and downlink settings. In addition, the basic principles of code-domain
NOMA are elaborated. Further, the article explains various resource allocation
techniques such as user pairing and power allocation for NOMA systems;
discusses the basic form of cooperative NOMA and its variants; and addresses
several opportunities and challenges associated with the compatibility of NOMA
with other advanced communication paradigms such as heterogeneous networks and
millimeter wave communications.
