  Next generation wireless networks require massive uplink connections as well
as high spectral efficiency. It is well known that, theoretically, it is not
possible to achieve the sum capacity of multi-user communications with
orthogonal multiple access. To meet the challenging requirements of next
generation networks, researchers have explored non-orthogonal and overloaded
transmission technologies-known as new radio multiple access (NR-MA)
schemes-for fifth generation (5G) networks. In this article, we discuss the key
features of the promising NR-MA schemes for the massive uplink connections. The
candidate schemes of NR-MA can be characterized by multiple access signatures
(MA-signatures), such as codebook, sequence, and interleaver/scrambler. At the
receiver side, advanced multi-user detection (MUD) schemes are employed to
extract each user's data from non-orthogonally superposed data according to
MA-signatures. Through link-level simulations, we compare the performances of
NR-MA candidates under the same conditions. We further evaluate the sum rate
performances of the NR-MA schemes using a 3-dimensional (3D) ray tracing tool
based system-level simulator by reflecting realistic environments. Lastly, we
discuss the tips for system operations as well as call attention to the
remaining technical challenges.
