  As 5G networks rolling out in many different countries nowadays, the time has
come to investigate how to upgrade and expand them towards 6G, where the latter
is expected to realize the interconnection of everything as well as the
development of a ubiquitous intelligent mobile world for intelligent life. To
enable this epic leap in communications, this article provides an overview and
outlook on the application of sparse code multiple access (SCMA) for 6G
wireless communication systems, which is an emerging disruptive non-orthogonal
multiple access (NOMA) scheme for the enabling of massive connectivity. We
propose to apply SCMA to a massively distributed access system (MDAS), whose
architecture is based on fiber-based visible light communication (FVLC),
ultra-dense network (UDN), and NOMA. Under this framework, we consider the
interactions between optical front-hauls and wireless access links. In order to
stimulate more upcoming research in this area, we outline a number of promising
directions associated with SCMA for faster, more reliable, and more efficient
multiple access in future 6G communication networks.
