  Non-orthogonal multiple access (NOMA) allows multiple users to share a
time-frequency resource block by using different power levels. An important
challenge associated with NOMA is the selection of users that share a resource
block. This is referred to as clustering, which generally exploits the channel
disparity (i.e. distinctness) among the users. We discuss clustering and the
related resource allocation challenges (e.g. power allocation) associated with
NOMA and highlight open problems that require further investigation. We review
the related literature on exploiting channel disparity for clustering and
resource allocation. There have been several misconceptions regarding NOMA
clustering including: 1) clustering users with low channel disparity is
detrimental, 2) similar power allocation is disastrous for NOMA. We clarify
such misunderstandings with numerical examples.
