  Various verticals in 5G and beyond (B5G) networks require very stringent
latency guarantees, while at the same time envisioning massive connectivity. As
a result, choosing the optimal multiple access (MA) technique to achieve low
latency is a key enabler of B5G. In particular, this issue is more acute in
uplink transmissions due to the potentially high number of collisions. On this
premise, in the present contribution we discuss the issue of delay-sensitive
uplink connectivity using optimized MA techniques; to this end, we perform a
comparative analysis of various MA approaches with respect to the achievable
effective capacity (EC). As opposed to standard rate (PHY) or throughput (MAC)
analyses, we propose the concept of the effective capacity as a suitable metric
for characterizing jointly PHY-MAC layer delays. The palette of investigated MA
approaches includes standard orthogonal MA (OMA) and power domain non
orthogonal MA (NOMA) in uplink scenarios, both considering random pairing and
optimized pairing alternatives. It further extends to encompass a recently
proposed third alternative, referred to as NOMA-Relevant (NOMA-R), which
extends OMA and NOMA approaches by flexibly selecting the MA technique. We show
that optimizing both user pairing and MA selection increases the network EC,
especially when stringent delay constraints are in place; thus a flexible MA is
a potentially preferable strategy for future low latency applications
