  This paper considers the forward error correction (FEC) code design for
approaching the capacity of a dynamic multiple access channel (MAC) where both
the number of users and their respective signal powers keep constantly
changing, resembling the scenario of an actual wireless cellular system. To
obtain a low-complexity non-orthogonal multiple access (NOMA) scheme, we
propose a serial concatenation of a low-density parity-check (LDPC) code and a
repetition code (REP), this way achieving near Gaussian MAC (GMAC) capacity
performance while coping with the dynamics of the MAC system. The joint
optimization of the LDPC and REP codes is addressed by matching the analytical
extrinsic information transfer (EXIT) functions of the sub-optimal multi-user
detector (MUD) and the channel code for a specific and static MAC system,
achieving near-GMAC capacity. We show that the near-capacity performance can be
flexibly maintained with the same LDPC code regardless of the variations in the
number of users and power levels. This flexibility (or elasticity) is provided
by the REP code, acting as "user-load and power equalizer", dramatically
simplifying the practical implementation of NOMA schemes, as only a single LDPC
code is needed to cope with the dynamics of the MAC system.
