  In this paper, we study a layered random access scheme based on
non-orthogonal multiple access (NOMA) to improve the throughput of multichannel
ALOHA. At a receiver, successive interference cancellation (SIC) is carried out
across layers to remove the signals that are already decoded. A closed- form
expression for the total throughput is derived under certain assumptions. It is
shown that the transmission rates of layers can be optimized to maximize the
total throughput and the proposed scheme can improve the throughput with
multiple layers. Furthermore, it is shown that the optimal rates can be
recursively found using multiple individual one-dimensional optimizations. We
also modify the proposed layered random access scheme with contention
resolution repetition diversity for reliable transmissions with a delay
constraint. It is shown to be possible to have a low outage probability if the
number of copies can be optimized, which is desirable for high reliability low
latency communications.
