In this paper, we review the key figures of merit to assess the performance
of advanced random access (RA) schemes exploiting physical layer coding,
repetitions and collision resolution techniques. We then investigate RA
modeling aspects and their impact on the figures of merit for the exemplary
advanced RA schemes: Contention Resolution Diversity Slotted ALOHA (CRDSA),
Irregular Repetition Slotted ALOHA (IRSA), Coded Slotted ALOHA (CSA) and
Enhanced Spread-Spectrum ALOHA (E-SSA). We show that typical simplifications of
the reception model when used to optimize RA schemes lead to inaccurate
findings, both in terms of parameter optimization and figures of merit, such as
the packet loss ratio (PLR) and throughput. We also derive a generic RA energy
efficiency model able to compare the schemes in terms of the energy required to
transmit a packet. The combination of achievable RA throughput at the target
PLR and energy efficiency, for the same average user power investment per frame
and occupied bandwidth, shows that E-SSA, which is an unslotted scheme,
provides the best overall performance, while, in terms of the slotted schemes,
CRDSA outperforms the more elaborated IRSA and CSA. This surprising results is
due to the fact that the IRSA and CSA optimization has so far been performed
using RA channel models that are not accurately reflecting the physical layer
receiver behavior. We conclude by providing insights on how to include more
accurate reception models in the IRSA and CSA design and optimization.Comment: To appear in IEEE Journal on Selected Areas in Communications -
Special Issue on Advances in Satellite Communication