  Recently, random access protocols have acquired a new wave of interest, not
only from the satellite communication community, but also from researchers
active in fields like Internet of Things and machine-to-machine. Asynchronous
(slot- and frame-wise) ALOHA-like random access protocols, are very attractive
for such applications, enabling low complexity transmitters and avoiding time
synchronization requirements. Evolutions of ALOHA employ time diversity through
proactive replication of packets, but the time diversity is not fully exploited
at the receiver. Combining techniques, as selection combining and maximal-ratio
combining, are beneficial and are adopted in the enhanced contention resolution
ALOHA (ECRA) scheme, presented here. A tight approximation of the packet loss
rate for asynchronous random access, including ECRA, well suited for the low
channel load region is derived. Finally, ECRA is evaluated in terms of spectral
efficiency, throughput and packet loss rate in comparison with recent
protocols, showing that it is able to largely outperform both slotted
synchronous and asynchronous schemes.
