  Proliferation of the urban Internet-of-Things (IoTs) for smart cities has
fuelled massive amounts of data over wireless cellular networks. Random access
(RA) system of wireless cellular networks, e.g., 5G New Radio (NR), based on
S-ALOHA system should cope with ever-growing IoT traffic. This work proposes
S-ALOHA system with time offsets (TOs), where one slot consists of K TOs and
one packet transmission time. The length of the overall TOs is a fraction of a
packet transmission time. In the system users (re)transmit to the boundary of a
TO randomly selected. This enables the base station (BS) to inform the users of
who transmits the first and the last packets in the slot with collision so that
the two users can retransmit successfully in the following two slots
respectively. Our throughput analysis compared to simulations shows that
adopting even with three and four TOs surpasses the throughput limit of S-ALOHA
system without TOs. Additionally, we propose two Bayesian-optimized backoff
algorithms for S-ALOHA system with TOs, with which users can apply
throughput-optimal (re)transmission probability or uniform backoff window even
in unsaturated traffic scenarios. Numerical results demonstrate that the
proposed backoff algorithms can achieve the throughput close to an ideal system
and drastically reduce the access delay compared to S-ALOHA system.
