  Radio access management plays a vital role in delay and energy consumption of
connected devices. The radio access in existing cellular networks is unable to
efficiently support massive connectivity, due to its signaling overhead. In
this paper, we investigate an asynchronous grant-free narrowband data
transmission protocol that aims to provide low energy consumption and delay, by
relaxing the synchronization/reservation requirement at the cost of sending
several packet copies at the transmitter side and more complex signal
processing at the receiver side. Specifically, the timing and frequency
offsets, as well as sending of multiple replicas of the same packet, are
exploited as form of diversities at the receiver-side to trigger successive
interference cancellation. The proposed scheme is investigated by deriving
closed-form expressions for key performance indicators, including reliability
and battery-lifetime. The performance evaluation indicates that the scheme can
be tuned to realize long battery lifetime radio access for low-complexity
devices. The obtained results indicate existence of traffic load regions, where
synchronous access outperforms asynchronous access and vice versa.
