  Next-generation internet-of-things (IoT) networks require extremely low
latency, complexity, and collision probability. We introduce the novel
partial-information multiple access (PIMA) scheme, a semi-grant-free (GF)
coordinated random access (RA) protocol for short packet transmission, with the
aim of reducing the latency and packet loss of traditional multiple access
schemes, as well as more recent preamble-based schemes. With PIMA, the base
station (BS) acquires partial information on instantaneous traffic conditions
in the partial information acquisition (PIA) sub-frame, estimating the number
of active devices, i.e., having packets waiting for transmission in their
queue. Based on this estimate, the BS chooses both the total number of slots to
be allocated in the data transmission (DT) sub-frame and the respective
user-to-slot assignment. Although collisions may still occur due to multiple
users assigned to the same slot, they are drastically reduced with respect to
the slotted ALOHA (SALOHA) scheme, while achieving lower latency than both
time-division multiple-access (TDMA) and preamble-based protocols, due to the
extremely reduced overhead of the PIA sub-frame. Finally, we analyze and assess
the performance of PIMA under various activation statistics, proving the
robustness of the proposed solution to the intensity of traffic, also with
burst traffic.
