  Partial-information multiple access (PIMA) is an orthogonal multiple access
(OMA) uplink scheme where time is divided into frames, each composed of two
parts. The first part is used to count the number of users with packets to
transmit, while the second has a variable number of allocated slots, each
assigned to multiple users to uplink data transmission. We investigate the case
of correlated user activations, wherein the correlation is due to the
retransmissions of the collided packets, modeling PIMA as a partially
observable-Markov decision process. The assignment of users to slots is
optimized based on the knowledge of both the number of active users and past
successful transmissions and collisions. The scheduling turns out to be a mixed
integer nonlinear programming problem, with a complexity exponentially growing
with the number of users. Thus, sub-optimal greedy solutions are proposed and
evaluated. Our solutions show substantial performance improvements with respect
to both traditional OMA schemes and conventional PIMA.
