Opportunistic Adaptive Non-Orthogonal Multiple Access in Multiuser Wireless Systems: Probabilistic User Scheduling and Performance Analysis

This paper designs a novel opportunistic adaptive non-orthogonal multiple access (OA-NOMA) strategy, where a base station (BS) employs NOMA to serve a near user (NU)-far user (FU) pair opportunistically scheduled from
$M$
NUs and
$K$
FUs. In particular, the NOMA transmission to the scheduled NU-FU pair adaptively operates in one of two modes:
Direct NOMA mode
, in which the BS directly serves the scheduled NU-FU pair with using NOMA;
Cooperative NOMA mode
, in which the scheduled NU receives the messages intended by both scheduled users from the BS, and then forwards the message intended by the scheduled FU. For the OA-NOMA strategy, a scheduling candidate acquisition method and a probabilistic user pair scheduling scheme are proposed to guarantee the transmission reliability and improve the scheduling fairness, respectively. To evaluate the scheduling fairness, we develop a max-min fairness criterion and show that the OA-NOMA strategy approximately achieves max-min fairness. The reliability of the OA-NOMA strategy is also evaluated in terms of outage probability and diversity order. For the outage probability, we derive an approximate expression and numerically verify its tightness. For the diversity order, we show that the proposed OA-NOMA strategy achieves a diversity order of
$M$
.