  With the envisioned massive Internet-of-Things (IoT) era, one of the
challenges for 5G wireless systems will be handling the unprecedented spectrum
crunch. A potential solution has emerged in the form of spectrum sharing, which
deviates from a monopolistic spectrum usage system. This article investigates
the medium access control (MAC) as a means of increasing the viability of the
spectrum sharing technique. We first quantify the opportunity of spectrum
access in a probabilistic manner, a method referred to as opportunity
probability (OP). Based on the OP framework, we propose a random MAC algorithm
in which the access of a node is randomly determined with its own OP value. As
a possible application of our OP based random MAC, we propose a hybrid half
duplex (HD)/full duplex (FD) communication where each pair decides the
duplexing mode according to the OP values of the two pair nodes. This approach
fits well with the spectrum sharing system since it enables a flexible
operation for the spectrum access according to the spectrum usage level. From
the numerical analysis, we validate the feasibility and verify the performance
enhancements by implementing an FPGA based real-time prototype. Measurements
and numerical results confirm that the proposed architecture can achieve up to
4 times higher system throughput than conventional LTE-TDD (time division
duplex) systems.
