  This paper analyzes the coexistence performance of Wi-Fi and cellular
networks conditioned on non-saturated traffic in the unlicensed spectrum. Under
the condition, the time-domain behavior of a cellular small-cell base station
(SCBS) with a listen-before-talk (LBT) procedure is modeled as a Markov chain,
and it is combined with a Markov chain which describes the time-domain behavior
of a Wi-Fi access point. Using the proposed model, this study finds the optimal
contention window size of cellular SCBSs in which total throughput of both
networks is maximized while satisfying the required throughput of each network,
under the given traffic densities of both networks. This will serve as a
guideline for cellular operators with respect to performing LBT at cellular
SCBSs according to the changes of traffic volumes of both networks over time.
