  The effort to extend cellular technologies to unlicensed spectrum has been
gaining high momentum. Listen-before-talk (LBT) is enforced in the regions such
as European Union and Japan to harmonize coexistence of cellular and incumbent
systems in unlicensed spectrum. In this paper, we study throughput optimal LBT
transmission strategy for load based equipment (LBE). We find that the optimal
rule is a pure threshold policy: The LBE should stop listening and transmit
once the channel quality exceeds an optimized threshold. We also reveal the
optimal set of LBT parameters that are compliant with regulatory requirements.
Our results shed light on how the regulatory LBT requirements can affect the
transmission strategies of radio equipment in unlicensed spectrum.
