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Licensed assisted access LTE (LAA-LTE) aggregates 5 GHz unlicensed bands with LTE's licensed bands via carrier aggregation, and adopts energy detection (ED)-based clear channel assessment (CCA) for protection of coexisting Wi-Fi devices. Since LAA-LTE requires the ED threshold should be set conservatively in the potential presence of Wi-Fi, the spatial spectrum reuse of the LAA-LTE will be much impaired. Such non-flexible thresholding has been introduced mainly due to ED's incapability of differentiating Wi-Fi frames from LTE frames. As a remedy, this paper proposes a lightweight but effective Wi-Fi frame detection method with which the LAA-LTE devices can capture a Wi-Fi preamble by only using the LAA-LTE's own time domain samples while incurring very small latency. Built upon the proposed method, we also propose the Wi-Fi energy tracking algorithm to identify the duration of a Wi-Fi frame, and a dynamic ED threshold selection algorithm. The proposed schemes were evaluated via the MATLAB simulations and USRP-based experiments, through which their efficacy has been confirmed, e.g., Wi-Fi frame detection probability up to 98.7%. Moreover, via extensive NS-3 based simulations with a multi-cell coexistence topology, we further revealed that the proposed mechanism not only enhances the spatial efficiency of the LAA-LTE achieving up to 23.68% more throughput than the legacy LAA-LTE but also protects coexisting Wi-Fi better. INDEX TERMS Algorithm design and analysis, cellular networks, computer simulation, wireless LAN, LAA-LTE, schmidl-cox detection.