To address the mobile data growth challenges, mobile operators need to access more spectrum resources. LTE in unlicensed spectrum (LTE-U) has been proposed to extend the usual operation of LTE in licensed spectrum to cover also unlicensed spectrum, mainly at 5 GHz band due to its wide spectrum availability. However, this extension poses significant challenges especially regarding the coexistence between LTE-U and legacy systems like Wi-Fi. In case of LTE-U adopts Time-Division Multiplexing (TDM) schemes to share the spectrum with Wi-Fi, we expect performance degradations of Wi-Fi networks. In this paper, we quantify the impact of TDM schemes on Wi-Fi performance in a coexistence scenario. We provide an analytical model to compute the probability of collision faced by Wi-Fi and its downlink throughput performance. This model provides an upper bound of the probability of collision that could be faced by a finite number of Wi-Fi stations. NS3 simulations show that the model estimates accurately the collision probability and the throughput experienced by Wi-Fi. The model is then used to study and compare different coexistence schemes showing for instance that the Wi-Fi frame size impacts globally the performance of Wi-Fi users.