With the rapid growth of mobile devices and popularity for data-intensive services, LTE Licensed-Assisted Access (LAA) has been proposed to allow coexistence of LTE and Wi-Fi in 5 GHz unlicensed band. Most existing works have been focusing on developing collision avoidance mechanisms to ensure the harmonious coexistence between Wi-Fi and LTE. In this paper, we argue that simply avoiding possible collisions caused by simultaneous transmissions does not fully utilize the unlicensed spectrum. We derive the optimal CCA threshold that enables as many simultaneous transmissions between coexisting LTE and Wi-Fi as possible and exploit successive interference cancellation to further increase the chance of concurrent transmissions. We introduce a Markov-model based approach to quantity the impact of energy detection, concurrent transmission and successful decoding probability on the throughput expressions of LAA and Wi-Fi. Extensive simulations have been presented to validate our proposed theoretical model. Our results show the throughput of Wi-Fi and LAA can be significantly improved under several typical coexistence scenarios.