Based on the License-Assisted Access (LAA) small cell architecture, the LAA coexisting with Wi-Fi heterogeneous networks provides LTE mobile users with high bandwidth efficiency as the unlicensed channels are shared among LAA and Wi-Fi. However, LAA and Wi-Fi interfere with each other when both systems use the same unlicensed channel in heterogeneous networks. In such a network, unlicensed band allocation for LAA and Wi-Fi is an important issue that may affect the quality of service (QoS) of both systems significantly. In this paper, we propose an analytical model and conduct simulation experiments to study four allocations for the unlicensed band: unlicensed full allocation (UFA), unlicensed time-division allocation (UTA), and UFA/UTA with buffering mechanism (UFAB and UTAB) for the LAA data packets. We evaluate the performance of these unlicensed band allocation schemes in terms of the acceptance rate of both LAA and Wi-Fi packet data in the LAA buffer queue. Our study provides guidelines for designing the channel occupation phase and the buffer size of the LAA small cell.