Optimal Channel Occupancy Duration and Subcarrier Allocation for LTE-U Systems

Deploying long term evolution (LTE) system into unlicensed spectrum for offloading faces with two challenges. (a) The system can not serve the users during its non-channel access period, which makes the metrics of instantaneous performance in traditional LTE system be no longer applicable for unlicensed LTE (LTE-U) system. That is, the user satisfaction can not be fairly evaluated. (b) The scheduled users to transmit / receive data within the same frame must end their data transmissions / receptions simultaneously so as to prevent Wi-Fi nodes from sensing channel idle and then accessing the channel. Therefore, the users with insufficient data are mandated to add padding bits, which deteriorates the channel utilization. To address the abovementioned issues, it is necessary to adjust the system's channel occupancy duration (COD) dynamically and allocate subcarriers to the users properly according to user satisfaction and channel condition. In this paper, we first define the unified channel utilization and user satisfaction. Then, we model a two-dimensional optimization problem, which maximizes the weighted sum of channel utilization and user satisfaction by optimizing COD in time domain and subcarrier allocation in frequency domain. By introducing a control factor, the tradeoff between channel utilization and user satisfaction can be achieved. By using Lyapunov optimization theory we transfer the original optimization problem into a new problem, so that it can be solved by using the instant data arrival and channel state information (CSI) of the current frame instead of knowing the statistical knowledge of CSIs and data arrivals of all frames in advance. Simulation results reveal that as compared with the baselines, our proposed heuristic algorithm for solving the new problem can flexibly balance channel utilization and user satisfaction, and can meet different applications on demand.