Lower-Complexity Power Allocation for LTE-U Systems: A Successive Cap-Limited Waterfilling Method

Unlicensed spectrum, around 5 GHz, will be introduced to Long Term Evolution (LTE) systems, referred to as LTE-Unlicensed (LTE-U), to combat the explosive growth of traffic volume in next 10 years. In this paper, the interference-controlled power allocation problem is studied for LTE-U systems, which can be inherently classified as orthogonal frequency division multiplexing (OFDM)-based cognitive radio (CR) systems, where optimal power allocation algorithms are currently available by resorting to computation-intensively numerical iterations with a moderate risk of divergence. In order to satisfy the rigorous algorithm requirements, i.e., convergent outputs of power allocation and running time of milliseconds, for practical LTE-U deployments, a cap-limited waterfilling method is proposed to regulate the interference to primary users one by one successively, by which not only a near-optimal solution can be obtained, but also the intractable iteration divergence and computation complexity issues can be excluded completely. Simulation results indicate the capacity performance of the proposed low-complexity method approaches to the optimal solution with a slight loss less than 5%, and is remarkably superior to the existing suboptimal methods.