Inter-cell Interference Management for Cellular Systems
Lee, Dae Won
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The wide spread of mobile smart phones and ever-increasing demand for more throughput by the users has put a sever burden on cellular networks. Limited bandwidth availability and crowded base station deployments of the cellular system pushes network operators to improve existing network efficiency. This means reducing inter-cell interference such that each base station can maximize performance. In goal of this dissertation is to investigate techniques that mitigate inter-cell interference for modern cellular systems and focuses on development of practical scheduling algorithms for inter-cell interference coordination that can be applied to currently deployed LTE Release 8 networks and development of more advanced inter-cell interference mitigation techniques based on coordinated beamforming. One of the classical methods of mitigating inter-cell interference is inter-cell interference coordination. Not only does it help improve cellular data coverage but also allow more efficient use of valuable wireless spectrum. This dissertation investigates soft frequency reuse for LTE systems. Systems employing soft frequency reuse is analyzed and it is found that classification of cell-edge users and cell-center users is critical. To improve performance, de-centralized scheduling algorithms that include optimal user classification and balances throughput and fairness among users is proposed. Additional sub-optimal methods to reduce the computational complexity is introduced. Simulation demonstrates that gains of the 5th percentile user throughput can be increased without loss to the overall cell average throughput compared to a non-cooperating system. Next, coordinated beamforming which require faster and tighter coordination among base stations is investigated. Multi-user and multi-stream coordinated beamforming based on maximization of the harmonic-sum of signal-to-interference-plus-noise ratio is proposed. Simulation results show that it improves cell-edge users compared with prior researched coordinated beamforming algorithms based on minimizing mean square error, maximizing uplink signal-to-interference-plus-noise ratio, and maximizing weighted sum-rate. Additionally, proposed coordinated beamforming is further extended to multi-carrier systems with per-antenna power constraints. Low complexity algorithm that can be applied to coordinated beamforming algorithms with whitened match filter structure is proposed and simulated. The key idea is to update a common antenna power regulating diagonal matrix, which determines the antenna power of a precoding matrix in each iteration. The algorithm can be scaled to any number of subcarriers because it only updates a common diagonal matrix for all subcarrier. Simulations show that proposed algorithm enables excellent antenna power efficiency with low number of iterations and converges quickly. Furthermore, it can significantly mitigate inter-cell interference and improve performance over non-coordinating system.