Research On Energy Saving Methods For Hybird Energy Powered Cellular Networks

To meet the rapidly growing demands for wireless data traffic, several orders of magnitude base stations(BSs) have been deployed, and thus the relevant energy consumption and carbon emissions have increased explosively. Therefore, it is imperative to construct the green wireless communication network, which aims at energy saving and carbon footprint reduction, for the 3G and beyond cellular communication system.Using renewable energy, such as solar and wind, to power cellular networks has been widely accepted as a promising avenue to reduce and optimize energy consumption and corresponding carbon footprints and operational expenditures due to their pollution-free and renewable nature. In the hotspot of the city, bringing renewable energy into the cellular network powered by on-grid energy will construct the Hybrid Energy powered cellular Networks(HybE-Net). In HybE-Net, BSs are previously powered by renewable energy, and when the harvested energy cannot provide sufficient power for the BS energy demand, on-grid energy is immediately switched on by the energy controller to guarantee the cell coverage and users’ quality of service. However, the fluctuation of renewable energy harvesting and the indeterminacy of the BS energy consumption may result in the imbalance of renewable energy utilization among BSs in HybE-Net. This makes renewable energy not completely useable, and energy saving and carbon footprint reduction cannot be achieved to capacity. Therefore, in this paper, the cellular networks powered by the solar energy and on-grid energy are taken as the example, and the optimization and control methods for energy saving were studied to solve the problem mentioned above. Additionally, the optimal design of renewable energy powered BS system was also studied, which can be used as a guideline for the practical deployment of the HybE-Net. The main contributions are described as follows:First, the optimal design of the renewable energy system is studied in this dissertation. Firstly, the dynamic energy flow behavior of solar energy powered BS is modeled by using an M/G/1/K stochastic energy queue. Based on the constructed model, dynamics of energy flow behavior, i.e., energy harvesting rate, energy consumption interval and state probability of energy, in solar energy powered BS system are analyzed mathematically by an imbedded Markov chain. Then, key design metrics(i.e., service outage probability, solar energy utilization efficiency and mean depth of discharge) are defined and analyzed, which can be employed to evaluate and quantify the system reliability, solar energy utilization, and durability. Finally, the sizing optimization problem under constraints of design metrics is formulated as the problem of minimizing the capital expenditure of the system, and the solution of the optimization problem is proposed. Simulation results demonstrate that the proposed method searches for an optimal trade-off between system economy and reliability, and the proposed modeling, design metrics, and sizing methods provide a theoretical basis for actual HybE-Net designs.Second, the admission control algorithm in HybE-Net is studied in this dissertation. Firstly, solar energy states and dynamic characteristics of BSs are analyzed using the diffusion approximation theory. Then, we comprehensively consider admission judgment parameters, such as data transmission rate, available bandwidth, and solar energy state of the BS, using the fuzzy logic theory. Meanwhile, the equilibrium metric of solar energy utilization in HybE-Net is employed to evaluate the influences on equilibrium of solar energy utilization of various BSs in network, which caused by accessing different BSs in the terminals. Finally, simulation results show that the proposed algorithm performs excellently in improving the equilibrium of solar energy utilization among BSs, on-grid energy saving and carbon emissions reduction. Furthermore, the accuracy of the proposed model and the adopted analytical method are verified by the simulation.Third,the coverage optimization algorithm in HybE-Net is studied in this dissertation. Firstly, models of solar energy harvesting and energy consumption are introduced, and the closed-form expression of the relationship between the BS’s energy consumption and its coverage area is derived mathematically. Then, in order to minimize on-grid energy consumption of the HybE-Net on guaranteeing quality of service, we formulate the coverage optimization problem as the on-grid energy consumption minimization problem. Further, considering the fluctuation of solar energy harvesting and the indeterminacy of energy consumption on the time and spatial dimensions, the formulated problem is decomposed into three sub-problems: per-link energy consumption minimization sub-problem, initial stored energy allocation sub-problem, and spatial traffic lattice generalized assignment sub-problem. Finally, the transmission power optimization, initial solar energy state allocation optimization and spatial traffic lattice assignment are proposed to solve these sub-problems. The experiments have been completed to verify the proposed algorithms’ feasibility and effectiveness.Last, the BS sleeping strategy in HybE-Net is studied in the dissertation. Firstly, essential conditions for the BS sleeping in HybE-Net are analyzed in view of guaranteeing quality of communication service and the effective energy saving. On basis of these conditions, the problem of base station sleeping in HybE-Net is formulated as an on-grid energy saving maximization problem, in which BSs’ sleeping decision must be made dynamically in order to improve the utilization of system resources and maximize the on-grid energy saving of the HybE-Net, as well as guarantee the quality of service requirement of users. Then, to solve this problem, we propose a BS sleeping algorithm based on coalition games, and the stability of the proposed algorithm is proved theoretically. Additionally, in order to avoid the frequent switch between sleeping and non-sleeping modes, the wake-up threshold based on solar energy state is introduced, and the optimal value of this wake-up threshold is derived mathematically. Finally, extensive experiments have been completed to verify the proposed algorithms’ feasibility and effectiveness in terms of on-grid energy saving.