Energy Efficiency In Large-scale MIMO System

Mobile communication technology has experienced great development since it appeared. On one hand, it supplies growing transmission rate and provides better user experience. On the other hand, its rapid development brings about escalating energy consumption and environmental pollution problems. Hence, green communication is put forwarded and regarded as the developing trend of the future communication.Large-scale MIMO is a new technology in the fifth generation mobile communication system, which equips with dozens or even hundreds of antennas and can not only significantly improves the system capacity without increasing bandwidth but also enhances the energy efficiency. The study on the energy efficiency of large-scale MIMO system is beneficial to solve the network energy consumption problem and realize the green communication.Despite the great improvement on system performance, large-scale MIMO also brings about some problems. For example, the circuit energy consumption is non-ignorable and the system hardware complexity and computational complexity becomes higher as well. Hence, how to reduce the complexity in large-scale MIMO system is a practical topic to study. Besides, the goal of traditional wireless communication system is to maximize spectrum efficiency, but there always exists a compromise between energy efficiency and spectrum efficiency because their trends are not always consistent. In order to balance these two performance metrics in wireless communication systems better, it is necessary to analyze the tradeoff relationship between them.The energy efficiency in large-scale MIMO system is the research topic in this thesis and can be divided into the following aspects.How to determine the optimal number of antenna rapidly and accurately in large-scale MIMO system under power limitation including transmit antennas selection and joint transmit and receive antennas selection is studied in this thesis. The upper bound and lower bound of the optimal number of antenna are derived in the case of transmit antennas selection and are used to determine the optimal antenna number with derivative-based binary search algorithm. Besides, a new ergodic capacity lower bound is deduced when the number of transmit antennas is equal to that of receive antennas and is utilized to ascertain the optimal number of antenna in the case of joint transmit and receive antennas selection. The simulation results show that the system complexity is reduced greatly in both cases at the price of small capacity loss.The energy-efficiency and spectrum-efficiency tradeoff problem in single-user large-scale MIMO system which employs two-dimensional antenna array is also analyzed in this thesis. The multi-objective optimization problem that maximizes energy efficiency and spectrum efficiency is transformed into single-objective optimization problem with Cobb-Douglas production function. And the resulting utility function is viewed as system performance index. Then the trends of energy efficiency and spectrum efficiency are analyzed with the variation of transmission power and the antenna number. The simulation results show that energy efficiency and spectrum efficiency can be compromised with tradeoff vector.