| With the rapid growth of wireless communications service, energy consumption of wireless network is growing at an alarming rate, green communication has attracted wide attention. For the sake of environmental protection and financial expenditure, the improvement of energy efficiency (EE) has become a more and more important problem in the future mobile cellular networks. Cooperative communication technology is the effective way to realize green communication, and can effectively improve the network energy efficiency. Therefore, researches on cooperative communications based on energy efficiency have the high academic value and practical significance.In the distributed antenna system (DAS) with a single cell, the different cooperative strategies with re-mote antenna units (RAUs) selection are investigated. Considering the distribution of the channel state infor-mation from RAUs to the user, using the knowledge of the theory of probability, the distribution of the user's signal to noise ratio (SNR) and the distribution of the ergodic capacity with various cooperative transmission modes are derived in theory, and the closed expression is given. By introducing the power consumption mod-el of a distributed antenna system, the system energy efficiency is further studied, the expression of energy efficiency is given, and the tradeoff between spectrum efficiency and energy efficiency is deduced, which provides scientific guidance for the future network optimization design.As for the user with single antenna configuration in multi-cell cooperative scenario, the system capacity and fairness based on zero-forcing pre-coding design are studied in the case of the total power constraint and per antenna power constraint, respectively. The fairness is analyzed for the two kinds of power constraints, and the closed solution is given. As for the multiple antennas configuration for the user, the receive antenna selection algorithm by maximizing the channel norm is used in order to reduce the computational complexity, the case for the user with multiple antennas configuration is transformed into the case for the user with single antenna configuration, the fairness algorithm for the user with single antenna is still applicable. The energy efficient transmission is studied in the case of the total power constraint and per antenna power constraint, respectively. The problem is a non-convex fraction optimization problem, an energy efficiency maximization iterative algorithm is proposed, which converts the fractional optimization problem into an equivalent integral optimization problem. In each iteration process, the optimal power allocation values can be obtained by the Lagrangian dual decomposition, so as to achieve the maximum energy efficiency.When the network traffic load is low, the energy saving problems based on base station sleeping and bandwidth expansion are investigated, respectively. With different quality of service requirements, an energy minimization problem based on base station sleeping is formulated, and user round robin base stations access algorithm is presented. The algorithm can significantly reduce the total energy consumption of the network and has a low complexity. In addition, the power saving algorithm based on bandwidth expansion in multi- user orthogonal frequency division multiplexing (OFDM) downlink systems is investigated. The original problem is a mixed optimization problem. Through variable substitution, the mixed optimization problem can be transformed into an integer optimization problem with respect to the allocated sub-carriers. Two kinds of sub-carriers allocation algorithms with low complexity are proposed for the integer optimization problem, i.e. iterative sub-carriers allocation for power minimization algorithm and modified move right iteration algorithm. The two algorithms allocate the sub-carriers to users with different ways. The performances of the two algorithms are better than that of the average sub-carriers allocation, and are the same as that of exhaustive search (ES) method.In the whole network system, multiple base stations cooperate in the form of cluster. The full coopera-tion of all base stations is unaffordable for system overhead, so how to partition the base stations in the system into different clusters to cooperate with a low complexity is a challenging issue. In this paper, a novel dy-namic clustering algorithm for multiple base stations cooperation in downlink is proposed, and system energy efficiency is investigated. Firstly, with equal power allocation per symbol and per antenna equal power con-straint, the formulas of spectral efficiency and energy efficiency for the case of ideal transmit and the case of actual transmit are derived, respectively. In addition, a novel dynamic clustering algorithm based on channel norm is presented. Energy efficiency of the proposed algorithm is better than that of the static clustering one and slightly worse than that of the decentralized algorithm but with a lower complexity. |
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