| The past decade has witnessed a rapid development in mobile communication networks,owing to the increasing number of new mobile communication devices,which led to an exponential growth in the wireless traffic volume.So,we need to provide a large number of access points for these devices.In order to achieve these goals,a large number of various low-power small cells,such as micro,pico,and femto base stations will be deployed to overlay the traditional macro cells.Then increasing the density at different type of BSs can constitute an ultra-dense heterogeneous networks.As a key technology of 5G,ultra-dense heterogeneous networks can not only meet the above requirements,but also dramatically improve the network capacity and wireless resource utilization.However,with the increment of BSs,the total energy consumption will also increase.Therefore,based on the green communication,this paper takes the ultra-dense heterogeneous networks as the research object,and discusses the base station density and the number of users when the energy efficiency of the system is maximized from the point of view of base station deployment and based on non-orthogonal multiple access technology,respectively.The main research results of this thesis are summarized as follows:Firstly,when the base station follows the independent Poisson point process,the two-layer downlink ultra-dense heterogeneous network model is established.The users in the network also follow the Poisson point process independent from each layer of base station and access the nearest base station.Then,taking the inter-tier interference and intra-tier interference into consideration,the performance of the network is analyzed and the expressions of spectrum efficiency and energy efficiency are derive based on stochastic geometry.One of the objectives of this paper is to maximize the energy efficiency of the system from the point of view of base station deployment.Therefore,we use the optimization theory to maximize the energy efficiency under the constraints of spectral efficiency,base station density and base station transmission power.Simulation results show that optimal base station density can be found to get maximum energy efficiency under the condition of satisfying minimum required spectral efficiency and the relationship of spectral efficiency and energy efficiency with base station density can be observed.Combining non-orthogonal multiple access technology with ultra-dense heterogeneous networks can greatly improve the spectral efficiency of the system,but also have a certain impact on the energy efficiency.Therefore,the user access the downlink two-layer downlink ultra-dense heterogeneous network by non-orthogonal multiple access.Moreover,the signal is sent to the user by the superposition coding technique at the transmitter.And each user signal is decoded by successive interference cancellation technique at the receiver.In the case of non-orthogonal multiple access and orthogonal multiple access,the energy efficiency of the system is analyzed,respectively.Another goal of this paper is to maximize energy efficiency from the point of view of the number of users.Therefore,the energy efficiency optimization problem is formulated subject to the constraint of allocated powers for users.In order to solve this optimization problem,we propose a fair power allocation algorithm for users to allocate power properly to ensure the throughput of cell-edge users.Simulation results show that non-orthogonal multiple access technology can enhance energy efficiency compared with orthogonal frequency division multiple access.And the fairness of the proposed power allocation approach is much better than that of the conventional fractional transmit power allocation.In addition,the energy efficiency of the macro base station is much lower than that of the small base station.So,from the energy efficiency point of view,the heterogeneous network is more energy-efficient and environmental friendly than the traditional single-layer cellular network. |
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