| With the rapid development of mobile internet and the explosive growth of mobile applications,wireless cellular networks are facing unprecedented challenges.On the one hand,data service gradually takes the place of traditional voice-oriented business,and becomes the leading role of wireless communication business,and a lot of data services happen indoors;on the other hand,the massive mobile devices and kinds of application services such as video,online games,and high-speed downloads have raised more and more requirements for the capacity,rate and time delay of cellular networks.The introduction of small cell such as femtocell,pico cell and relay into the traditional macro cellular network which makes the cellular network heterogeneous has been widely considered to be one of the most effective ways to address the above challenges.However,the introduction of different kinds of BSs into heterogeneous cellular networks(HCNs)directly results in the increase of the associated energy consumption.Therefore,exploring a method to accurately analyze the performance of HCNs and the corresponding energy efficiency(EE)optimization problem become a hot topic in the field of wireless communication.In this dissertation,the stochastic geometry theory,a mathematical modeling method for the spatial point distribution,is used as a mathematical analysis tool to model and analysis HCNs and EE optimization.On this basis,the EE optimization of HCNs based on BS sleeping strategy is further explored,and the sleeping probability of small cell under the dual constraints of average connection ratio(ACR)and transmission rate is optimized.The main contributionsof this thesis are listed as follows:(1)Based on the three-dimensional Poisson Point Process(3-D PPP)theory,the performance of small cell networks is analyzed and the performance expression is derived.According to the spatial distribution of small cell in dense urban areas,the traditional 2-D PPP interference modeling method is extended to three dimensions,and a small base station interference model based on 3-D PPP is constructed.Based on this model,the network performance metrics,such as the uplink and downlink coverage probability and the average rate of the small cell networks,are analyzed.For downlink,this dissertation assumes that the small cells in a certain area form a 3-D PPP with certain parameter.Under the support of point process theory,the general expression of the coverage probability and average rate of the target user and the closed form in special case are derived.For uplink,this dissertation assumes that the small cells and the users form the independent 3-D PPPs of their respective parameters and derives the theoretical expression of the uplink coverage probability and the transmission rate of the target cell with the uplink power control strategy based on the fractional power control factor.Simulation results show that compared with the traditional 2-D PPP analysis results,the 3-D PPP based analysis method proposed in this dissertation can better describe the spatial distribution characteristics of the small cell networks,which provides a closer lower bound for the coverage performance of the small base station cellular networks.(2)The performance of small cell networks considering the mix interference of uplink and downlink is investigated.On the basis of the analysis of the existing FDD scene and uplink/downlink interference modeling,the system performance of the small cell network under the TDD system is further studied under the condition of the uplink and downlink mix interference.The 3-D PPP theory is used to model and analyze the mix interference on the dynamic TDD small cell networks,the coverage probabilities of the target cells in the uplink and downlink scenarios are deduced,and the spectral efficiency and energy efficiency are derived.Through simulation analysis,the theoretical results are consistent with the simulation results of dynamic TDD small cell networks,which verify the accuracy of this model.(3)The system performance of heterogeneous cellular networks with different distribution characteristics of macro base station and small cell is analyzed.On the basis of 3-D PPP analysis for the small cell networks,according to the plane distribution of the macro BS at the top of the floor and the spatial distribution of the small cell inside the floor,the performance of heterogeneous cellular networks using 2-D PPP and 3-D PPP is analysed.The macro BS and the small cell in the heterogeneous network are respectively assumed as 2-D PPP and 3-D PPP,which form their parameters respectively.Based on the analysis the probability that a typical user is associated with macro BS and the small cell,and the pdf of the distance between the target user to its serving BS,the coverage probability of the target user associated with the macro BS and the small cell is derived.The full probability formula is used to calculate the target user's downlink coverage probability in the whole HCNs,and the Shannon's formula is used to deduce the downlink transmission rate.The simulation results show that the hybrid analysis method of 2-D PPP and 3-D PPP proposed in this dissertation can better reflect the distribution characteristics of the different BS in the HCNs than the single 2-D PPP model,especially for the spatial distribution characteristics of the small cell.(4)EE optimization of small cell networks based on sleeping strategy.Small cell networks are deployed according to the peak traffic load,which cause energy waste during low traffic periods.In this dissertation,we focus on the EE problem in small cell networks in which a large number of small cells are spatial deployed in dense urban area such as office building,shopping mall and etc.We optimize the EE through small cell sleep strategy under ACR and average user rate constrains.First,based on the 3-D PPP analysis of the small cell network,the mathematical expression of the ACR in the small cell network is further derived.Second,the monotonicity of average user rate and ACR are analyzed in detail.Then,based on the results of monotonicity analysis,the optimal small cell sleep probability is calculated to satisfy ACR and average user rate constrain respectively.At last,we formulate a network energy consumption minimization problem subject to both ACR and average user rate constraints to determine the sleep probability.In addition;we also formulate an optimal maximum connections number of small cell that minimizes the energy consumption subject to the joint constraint of ACR and average rate.Numerical results show that our 3-D PPP model is more accurate than the traditional 2-D in both average rate and ACR performance and the small cell networks' energy consumption can be reduce about 21% of the total energy consumption with the sleep strategy in this dissertation. |
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