Investigations On Energy Efficiency Optimization Techniques For Ultra-Dense Networks

With the popularity of the mobile smart terminals and the widespread of the wireless multimedia data traffic,the traffic demand of the wireless communication network grows explosively.In order to meet future demand,the communication industry proposes to maximize the reuse of the spectrum and enhance the system performance by introducing ultra-dense networks(UDN).However,the densely deployment of the low-power small cells in UDN will increase the energy consumption of the system.Therefore,energy saving technology aiming at achieving optimal tradeoff for energy consumption and system performance becomes a crucial research topic in UDN to minimize the system energy consumption under the constraint of delay performance.Small cell sleeping strategy is an important technology to save energy through switching off low traffic load base station.However,turning off low traffic load base station leads to decreased system capacity and increased delay.Therefore,there exists a tradeoff between system energy consumption and delay in the design of the sleeping strategy.Energy-delay tradeoff(EDT)based small cell sleeping strategy to minimize the system energy consumption while guaranteeing the delay in UDN is investigated to achieve optimal tradeoff between quality of service for user and energy consumption.The joint optimization problem of small cell sleeping strategy and user association as well as the joint optimization problem of small cell sleeping strategy and caching technology are investigated.In addition,EDT based small cell on/off strategy for wireless bakhaul ultra-dense network is discussed.The main contributions of this dissertation are as follows:The joint optimization problem of small cell sleeping strategy and user association based on EDT in UDNs is investigated.The EDT problem in UDN is analyzed,and flexible user association and dynamic base station sleeping strategy to minimize system energy consumption while ensuring delay performance are proposed.The tradeoff between quality of service for user and energy consumption is achieved.An M/G/1/N processor sharing vacation queueing is introduced to model small cell with sleeping strategy and cell association.The general expressions for mean delay and energy consumption are derived via analyzing the queueing model,and the impacts of sleeping and user association strategy on the mean delay and system energy consumption are discussed.Then EDT problem is formulated as a cost minimization problem,and the optimal sleeping time and association radius are analyzed via Lambert W function.Assume the maximimum number of user associating with one small cell is no more than N.The optimal sleeping time for one and infinite number of users are derived,since there is no closed-form expression for the optimal sleeping time for arbitrary value of N.Simulation results show that the optimal sleeping time for infinite number of users is a reasonable approximation for arbitrary number when it is more than one,and also the the analysis is confirmed by the simulation results.The optimal sleeping mechanism for cache-aided UDN is investigated.Considering small cell with caching space,each small cell can cache a part of contents in advance.Both video content and file content are considered.All video contents are cached at small cell base stations and delivered by them.Part of file contents are cached at small cell base stations,and macro cell base station cooperates with cache-aided small cell base station and provides service for file content.The general expressions of system average energy consumption and transmission delay for video and file content are derived.Then,EDT problem is formulated as finding the optimal sleeping scheme to minimize the system energy consumption under network stability condition.Lyapunov technique is used to analyze the bound of delay performance and the stability condition is incorporated in delay-related control parameter.Thus,the EDT problem is transformed into joint optimization problem of delay and energy consumption under a given weighting factor.Considering the traffic state of user and the channel condition,a delay-constrained sleeping algorithm is proposed to minimize system energy consumption while guaranteeing delay performance.Simulation results show that the proposed algorithm with better performance benefitting from caching technology,it can not only improve delay performance,but also bring about energy saving gain.A joint optimization problem of collaborative caching and sleeping strategy based on EDT for device-to-device(D2D)-aided UDN is investigated.D2 D transmitter and small cell base station use heterogeneous caching scheme to select a part of file contents for caching.System energy consumption and transmission delay are analyzed based on stochastic traffic model,and then joint optimization problem is formulated as a cost minimization problem.To solve the joint optimization problem,we decompose it into sleeping sub-problem and collaborative caching sub-problem.For sleeping subproblem,the optimal sleeping ratio is first derived,then,a delay-aware sleeping strategy is proposed to obtain the local optimal sleeping scheme.Simulation results show that it can converge to the global optimal solution.For the collaborative caching subproblem,the suboptimal solution is obtained by distributed iterative method.In each iteration step,the suboptimal caching ratios of each file cached at small cell base station are derived by solving the combinatorial optimization problem with given D2 D caching strategy under Karush-Kuhn-Tucker conditions.And then,the combinatorial optimization problem for D2 D transmitter is solved based on the derived local optimal small cell caching scheme.Simulation results show that the proposed algorithms can coverage to the global solution,and it also achieves an excellent balance between complexity and accuracy.Small cell sleeping strategy based on EDT in wireless backhaul UDN is investigated.A theoretical framework for EDT in wireless-backhauling UDN is proposed,where user association probability and transmission probability of base station are derived.The expressions of system energy consumption and packet delay are obtained,and the impacts of base station sleeping ratio on system energy consumption and packet delay are discussed.EDT problem is formulated as a cost minimization problem to select the optimal set of sleeping small cells.To solve the EDT optimization problem,a locally optimal sleeping ratio for EDT is obtained by dynamic gradient iteration algorithm,and we prove it can converge to global optimal sleeping ratio.Then,queue-aware and channel-queue-aware sleeping strategy are proposed to find the optimal set of sleeping small cells according to the optimal sleeping ratio.Simulation results show that channel-queue-aware sleeping strategy has better performance than those schemes that only consider the traffic state or channel condition.