Energy Efficient User Association And Transmission Design In Multicell Networks

To reduce energy consumption and improve energy efficiency(EE)in cellular networks,green network has attracted a great deal of attentions.However,compared with spectral efficiency(SE)optimization,the objective in EE optimization has a more complicated fractional form,which makes it hard to obtain the optimal solution in most scenarios.Especially in practical applications,channel state information(CSI)is in general estimated at receiver,then the receiver sends CSI estimate back to transmitter.This procedure inevitably produces estimation error and leads to latency.Hence,it is difficult to obtain perfect CSI at base station(BS).This thesis considers robust energy efficient transmission design under CSI impairments.Robust precoding optimization is firstly studied via maximizing single-cell EE when only partial CSI is available at transmitter.Three difficult scenarios,namely,multiple-input single-output(MISO),multiple-input multiple-output(MIMO)and relay are considered.For MISO scenario,we obtain the optimal robust energy efficient precoding strategy,which admits an explicit closed-form solution.For MIMO channel,it is proved that the optimal precoding matrix satisfies a channel-diagonalizing structure.Accordingly,we trans-form the primal complicated matrix-value problem into a simple,but equivalent,power optimization.The optimal power allocation can then be found efficiently with a water-filling interpretation.The primal relay precoding for EE optimization is,however,complicated and we transform it into a one-dimension search for efficient solutions.Specifically,we further present simpler closed-form solutions for some asymptotic scenarios.Subsequently,the robust energy efficient transmission design is then extended to multicell networks.Different from single-cell networks,the objective of robust energy efficient beamforming optimization in multicell networks has a nonconvex form because the beamforming variables are coupled with CSI error.Inorder to solve it efficiently,we resort to a lower bound and cast it as alternating optimization with semidefinite program(SDP).In particular,the iterative process can be implemented parallelly at multiple BSs,allowing distributed operations with reduced latency.In order to meet the increasing wireless service demands,traditional cellular structure composed of macrocell BSs is evolving into an irregular network topology equipped with low transmission power nodes,which is referred as a heterogeneous network(HetNet).Along with the rise of HetNets,EE is also gradually considered to be an important criteria of cellular networks.The thesis finally studies energy efficient asso-ciation with the optimal selection of blank resource blocks(RBs)fraction.Without any individual quality of service(QoS)requirement,we obtain a closed-form solution which provides useful insights for system design.By further incorporating QoS constraints,we transform the primal fractional EE problem into a linear program(LP)with efficient solutions.In addition,the assumption that a user in a given RB will be served by at most one BS is verified by both theory and simulation results,which indicates that practical systems can obtain approximately the optimal network EE by simply constraining a user served by one BS.