Loop Interference Suppression In Full-uplex Wireless Relays

The full-duplex wireless relays can providecost-effective coverage extension andhigh spectral efficiency. Especially, in order to improve the communication data rate forthe3G and4G mobile communication system,much attention are attached to full-duplexwireless relays and its networks.3GPP TS36.216(Release11) also provides relevanttechnical specification of relay station. However, simultaneous reception andtransmission in the same frequency results in loop interference which distorts theretransmitted signal and makes the relay prone to oscillation or decreases the channelcapacity of communication system.In this thesis, according to the typical two hop relay system, establish the systemmodel,and then discuss the causeand influence of loop interference. Based on theexisting relay system with loop interference, the loop interference suppressiontechniques from the time-domain, spatial-domain and the combination of them in theSISO and MIMO full-duplex wireless relays are studied. Make simulation and summaryof the classical loop interference cancellation and suppression techniques, and thenimprove and propose new algorithmadapting to more complex channel environment.The concrete contents include:Introduce the classification of relays. Establishfull-duplex wireless relay systemmodel including base station, relays and user station.Present the self-excitationcondition of full-duplex SISO relay going to prone caused by loop interference, andanalysis the influence of loop interference on channel capacity offull-duplex MIMOrelays. Then, from the principle and structure, verify the effectiveness of the currentloop interference cancellation and suppression technology.Research the loop interference cancellation techniques from time-domain: LMSalgorithm、NLMS algorithm and RLS algorithm in full-duplex SISO relays. The abovethree algorithms utilize the estimatedloop channel to subtract the interference from thereceived signal of the relays. LMS algorithm uses statistic and signal correlation toestimate loop channelwith slow convergence speed and poor tracking performance butlow implementation complexity. NMLS algorithm,the normalized LMS algorithm, can obtain better convergence rate. When the signalfrom thebase station to relaysisin thepresence of multipath interference, the normalized loop channel estimation error is highwithLMS or NLMS algorithm. In front of the problem, we propose improvedalgorithm.Base on linear fitting to loop channel estimation,RLS algorithm can havefaster convergence rate and accuracy of convergence. Mainly concern the performanceof the three time-domain cancellation techniqueson convergence, convergence speedand its precision. More, Comparethe performance between LMS algorithm and RLSalgorithm as well as the occupation of resources. In the simulations, how theinitialization parameter of algorithmsaffects the performance of the algorithm is alsopresented.Present multi-channel LMS(MC-LMS)algorithmand spatial suppression techniquesfor cancellingor mitigating loop interference in full duplex MIMO relays. MC-LMSalgorithm is root inthe SISO LMS algorithm. Put forward and analyze the performanceof MC-LMS facing two typical MIMO channel: Rayleigh channel and Rician channel.Finally get the tracking performance of LMS algorithm. Discuss the spatial domainsuppression techniques, and propose null-space projection and subspace projectiontechniques to design the transmit filter and the receive filter for mitigating loopinterference as much as possible or even completely based on the zero forcing algorithm,while the multipath propagation of loop interference channel is taken into account.Simulation results show the proposed space projection algorithm outperforms theprevious schemes. Especially when the loop channel matrix is full rank, the proposedalgorithm also can completely suppress the loop interference. More, compare thetime-domain algorithm and spatial suppression techniques, and then verify thecombinationstrategies of them to mitigate the loop interference complementarily.Finally, the contents of this thesis are summarizedand put forward the suggestionson furtherstudy.