Research And Realization Of Feedback Interference Cancellation Technology For WCDMA Repeater

In the wireless communication system, the repeater plays a very importantrole. No matter what system of wireless communication, it will still exist blind areaand the weak area of system coverage. And in some areas of few users, such asremote rural and mountainous area, repeater is a kind of optimization scheme toenhance the user experience and to extend network coverage for service provider.Also repeater has the characters of low cost, networking flexible and effective tooptimize and improve service quality.Wireless repeater is space coupled bidirectional amplification equipment, andbecause of the transceiver with the same frequency, the receiving antenna willreceive the feedback interference signals from the transmitting antenna along withthe useful signals from the base station at the same time. When the isolation betweenrepeater’s transmitting and receiving antenna don’t meet the condition that greaterthan repeater’s gain more than15dB, the interference signal will seriously affect thequality of repeater’s output signal, and will lead to call dropped in the covered area.Even more, the repeater will be burned out. Traditional wireless repeater can onlyincrease the distance between transmitting and receiving antennas, adjust thedirection angle, and other means to improve physical isolation degree in order tosolve this problem. And it means that increase the difficulty of the engineeringinstallation. When it is hard to improve the physical isolation, the gain of the repeaterhas to been reduced to match the installation condition. Thus reduces the coverage.After research and compared with several adaptive filtering algorithm, anenhanced variable step size LMS algorithm with fast converging speed and littlesteady-state error is proposed. Then a serial of classic feedback interferencecancellation schemes at home and aboard are introduced in this paper. Focus onsolving the feedback interference problems of WCDMA wireless repeater，a set ofthe feedback interference cancellation scheme is proposed. First of all, use trainingsequence that is not related with WCDMA signal for space channel initial estimationto get the delay and amplitude of echo signals. Secondly, design adaptive feedback interface cancellation filter based on LMS algorithm to track and offset the echosignals. Then the scheme is simulated and analyzed on MATLAB/Simulink platform.It resulted that the interference cancellation system scheme performed well.According to the proposed scheme, the hardware plan based on the XilinxVirtex6platform is designed which contain the whole downlink of WCDMA digitalwireless repeater with interference cancellation system function. And on thisplatform, we set up ICS module test platform and simulated echo interferenceproblem in the laboratory environment. When input downlink single carrierWCDMA signal, we test and verify the improvement of isolation based on thescheme by adjusting the value of adjustable attenuator. Test results show that thescheme of the ICS module can effectively offset echo interference signal. When theisolation is less than the gain20dB, the key specifications of output signals such asEVM and ACPR all meet the WCDMA standard. It means that it improved isolationby35dB, greatly reduce the repeater’s installation requirement of isolation. It willsave cost and realize more gain and output with the same isolation degree, and willbe convenient to install.