Design And Implementation Of Interference Cacellation System In 3G Repeaters

Adaptive interference cancellation technology first appeared in American military projects. With the development of digital signal processing, interference cancellation system is now being widely used in telecommunications, such as the underlying discussed 3G repeater, which is a type of relaying equipment used to provide wider coverage. Repeater retransmits a signal on the same frequency as it receives, so that no frequency translation is required as is the case in RF.However,due to short distance induced unwanted multipath coupling between the receiving and transmitter antennas,the device can also receive its own output,thus causing expected signal is buried in the multipath echoes as well as destroying the frequency characteristics of the retransmitted signal.In this paper, interference cancellation system is implemented in 3G repeater to combat its inherent co-channel interference.Firstly, this paper introduces the basic theory of ICS technology and basic structure of 3G repeater. Secondly the inherent co-channel interference of repeater is analyzed. The stability of least-mean-square (LMS) algorithm for implementing Interference Cancellation function is also verified through three port echo cancellation system. The principle of interference cancellation algorithm along with its corresponding simulation platform, according to strict LMS and NLMS algorithm simulation, is discussed and presented respectively. This paper also presents an improved algorithm of Variable-step-LMS (VLMS) based on the simulation results and hardware implementation complexity. The advantage of this algorithm is verified by VLMS float-point simulation and furthermore its fix-point simulation is implemented according to actual requirements. As results of above, FPGA implementation method along with design specification of some key-modules are presented. At the end of this paper, the construction of WCDMA Interference cancellation hardware platform along with hardware verification result of all three different implementation skills is given. The algorithm bring EVM depravation within 5% even under the SIR of-20db, which satisfies the performance criterion of 3G repeater,grately reduce the isolation requirement of repeater.From the result one can easily find that the inherent co-channel interference in 3G repeater is reduced greatly with the ICS algorithms. ICS technology guarantees a significant improvement on 3G repeater, which is meaningful for the practical projects.