Research And FPGA Implementation Of RAKE Receiver For Spread Spectrum Communication System

In recent years,Unmanned Aerial Vehicle(UAV)have played an important role in the military and civilian field,such as reconnaissance,long-range strike,aerial photography,aerial survey and data acquisition,etc.UAV’s communication is mainly interfered by multipath effect in the complex near earth communication environment.To solve this problem,based on the study of direct spread spectrum communication system of UAV,this paper designs a spread spectrum communication Rake receiving system that can resist multipath interference.First of all,this paper proposes to use PMF-FFT method to capture the transmission of the spread spectrum signal in the multipath channel,solve the PseudoNoise code phase drift caused by multipath interference,in turn,led to capture failure problem,and the combination of frequency estimation and multipath search to the module,compared to a serial capture time domain method to capture and multipath search work done faster,but in the SNR of-15 d B,acquisition probability above 95%.Secondly,in order to solve the problem of low carrier tracking accuracy in the case of frequency slant up Doppler frequency shift,a two-loop carrier tracking method is proposed.This method combines the advantages of the fast tracking carrier frequency error of the second-order FLL and the zero steady-state phase error of the third-order PLL to form a carrier tracking loop that works in parallel with the second-order FLL and the third-order PLL,which can track the carrier phase of the received signal quickly and accurately under the condition of a large dynamic Doppler frequency shift.Third,Rake receiving system adopts the maximum ratio method to merge multipath signals,which can maintain stable communication under the condition of multipath interference.Compared with the direct spread spectrum signal receiving system,Rake receiving system can improve the performance by about 2d B when the BER is 10^(-4).Finally,AD9361 chip is used as a wideband digital modulator in this paper,which can realize the miniaturization and low power consumption of baseband front-end hardware design.Finally,this paper carries out desktop test and field experiment aiming at the design index of this scheme.The experiment shows that this scheme can receive spread spectrum signal subject to multipath interference and meet the requirements of index.