Research And Implementation Of The Method Based On High Order QAM Carrier Recovery

Quadrature amplitude modulation (Quadrature Amplitude Modulation, QAM)is a kind of amplitude modulation mode in which the two orthogonal carrier signal，I component and Q component an be modulated, which is phase and amplitudeKeying whose amplitude and phase all carry important information. Because of itsutilization rate of spectrum,QAM signal is highly and widely used in modern digitalsystem, but in the process of signal transmission, due to these unavoidablefactors,such as signal transmission delay,local carrier phase error and so on willlead to the carrier offset, which makes the system error_rate rising,phase deviationof a carrier makes the QAM constellation skew,while frequency deviation makesthe QAM constellation rotation,so the whole system can’t be demodulated correctlyat the receiving end.In order to recover the deviation of phase and frequency of thecarrier accurately in the receiver,the carrier recovery module becomes an integralpart of the digital receiver system.Because of the sensitivity of the high QAM modulation and demodulationsystem carrier offset, A recovery method of high order QAM has been used basedon pilot，frequency sweeping loop and carrier recovery loop.Frequency sweepingloop is composed of three modules,which are frame detection module, frequencysweep module and frequency correction module.Carrier recovery loop contains thepolarity decision algorithm module and Decision-Directed(DD) module. Frequencysweep and polarity decision algorithm both have large range of frequency offsetacquisition,they can be used in carrier frequency offset acquisition stage, which notonly overcome the difficulties of general sweep algorithm often residual a largefrequency offset, but also solve the problem of polarity algorithm often have littleavailable corner; DD algorithm has a smaller phase jitter,but also a smaller rangeof frequency offset capturing,usually3%of the symbol rate, which is alwaysused for tracking stage for the frequency offset,correcting the residual frequencyoffset of the capturing stage.At the same time,the pilot also accelerates thesynchronization speed of the carrier loop.Based on the above algorithm, by writtingC code and RTL code of each module to test and verify whether they satisfyperformance requirements, the algorithm for carrier recovery in the paper is not only applicable to square QAM modulation constellation,but also suitable for crossconstellation, the highest modulation order can be up to1024QAM. Then load thedata results generated in C program into Matlab, through the indicators, such asconstellation before and after carrier recovery, mse and so on, show that thealgorithm has a reliable performance,a large range of frequency offsetcapturing,which is up to10%of the symbol rate,a short capturing time, asmall phase jitter and a high stability, which is able to recover the carrier quicklyand accurately.