Research On Blind Carrier Recovery Algorithms For Single Carrier 4096-QAM System

As an effective complement to optical fiber transmission,the large capacity point-to-point microwave transmission link is urgent for the development of 4G/5G mobile communications.Using high-order QAM such as 4096-QAM is the most straightforward and effective means of increasing the capacity of the microwave systems.However,the high-order modulation constellations are more denser and their Euclidean distance is smaller,so they are more sensitive to the frequency offset and phase offset.In particular,the phase noise in the link may cause the appearance of error floor,and becomes non-negligible in high-order QAM systems.All these are new challenges to traditional carrier recovery technologies.This paper focuses on the carrier recovery technology for 4096-QAM,especially the blind carrier recovery algorithms.The main contributions are as follows:Firstly,this paper studies the basic principles and characteristics of classic blind carrier recovery algorithms,and analyzes their applicability in the 4096-QAM system.Especially,the fast frequency offset acquisition algorithm proposed by Lee et al.estimates the frequency offset in open-loop,then the phase-frequency detector tracks and compensates the small frequency error in closed-loop mode.The carrier recovery loop converges fast.However,the polarity decision phase detector in this algorithm has a relatively large computational complexity.This paper deduces the formula of the polarity decision phase detector,obtaining a simple phase discrimination structure,and proposes a simplified algorithm.Simulations show that the proposed algorithm reduces the complexity of the hardware implementation and has slightly faster acquisition speed.Considering 4096-QAM signals at SNR=35dB,the normalized acquisition frequency range is-0.1 to 0.1.When the normalized frequency offset is less than 0.08,the convergence time of carrier recovery is less than 50,000 symbols.When the phase noise is ideally compensated,the bit error rate?BER?performance of the proposed algorithm is almost consistent with the theoretical value.Secondly,this paper summarizes the common phase noise models and analyzes the phase noise influence on M-QAM signals.The Directed Decision-Extended Kalman Filting?DD-EKF?algorithm has been successfully applied in optical fiber communication in the past five years,which can estimate the frequency offset while tracking the phase noise or the cross-polarization interference,etc.However,the current research mainly focuses on QPSK or 16-QAM.In this paper,we add a mode selection module to the DD-EKF,which can adjust parameters to the current system state.This proposed algorithm is suitable for 4096-QAM.The simulations show that the proposed algorithm can correct the frequency offset of 12%of the symbol rate at most.The BER is almost the same as the theoretical value without phase noise,and when considering the phase noise,the performance loss is only about 0.5dB at BER of 10^-4.