The Research On Channel Equalization And Phase Noise Compensation Methods For PDM CO-FBMC-OQAM Systems

The rapid development of new Internet services has led to higher requirements for the rate and quality of service of communication networks.Compared with single-carrier communication systems,multi-carrier communication systems have the advantages of high spectral efficiency and simple equalization methods,which can better meet the demand for rate and transmission quality in modern communication networks.The filter bank multicarrier(FBMC)system with offset quadrature amplitude modulation(OQAM)is a multicarrier communication technology that has received much attention in recent years.Compared with conventional orthogonal frequency division multiplexing(OFDM)systems,FBMC-OQAM has smaller out-of-band energy leakage and higher spectral efficiency;therefore,it is considered as one of the possible alternatives to OFDM.Coherent optical FBMC-OQAM system(CO-FBMC-OQAM)is a fiber optic communication technology based on FBMCOQAM,which when combined with polarization division multiplexed(PDM)can multiply the transmission capacity.Optical fiber channels introduce dispersion,polarization mode dispersion and laser phase noise,and how to eliminate the effects of these physical impairments using digital signal processing algorithms is crucial to enhance the transmission quality of COFBMC-OQAM.However,the CO-FBMC-OQAM system maintains the orthogonality condition only in the real domain,and there is always inherent imaginary interference.Some widely used physical impairment cancellation methods cannot be directly applied to COFBMC-OQAM,and the related algorithms need to be redesigned.The object of this paper is the frequency domain channel estimation and phase noise compensation methods for polarization division multiplexed CO-FBMC-OQAM communication system(PDM-COFBMC-OQAM),and the research focuses on the following three aspects:(1)This paper analyzes in detail the advantages and disadvantages of the already proposed frequency-domain channel estimation methods.To improve the channel estimation accuracy,PDM-CO-FBMC-OQAM usually uses periodic repetition of the pilot to increase the pseudo pilot frequency power.On the one hand,this scheme leads to an increase in the power-peak-toaverage ratio(PAPR)of the time-domain signal,which affects the nonlinear transmission quality.On the other hand,more zero subcarriers need to be inserted between the pilot and the payload to form a protection interval,and too many redundant frequency domain symbols reduce the spectral efficiency.To solve the problem that the channel estimation process leads to further degradation of PAPR,this paper designs an improved frequency domain channel estimation method for PDM-CO-FBMC-OQAM system using the dual-dependent pilot principle.The method has a randomly selected pilot value between +1 and-1,and the amplitude of the pseudo pilot can reach 2.3136.The PAPR and channel estimation performance of the method are investigated in detail using numerical simulations,and the results confirm that it does not make the PAPR worse while ensuring a high enough channel estimation accuracy.(2)To reduce the protection interval and further improve the spectral efficiency,a high spectral efficiency channel estimation method is proposed in this paper for the PDM-COFBMC-OQAM system.The method uses the polarization division multiplexed fiber channel model and the symmetry law of the inherent imaginary interference to design a Full-Load realvalued pilot scheme,which is combined with the interpolation method to reduce the number of frequency domain symbols to be occupied by the training sequence on each polarization state to four.In addition,the pilot values used in this method are also randomly selected,which has good power peak-to-average ratio performance.The PAPR and channel estimation performance of the method are investigated in detail using numerical simulations,and the results confirm that it can effectively achieve channel equalization while improving spectral efficiency.(3)The laser phase noise has a more serious impact on CO-FBMC-OQAM due to the real domain orthogonality condition and inherent imaginary interference.The commonly used blind phase search algorithm is too computationally intensive and prone to insufficient search range.In this paper,the real-valued signal rotation of FBMC-OQAM caused by phase noise on the complex plane is analyzed in detail.Based on this,a phase noise compensation method based on real-imaginary alternating pilot is proposed.The method designs a new real-imaginary alternating pilot pattern.In the complex plane,the phase noise rotates the real-imaginary pilot,and the rotation angle of the real/imaginary pilot with respect to the real/imaginary axis is the estimated value of the phase noise.The results of numerical simulations show that the method can effectively compensate for the common phase error caused by the laser phase noise.