In this paper, we introduce a nonlinear phase noise suppression method for Coherent Optical Orthogonal Frequency- Division Multiplexing (CO-OFDM) systems based on Gaussian Basis Expansion (GBE). In comparison with previous phase noise suppression approaches, it enhances the system's tolerance to nonlinear phase noise evidently, effectively mitigating the nonlinear phase noise in CO-OFDM systems. The proposed method is theoretically analyzed and its efficacy is validated through simulations using VPI Software.
KEYWORDS: Orthogonal frequency division multiplexing, Optical transmission, Tolerancing, Receivers, Optical fibers, Matrices, Signal to noise ratio, Data transmission, Optical communications, Laser systems engineering
In this paper, we develop a Gaussian basis expansion (GBE)-based phase noise suppression method for coherent optical orthogonal frequency-division-multiplexing (CO-OFDM) systems. The GBE method displays notable advantages over conventional phase noise suppression approaches as it substantially enhances the system tolerance to laser phase noise. These theoretical benefits are further confirmed through numerical simulations conducted in CO-OFDM systems utilizing a 16-quadrature-amplitude-modulation format.
Compared to conventional orthogonal frequency-division multiplexing (OFDM), orthogonal frequency-division multiplexing offset quadrature amplitude modulation (OFDM/OQAM) system relaxes the orthogonal condition from the complexed field to real field, cyclic prefix (CP) could thus be removed to promote spectral efficiency. However, there exists intrinsic imaginary interference (IMI) for OFDM/OQAM systems when faced with multiple-path fading channel. Therefore, developing effective channel estimation method to suppress IMI effect is very important for maintaining the performance of the OFDM/OQAM systems. Recently, numerical discussions on channel estimation method for coherent optical OFDM/OQAM (CO-OFDM/OQAM) have been reported. For these works, although frequency domain channel estimation method based on the using of pseudo pilot have been established, the correlation of the noise on each subcarrier brought by the half period time offset between the real and imaginary part, have not been taken into consideration. In this case, the channel estimation accuracy might not be optimal. In this paper, we proposed intra symbol frequency domain averaging (ISFDA) channel estimation method based on the maximum-likelihood algorithm for this system to decrease the interference induced by the correlation property. We analyzed the correlation property of the received amplified spontaneous emission (ASE) noise on each subcarrier for CO-OFDM/OQAM system and performed weighed averaging method for the frequency domain channel transmission matrix. As shown in the theoretical analysis and simulation results, the interference induced by the correlation property of the ASE noise could be suppressed significantly thanks to the using of the ISFDA method.
For the advantage of high spectral efficiency and lower out of band power radiation, orthogonal frequency-division multiplexing offset quadrature amplitude modulation (OFDM/OQAM) system have aroused wide interest. For OFDM/OQAM, prototype filter with promising time-frequency localization properties have been employed to suppress the out of band radiation. Time offset between the real and imaginary part of the QAM symbol and the phase offset module guaranteed the real filed orthogonal condition of the sub-carriers for OFDM/OQAM. Most recently, numerical experimental demonstrations for coherent optical OFDM/OQAM (CO-OFDM/OQAM) have been reported. Polarization-division-multiplexed structure could promote the spectral efficiency of CO-OFDM/OQAM further. However, chromatic dispersion (CD) and polarization mode dispersion (PMD) effect brought serious intrinsic imaginary interference (IMI) to PDM CO-OFDM/OQAM. Thus, effective channel estimation methods have to be developed to combat IMI and maintain system performance. Although numerical frequency domain channel estimation method based on the concept of pseudo pilot have been discussed, the correlation of the amplified spontaneous emission (ASE) noise brought by the time offset module decreased the channel estimation accuracy. To our best knowledge, channel estimation method based on weight averaging have not been discussed for PDM CO-OFDM/OQAM system. In this paper, we proposed joint intra symbol frequency domain averaging (JISFDA) channel estimation method based on the maximum-likelihood criterion for the PDM CO-OFDM/OQAM system. Compared to the conventional channel estimation method which only use single subcarrier for channel estimation, the JISFDA method considers the correlation of the ASE noise on each subcarrier and utilizes weight averaging method for the channel estimation matrix. As shown in the theoretical analysis and Montel Carlo simulation results, the interference induced by CD, PMD, and the correlation property of the ASE noise could be suppressed significantly with the using of the JISFDA method.
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