一种基于星座恢复的相干光OFDM系统非线性抑制方法

鉴于相干光正交频分复用系统（CO-OFDM）易受非线性效应影响,严重制约系统传输性能, 提出一种基于星座恢复的降低系统峰均值功率比（PAPR）的方法。对系统模型及作用机理进行分析,仿真结果显示,以10 Gbit/s的速率通过单模光纤传输720 km,基于星座恢复的CO-OFDM系统Q值较无非线性抑制的CO-OFDM系统有3 dB提高;另外,系统最佳限幅比例系数受色度色散影响,随着色散因子的增大,星座恢复效果也逐渐减弱,在传输240 km条件下,色散因子为12 ps/nmkm较6 ps/nmkm,限幅比例系数从0.9降低至0.8。     

An improvement on constant modulus algorithm for polarization demultiplexing in optical coherent receivers

In order to solve the problem of ill-convergence for constant modulus algorithm (CMA) in polarization division multiplexing system with optical coherent receivers, a modified CMA based on bit error rate (BER) aiding to control initial tap setup, named as BER-Aiding CMA (BA-CMA), is proposed in this paper. By analyzing the principle of CMA for adaptive digital equalization and polarization demultiplexing, the convergence behavior that leads to undesirable result is evaluated and therefore BA-CMA is proposed. Simulation results show that the proposed algorithm is able to overcome the undesirable convergence effectively and keep inherent advantages of CMA at the same time.     

Ultra-broadband OFDM signal generation for 1-Tb/s super-channel coherent optical OFDM transmission

A 320.6-GHz multi-band optical OFDM signal generation scheme consisting of continuous 4,104 subcarriers is demonstrated for 1-Tb/s coherent optical OFDM (CO-OFDM) transmission. The 36 uncorrelated orthogonal bands are generated using a recirculating frequency shifter (RFS) with single laser by adjusting the loop delay of RFS to be an integer multiple of OFDM symbol period. The principle and performance of this generation scheme is studied theoretically and experimentally. The 1-Tb/s CO-OFDM signal is successfully received after 600-km SSMF fiber transmission without any dispersion compensation.     

Joint nonlinearity and chromatic dispersion pre-compensation for coherent optical orthogonal frequency-division multiplexing systems

This paper introduces a joint nonlinearity and chromatic dispersion pre-compensation method for coherent optical orthogonal frequency-division multiplexing systems. The research results show that this method can reduce the walk-off effect and can therefore equalize the nonlinear impairments effectively. Compared with the only other existing nonlinearity pre-compensation method, the joint nonlinearity and chromatic dispersion pre-compensation method is not only suitable for low-dispersion optical orthogonal frequency-division multiplexing system, but also effective for high-dispersion optical orthogonal frequency-division multiplexing transmission system with higher input power but without optical dispersion compensation. The suggested solution does not increase computation complexity compared with only nonlinearity pre-compensation method. For 40 Gbit/s coherent optical orthogonal frequency-division multiplexing 20×80 km standard single-mode fibre system, the suggested method can improve the nonlinear threshold (for Q > 10 dB) about 2.7, 1.2 and 1.0 dB, and the maximum Q factor about 1.2, 0.4 and 0.3 dB, for 2, 8 and 16 ps/(nm·km) dispersion coefficients.     

An improved least square channel estimation algorithm for coherent optical OFDM system

This paper presents an improved processing added to conventional least square (LS) channel estimation to modify its performance for coherent optical orthogonal frequency division multiplexing (CO-OFDM) system. By testing selected limitation factors of the existing algorithms, the influence of our improved algorithm to the performance of CO-OFDM system were studied and compared with other published algorithms. The simulation results of the study demonstrated that the proposed approaches achieved better channel estimation performance and are considered as a more appropriate alternative for CO-OFDM system with the tradeoff between complexity and performance.