偏振复用差分相移键控信号的数字相干解调与偏振解复用算法研究

利用光纤偏振分束器和3 dB耦合器搭建偏振分集90°光学混频器,采用单端接收实现了偏振分集的相干接收机。实现了10 Gb/s偏振复用差分相移键控(DPSK)光信号,经过280 km普通单模光纤(SSMF)和掺铒光纤放大器(EDFA)传输后的数字相干接收,测量无误码。研究了数字相干光接收中的各种信号处理算法。采用数字信号处理算法完成载波相位估计,数字滤波器补偿光纤色散,恒模算法进行自适应数字偏振解复用等。研究了90°光学混频器的非理想正交特性,运用统计方法补偿了光学混频器偏离90°的误差,此算法和信号格式以及光纤色散等因素无关。使用恒模算法实现数字偏振解复用,该算法收敛时间小于0.5μs。     

基于相位调制器产生六倍频光毫米波的OFDM信号光传输系统研究

提出了两个并行相位调制器产生六倍频光毫米波并传输正交频分复用信号的全双工光纤无线通信系统.将2.5 Gbt/s的正交频分复用信号调制到60 GHz光毫米波上.从理论上分析了毫米波产生的原理,并分别模拟了毫米波信号承载OOK信号和正交频分复用信号在光纤中的传输性能.从而验证系统的可行性.仿真模拟结果表明,在光纤无线通信系统中,正交频分复用比非归零码更具有优势.     

基于相位调制器产生六倍频光毫米波的OFDM信号光传输系统研究

提出了两个并行相位调制器产生六倍频光毫米波并传输正交频分复用信号的全双工光纤无线通信系统.将2.5Gbt/s的正交频分复用信号调制到60GHz光毫米波上.从理论上分析了毫米波产生的原理,并分别模拟了毫米波信号承载OOK信号和正交频分复用信号在光纤中的传输性能.从而验证系统的可行性.仿真模拟结果表明,在光纤无线通信系统中,正交频分复用比非归零码更具有优势.     

一种提高强度调制直接检测基带光双边带正交频分复用系统传输性能的预处理技术

为了减轻电光调制器的非线性效应以及光纤色度色散给直接检测光正交频分复用系统性能带来的损伤,对两者的损伤机理进行了理论分析.在此基础上提出了在发射中使用一种针对调制器的非线性采用预失真技术,而高频子载波功率衰减采用预增强的联合预处理技术来改善系统的性能.运用数值仿真法对所提出的方法进行了验证,结果表明在光背靠背传输系统中调制指数最优时,接收信号的误差矢量幅度值可以得到2dB的改善.随着光纤传输距离的增长,正交频分复用信号高频子载波功率衰减程度加剧而使得信噪比恶化,从而导致系统误码性能急剧下降.使用所提出的预处理技术的计算复杂程度低,可以使接收机的灵敏度得到明显的改善.     

一种光正交频分复用系统的联合相位均衡方法

正交频分复用（OFDM）技术加相干接收与数字信号处理法(DSP)的组合是超长距离光通信的理想模型。光OFDM系统对相位噪声十分敏感,必须对相位噪声进行补偿。提出一种基于正交小波基变换的光OFDM系统的联合相位均衡方案。该方案将块状导频周期性地插入OFDM信号,在接收端利用导频信息首先消除各个子载波的公共相位误差,然后采用自适应均衡方式消除每个子载波自身相位误差。仿真结果表明,对于二进制正交振幅键控(4QAM)调制信号,在采用常规的G.652光纤、100 Gb/s的相干光OFDM系统中,该联合相位补偿方法可使信号在满足传输系统的误码性能要求下,传输距离达到1000 km。     

40 Gbit/s 相干偏振复用QPSK传输中基于Manakov方程的非线性补偿

 在40 Gbit/s相干偏振复用正交相移键控（QPSK）传输系统中,为了补偿由于光纤中非线性效应引起的传输信号损伤,采用了基于Manakov方程的反向传播非线性补偿算法.传统的基于标量非线性薛定谔方程（NLSE）的反向传播算法忽略了偏振模色散（PMD）的作用,因此在偏振复用系统中不能补偿由于PMD引起的信号损伤|而基于Manakov方程的数字信号处理方法能够对PMD与克尔非线性效应的耦合作用进行补偿.从仿真与实验两个方面对此方法在40 Gbit/s相干偏振复用QPSK传输系统中的补偿效果进行了验证.结果均表明,与NLSE相比,基于Manakov方程的反向传播算法在400 km长距离QPSK传输中显示出更好的性能.在光信噪比（OSNR）为18 dB时,基于Manakov方程的反向传播算法得到的Q值与NLSE相比提高约3dB.     

承载正交频分复用信号的58GHz光载毫米波波分复用光纤无线通信系统

实验研究了一种采用马赫-曾德尔强度调制器和光交叉复用器(IL)产生58 GHz光载毫米波传输正交频分复用(OFDM)信号的波分复用光纤无线通信(WDM-ROF)系统。中心站的4路连续光波耦合后输入射频(RF)信号频率为29 GHz的强度调制器进行双边带(DSB)调制,再用另一个强度调制器将2.5 Gb/s的OFDM信号调制到DSB信号上。经20 km单模光纤(SMF)传输至基站,通过IL将中心载波和一阶边带分离。经可调谐光滤波器(TOF)滤取所需信道的一阶边带,由高速光电检测器产生58 GHz的电毫米波,利用相干解调恢复下行OFDM基带数据信号。实验结果表明,在无色散补偿和误码率为10-3的条件下,下行OFDM信号经光纤传输20 km后的功率代价小于0.5 dB,而且星座图依然清晰。     

基于压扩变换的60GHz正交频分复用光载无线通信系统实验研究

提出并实验研究了在60GHz正交频分复用(OFDM)-光载无线通信(ROF)系统中采用压扩变换技术降低OFDM信号峰值平均功率比(PAPR),并提高系统的接收灵敏度。实验结果表明,携带基于压扩变换的2.5Gb/s60GHz的OFDM-ROF信号经标准单模光纤传输50km后,在无色散补偿和误码率为10-3的条件下,当入纤功率为12dBm时,系统接收灵敏度提高约2.3dB,并且当互补累积分布函数(CCDF)为10-4时,OFDM信号的PAPR下降约3dB。     

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。     

偏分复用系统中偏振模色散补偿与偏分解复用一体化方案

本文建立了偏分复用系统中偏振模色散与信号偏振态变化引起信道串扰的数学模型, 分析了偏振模色散对偏分复用信道射频功率的影响, 并提出了适用于偏分复用系统的光域偏振模色散补偿与偏分解复用同时进行的方案: 用信道的射频功率作为反馈控制信号, 监测链路中偏振模色散和偏振态变化引起的信道串扰的大小, 用改进的粒子群优化算法对偏振控制器进行自适应控制, 同时完成偏振模色散补偿与偏分解复用. 在112 Gb/s偏分复用-差分正交相移键控(PDM-DQPSK)传输系统中仿真验证了该方案的有效性. 结果表明该方案可以使112 Gb/s-PDM-DQPSK传输系统完成自适应偏分解复用的同时, 在1 dB的光信噪比代价下, 使系统对偏振模色散的容忍度提高20 ps. 关键词： 偏分复用系统 信道串扰 偏振模色散 偏分解复用     

Pilot-based cross-phase modulation compensation for coherent optical orthogonal frequency division multiplexing long-haul optical communications systems

A pilot-based nonlinearity compensator (PB-NLC) is shown in this Letter to be an effective method for compensating cross-phase modulation (XPM) in coherent optical orthogonal frequency division multiplexing (CO-OFDM) systems. An unmodulated pilot tone is transmitted at the center of each OFDM channel to detect phase errors caused by the Kerr effect, which converts intensity fluctuations from all channels to phase errors. The pilots are then used to cancel the XPM phase errors for each OFDM channel at the receiver after each channel's self-phase modulation (SPM) has been compensated, using its intensity waveform to determine its SPM. Numerical simulations of a 58 Gb/s single polarization 2375 km system with inline dispersion compensation show that the signal quality, Q, at the optimal launch power is increased by 2.4 dB if SPM compensation is used before the PB-NLC. This contrasts with only a 0.9 dB improvement if the PB-NLC is used without an SPM compensator for the same link. This shows the PB-NLC can effectively mitigate XPM but not SPM.     

Fiber nonlinearity mitigation by PAPR reduction in coherent optical OFDM systems via biased clipping OFDM

A new method incorporating biased clipping orthogonal frequency division multiplexing (OFDM) is presented, which mitigates fiber nonlinear effects in a long-haul coherent optical OFDM (CO-OFDM) system. Under the scheme of the method, the wanted signal carried by odd subcarriers is orthogonal to clipping noise and a Mach-Zehnder modulator (MZM) performs the optimal OFDM signal up-converter from the radio frequency (RF) domain to the optical domain. Analysis and simulation results show that fiber nonlinear effects can be effectively mitigated by reducing the peak-to-average power ratio (PAPR) in biased clipping CO-OFDM system. The nonlinearity threshold (NLT) is improved by 5 dB with a reach of 240 km. With a fiber length up to 800 km, system Q value is improved by approximately 2.3, 1.2, and 0.6 dB at a chromatic dispersion of 6, 12, and 16 ps/(nm·km), respectively. Additionally, system Q reaches the maximum when direct currect (DC) bias is equal to the mean value of the OFDM waveform.     

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.     

Performance comparison between digital back propagation (DBP) and pilot-aided method for fiber nonlinearity compensation in different fiber links

We numerically investigate and compare the performance of fiber nonlinearity compensation using digital back propagation (DBP) method and pilot-aided method in coherent optical transmission systems using different fiber links. Simulations for wavelength division multiplexed (WDM) 112 Gb/s polarization division multiplexed quadrature phase shift keying (PDM-QPSK) systems with dispersion unmanaged (no DM) and dispersion managed (DM) fiber links are implemented. System Q-factor and maximum transmission distance at bit error rate (BER) of 3.8 ÿ 10^?3 are calculated for performance comparison. The results show that, for system with no DM fiber link, DBP method outperforms pilot-aided method, because DBP method has better performance for intra-channel fiber nonlinearity compensation. However, for system with DM fiber link where inter-channel fiber nonlinearity plays an important role, pilot-aided method performs better than DBP method, because of its ability for inter-channel fiber nonlinearity compensation.     

Normalized LMS digital filter for chromatic dispersion equalization in 112-Gbit/s PDM-QPSK coherent optical transmission system

High bit rates optical communication systems pose the challenge of their tolerance to linear and non-linear fiber impairments. Coherent optical receivers using digital signal processing techniques can mitigate the fiber impairments in the optical transmission system, including the chromatic dispersion equalization with digital filters. In this paper, an adaptive finite impulse response filter employing normalized least mean square algorithm is developed for compensating the chromatic dispersion in a 112-Gbit/s polarization division multiplexed quadrature phase shift keying coherent communication system, which is established in the VPI simulation platform. The principle of the adaptive normalized least mean square algorithm for signal equalization is analyzed theoretically, and at the meanwhile, the taps number and the tap weights in the adaptive finite impulse response filter for compensating a certain fiber chromatic dispersion are also investigated by numerical simulation. The chromatic dispersion compensation performance of the adaptive filter is analyzed by evaluating the behavior of the bit-error-rate versus the optical signal-to-noise ratio, and the compensation results are also compared with other present digital filters.     

Investigation of Hybrid Optical Amplifiers for Dense Wavelength Division Multiplexed System with Reduced Spacings at Higher Bit Rates

Abstract

This article proposes various combinations of optical amplifiers for a dense wavelength division multiplexed system and investigates the impact of reduced channel spacing at high bit rates in terms of quality factor, bit error rate, eye closure, and output power. It is reported that the hybrid optical amplifier (Raman–erbium-doped fiber amplifier [EDFA]) provides better results with a maximum covered single span distance (220 km) at channel spacing of 6.25 GHz. The maximum acceptable bit rate for the 12.5-GHz channel spacing dense wavelength division multiplexed system is also investigated, and the recommendation is provided that for the Raman–EDFA, Raman–EDFA–Raman, EDFA–Raman–EDFA, and EDFA–semiconductor optical amplifier–EDFA, the operating bit rate should not be greater than 20, 16, 19, and 20 Gbps, respectively.     

Coherent optical OFDM scheme with inter-carrier interference self-cancellation and common phase error compensation

Schemes integrating inter-carrier interference （ICI） self-cancellation and common phase error （CPE） com- pensation for coherent optical orthogonal frequency division multiplexing （CO-OFDM） systems are investi- gated. The purpose of our research is to counteract the impacts of laser phase noise and fiber nonlinearity. We propose two ICI self-cancellation-based CO-OFDM schemes, and adopt a pilot-aided decision feedback （DFB） loop for CPE compensation. The proposed schemes are compared with conventional CO-OFDM schemes at the same spectral efficiency. Simulations show that our schemes can not only enhance laser linewidth tolerance of the CO-OFDM system, but also present strong robustness against fiber nonlinearity.     

Phase pre-emphasis for PAPR reduction in optical OFDM systems based on OIDFT or time lens

Phase pre-emphasis is theoretically studied and introduced to reduce peak-to-average power ratio （PAPR） in optical orthogonal frequency division multiplexing （OFDM） systems. In intensity modulated （IM） systems, simulations show noticeable PAPR reductions： 4.14 dB （N = 16） and 15.48 dB （N = 512） in time lens-based OFDM, N is the number of subcarriers. An equation is developed to calculate phase values and is proved to be effective. Optical implementing methods are proposed and analyzed. In a time lens-based OFDM system, phase pre-emphasis reduces fiber nonlinearity and results in a 5.2-dB increase of launch power at the bit error rate （BER） of 10 ？6 . Simulations also show similar PAPR reduction and fiber nonlinearity mitigation in optical inverse discrete Fourier transformer （OIDFT） based OFDM systems.     

Enhanced performance for 10 Gb/s long reach RSOA based WDM-PON by using power pre-emphasized OFDM signal

An extended reach 10 Gb/s wavelength division multiplexing passive optical networks (WDM-PONs) system based on reflective semiconductor optical amplifier (RSOA) is proposed by using power pre-emphasized orthogonal frequency division multiplexing (OFDM) signal. Experimental results show that the proposed technique can effectively enhance the system performance against the limited bandwidth and chirp induced fading effect from direct modulation of RSOA. The receiver sensitivity is improved by 5 dB at the limit of BER for forward error correction (FEC) code over the 60 km and 85 km fiber transmission without any dispersion compensation module.     

Spectral efficient hybrid wireless optical broadband access network (WOBAN) based on transmission of wireless MIMO OFDM signals over WDM PON

In this paper, a spectral efficient hybrid wireless optical broadband access network (WOBAN) is proposed and demonstrated based on the transmission of wireless multi-input multi-output orthogonal frequency division multiplexing (MIMO OFDM) signals over wavelength division multiplexing passive optical network (WDM PON). By using radio over fiber (ROF) techniques, the optical fiber is well adapted to propagate multiple wireless services having different carrier frequencies. It is a known fact that multiple wireless signals having the same carrier frequency cannot propagate over a single optical fiber at the same time, such as MIMO signals feeding multiple antennas in fiber wireless (FiWi) system. A novel optical single-sideband frequency translation technique is designed and simulated to solve this problem. This technique allows four pairs of wireless MIMO OFDM signals with the same carrier frequency for each pair to be transmitted over a single optical fiber by using one optical source per wavelength. The crosstalk between the different MIMO channels with the same frequency is eliminated, since each channel is upconverted on specified wavelength with enough channel spacing between them. Also the maximum crosstalk level between the different MIMO channels with different frequencies is very low around ?76 dB. The physical layer performance of the proposed WOBAN is analyzed in terms of the bit error rate (BER), error vector magnitude (EVM), and signal-to-noise ratio (SNR). The proposed WOBAN achieves 7.68 Gb/s data rate for 20 km for the optical back-end and 240 Mb/s for the outdoor wireless front-end.