补偿CO-OFDM传输中10.7 Gb/s NRZ-OOK邻道光纤非线性

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"Fiber nonlinearity compensation for CO-OFDM transmission with 10.7-Gb/s NRZ-OOK neighbors" 这篇论文关注的是在光通信领域中的一个问题，特别是关于相干光正交频分复用（CO-OFDM）系统在升级波分复用（WDM）系统时遇到的光纤非线性挑战。由于传统的非归零开关键控（NRZ-OOK）通道的存在，光纤非线性限制了CO-OFDM技术的应用。作者提出了一种新颖的方法来补偿这种非线性效应，即通过结合基于数字信号处理（DSP）的自相位调制（SPM）后补偿和基于导频的交叉相位调制（XPM）补偿。光纤非线性是光通信中的一个主要问题，它会导致信号失真，从而限制了传输距离和数据速率。在本文中，研究者考虑的场景是10.7-Gbps NRZ-OOK邻道的存在，这些邻道会加剧CO-OFDM系统的非线性效应。NRZ-OOK是一种常用的光调制格式，其中“1”和“0”状态通过光功率的有无来表示，这种格式在现有的WDM系统中广泛使用。针对这个问题，论文提出的解决方案是采用一种混合补偿策略。首先，利用DSP进行SPM后补偿，这是针对光纤中的自相位调制效应的一种处理方式，SPM是由光信号自身强度引起的相位变化。通过DSP，可以分析并校正由SPM导致的信号失真。然后，结合使用导频（pilot tone）辅助的XPM补偿，XPM是非线性效应中的一种，它发生在两个或多个信号在光纤中同时传输时，一个信号的相位会受到另一个信号的影响。导频在这里起到了参考信号的作用，帮助识别和校正由XPM引起的相位变化。论文通过模拟研究验证了这种方法的有效性，尽管具体的结果和性能提升未在摘要中详述。不过，可以推测，这种补偿策略提高了CO-OFDM系统与NRZ-OOK邻道共存时的性能，可能包括增加了传输距离、改善了误码率性能以及增强了系统的总体稳定性。总结来说，这篇研究论文提出了一个创新的补偿方法，以应对CO-OFDM系统与NRZ-OOK邻道共同工作时的光纤非线性问题，通过结合使用DSP和导频技术，有望提升现有WDM系统的升级潜力和效率。

COL 10(11), 110602(2012) CHINESE OPTICS LETTERS November 10, 2012

Fiber nonlinearity compensation for CO-OFDM

transmission with 10.7-Gb/s NRZ-OOK neighbors

Mingliang Deng (

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, Xingwen Yi (

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), Jing Zhang (

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Hongbo Zhang (

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Key Laboratory of Optical Fiber Sensing and Communication Networks, Ministry of Education,

University of Electronic Science and Technology of China, Chengdu 611731, China

∗

Corresponding author: dengmingliang9@163.com

Received April 18, 2012; accepted June 20, 2012; posted online September 28, 2012

Fiber nonlinearity limits the use of coherent optical orthogonal frequency division multiplexing (CO-

OFDM) to upgrade wavelength-division multiplexing (WDM) systems using legacy non-return-to-zero-on-

off-keying (NRZ-OOK) channels. This letter proposes to compensate for the fiber nonlinearity of CO-

OFDM with NRZ-OOK neighbors by combining digital signal processing (DSP)-based self-phase modula-

tion (SPM) post-compensation with pilot-tone-based cross-phase modulation (XPM) compensation. The

simulation results demonstrate that the optimum low-pass filter bandwidth for pilot-tone-based XPM com-

pensation depends on the pilot-to-signal ratio value and launch optical power. Our method allows a 4-dB

increase in the launch power for a 40-Gb/s single polarization CO-OFDM channel placed in the middle of

six 10.7-Gb/s NRZ channels in a 50-GHz space and 1 200-km WDM system.

OCIS codes: 060.1660, 060.4370, 060.4080, 060.2330.

doi: 10.3788/COL201210.110602.

Coherent optical orthogonal frequency division multi-

plexing (CO-OFDM) is immune to linear impairments

in optica l fiber transmissions, such as chromatic disp e r-

sion (CD) and polarization mode dispersion (PMD)

[1,2]

However, fib er nonlinearities for long-haul CO-OFDM

transmission systems restrict the optical power launched

into e ach fiber span and the tr ansmission distance

[3,4]

Thus, several studies have been devoted to the nonlin-

earity compe nsation for CO-O FDM systems

[5−7]

In legacy waveleng th-divis ion multiplexing (WDM) op-

tical networks, no n-return-to-zero-on-off-keying (NRZ-

OOK) is the dominant modulation format, and the pe-

riodic dispersion map is used to compensate for CD.

In these optical networks, CO-OFDM can be used to

upgrade a few wavelengths. The mixed modulation for-

mats can then coexist in one optical link. However, the

fiber nonlinea rity p e rformance of CO-O FDM is poor in

such networks due to self-phase modulation (SPM)

[8]

and

cross-pha se modulation (XPM) from the NRZ neighbors

operated at 10.7 Gb/s

[9]

. Therefore, fiber nonlinearity

compensation for the CO-OFDM channel is useful. Us-

ing the phase modulation proportional to the bandwidth-

limited received intensity o n all channels to cancel cross-

phase modula tio n XPM caused by NRZ neighbor s was

proposed in Ref. [10], and the nonlinearity threshold was

improved to about 2 dB.

In this letter, the effectiveness of digital signal pro-

cessing (DSP)-base d SPM post-compensation and pilot-

tone (PT)-based XPM compensation is demonstrated to

mitigate the fiber nonlinear impairments of CO-OFDM

transmission w ith 10.7-Gb/s NRZ neighbors in the con-

ventional WDM system. The optimum low-pass filter

(LPF) bandwidth for PT-based XPM compensation was

also s hown to depend on the pilot-to-signal ratio (PSR)

at different launch optical powers, which is different from

the PT-aided phase noise compensation in Ref. [2]. In a

numerical simulation of a 50-GHz spacing and 1 200-km

WDM tr ansmission with inline dispersion compensation,

the optimum launch power for a 40-Gb/s CO-OFDM

channel placed in the middle of six 10.7-Gb/s NRZ chan-

nels is increased by 4 dB, and the Q-factor can also be

improved by 1.5 dB at the optimal operational power.

Meanwhile, no penalty was found in the amplified si-

multaneous emission (ASE) limited linear region using

optimum PSR values and LPF bandwidths for PT ex-

traction.

The fiber nonlinear impairments of the CO-OFDM

channel with NRZ neighbors predominantly consist of

SPM caused by intensity ﬂuctuation of the CO-OFDM

channel and the XPM induced by NRZ neighbors. SPM

can be partially removed by imposing a phase modulation

proportional to the instantaneous received CO-OFDM

channel power before the fast Fourier transform (FFT)

in the OFDM receiver

[11]

. This phase modulation c an

be expressed as S

(t) = S

(t)e

−jβ|S

(t)|

, where S

(t)

is the SPM mitigated CO-OFDM signal, S

(t) is the

time domain signal o f the CO-OFDM channel at the

receiver, and β is a phenomenological nonlinear factor

that can be estimated without the fiber link information.

The optimal β can b e estimated, for instance, based on

the maximum Q- factor of the received constellation or

the minimum rece ived bit error r ate (BER). Due to the

CD-caused walk-off of the subcarriers in OFDM signa l,

the intensity ﬂuctuations of the highest frequency sub-

carriers have a minimal contribution to the nonlinear

degradation

[12]

. Thus, a digital L PF is introduced to

restrict the ba ndwidth of the compensation signal. The

block diagram and algo rithm of SPM compensation are

depicted in Fig. 1.

PT-based nonlinearity comp e ns ation is similar to the

PT-aided phase noise compensation in Ref. [2]. This

compensation was proposed for the case o f fiber non-

linearities in CO-OFDM WDM systems in Refs. [13–

15]. The PT-based nonlinearity compensation is only

1671-7694/2012/110602(4) 110602-1

 2012 Chinese Optics Letters

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