Challenges to coherent optical
communication systems and their
mitigation techniques –A Review
MONIKA MEHRA*
Ph.D. Research Scholar,
I.K.Gujral Punjab Technical University, Jalandhar, Punjab, India [email protected]
Dr. HARSH SADAWARTI
Campus Director ,CT Group Of Institutions ,Jalandhar, India [email protected]
Dr.M.L.SINGH
Professor, Department of Electronics Technology, Guru Nanak Dev University, Amritsar, India
Abstract: Optical communication systems often arise as the most efficient solution to transmit at high data rates and also over long distances. Recent trends in optical communication are focusing on spectrally efficient systems in order to meet the demands for bandwidth hungry applications. To improve spectral efficiency , advanced modulation techniques with digital coherent detection has been extensively explored by researchers. Advanced modulation formats requires increased SNR and leads to greater transmission impairments which decreases the distance over which the increased capacity can be provided. Higher spectral efficiency also requires tightly spaced wavelength-division multiplexed (WDM) channels to optimize the operational bandwidth of optical amplifiers. Therefore, the mitigation or compensation of fiber impairments is important to increase transmission capacity. In this paper, we review the different challenges that limit the performance of coherent optical communication systems and different mitigation techniques to increase capacity of these systems.
Keywords: Advanced modulation formats, Dual polarization coherent systems, Manakov equation, nonlinear impairments
1.Introduction
2. Advanced modulation formats
For direct detection systems , modulation formats based on binary signals such as On-Off Keying (OOK) , differential BPSK or differential QPSK were dominating due to its easy generation and detection with low complexity. With the growing demand of spectral efficiency[9], alternative modulation formats were explored by the researchers where information can be transmitted by modulating the phase of the optical carrier such as phase shift keying(PSK) and quadrature amplitude modulation(QAM) and spectral efficiency can be increased to 1bit/s/Hz/polarization . As the bandwidth supported by the EDFA (C-band ≈ 1530–1565 nm ) was exhausted, research on efficient utilization of the available spectrum was initiated[10]. With the revival of coherent detection systems in which both the amplitude and phase information can be easily accessed , the research on advanced modulation formats gained momentum as the phase tracking was performed in the digital domain which reduce the use of complicated phase locked loops in differential detection systems. With each symbol encoding m bits, advanced modulation formats can achieve SE up to m bits/s/Hz/polarization. Commonly used advanced modulation formats in coherent communication systems are QPSK, 8-PSK, 8-QAM , 16-QAM and 64-QAM. As coherent detection systems have the ability for polarization demultiplexing of dual polarization signals in the electrical domain by high-speed DSP, dual polarized modulation formats such as DP-QPSK and DP-mQAM can further improve the spectral efficiency. But increase in spectral efficiency appears at the expense of decreased tolerance towards linear and nonlinear fiber impairments. As compensation of linear fiber impairments is possible with DSP, the research on compensation of nonlinear fiber impairments is intensified.
3. Dual polarization Coherent optical system
Figure1: Block diagram of coherent optical system[11]
Block diagram of dual polarization coherent optical system is shown in the figure1 [11] .Coherent transmitter up-converts the baseband signal to the optical field. In the transmitter, signal from continuous wave laser is split into two polarized signals .It consists of I/Q modulator having two Mach Zehnder modulators (MZMs) and a 900 phase shifter where MZMs modulate the in phase (I)and quadrature(Q) components of the each polarized
signal on to the optical carrier with 900phase shift for orthogonality between I and Q components.These two
4.Transmission impairments
Fig2: Nonlinear impairments in fiber optic system[1]
When an information-carrying optical signal transmits along the fiber link in coherent optical systems , it gets attenuated by the fiber loss and phase, amplitude and polarization of the signal are altered mainly due to the linear and nonlinear transmission impairments .The important linear impairments are amplified spontaneous emission (ASE) noise, chromatic dispersion(CD), polarization mode dispersion(PMD. Nonlinear transmission impairments arise due to Kerr nonlinearity which imposes a fundamental limit on channel capacity. Fiber nonlinear impairments results in severe degradations on single channel and WDM coherent optical systems. For WDM transmission systems, fiber nonlinear impairments consist of inter-channel impairments and intra-channel impairments. Inter-channel nonlinear effects refer to the interference between different wavelength channels, which include the cross-phase modulation and the four-wave mixing. Intra-channel nonlinear effects indicate the interference between different modules in the same wavelength channel, which include self-phase modulation, intra-channel XPM and intra-channel FWM. Inter-channel nonlinearities are dominant for lower bit-rate transmission systems, and intra-channel nonlinearities are dominant for higher bit-rate transmission systems. Figure 2 shows the fiber nonlinearities in the fiber optical systems [1]
4.1 Signal propagation in a fiber
For single polarization optical signals, the nonlinear Schrödinger equation (NLSE) describes the deterministic transmission effects of optical fiber propagation but for dual polarization signals, the propagation of light is modeled using the Manakov equation in an optical fiber[12]
| | (1)
where A= , is the complex envelope of the two polarization components of the optical field, is the electric field component of a signal propagating in one polarization, is the electric field of the component propagating in the orthogonal polarization, α is the loss coefficient, γ is the nonlinear coefficient , is group velocity dispersion coefficient, z is distance of propagation.Chromatic dispersion effect is measured by dispersion parameter D where . In the above Manakov equation the term with the nonlinear parameter γ represents the nonlinear effects due to Kerr nonlinearity which arises due to power dependent refractive index of fiber and describes intra channel nonlinear effects .
For WDM system with two channels a and b having non over lapping spectrum “Eq. (1)” can be written as
| | | | (2)
Where ∗, ∗ is the transpose conjugate of A. In WDM systems, signal in the required channel
5. Compensation Techniques
Various methods of compensating fiber transmission impairments have been proposed, both in optical and electronic domain. Recent advances of DSP algorithms in coherent systems have enabled the digital compensation of various transmission impairments. Digital Back-propagation (DBP) algorithm has become a prominent nonlinear impairment compensation method in coherent optical systems .It is implemented by using split-Step Fourier method. Polarization division multiplexing is used in coherent system to increase the spectral efficiency. Fiber nonlinearities in Dual Polarized coherent systems can be compensated by back propagation method used in Manakov equation. The interaction of dispersion and fiber nonlinear effects in optical systems limit the achievable capacity and transmission distance. Digital back propagation (DBP) has a ability to compensate linear as well as nonlinear effects of optical fiber by converting the signal into digital signal and passing it through virtual fiber having dispersion , loss and nonlinear coefficient with same magnitude but opposite sign as those of optical fiber. Roberts et al. in 2006 [13] proposed nonlinear pre compensation using DBP for a single channel optical system .Savory et al in 2007 [14] first studied Electronic backward propagation as a transmitter- based compensation method. Due to better flexibility i.e application of adaptive compensation techniques in post compensation as compared to pre compensation. Li et al. [15] and Ip and Kahn [16] applied DBP in WDM systems using post-compensation ..Mateo and Li [17] in 2009 proposed a split step DBP scheme which was based on an enhanced coupled NLSE for full post compensation of inter channel XPM with partial compensation of FWM. . Mateo et al.[18] in 2010 explored an advanced split-step DBP method using coupled NLSE for inter channel nonlinearities compensation . Du and Lowery in 2010 [19] proposed a filtered DBP scheme to reduce both the required sampling rate and number of steps. Zhu and Li in 2011[20] proposed a folded DBP scheme for periodically dispersion managed optical system. Zhu and Li in 2011 [21] investigated an improved folded DBP that works even when RDSP is large, called dispersion-folded (D-folded) DBP.S. Liu et al. in 2014 [22] verified experimentally dispersion folded DBP for the mitigation of intra-channel and inter-channel nonlinear impairments . Yaman and Li in 2010 [23] investigated the implementation of DBP in PDM transmission systems. Mateo et al. in 2011 [24] reported an improved split step DBP for PDM transmission systems. Recently, a combination of DBP and OPC has been suggested which relaxes both the required DBP accuracy and the OPC placement, and offers a performance benefit. In spite of the high computational complexity, DBP has been proposed as a universal technique1 for jointly compensating for the linear and nonlinear impairments and is often used to benchmark against other detectors .Despite the high effectiveness of DBP based techniques to compensate the fiber nonlinearity, their main drawback comes from the high complexity requirements, due to the large number of iterations, for a practical implementation[25] Several investigations have been done to reduce the complexity requirement by different algorithms[26-28]. Algorithms based on DSP with DBP-SSFM have been investigated for fiber nonlinear impairment compensation. D.Rafique et al in 2011[14] proposed weighted SSFM which reduce the complexity as compared to standard SSFM M. Secondini et al in 2015[29] proposed enhanced SSFM (ESSFM) which , allow the decrease of computational effort over SSFM, with the same performance. To further reduce the complexity, VSNE (Volterra Series Non linear equalizer) based DBP was proposed by F. P. Guiomaret et al in 2013 and 2015 [30-31]. Volterra series is a numerical tool that enables to model ,calculate and compensate linear and nonlinear impairments with good accuracy. It is based on a nth order truncated polynomial expansion of the input-output relationship for a nonlinear system which includes the memory effects and is represented by a series of nonlinear kernel functions where the input output relationship is represented by a transfer function. Volterra series approach, helps to estimate the signal to noise ratio (SNR) of the received constellation with respect to different nonlinearities effect.
6. Conclusions
Coherent optical systems with advanced modulation formats can increase the spectral efficiency as well as overall capacity as the information is not only encoded on the amplitude but also on the polarization state and phase of the optical signal but the use of advanced modulation formats requires higher input power which leads to increased nonlinear effects in the system .Different compensation techniques can be used for compensation of nonlinear impairments . Digital back propagation (DBP) technique has a ability to compensate linear as well as nonlinear effects of optical fiber by converting the signal into digital signal and passing it through virtual fiber having dispersion, loss and nonlinear coefficient with same magnitude but opposite sign as those of optical fiber. The main drawbacks of DBP are the computational complexity due to the large number of iterations, for a practical implementation. Computational complexity of DBP technique can be further reduced by the use of VSNE so we can work in the area to further reduce the computational complexity of DPB by the use of different algorithms .
Acknowledgments
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