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interesting for increasing the transmitted capacity, it suffers from decreased PMD tolerance
(Nelson et al., 2000; 2001) and increased polarization induced cross-talk (X-Pol), due to the
polarization-sensitive detection (Noe et al., 2001) used to separate the POLMUX channels.
Previous investigations on DBP demonstrate the results for the WDM channels having the
same polarization and solving the scaler NLSE equation is adequate. In (Yaman et al.,
2009) it is depicted that the same principles can be applied to compensate fiber transmission
impairments by using DBP but a much more advanced form of NLSE should be used which
includes two orthogonal polarization states (
E
x
and
E
y
), i.e. Manakov equation. Polarization
mode dispersion (PMD) is considered negligible during investigation. In this article the results
depict that back-to-back performance for the central channel corresponds to a Q value of
20.6 dB. When only dispersion compensation is applied it results in a Q value of 3.9 dB. The
eye-diagram is severely degraded and clearly dispersion is not the only source of impairment.
Whereas, when DBP algorithm is applied the system observed a Q value of 12.6 dB. The results
clearly shows efficient compensation of CD and NL by using the DBP algorithm. In (Mussolin
et al., 2010; Rafique et al., 2011b) 100Gbit/s dual-polarization (DP) transmission systems are
investigated with advanced modulation formats i.e. QPSK and QAM.
Another modification in recent times in conventional DBP algorithm is the optimization of
non-linear operator calculation point (
r
). It is demonstrated that DBP in a single-channel
transmission (Du et al., 2010; Lin et al., 2010b) can be improved by using modified
split-step Fourier method (M-SSFM). Modification is done by shifting the non-linear operator
calculation point
Nlpt
(
r
) along with the optimization of dispersion
D
and non-linear
coefficient
to get the optimized system performance (see section 3.2.2 of this chapter). The
modification in this non-linear operator calculation point is necessary due to the fact that
non-linearities behave differently for diverse parameters of transmission, i.e. signal input
launch power and modulation formats, and hence also due to precise estimation of non-linear
phase shift
γ
φ
NL
from span to span. The concept of filtered DBP (F-DBP) (Du et al., 2010) is also
presented along with the optimization of non-linear point (see section 3.2.3 of this chapter).
The system performance is improved through F-DBP by using a digital low-pass-filter (LPF)
in each DBP step to limit the bandwidth of the compensating waveform. In this way we can
optimize the compensation of low frequency intensity fluctuations without overcompensating
for the high frequency intensity fluctuations. In (Du et al., 2010) the results depict that with
four backward propagation steps operating at the same sampling rate as that required for
linear equalizers, the Q at the optimal launch power was improved by 2 dB and 1.6 dB for
single wavelength CO-OFDM and CO-QPSK systems, respectively, in a 3200 km (40x80km)
single-mode fiber link, with no optical dispersion compensation.
Recent investigations (Ip et al., 2010; Rafique et al., 2011b) show the promising impact of DBP
on OFDM transmission and higher order modulation formats, up to 256-QAM. However
actual implementation of the DBP algorithm is now-a-days extremely challenging due to
its complexity. The performance is mainly dependent on the computational step-size (
h
)
(Poggiolini et al., 2011; Yamazaki et al., 2011) for WDM and higher baud-rate transmissions.
In order to reduce the computational efforts of the algorithm by increasing the step-size (i.e.
reducing the number of DBP calculation steps per fiber span), ultra-low-loss-fiber (ULF)
is used (Pardo et al., 2011) and a promising method called correlated DBP (CBP) (Li et
al., 2011; Rafique et al., 2011c) has been introduced (see section 4.1 of this chapter). This
method takes into account the correlation between adjacent symbols at a given instant using
a weighted-average approach, and an optimization of the position of non-linear compensator
