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followed by a non-linear operator ( N ) (see section 3.2.1 of this chapter). In this investigation
the results depict that the efficient performance of DBP algorithm can be obtained if there is
no dispersion compensating fiber (DCF) in the transmission link. This is due to the fact that
in the fully compensated post-compensation link the pulse shape is restored completely at the
input of the transmission fiber in each span. This reduces the system efficiency due to the
maximized accumulation of non-linearities and the high signal-ASE (amplified spontaneous
emission) interaction leading to non-linear phase noise (NLPN). So it is beneficial to fully
compensate dispersion digitally at the receiver by DBP. The second observation in this article
is about the oversampling rate which improves system performance by DBP.
A number of investigations with diverse transmission configurations have been done with
coherent detection and split-step Fourier method (SSFM) (Asif et al., 2010; Mateo et al., 2011;
Millar et al., 2010; Mussolin et al., 2010; Rafique et al., 2011a; Yaman et al., 2009). The results
in these articles shows efficient mitigation of CD and NL. In (Asif et al., 2010) the performance
of DBP is investigated for heterogeneous type transmission links which contain mixed spans
of single mode fiber (SMF) and non-zero dispersion shifted fiber (NZDSF). The continuous
growth of the next generation optical networks are expected to render telecommunication
networks particularly heterogeneous in terms of fiber types. Efficient compensation of fiber
transmission impairments is shown with different span configurations as well as with diverse
dispersion mapping.
All the high capacity systems are realized with wavelength-division-multiplexed (WDM) to
transmit multiple-channels on a single fiber with high spectral efficiency. The performance
in these systems are limited by the inter-channel non-linearities (XPM,FWM) due to the
interaction of neighbouring channels. The performance of DBP is evaluated for WDM systems
in several articles (Gavioli et al., 2010; Li et al., 2008; Poggiolini et al., 2011; Savory et al.,
2010). In (Savory et al., 2010) 112Gbit/s DP-QPSK transmission system is examined and
investigations demonstrate that the non-linear compensation algorithm can increase the reach
by 23% in a 100GHz spacing WDM link compared to 46% for the single-channel case. When
the channel spacing is reduced to 50GHz, the reach improvement is minimal due to the
uncompensated inter-channel non-linearities. Whereas, in (Gavioli et al., 2010; Poggiolini et
al., 2011) the same-capacity and bandwidth-efficiency performance of DBP is demonstrated in
a ultra-narrow-spaced 10 channel 1.12Tbit/s D-WDM long haul transmission. Investigations
show that optimum system performance using DBP is obtained by using 2, 4 and 8 steps per
fiber span for 14GBaud, 28GBuad and 56GBaud respectively. To overcome the limitations by
inter-channel non-linearities on the performance of DBP (Mateo et al., 2010; 2011) proposed
improved DBP method for WDM systems. This modification is based on including the effect
of inter-channel walk-off in the non-linear step of SSFM. The algorithm is investigated in a
100Gbit/s per channel 16QAM transmission over 1000km of NZDSF type fiber. The results are
compared for 12, 24 and 36 channels spaced at 50GHz to evaluate the impact of channel count
on the DBP algorithm. While self-phase modulation (SPM) compensation is not sufficient in
DWDM systems, XPM compensation is able to increase the transmission reach by a factor
of 2.5 by using this DBP method. The results depicts efficient compensation of cross-phase
modulation (XPM) and the performance of DBP is improved for WDM systems.
Polarization multiplexing (POLMUX) (Evangelides et al., 1992; Iwatsuki et al., 1993) opens
a total new era in optical communication systems (Fludger et al., 2008) which doubles
the
capacity
of a wavelength channel
and the spectral efficiency by
transmitting
two
signals via orthogonal states of polarization (SOPs).
Although POLMUX is considered
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