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insertion occurring at the N2 node, for 10 km and the worst-case scenario of in-band
and 60 km spans, respectively. This occurs
because when the signal is inserted at the N2
node, at the optimal power per channel, the
accumulated ASE noise and NLI powers are
higher.
CROSSTALK
The signal performance in optical networks
can be degraded by optical linear crosstalk,
which existence is mainly due to imperfect
isolation of the components within the nodes,
such as WSSs. The imperfect isolation causes
signal power leakages along the entire network
that impair and degrade a selected signal
along its network path. The selected signal is a
signal centered at a specific wavelength, which
is chosen to assess the impact of crosstalk
along its entire optical path, from its insertion
in the network to its network drop.
When the interfering signals have a different
nominal wavelength from the selected signal,
it corresponds to out-of-band crosstalk, which
can be removed by filtering at the receiver. In-
band crosstalk occurs when different optical
sources produce interfering signals with the
same nominal wavelength as the selected
signal, which can not be removed after
interfering. Before evaluating the impact of
crosstalk in a network, it is necessary to
characterize the number of interfering terms
and the crosstalk level of each interfering term.
The crosstalk is considered first order when
an optical signal overcomes the isolation of
one optical component. When it overcomes the
isolation of two optical components, the
crosstalk is considered second-order, and its
contribution to the performance degradation is
lower. Typically, second-order interfering terms
are neglected in a crosstalk analysis. The
number of interfering terms depends on the
ROADM or FD&W node degree. As the
horseshoe topology is considered, the nodes
N1 are 2-degree, the nodes N2 are 5-degree,
crosstalk is considered when the wavelength
corresponding to the selected signal appears
in all possible inputs and add ports of the
network nodes. It is also necessary to take into
account that the nodes N2 are 5-degree CD
ROADM R&S nodes, and therefore the number
of interferers will be different from those
arising in the nodes N1. As the nodes N2 are
integrated into a metro-core network whose
physical topology is meshed, they lead to four
second-order interfering terms at the output of
node N2.
Concerning the N1 FD&W ChD node
architecture, each wavelength is only used
once in an optical path, and, therefore, there is
no frequency reuse. Hence, the wavelength
being added to a network node is always
different from the wavelength dropped in that
same node. When a signal is added, there is no
in-band crosstalk at node N1. Only
wavelengths that are expressed reach the
output stage, as well as the added signals that
have a different wavelength from the
expressed signals. Thus, in networks with
horseshoe physical topology with N1 FD&W
ChD node architectures, the selected signal is
only impaired by in-band crosstalk introduced
by the last node N2.
In Table 2, the number of interfering terms is
presented, considering the two-node N1
architectures CD ROADM B&S and FD&W ChD,
per node and for the total network, which has 9
nodes N1. There are considerably more
interfering terms in the CD ROADM B&S case
than in the FD&W ChD case because, in the last
architecture, only the node N2 contributes to
the total number of interfering terms in the
network. The 13 interfering terms associated
with networks with N1 CD ROADM B&S nodes
result from the accumulation of in-band
crosstalk along with nine nodes N1 and the last
node N2.
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