  Inter-core crosstalk is one of the most serious impairments for signal
transmission in a multi-core fiber (MCF) optical network. On the other hand,
because of wide deployment of data centers (DCs), we are seeing an increasing
bidirectional traffic demand asymmetry, which leads to significant capacity
wastage in designing and operating an optical transport network. To alleviate
these effects, for an MCF optical network, we propose to assign fiber cores in
an MCF in an asymmetric and counter-propagating manner. This can not only
significantly reduce inter-core crosstalk between counter-propagating fiber
cores but also flexibly assign different numbers of fiber cores in the opposite
directions of a fiber link, thereby overcoming network capacity wastage due to
the bidirectional traffic demand asymmetry. To evaluate the benefits of the
proposed strategy, we consider the routing, spectrum, and core assignment
(RSCA) problem for the MCF optical network. An integer linear programming (ILP)
model and an auxiliary graph (AG) based heuristic algorithm are developed to
optimize network spectrum resource utilization. Simulation studies show the
effectiveness of the proposed core counter-propagation strategy, which can
significantly outperform its counterpart, i.e., the co-propagation scheme, in
terms of the total number of MCFs used and average inter-core crosstalk. In
addition, the proposed RSCA heuristic algorithm is efficient to perform close
to the ILP model, which can minimize the number of MCFs used and crosstalk
between neighboring cores.
