  Mode-division multiplexing (MDM) is seen as a possible solution to satisfy
the rising capacity demands of optical communication networks. To make MDM a
success, fibers supporting the propagation of a huge number of modes are of
interest. Many of the system aspects occurring during the propagation can be
evaluated by using appropriate models. However, fibers are a nonlinear medium
and, therefore, numerical simulations are required. For a large number of
modes, the simulation of the nonlinear signal propagation leads to new
challenges, for example regarding the required memory, which we address with an
implementation incorporating multiple GPU-accelerators. Within this paper, we
evaluate two different approaches to realize the communication between the GPUs
and analyze the performance for simulations involving up to 8 Tesla GPUs. We
show results for a MDM transmission system utilizing the extremely large but
practically very relevant number of 120 spatial modes as an application example
and analyze the impact of the nonlinear effects on the transmitted signals.
