  The free-space optical (FSO) communication links with orbital angular
momentum (OAM) multiplexing have been demonstrated that they can largely
enhance the systems' capacity without a corresponding increase in spectral
bandwidth, but the performance of the system is unavoidably disturbed by
atmospheric turbulence (AT). Different from the existed AT disturbance, the
OAM-multiplexed systems will cause both the burst and random errors for a
single OAM state carrier and the `crosstalk' interference between the different
OAM states carriers. In this paper, we propose a turbulence mitigation method
to improve AT tolerance of OAM-multiplexed FSO communication links. In the
proposed scheme, we use channel codes to correct the burst and random errors
caused by AT for a single OAM state carrier; And we use wavefront correction
method to correct the `crosstalk' interference between the different OAM states
carriers. The improvements of AT tolerance are discussed by comparing the
performance of OAM-multiplexed FSO communication links with or without channel
coding or Shark-Hartmann wavefront correction method. The numerical simulation
results show that the OAM-multiplexed FSO communication links have enhanced
their AT tolerance. The usage of channel codes and wavefront correction methods
together has improved greatly the performance of OAM-multiplexed FSO
communication links over atmospheric turbulence.
