  In this paper, we propose a comprehensive research over triple hop
all-optical relaying free-space optical (FSO) systems in the presence of all
main noise sources including background, thermal and amplified spontaneous
emission (ASE) noise and by considering the effect of the optical degree-of-
freedom (DoF). Using full CSI relaying, we derive the exact expressions for the
noise variance at the destination. Then, in order to simplify the analytical
expressions of full CSI relaying, we also propose and investigate the validity
of different approximations over noise variance at the destination. Finally, we
evaluate the the performance of considered triple-hop all-optical relaying FSO
system in term of ergodic capacity.
