  We investigate transmission protocols for relay-assisted free-space optical
(FSO) systems, when multiple parallel relays are employed and there is no
direct link between the source and the destination. As alternatives to
all-active FSO relaying, where all the available relays transmit concurrently,
we propose schemes that select only a single relay to participate in the
communication between the source and the destination in each transmission slot.
This selection is based on the channel state information (CSI) obtained either
from all or from some of the FSO links. Thus, the need for synchronizing the
relays' transmissions is avoided and the slowly varying nature of the
atmospheric channel is exploited. For both relay selection and all-active
relaying, novel closed-form expressions for their outage performance are
derived, assuming the versatile Gamma-Gamma channel model. Furthermore, based
on the derived analytical results, the problem of allocating the optical power
resources to the FSO links is addressed, and optimum and suboptimum solutions
are proposed. Numerical results are provided for equal and non-equal length FSO
links, which illustrate the outage behavior of the considered relaying
protocols and demonstrate the significant performance gains offered by the
proposed power allocation schemes.
