  Hybrid radio frequency (RF)/free space optical (FSO) systems are among the
candidate enabling technologies for the next generation of wireless networks
since they benefit from both the high data rates of the FSO subsystem and the
high reliability of the RF subsystem. In this paper, we focus on the problem of
throughput maximization in the parallel hybrid RF/FSO relay channel. In the
parallel hybrid RF/FSO relay channel, a source node sends its data to a
destination node with the help of multiple relay nodes. Thereby, for a given
relay, the source-relay and the relay-destination FSO links are orthogonal with
respect to each other due to the narrow beam employed for FSO transmission,
whereas, due to the broadcast nature of the RF channel, half-duplex operation
is required for the RF links if self-interference is to be avoided. Moreover,
we consider the two cases where the relays are and are not equipped with
buffers. For both cases, we derive the optimal relay selection policies for the
RF and FSO links and the optimal time allocation policy for transmission and
reception for the RF links. The proposed optimal protocols provide important
insights for optimal system design. Since the optimal buffer-aided (BA) policy
introduces an unbounded end-to-end delay, we also propose a suboptimal BA
policy which ensures certain target average delays. Moreover, we present
distributed implementations for both proposed optimal protocols. Simulation
results demonstrate that a considerable gain can be achieved by the proposed
adaptive protocols in comparison with benchmark schemes from the literature.
