Buffer-Aided Relaying Protocols for Cooperative FSO Communications

In this paper, we consider the problem of relay-assisted free space optical (FSO) communications in the case where the relays are equipped with buffers of finite size. The high directivity of the FSO links clearly distinguishes cooperative FSO networks from their radio frequency (RF) counterparts thus motivating the design of FSO-specific buffer-aided (BA) cooperative protocols. We propose three novel decode-and-forward relaying protocols that are adapted to the nature of FSO transmissions and are capable of achieving different levels of tradeoff between outage probability, average packet delay and system complexity: 1) the BA selective relaying protocol that can be implemented in the presence of channel state information (CSI) and that outperforms the RF max-link protocol with a reduced delay; 2) the BA all-active relaying protocol that can be implemented in the absence of CSI and constitutes the simplest protocol with the best delay performance at the expense of a degraded outage performance; and 3) the BA load-balanced selective protocol where supplementary FSO communications are triggered along the inter-relay links for a more balanced distribution of the packets among the buffers. While the last protocol incurs the highest signaling complexity, it results in significant performance gains with a delay that is comparable to that of the BA selective protocol. A Markov chain analysis is adopted for evaluating the system outage probability and the average packet delay where the corresponding state transition matrices are derived in the cases of both symmetrical and asymmetrical networks.