Cooperative FSO Systems: Performance Analysis and Optimal Power Allocation

Abstract

Abstract-In this paper, we investigate the cooperative diversity technique as a candidate solution for combating turbulence-induced fading over free-space optical (FSO) links. In particular, we propose a novel cooperation strategy that is suitable for quantum-limited FSO systems with any number of relays and we derive closed-form expressions for the error performance of this strategy. In scenarios where the channel-state-information (CSI) is available at the different nodes, we propose an optimal power allocation strategy that satisfies the Karush-Kuhn-Tucker (KKT) conditions and that further boosts the performance of FSO networks. It turned out that this closed-form optimal solution corresponds to transmitting the entire optical power along the "strongest link" between the source and the destination nodes. A simple procedure is proposed for selecting this link and for distributing the power among its different hops.

Performance Analysis of FSO Communications With Diversity Methods: Add More Relays or More Apertures?

“…Note that the optimal power allocation strategy (PAS) in the case of parallel-relaying corresponds to selecting the "best" S-R-D link and optimally allocating the power among the two hops of this link [16]. Consequently, the difference between the optimal PAS and the considered selective-relaying scheme resides in adapting the values of r 0,n and r n,Nr +1 to the strengths of the two hops rather than setting r 0,n = r n,Nr+1 = 1 2 as in (54).…”

“…It is worth noting that this diversity gain is associated with a reduced system complexity where adding apertures to the transmitter or to the receiver is much more cost-effective than deploying relays thus proving the inadequacy of this last approach to FSO systems. On the other hand, if the relays are independent entities that have their own information to exchange with S and D as in [5]- [11], [16], [17], then taking advantage of the presence of these relays (that can not be removed since they are not deployed solely for assisting S) can not but improve the reliability of the communications between S and D. Other contributions include deriving the diversity gains of MIMO systems with RC and TLS as well as the coding gain of TLS compared to RC which constitute novel findings as well. We also study the impact of aperture averaging on the derived results.…”

Optical Wireless Scattering Channel Estimation for Photon-Counting and Photomultiplier Tube Receivers

“…In a nutshell, those contributions address either the transmission capacity limits or the practical schemes that are capable of approaching those limits. Recently, a range of advanced transmission protocols have also been investigated, including the protocols designed for relay channels [13], [14], [15], [16], [17]. The channels' correlation and optimal linear receivers designed for single-input multi-output channels have been studied in [18] and [19], [20], respectively.…”

New Results in DF Relaying Schemes Using Time Diversity for Free-Space Optical Links

“…This is a well known technique employed in radio-frequency (RF) systems, wherein more attention has been paid to the concept of user cooperation as a new form of diversity for future wireless communication systems [12]. Recently, several works have investigated the adoption of this technique in the context of FSO systems [13][14][15][16][17][18][19], being recognized as a very promising solution for future ad-hoc optical wireless systems. In [14,15] a 3-way FSO communication setup is proposed to implement a cooperative protocol in order to improve spatial diversity without much increase in hardware, being evaluated the error-rate performance by using the photon-count method as well as the outage performance for both amplify-and-forward (AF) and decode-and-forward (DF) strategies.…”