IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS, VOL. 7, NO. 12, DECEMBER 2008 5441 Relay-Assisted Free-Space Optical Communication Majid Safari, Student Member, IEEE, and Murat Uysal, Senior Member, IEEE Abstract—In this paper, we present relay-assisted transmission as a powerful fading mitigation tool for free-space optical systems operating in atmospheric turbulence channels. We study both serial (i.e., multi-hop transmission) and parallel (i.e., cooper- ative diversity) relaying encoupled with amplify-and-forward and decode-and-forward modes. We consider an aggregated channel model which takes into account both path-loss and turbulence-induced log-normal fading. Since fading variance is distance-dependent in free-space optical systems, relay-assisted transmission takes advantage of the resulting shorter hops and yields significant performance improvements. We derive outage probability of the relaying schemes under consideration which are further confirmed through Monte-Carlo simulations. Our outage probability analysis demonstrates that an impressive performance improvement of 18.5 dB is possible with the use of a single relay at a target outage probability of 10 -6 . Index Terms—Free-space optical systems, cooperative diver- sity, fading channels. I. I NTRODUCTION F REE-SPACE optical (FSO) communication refers to ter- restrial line-of-sight optical transmission through the at- mosphere. This technology has recently attracted a renewed interest within the research community although its roots can be traced back to Alexander Graham Bell’s ”photophone” [1]. In this first FSO experiment carried out on February 18, 1880, Bell was able to transmit voice signals through a modulated beam of light via atmosphere for a distance of about 200 m. Although Bell’s photophone never came out as a commercial product, it has successfully demonstrated the potential of FSO transmission. Today’s FSO systems use either lasers or LEDs (light emit- ting diodes) to transmit a modulated beam of visible/infrared light [2]. These systems are license-free with high-bandwidth capacity providing a cost-effective and easy-to-install alterna- tive to fiber optics. They further provide an inherent security due to the nature of their directional and narrow beams which make eavesdropping and jamming nearly impossible. With its unique features, FSO communication is appealing for a number of applications including last-mile access, fiber back- up, back-haul for wireless cellular networks, and disaster recovery [3]. Manuscript received December 1, 2007; revised March 2, 2007; accepted March 31, 2007. The associate editor coordinating the review of this paper and approving it for publication was N. Arumugam. This paper was presented in part at the Asilomar Conference on Signals, Systems, and Computers, Monterey, CA, USA, November 2007. The work of M. Uysal is supported in part by a Natural Sciences and Engineering Research Council of Canada (NSERC) Special Research Opportunity Grant (SROPJ305821-05). The authors are with the Department of Electrical and Computer Engi- neering, University of Waterloo, Waterloo, ON, N2L3G1, Canada (e-mail: m3safari@uwaterloo.ca, muysal@ece.uwaterloo.ca). Digital Object Identifier 10.1109/T-WC.2008.071352 Despite the major advantages of FSO, its widespread use has been hampered by its rather disappointing performance for long-range links. For link ranges longer than 1 km, at- mospheric turbulence-induced fading becomes a major perfor- mance limiting factor in FSO systems [4]. A number of fading- mitigation techniques have been proposed in the literature including error-correcting codes [5], [6], maximum-likelihood sequence estimation [7], and spatial diversity [8]–[11]. Among those, spatial diversity is particularly attractive with its lower complexity. Spatial diversity involves the use of multiple trans- mit and/or receive apertures and has been extensively studied in the context of wireless radio-frequency (RF) communication before it was applied to FSO communication. In this paper, we borrow another well-studied concept - cooperative diversity - from wireless RF literature to apply within the context of FSO communications. Cooperative diversity has been recently introduced as an alternative way of realizing spatial diversity advantages [12]– [14]. The main idea behind cooperative diversity is based on the observation that in a wireless RF channel, the signal transmitted by the source node is overheard by other nodes, which can be defined as partners or relays. The source and its partners can jointly process and transmit their information, creating a virtual antenna array although each of them is equipped with only one antenna. Multihop transmission is an alternative relay-assisted transmission scheme which employs the relays in a serial configuration [15], [16]. Such schemes are typically used to broaden the signal coverage for limited- power transmitters and do not offer performance improvement against fading effects in wireless RF environments, i.e., it does not increase the diversity order [12]. To the best of our knowledge, relay-assisted FSO trans- mission was first proposed by Acampora and Krishnamurthy in [17]. Their work, however, has a networking perspective and does not address the physical layer aspects which our paper aims to focus on. In [18], Akella et.al. have studied the bit error rate performance of a decode-and-forward FSO multi-hop scheme. Their channel model considers only path- loss and ignores the fading effects. In [19] and [20], Tsiftsis et.al. have considered K and Gamma-Gamma atmospheric- induced fading models without explicitly taking into account the path-loss and evaluated outage probability for a multi-hop FSO system. Their results demonstrate the usefulness of relay- assisted transmission as a method to broaden the coverage area, but do not highlight its use as a fading-mitigation tool which is demonstrated in our paper. In this paper, we study relay-assisted FSO communication system based on different configurations of relays whether they are employed in serial (i.e., multi-hop transmission) or in parallel (i.e., cooperative diversity). 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