One of the potential solutions to the radio frequency (RF) spectrum scarcity problem is optical wireless communications (OWC), which utilizes the unlicensed optical spectrum. Long-range outdoor OWC are usually referred to in the literature as free-space optical (FSO) communications. Unlike RF systems, FSO is immune to interference and multi-path fading. Also, the deployment of FSO systems is flexible and much faster than optical fibers. These attractive features make FSO applicable for broadband wireless transmission such as optical fiber backup, metropolitan area network, and last mile access. Although FSO communication is a promising technology, it is negatively affected by two physical phenomenon, namely, scintillation due to atmospheric turbulence and pointing errors. These two critical issues have prompted intensive research in the last decade. To quantify the effect of these two factors on FSO system performance, we need effective mathematical models. In this work, we propose and study a generalized pointing error model based on the Beckmann distribution. Then, we aim to generalize the FSO channel model to span all turbulence conditions from weak to strong while taking pointing errors into consideration. Since scintillation in FSO is analogous to the fading phenomena in RF, diversity has been proposed too to overcome the effect of irradiance fluctuations. Thus, several combining techniques of not necessarily independent dual-branch free-space optical links were investigated over both weak and strong turbulence channels in the presence of pointing errors. On another front, improving the performance, enhancing the capacity and reducing the delay of the communication link has been the motivation of any newly developed schemes, especially for backhauling. Recently, there has been a growing interest in practical systems to integrate RF and FSO technologies to solve the last mile bottleneck. As such, we also study in this thesis asymmetric an RF-FSO dual-hop relay transmission system with both fixed and variable gain relay.

The exponential increase in the number of mobile users, coupled with the strong demand for high-speed data services results in a significant growth in the required cellular backhaul capacity. Optimizing the cost efficiency while increasing the capacity is becoming a key challenge to the cellular backhaul. It refers to connections between base stations and mobile switching nodes over a variety of transport technologies such as copper, optical fibers, and radio links. These traditional transmission technologies are either expensive, or cannot provide high data rates. This work is focused on the opportunities of free-space-optical (FSO) technology in next generation cellular back- haul. FSO is a cost effective and wide bandwidth solution as compared with the traditional radio-frequency (RF) transmission. Moreover, due to its ease of deployment, license-free operation, high transmission security, and insensitivity to interference, FSO links are becoming an attractive solution for next generation cellular networks. However, the widespread deployment of FSO links is hampered by the atmospheric turbulence-induced fading, weather conditions, and pointing errors. Increasing the reliability of FSO systems, while still exploiting their high data rate communications, is a key requirement in the deployment of an FSO-based backhaul. Therefore, the aim of this work is to provide different approaches to address these technical challenges. In this context, investigation of hybrid automatic repeat request (HARQ) protocols from an information-theoretic perspective is undertaken. Moreover, performance analysis of asymmetric RF/FSO dual-hop systems is studied. In such system models, multiple RF users can be multiplexed and sent over the FSO link. More specifically, the end-to-end performance metrics are presented in closed-form. This also has increased the interest to study the performance of dual-hop mixed FSO/RF systems, where the FSO link is used as a multicast channel that serves different RF users. Having such interesting results motivates further the analysis of dual-hop FSO fixed-gain relaying communication systems, and exact closed-form performance metrics are presented in terms of the bivariate H-Fox function. This model is further enhanced through the deployment of a multihop FSO relaying system as an efficient technique to mitigate the turbulence-induced fading as well as pointing errors.