Performance analysis of FSO cooperative communications with rf inter-connected relays in the absence and presence of channel state information. (c2018)

Abstract

Free Space Optics (FSO) communication systems have been heavily studied in the literature during the recent decade. Compared to the widely used and traditional Radio Frequency (RF) communication systems, FSO systems are believed to achieve much higher data rates and better performance in terms of feasibility and practicality to compile with the worldwide increased speed need and uprising fast technologies. However, as the case for any new technology, various limitations and challenges such as bad weather conditions, atmospheric turbulence, scintillation, and building sways, face FSO technology. As a result, continuous research is being conducted in the field to overcome these challenges and enhance the system reliability and performance. Among the explored solutions, deploying cooperative schemes in the context of the FSO communication systems was proven to add another degree of freedom for the existent systems. More specifically, diversity methods and inter-relay cooperative schemes are the leading between the discussed techniques as they suggest exploiting the existing relays in the system without any extra needed hardware or resources. Parallel relaying without inter-relay cooperation (NIRC), unidirectional inter-relay cooperation (IRC1), and bidirectional inter-relay cooperation (IRC2) are three main categories that have been studied recently which exploit the existence of adjacent relays in the system to cooperate in the communication process. NIRC schemes use two types of FSO links in the communication system: source to relay Sð€€€R and relay to destination Rð€€€D links, while IRC schemes benefit additionally from the existence of inter-relay Rð€€€R links connecting each pair of adjacent relays. However, none of the done work in the field tackles the possibility of talking advantage of another potential inter-relay links other than those connecting adjacent pairs. Therefore, the work in this thesis proposes a novel method which deploys a full inter-relay connectivity scheme labeled as Allto- All relaying scheme. Conversely, some limitations face the proposed scheme such as the difficulty of ensuring clear line of sight (LOS) between all the relays in the system which is required for deploying the FSO links. Moreover, the RF links still offer higher reliability in bad weather conditions and at the same time are of broadcasting nature which is more practical for the proposed All-to-All scheme. Moreover, RF links are used as backup for the FSO links of practical FSO systems which exist in the market today. Therefore, we propose in this thesis a novel mixed RF/FSO All-to-All connected system which combines the advantages of both FSO and RF technologies. The proposed system ensures the high data rates empowered by the FSO links deployed for Sð€€€R, Rð€€€D, and Sð€€€D links. Moreover it maintains high reliability by activating the Rð€€€R RF links whenever needed in the communication process. In fact, the proposed system is encouraging as it does not require any extra hardware or resources. In this thesis, we describe the proposed system components, functionalities, requirements, strategies, advantages and the gains carried with it compared to existing systems and relaying schemes. We also present two protocols that the system can work with: (1) All-Active relaying where all the FSO links are activated to participate in the cooperation strategy and (2) Selective relaying where only the strongest path is selected for the communication process. Selective relaying is showed to achieve higher gains compared with All-Active but with the cost of prior knowledge of channel state information (CSI). Therefore, a detailed comparison and analysis are presented in this thesis at two levels: (1) The proposed system vs existing systems and (2) All-Active relaying vs Selective relaying. The conducted analysis is carried out in terms of outage probability analysis and diversity order analysis where the first studies the effects of implementing the proposed system under the different relaying protocols in lowering the outage probability of the system thus increasing its reliability. In addition, the diversity order analysis presents another performance measurement of the system behavior under different relaying schemes and techniques. The results show superiority of the proposed mixed RF/FSO All-to-All scheme over other existing relaying schemes such as NIRC, IRC1, and IRC2, in addition to the superiority of the Selective relaying protocol over the All-Active relaying protocol. The results are studied theoretical in three aspects: exact, approximate, and simulated analysis, and supported by experimental modeling with numerical results which validate the theoretical analysis. The flow of work in this thesis is given as follows: Chapter one presents a review of the communication technologies with special highlights on the FSO generalities and the implemented diversity methods. Then, chapter two explains the mathematical tools and concepts such as conditional probability approach, minimum cut set method, and Monte Carlo simulation that are used throughout the whole thesis. Moreover, the system model along with all used channel models and corresponding equations are presented in chapter three. The core of the thesis is presented in chapters four and five where a detailed outage probability analysis and diversity order analysis are presented, respectively. The presented analysis in both chapters covers both special cases and general cases for any number of relays under both relaying protocols: All- Active and Selective. A proper comparison is conducted between the relevant schemes under different network setups and correspondingly valid conclusions are derived theoretically and then verified numerically in chapter six. Finally, the presented work is summarized and potential future work is suggested at the end of this thesis in chapter seven.