Impact of inter-relay cooperation on the performance of FSO systems with any number of relays. (c2015)

Abstract

In recent times, FSO (Free Space Optics) technology placed itself as a powerful means to transport data in communication systems. Its great potential to provide relatively high data rates services amongst many other advantages led to a surge of interest and research in this field of communication. However, the chief element that hinders and limits many aspects of FSO technology is mostly the random aspect of the atmosphere such as temperature fluctuations, wind, fog, rainfall… Overcoming Atmospheric turbulence-induced fading has been mainly the major key research field. Many proposals have been discussed and considered in tackling this problem. Some proposals targeted the hardware aspect of the photodetectors and transmitters. Others suggested for example the usage of extra standby power in case of fog. While some proposals aimed at taking advantage of the network topology and tackle this problem from an architectural point of view. The work in this thesis falls under the latter proposal and is pertinent to combating turbulence-induced fading in FSO systems in the context of cooperative diversity techniques in FSO systems. Unlike point-to-point communications where the source directly transmits the signal to the destination, in cooperative communications, the source takes advantage from the presence of neighboring nodes for increasing the chances of the information signal to reach the destination. These neighboring nodes will be denoted by relays in what follows. Typically, parallel relaying corresponds to a two-phase a communication scheme where the signal is first sent to the relays (and destination) while in the next phase the relays forward the received signals to the destination. This will be referred to as No Inter- Relay Cooperation (NIRC) technique. For the Inter-Relay Cooperation Technique (IRC) that was introduced recently, the relays inter-cooperate with each other before transmitting the message to the destination. In other words, IRC corresponds to a three phase source-relay, relay-relay and relay-destination scheme. Two variants of IRC are possible; one is unidirectional and will be named IRC1 and another bidirectional and will be named IRC2. The well-known NIRC technique will be mainly used as a benchmark to test the performance of the proposed techniques IRC1 and IRC2. We must note that the IRC techniques are worth being explored due to the fact that they take advantage of the already existing communication interoperability link; hence no extra resources need to be deployed. We are only benefitting from already available resources at a cost of an increased system complexity. The NIRC, IRC1 and IRC2 techniques will be thoroughly investigated throughout this thesis. A proper analysis will be conducted to weigh the gain stemming from adopting each one of them and under which conditions and what will be the impact they have on the performance of the FSO systems. While previous contributions targeted the performance of IRC1 and IRC2 with only two relays [1], these techniques will be explored with any number of relays in this work. Chapter one serves basically as a general introduction to FSO technology, citing its main limitations, advantages and diverse applications. It will also introduce the fading mitigations techniques and the main concept behind IRC. The channel model under which the IRC techniques are analyzed is also represented in this chapter as well as the modulation technique used. The outage probability and diversity orders analyses are conducted in chapters two and three respectively while chapter four illustrates the results of simulating the outage probabilities of each technique and validates the theoretical study. Proper conclusions are derived and the conditions under which each technique is more beneficial are examined. Finally, chapter five summarizes this work and the gives some insights as to how we can further develop and ameliorate the FSO systems.