The main topic of this thesis as previously stated is pertinent to combating turbulence-induced fading in FSO systems in the context of cooperative diversity techniques in FSO systems. The techniques studied in this thesis included the well-known NIRC scheme and the newly proposed more complex IRC1 and IRC2 schemes. The latter two cases aim to enhance the already existing cooperative diversity techniques by exploring the additional inter-relay links and employing them in transmitting the signal from the source to the destination.
As we have seen in the diverse sections of this thesis, by conducting a proper analysis on the outage probabilities and diversity orders, we have evaluated the gain stemming from exploiting the inter-relay links in FSO communication systems with N relays (any number of relays). In other words, we tried to analyze the additional advantages of exploiting the inter-Relay links and employing them to assist the source S in its communication with the Destination D, thus creating additional dependent paths to the well know NIRC scheme.
We can conclude that communicating over the existing relay-relay links constitutes an additional degree of freedom that can be exploited to enhance the achievable diversity orders and performance levels.
However the gain arising from using IRC1 or IRC2 is highly dependent on the network topology. In some scenarios, exploiting the inter-relay links is not efficient and doesn’t add up much to the NIRC technique.
Whilst in other cases, it proves to be a considerate enhancement over the NIRC case.
Hence special consideration needs to be paid to the engineering of such systems.
Even in the scenarios where inter-relay cooperation is capable of increasing the diversity order, the achievable gains are highly dependent on the particular network topology. In some cases, the minor gains in the diversity order do not justify the upsurge in the system complexity that results from implementing the IRC techniques; in other cases, significant gains can be reached stressing on the huge potential of IRC techniques.
This study is valid for any number of interoperability relays. We explored this new dimension under the strategy where all relays are active (No- CSI). However, IRC1 & IRC2 strategies can be further explored in the presence of CSI. In fact, selective-relaying is superior to all-active relaying at the expense of an increased complexity since the CSI needs to be acquired. Moreover, by exploring the IRC techniques under CSI, we can also ensure that the power would be more preserved since it won’t be split on all links and it will be used for only one path. As a brief definition, selective-relaying protocols (CSI) correspond to transmitting all information symbols along the unique strongest path
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between S and D unlike the all-active relaying (No-CSI) characterized by its remarked simplicity since it can be implemented without the need of acquiring any form of channel state information.
The main challenge in employing the IRC techniques under No-CSI would be in synchronizing all the signals falling into a certain node. Hence trying to tackle this problem from a hardware point of view and at the level of the photo detector is encouraged as a next step. In addition, DF (decode and forward) or amplify and forward (AF) techniques can be implemented in each node. This way each relay will treat the signal and try lowing its BER instead of dully forwarding it to the next node.
IRC techniques have a great potential and could be also merged with other techniques in order to reach the next level in fighting turbulence-induced fading in FSO systems.
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