LEE- Bilgi ve Haberleşme Mühendisliği-Doktora

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http://hdl.handle.net/11527/19202

Gözat

Yazar "Ibrahim, Abdulgani Abshir" ile LEE- Bilgi ve Haberleşme Mühendisliği-Doktora'a göz atma
  • Öge
    Physical layer design of energy efficient and secure FSO links for next generation communication systems
    (Graduate School, 2025-05-23) Ibrahim, Abdulgani Abshir ; Ata Durak, Lütfiye ; Ata, Serdar Özgür ; 708182011 ; Information and Communications Engineering
    The ever-increasing demand for extremely high data rate networks has prompted the research community to explore new technologies capable of meeting this need. Free-space optical (FSO) communication systems are a promising technology for next-generation wireless networks due to their attractive features, such as ultra-high data rates, license-free spectrum, ease of deployment, and low cost. However, despite these appealing characteristics, the performance of FSO communication systems is hindered by several detrimental factors. Weather conditions, such as fog and rain, can significantly degrade FSO system performance. Atmospheric turbulence fading, caused by random variations in the atmosphere, also attenuates the power of the transmitted optical signal. Pointing errors, which arise from violations of the line-of-sight requirement between transmitting and receiving apertures, further degrade FSO performance. Another limiting factor is channel estimation errors, which stem from imperfect knowledge of the channel state information at the receiver. This thesis presents three distinct studies concerning the physical layer of FSO communication systems. In the first study, we investigate the performance of FSO communication systems over an imprecise Málaga turbulence-induced fading channel, which is a generalized form of both the Gamma-Gamma and K turbulence channels. The effect of pointing errors is also considered. In our analysis, we first derive the probability density function (PDF) and cumulative distribution function (CDF) of the channel's fading coefficient in the presence of channel estimation errors. We then obtain exact closed-form expressions for the average bit error rate (BER), outage probability (OP), and ergodic channel capacity to quantify the performance of the system under consideration. Additionally, to provide further insight into system performance in the high signal-to-noise ratio (SNR) regime, we derive asymptotic expressions for the BER and OP. Furthermore, the analytical results are successfully validated through Monte Carlo simulations. The results demonstrate the detrimental effects of the channel estimation errors on the performance of FSO communication systems. For instance, an estimation error of 5% results in approximately an 8 dB SNR loss at a BER of 5 × 10^−2. In the second study, spatial diversity techniques are proposed for FSO communication systems to combat the deteriorating effects, such as atmospheric turbulence and pointing errors. The performance of FSO communication systems with the Alamouti encoding scheme over the Málaga turbulence channel is investigated. We first derive the PDF of the end-to-end channel gain under atmospheric turbulence and pointing error conditions. Then, by capitalizing on this PDF, closed-form expressions for the average BER and OP of the proposed system are obtained. Additionally, to provide more insight, the asymptotic expressions for the average BER and OP are also derived. In the analysis, intensity modulation/direct detection and heterodyne detection techniques are considered, allowing the obtained results to cover both cases. Furthermore, the analytical results are successfully validated through Monte Carlo simulations. Our results highlight the performance gains that can be achieved when the Alamouti encoding scheme is employed in FSO communication systems. In the final study, we present a dual-hop decode-and-forward relaying-based FSO communication system. We consider utilizing simultaneous lightwave information and power transfer (SLIPT) with a time-splitting technique at the relay, where the direct current component of the received optical signal is harvested as transmit power for the relay. It is assumed that the FSO links experience Málaga turbulence channels with pointing errors. In order to evaluate the performance of the proposed communication system, closed-form expressions for OP, ergodic capacity, average BER, and throughput are derived. Additionally, to analyze the physical layer security of the proposed system, closed-form expressions for secrecy outage probability and strictly positive secrecy capacity are obtained. Finally, the accuracy of the derived analytical expressions is validated through Monte Carlo simulations. Results show that our proposed system model outperforms its non-SLIPT counterpart.