Theses supervised by Prof. Dr. Murat Uysal
36 theses · Afyon Kocatepe University, Özyeğin University
Göktürk-1 uydu görüntülerinde pus giderme
Bu tez çalışmasında, Göktürk-1 uydu görüntülerinde pus kaynaklı atmosferik bozulmaların yol açtığı problemler ele alınmıştır. PCI Geomatica yazılımının sağladığı ATCOR atmosferik düzeltme yöntemi kullanılarak gerçekleştirilen pus giderme işlemi, başarılı bir şekilde atmosferik bozulmaları azaltarak görüntülerin kalitesini önemli ölçüde artırmıştır. Atmosferik düzeltme sonrasında, PCI Geomatica'nın Ortho Engine modülü kullanılarak yüksek çözünürlüklü ortofoto üretimi gerçekleştirilmiştir. Bu aşama, pus etkilerinden arındırılmış, coğrafi referanslı ve hassas görüntülerin elde edilmesine olanak sağlamıştır. Ortaya çıkan ortofoto, coğrafi bilgi sistemleri (CBS) uygulamalarında, arazi analizlerinde ve çeşitli uzaktan algılama projelerinde kullanılacak güvenilir bir veri kaynağı sunmaktadır. Tez kapsamında ayrıca, PCI Geomatica'nın Mosaic Tool ara yüzünde gerçekleştirilen görüntü zenginleştirme süreci detaylı bir şekilde incelenmiştir. Mosaic Tool, çeşitli görüntüleri birleştirme ve daha yüksek kaliteli bir görüntü oluşturma konusunda kullanılan bir araçtır. Bu aşama, ortofotolardan elde edilen verilerin daha derinlemesine analiz edilmesini sağlamış ve görüntü kalitesini önemli ölçüde artırmıştır. Bu tez çalışması, Göktürk-1 uydu görüntülerindeki atmosferik bozulmaların etkin bir şekilde giderilmesi ve elde edilen verilerin daha fazla değer katması için uygulanan aşamaları detaylı bir şekilde sunmaktadır. Ayrıca, kullanılan yöntemlerin pratik uygulamaları, elde edilen sonuçların önemi ve bu süreçlerin coğrafi bilgi sistemleri, tarım, çevre izleme gibi alanlarda nasıl kullanılabileceği üzerine vurgular yapılmıştır. Bu çalışma, uzaktan algılama alanında atmosferik düzeltme konusunda gerçekleştirilen kapsamlı bir metodolojinin potansiyelini vurgulamakta ve bu alandaki gelecekteki araştırmalara katkı sağlamayı amaçlamaktadır.
Derin öğrenme ve makine öğrenme algoritmaları kullanılarak orman yangını risk tahmini
Ormanlar, karasal ekosistemlerde canlılar için hayati önem taşıyan unsurlar olup toprak, su ve iklim gibi doğal faktörlerin bütünlüğünü ve sürdürülebilirliğini sağlamaktadır. Orman yangınları, sosyal ve ekonomik etkilerinin yanı sıra ekosistemin bozulmasına da neden olan afetlerdendir. Bu araştırmada orman yangınlarını etkileyen faktörler uzaktan algılama ve coğrafi bilgi sistemleri teknikleriyle antropojenik, meteorolojik, topoğrafik ve vejetasyon üst başlıklarında 21 adet bağımsız değişken olarak belirlenmiştir. Bağımlı değişken olan geçmiş orman yangınları ve bağımsız değişkenler 2017-2023 yılları arasında 7 yıl boyunca Mayıs-Eylül ayları arasında aylık zamansal çözünürlükte sınıflandırılmış ve haritalandırılmıştır. Üretilen haritalardan değer çıkarımı yapılarak 670x22 satır/sütundan oluşan I. veri seti ile 2100x22 satır/sütundan oluşan II. veri seti model eğitimleri için oluşturulup risk tahmini için de 689419x21 satır/sütundan oluşan çalışma alanı coğrafi veri seti haline getirilmiştir. Veri setleri Google Colab ara yüzünde Python yazılım dili kullanılarak keşifsel veri analizi teknikleriyle görselleştirilmiş ve korelasyon analizleri gerçekleştirilmiştir. Model eğitimleri tamamlandıktan sonra tahmin aşamasına geçilmiş ve 7 adet makine öğrenimi algoritmalarından en iyi doğruluk metriklerine ulaşılan Rastgele Orman (0,94) ve Aşırı Gradyan Artırma (0,95) algoritmaları ile orman yangını risk haritaları üretilmiştir. Derin Öğrenme modeli olan Sıralı (sequential) model ile de II. veri seti kullanılarak 0,95 test doğruluğu ile eğitilmiş modelden orman yangını risk tahmin haritası üretilmiştir. Bu çalışmanın sonucunda, orman yangını risk tahmini için kullanılan veri seti boyutlarının, orman yangınları ile ilişkilendirilebilen bağımsız değişkenler ile artırılarak daha doğru ve anlamlı sonuçlara ulaşılabileceği bununla birlikte rastgele orman ve aşırı gradyan artırma gibi topluluk modellerine dayalı algoritmalar ve daha büyük veri setleri için derin öğrenme tekinlerine dayalı algoritmaların orman yangını risk tahminlerinde daha başarılı olduğu ortaya konmuştur.
GÖKTÜRK-1 stereo uydu görüntülerinin 1/5000 ve 1/25000 ölçekli fotogrametrik harita üretiminde kullanılabilirliğinin araştırılması
Yüksek çözünürlüklü uydu görüntüleri kullanarak harita üretimi günümüzde de güncel araştırma konuları arasındadır. Ancak uydu görüntülerinden harita üretiminde göz önünde bulundurulması gereken en önemli kriterler konum doğruluğunun yanında maliyet ve zaman etkenleridir. Bu çalışmada ise Türkiye'nin sahip olduğu metre altı çözünürlüklü yer gözlem uydusu olan Göktürk-1 stereo uydu görüntüleri kullanılarak 1/5000 ve 1/25000 ölçekli stereo fotogrametrik kıymetlendirme haritası yapılarak hem yatay ve düşey doğruluk bakımından incelenmiş hem de belirtilen ölçekli haritalarda bulunması gereken detayların tespit teşhis ve tanınma durumu analiz edilmiştir. Ayrıca fotogrametrik haritada çizilen parsellerin gayrimeskûn alanda kadastro çalışmaları için altlık olarak kullanılabilirliği incelenmiştir. Son olarak çalışma alanında görüntü iyileştirme uygulamaları yapılarak elde edilen sonuçların fotogrametrik kıymetlendirmeye etkisi incelenmiştir. Elde edilen sonuçlara göre Göktürk-1 stereo uydu görüntülerinin hem yatay ve düşey doğruluk bakımından hem de içerdiği detaylar bakımından 1/5000 ve 1/25000 ölçekli fotogrametrik harita üretiminde kullanılabileceği görülmüştür. Ayrıca görüntü iyileştirme uygulamalarının görüntü kıymetlendirme aşamasında operatörün detay tespit ve teşhis edebilme yeteneğine olumlu etkisi olduğu ve bunun sonucunda da arazi bütünlemesi süresini etkileyebileceği değerlendirilmektedir.
Fotorealistik dijital ikiz ile sanal gerçeklik entegrasyonu ve yapısal denetimde kullanımı
Dijital ikiz teknolojisi son yıllarda önemli bir ivme kazanmış ve çeşitli amaçlar için giderek daha popüler hale gelmiştir. Dijital ikiz, canlandırma, izleme ve kestirimci bakım dahil olmak üzere çeşitli amaçlar için fiziksel bir nesnenin sanal bir temsilini oluşturma sürecidir. Dijital ikiz oluşturmanın temel zorluklarından biri, temsil edilen fiziksel nesne hakkında doğru ve güncel 3B veriler elde etmektir. Fiziksel nesne hakkında gerçek dokusal bilgiler de dahil olmak üzere 3B verileri toplamada en uygulanabilir yöntem fotogrametridir. Fotogrametri yöntemi kullanılarak fotorealistik 3B modellerden üretilen dijital ikizler en uygun çözümleri belirlemek için kullanılabilir. Özellikle maliyetli yerinde incelemeler yerine yapısal denetimlerde fotorealistik dijital ikizlerin kullanılması alternatif bir yöntem oluşturmaktadır. Bu yöntemle elde edilen dijital ikizlerin sanal gerçeklik teknolojileri ile entegrasyonu ile özellikle sanal ortamda interaktif denetimlere ve analizlere imkân vermektedir. Bu sayede dijital ikizlerin daha etkili bir şekilde kullanılması kolaylaşmakta ve fiziksel nesnelerin daha etkileşimli ve ayrıntılı bir şekilde incelenmesi sağlanmaktadır. Bu tezde, fotorealistik 3B model ile fiziksel nesnenin diğer yapısal bilgilerinin entegre edilmesiyle oluşturulan dijital ikiz, sanal gerçeklik teknolojileriyle entegre edilerek farklı inceleme perspektifleri sunulmaktadır. Geleneksel denetim yöntemlerine göre daha hızlı ve daha verimli olan bu yöntemler, özellikle yapısal denetimlerde hasar analizleri için önemlidir. Önerilen metodoloji farklı yapı türleri üzerinde test edilmiştir. Sonuç olarak, fotogrametrik 3B model ve otomatik olarak tespit edilen çatlaklar birleştirilerek bir hasarla arıtılmış dijital ikiz (3B model+çatlak) oluşturulmuştur. Ayrıca yapısal denetimlere farklı bir bakış açısı sunmak için termal fotoğraflar ile izleme ve inceleme çalışması da yapılmıştır. Üretilen dijital ikizler sanal gerçeklik ortamında farklı kullanıcılara aktarılarak etkileşimli çevrimiçi denetimlere olanak sağlanmış ve yerinde yapısal denetimlere alternatif bir çözüm sunulmuştur.
PS-InSAR ve derin öğrenme ile yerkabuğu hareketlerinin kestirimi
Yerkabuğu hareketleri, özellikle antropojenik etkilerle artan bir problem olup, sürdürülebilir kaynak yönetimi ve afet risk azaltımı için bu hareketlerin izlenmesi ve gelecekteki davranışlarının tahminlenmesi kritik öneme sahiptir. Bu doktora tezi, Afyonkarahisar-Bolvadin'deki yerkabuğu hareketlerinin uzun dönemli analizini ve geleceğe yönelik tahminlerini, PS-InSAR ve derin öğrenme tekniklerini entegre ederek gerçekleştirmeyi amaçlamıştır. Çalışmada, 2015-2024 arasını kapsayan 240 adet Sentinel-1A SAR görüntüsü PS-InSAR tekniğiyle işlenerek yıllık LOS hareket hızı zaman serileri üretilmiştir. Analizler, bölgede +1 ila -14 mm/yıl arasında değişen, genel olarak çökme yönlü ve özellikle yeraltı suyu çekiminin yoğun olduğu güneybatı kesimlerde yoğunlaşan hareketler tespit etmiştir. Bu zaman serileri; LSTM, GRU, TCN, CNN ve Transformer derin öğrenme modelleriyle, Yinelemeli Çok Adımlı Tahmin (RMF) stratejisi kullanılarak 2025-2026 yılları için modellenmiştir. Bulgular, tüm modellerin tek adımlı tahminlerde yüksek başarı (R² > 0.92) gösterdiğini, CNN mimarisinin ise en iyi performansı sergilediğini ortaya koymuştur. RMF projeksiyonları, çökme eğiliminin devam edeceğini öngörmüş, CNN ve LSTM modelleri çok adımlı tahminlerde daha stabil bulunmuştur. Bu çalışma, PS-InSAR ve derin öğrenme entegrasyonunun yüzey hareketlerinin izlenmesi ve anlamlı gelecek tahminleri için potansiyelini göstermiş olup, bölgesel planlama ve yönetim stratejilerine bilimsel bir zemin sunmaktadır. Yardımcı parametrelerin dahil edilmesi ve yer bazlı doğrulama gelecek çalışmalar için önerilmektedir.
Derin öğrenme ile farklı veri kaynaklarından elde edilen görüntülerden bina tespiti
Uzaktan algılama teknolojilerindeki hızlı gelişmeler ve yüksek çözünürlüklü uydu görüntüleri ile hava araçlarının yaygınlaşması, kentsel alanlarda bina tespiti gibi uygulamaları daha etkin hale getirmiştir. Derin öğrenme yöntemleri, bu süreçte geleneksel yöntemlere kıyasla daha hızlı, hassas ve ölçeklenebilir çözümler sunarak şehir planlaması, afet yönetimi ve çevresel analizler gibi alanlarda kritik bir rol üstlenmiştir. Bu tez çalışmasında, nesne tespiti problemleri için yaygın olarak kullanılan YOLO (Sadece Bir Kez Bak) algoritmasının iki farklı sürümü olan YOLOv8m ve YOLOv11m ile Mask-RCNN (Bölge Tabanlı Evrişimsel Sinir Ağının Maskelenmesi) algoritmaları, iki farklı veri kaynağından (Harita Genel Müdürlüğü ortofotosu ve WorldView03 uydu görüntüsü) elde edilen görüntüler üzerinde bina tespiti çalışması için test edilmiştir. YOLO algoritmaları Google Colab ortamında, Mask-RCNN algoritması ise ArcGIS Pro uygulamasında değerlendirilmiştir. Sonuç olarak, YOLO ve Mask-RCNN algoritmalarının bina tespiti alanındaki potansiyeli ortaya konulmuş ve bu çalışmanın, uzaktan algılama teknolojileri ile yapay zeka tabanlı çözümlerin entegrasyonuna katkı sunması hedeflenmiştir.
Performance analysis and optimization of relay-assisted free space optical communication systems
Free-space optical (FSO) communication has received an increasing attention in recent years with their ability to achieve ultra-high data rates (at the order of multiple gigabits per second) over unlicensed optical spectrum. A major degrading factor, particularly in long links, is the atmospheric turbulence induced fading. Smartly exploiting the fact that fading variance is distance-dependent in FSO channel, relay assisted transmission takes advantage of the resulting shorter hops and yields significant performance improvements. In this thesis, we make several contributions to the performance analysis and design of relay-assisted FSO systems. In the first part, we investigate how to determine optimal relay locations in serial and parallel FSO relaying as to minimize the outage probability and quantify performance improvements obtained through optimal relay placement. In the second part, we investigate the combined use of serial and parallel relaying for FSO mesh networks. We derive the outage probability expressions of FSO mesh networks and demonstrate performance improvements with respect to both stand-alone serial and parallel relaying schemes. We also present a diversity gain analysis and quantify the achievable diversity orders in terms of the number of relays and turbulence channel parameters. Finally, we investigate the use of all-optical relaying removing the need for optical-electrical and electrical-optical conversions. Building on all-optical relay assumption, we investigate the outage performance of a dual-hop FSO link taking into account practical limitations such as amplifier noise and filtering effects.
Su altı akustik haberleşme sistemlerinin bilgi kuramı perspektifinden başarım analizi ve eniyilemesi
Underwater wireless communication is a rapidly growing field of research and engineering as its applications, which were once exclusively military, are extending into commercial fields. The need for underwater wireless communications exists in a wide range of applications including offshore oil field exploration/monitoring, oceanographic data collection, maritime archaeology, seismic observation, environmental monitoring, disaster preventing, port and border security among many others. Although capacity calculations for terrestrial radio-frequency channels have been extensively studied, the literature on the capacity of underwater acoustic (UWA) channels is sporadic with many remaining open questions. Aiming to fill research gaps in this growing field, this thesis makes several contributions to the information theoretical performance analysis of point-to-point and relay-assisted UWA systems. A single-carrier communication architecture and sparse Rician frequency-selective UWA channel with intersymbol interference (ISI) is considered in our work. We assume non-white Gaussian distribution to model the ambient noise and consider Francois-Garrison path loss formula to take into account the effects of environmental parameters such as temperature, salinity, pressure as well as system parameters such as distance and frequency. We develop an equivalent channel model for UWA channel with ISI under consideration and show that the capacity of the equivalent channel converges to that of the operating channel in the limit of infinite block length. Using these results, we first obtain a capacity expression for the UWA channel and demonstrate the dependency on channel parameters such as the number and location of significant taps and power delay profile, and environmental parameters such as temperature, salinity, and pressure. Then, we use this expression to determine the optimal carrier frequency, input signaling, and bandwidth. A closed-form formula for the optimum carrier frequency is further obtained. In the second part of the thesis, we extend our results to cooperative UWA systems and obtain achievable rates of single-carrier cooperative UWA systems with orthogonal decode-and-forward (DF) relaying. We take into account the effect of relay geometry in the derivations of achievable rates, and use the derived expressions to optimize the location of the relay.
Kodlu devamlı faz modülasyonu taktik alıcı-vericileriçin
Satisfying the various demands of tactical transceivers is one the common challenges in military communications. Different modulation, coding and spread spectrum techniques are needed to meet these requirements. Although FSK is the most popular modulation technique used for tactical waveforms and frequently employed in military walkie-talkies, Continuous Phase Modulation (CPM) is superior to this conventional method due to its spectral efficiency feature. High spectral efficiency and constant envelope are the speci fications that make this modulation technique attractive for tactical waveforms. Serial concatenation of CPM with Low Density Parity Check Codes (LDPCs) provides a superior performance due to coding gains of this powerful code family. The choice of proper combination of LDPC and CPM is crucial to satisfy the targeted tactical waveform requirements of spectral efficiency, power efficiency and receiver complexity. In this thesis we address the problem of selecting the optimal transmission parameters in an LDPC-coded CPM system and investigating the performances under different jamming scenarios. The parameters are chosen to satisfy a given spectral efficiency subject to a constraint on the demodulator complexity. Speci cally, we consider modulation index, alphabet size, pulse shape duration and code rate as transmission parameters. Utilizing a systematic search procedure, we identify the proper transmission parameters to achieve a targeted spectral efficiency and error rate. Thereafter, the selected schemes are tested under attack of different frequency jamming signals.
Çoklu-atlama hibrit fso/rf haberleşme sistemleri
The reliability of free-space optical (FSO) links is severely restricted by weather-dependent atmospheric phenomena including absorption, scattering and turbulence. An efficient solution is to build hybrid links where a radio frequency (RF) link is incorporated in parallel to the FSO link. With respect to weather conditions, FSO links are mostly degraded by fog and turbulence-induced fading (scintillation) whereas RF links particularly suffer from rain scatter. Therefore, hybrid FSO/RF systems can offer significant enhancements in the system reliability for all weather conditions by means of the complementary nature of the underlying links. In this thesis, we investigate the outage performance of point-to-point hybrid FSO/RF systems as well as multi-hop hybrid FSO/RF systems with soft-switching. Starting from the point-to-point system, we model the link selection required for soft-switching between the FSO and RF channels in different weather conditions by a finite-state Markov chain (FSMC) process. Then, based on the proposed model, we derive a closed-form outage probability expression in terms of various system and weather-dependent parameters. Thereupon, we extend the scope of our analysis to multi-hop hybrid FSO/RF systems. We present the outage probability and diversity gain analysis for multi-hop hybrid FSO/RF systems. Our results clearly demonstrate the robust performance of the soft-switching hybrid FSO/RF systems deployed either as the point-to-point or multi-hop configuration in dealing with different weather conditions.
Çok röleli serbest uzay optik haberleşme sistemlerinin başarım analizi
Free space optical (FSO) communication has attracted a great attention in recent years because of their high bandwidth capacity in unregulated spectrum which makes them a cost-effective and easy-to-install alternative to fiber optics. Moreover they provide high immunity to interference and jamming due to directional and narrow beams. FSO systems are appealing for number of applications including last-mile access, fiber back-up, back-haul for wireless cellular networks, and disaster recovery among others. The major degrading factor in these systems is atmospheric turbulence induced fading particularly for long range links. Relay-assisted FSO systems provide significant performance gains by using the advantage of shorter hops and the fact that fading variance is distance dependent in FSO channels. In this thesis, we investigate the performance of multi-relay FSO systems in various settings. In the first part of thesis, we consider an all-optical relaying system to remove the need for optical -electrical- optical conversions. We explore the benefits of relay selection and investigate its outage performance over Gamma-Gamma distributed turbulence channels. In the second part of thesis, we assume that source is equipped with multiple transmit apertures employing the so-called spatial pulse position amplitude modulation (SPPAM). We investigate an upper bound for bit error rate of this system with multiple parallel relays over log-normal distributed channels.
Röle destekli OFDM tabanlı görünür ışık haberleşmeleri
In this study, a relay-assisted visible light communication (VLC) system where an intermediate light source cooperates with the main light source is investigated. Specifically, two light sources in an office space; one is the information source employed on the ceiling and the other one is a task light, are considered. The system uses DC biased optical orthogonal frequency division multiplexing (DCO-OFDM) where the task light performs relaying to assist the communication and operates in half-duplex (HD) mode for both amplify-and-forward (AF) and decode-and-forward (DF) relaying. The bit error rate (BER) performance of the relay-assisted VLC system is investigated and the performance is further optimized through optimal AC/DC power allocation under illumination constraints. The DC power allocation is determined by sharing the number of light emitting diode (LED) chips between the terminals to satisfy a desired luminance ratio. The AC power allocation decides the fraction of the information signal power to be consumed at the terminals in order to minimize the BER. Numerical results reveal that cooperation brings significant performance gains over direct transmission.
Gelişmiş tek kutuplu optik ortogonal frekans bölmeli çoğullama (eU-OFDM) temelli röle-destekli görünür ışık haberleşmesi (VLC)
Due to spectrum scarcity of the conventional radio frequency (RF) and the higher demand for wireless services, visible light communication (VLC) has emerged as a complementary solution to RF technology for ubiquitous wireless connectivity. In the first part of this thesis, we consider a relay-assisted VLC system in an indoor environment where an intermediate light source cooperates with the main light source through an orthogonal amplify-and-forward (OAF) relaying protocol in order to transmit high-speed data to the destination terminal. The optical channel considered in this scenario is based on the IEEE802.15.7r VLC reference channel model for an office space. The information source (ceiling light) is connected to the backbone network, desk light acts as a relaying terminal and the receiving terminal is a photodetector attached to a computer. All the three network terminals are equipped with signal processing units. The proposed relay-assisted VLC system employs the enhanced unipolar optical orthogonal frequency division multiplexing (eU-OFDM) modulation approach. Our performance results show that the relay-assisted system achieves a significant gain over the direct transmission. In the second part of this thesis, a serial multi-hop VLC network is considered. We evaluate the BER performance analysis of multi-hop VLC links through Monte Carlo simulation and validate the simulation results with their respective theoretical counterparts.
Ağ kodlamalı işbirlikli OFDMA sistemleri için çeşitleme-çoğullama ödünleşimi
Cooperative communication (CC) exploits the broadcasting nature of wireless transmission and creates a distributed multiple-input multiple-output (MIMO) system among the wireless nodes which are physically separated from each other. Earlier works on cooperative communication typically build on time division multiple access (TDMA). However, it is well known that conventional CC systems experience throughput loss due to additional time slots required for relaying phase which particularly become prohibitive with the increasing number of relay nodes. Network coded cooperative (NCC) systems have been proposed as a powerful alternative with significantly higher throughput efficiency. In an NCC system, the relay nodes perform either XOR operation or form a linear combination of the messages received from distinct sources. Through proper combining, network coding improves the spectral efficiency over conventional unidirectional cooperative communication systems. In the first chapter of this thesis, we provide an overview of NCC systems and discuss their advantages over conventional CC systems in terms of diversity multiplexing tradeoff (DMT). All existing works on the DMT analysis of NCC systems assume perfect channel state information (CSI) at relay and destination nodes and focus on asymptotical analysis. It is known that asymptotical DMT analysis overestimates the performance since most communication systems operate in low and moderate signal-to-noise ratio (SNR) regime. To characterize DMT within more practical SNR regime, finite-SNR DMT is a more meaningful performance measure. In the second chapter, we investigate the outage probability and finite-SNR DMT performance for NCC wireless networks in the presence of imperfect CSI. We show that our results are generalized versions of those earlier presented in the literature and coincide with them in high SNR regime. Our analysis further reveals that the DMT is highly dependent of channel estimation quality particularly at finite-SNR values. In an effort to have further gains over the initial works, which build upon the assumption of TDMA, the combined use of orthogonal frequency division multiplexing (OFDM) with NCC has been proposed in the literature. In the third chapter, we consider orthogonal frequency division multiple access (OFDMA), an extension of the OFDM to a multiuser system where subsets of carriers are assigned to different users. We derive a closed-form expression for the outage probability of the system over Rayleigh fading channels and present the DMT analysis. Our results demonstrate that NCC-OFDMA system is able to fully exploit both frequency and spatial diversity. In the third chapter, we investigate only asymptotic DMT analysis of NCC-OFDMA systems over Rayleigh fading channels. In the fourth chapter, we investigate both finite-SNR DMT and asymptotic DMT performance over Rician fading channels. Such an analysis helps us see the difference of performances over Rayleigh and Rician fading channels. Otherwise, DMT results for Rayleigh and Rician channels coincide each other for asypmtotical SNR values.
Morötesi haberleşme için çeşitleme teknikleri
Strong molecular and aerosol scattering in ultraviolet (UV) wavelengths enable mechanisms where transmitter can communicate with the receiver in the absence of line of sight (LOS) link. Recent advances in solid state technologies have enabled the production of efficient semiconductor ultraviolet (UV) LED sources and detectors which led to a renewed interest in UV communication. Non-line-of-sight (NLOS) communication is particularly desirable to relax or eliminate pointing, acquisition and tracking requirements. This thesis investigates spatial diversity techniques for NLOS UV communication systems. Particularly, we explore cooperative diversity in the form of relaying and multi-input multi-output (MIMO) communications to extract spatial diversity advantages. In the first chapter, we provide an overview of NLOS UV communication discussing its advantages and applications and present a literature survey. In the second chapter, we present the NLOS UV channel model used in this thesis. In third chapter, we consider a cooperative UV system with orthogonal cooperation protocol and use DC biased optical orthogonal frequency division multiplexing (DCO-OFDM) as the underlying physical layer. Under the assumption that turbulence can be ignored, we study both amplify-and-forward (AF) and detect-and-forward (DF) relaying. We analyze BER performance of the cooperative OFDM UV system under consideration and optimize the performance through optimal power allocation. We also consider a variable-rate cooperative OFDM UV system and investigate bit loading (i.e., use of different modulation orders per subcarrier) in an effort to maximize the throughput. In the fourth chapter, we further consider the effects of turbulence and investigate the performance of a multi-hop UV system with DF relaying. Based on the asymptotic outage expressions, we present a diversity gain analysis and obtain the diversity order as a function of link distance and system parameters. In fifth chapter, the performance of MIMO UV systems over turbulence channels is studied. We derive BER expressions for MIMO UV systems over NLOS log-normal turbulence channels. We also investigate the performance of single-input multiple-output (SIMO) and multiple-input single output (MISO) as special cases.
Açık uzay optik haberleşmesinin performans değerlendirmesi için özel tasarım atmosferik emülatörü
The properties of atmospheric chamber and measurement methods that are used in this system are explained and analyzed in detail. We present our own custom design atmospheric channel emulator for FSO system evaluations and experimentally investigate the performance of FSO links in different weather conditions. First, we present a link budget and quantify the geometrical loss. Then, we emulate foggy,rainy and turbulent weather conditions inside the chamber and quantify the absorption loss for different wavelengths. Based on the experimental data, we also propose a new and accurate model for the prediction of fog attenuation coefficient.
Adaptif OFDM tabanlı akustik su altı iletimi: Sistem tasarımı ve deneysel doğrulama
In this study, we design and implement an adaptive Underwater Acoustic (UWA) orthogonal frequency division multiplexing (OFDM) system employing adaptive modulation and transmit power. Recently, UWA communications gain more attractions as there has been a growing interest in exploring the communications possibilities for underwater applications such as oil field monitoring, oceanographic data collection, maritime archaeology and disaster prevention. However, the harsh characteristic of underwater environments imposes various difficulties for communication in this medium. Thus, proposing a reliable UWA communication system which has a high data rate can improve these applications performance significantly. Since UWA communications suffer from low data rates, we propose an adaptive OFDM communication system to support higher data rates over UWA channel. OFDM is an attractive technique for underwater channels due to the fact that inter-symbol-interference resulted from frequency selective channels can be removed completely. Additionally, adaptive communications achieve significant improvements in transmission data rate. For the sake of implementation, we take advantages of LabVIEW software to generate OFDM blocks. The generated data is transmitted to the receiver side with the aid of Software defined radios (USRPs) and acoustic front-ends. The received data at the acoustic hydrophones are transferred to the USRPs and then processed in LabVIEW for information detection. We conduct our experiments at the pool to investigate the performance of the proposed system over a real UWA channel.
Görünür ışık haberleşmesi için kanal modelleme ve nitelendirme: içmekan, araçsal ve sualtı kanallar
Despite the increasing attention on visible light communications (VLC) systems, there is a lack of proper visible light (VL) channel models. This is a serious concern since channel modeling is the very first step for efficient, reliable, and robust VLC system design. This dissertation focuses on channel modeling and characterization study for indoor, vehicular and underwater VLC. Our study is based on Zemax®; a commercial optical and illumination design software. Although the main purpose of such software is optical system design, we take advantage of the ray tracing features of this software which allows an accurate description of the interaction of rays emitted from the lighting source within a specified confined space. The simulation environment is created in Zemax® and enables us to specify the geometry of the environment, the objects within as well as the specifications of the sources (i.e., LEDs) and receivers (i.e., photodiodes). For a given number of rays and the number of reflections, the non-sequential ray tracing tool calculates the detected power and path lengths from source to detector for each ray. These are then imported to Matlab® and processed to yield the channel impulse response (CIR). In contrary to existing works which are mainly limited to ideal Lambertian sources and purely diffuse reflections, our approach is capable to obtain CIRs for any non-ideal sources as well as specular and mixed specular-diffuse reflections. Furthermore, we can precisely reflect the presence of objects and wavelength-dependent reflection characteristics of surface materials in channel study. In the first part of this thesis, we propose a realistic indoor channel modeling approach and carry out a detailed channel characterization study. We also investigate the effect of user mobility and receiver orientation on CIRs. In the second part of this thesis, we present VLC channel models for vehicle-to-vehicle (V2V) and vehicle-to-infrastructure (V2I) taking into account the asymmetrical pattern of headlamp and street lights, reflections from road surfaces and weather conditions. We further develop a closed-form path loss expression for V2V VLC channel for different weather conditions. In the last part of this thesis, we carry out a detailed underwater optical channel modeling and characterization study taking into account the reflection characteristics of the sea surface and sea bottom as well as the water characteristics, i.e., extinction coefficient, and scattering phase function of particles. We develop a closed-form path loss expression as an explicit function of water type, beam divergence angle and receiver aperture diameter and validate the accuracy of the proposed expression through Monte Carlo simulation results.
Dikey sualtı görünür ışık haberleşmesi bağlarının performans analizi
Demand for high-bandwidth underwater applications such as image and real-time video transmission, has been increasing parallel to the increase in maritime activities such as environment monitoring, navigation, ocean-pollution control, tactical surveillance, oil and gas field exploration, and coastal security. To satisfy this demand, underwater visible light communication (UVLC) has emerged as a high-capacity alternative to acoustic and RF signaling. One of the major impairments in UVLC systems is turbulence-induced fading as a result of temporal variations in temperature and salinity. Furthermore, unlike the horizontal links modeled with fixed turbulence strength, vertical links experience varying turbulence strength based on the depth-dependent temperature and salinity profiles. In this thesis, we consider a multiple-input multiple-output (MIMO) UVLC link over a vertical turbulence channel, which is modeled as the concatenation of multiple layers. Under the assumption of cascaded log-normal channel model, we derive the outage probability of vertical MIMO UVLC links and provide a quantitative analysis for the diversity gain in terms of the number of transmitter/receivers.
Lighting design considerations and channel modeling for visible light communications
There has been an extensive work on visible light communication (VLC) mainly due to scarcity in radio frequency (RF) spectrum and urgent need for a new wireless communication system in the near future. VLC considers the dual use of LED luminaires as the light sources for illumination as well as signal sources for wireless communication applications. However, to the best of our knowledge, the existing works mainly overlook lighting design considerations which might have significant effects on the performance of VLC systems. In addition, there is a lack of proper VLC channel models which take into account wiring and cabling topologies. In the first part of this thesis, we discuss some main indoor lighting design considerations and their implications for VLC applications. In particular, we consider some practical cabling topologies of LED luminaires and wiring topologies of LED chips within the luminaires. Based on non-sequential ray tracing, we obtain channel impulse responses (CIRs) and quantify the impact of cabling and wiring delays. In the second part of thesis, we consider outdoor applications of VLC. We first discuss outdoor lighting design considerations and their possible implications on VLC system design. Based on ray-tracing approach, we obtain CIRs for an infrastructure-to-pedestrian (I2P) outdoor VLC scenario. Using these CIRs, we calculate channel DC gain, path loss, root mean square (RMS) delay spread and mean excess delay values.
Performance characterization of vehicular visible light communications
In Intelligent Transportation Systems (ITSs), visible light communication (VLC) has emerged as a powerful candidate to enable wireless connectivity in vehicle-to-vehicle (V2V) and vehicle-to-infrastructure (V2I) links. VLC has been proposed as an alternative or complementary technology to radio frequency vehicular communications. While VLC has been studied intensively in the context of indoor communications, its application to vehicular networking is relatively new. Light emitting diodes (LEDs) are increasingly used in automotive exterior lighting. The expected wide availability of LED-based front and back lights makes VLC a natural vehicular connectivity solution. A major challenge in vehicular VLC systems is their relatively poor performance in adverse weather conditions such as severe fog. In this thesis, we evaluate the performance limits of vehicular VLC systems. In the first part of this thesis, we determine the maximum achievable distance to ensure a given bit error rate (BER) assuming positive-intrinsic-negative (PIN) diode as a receiver. We further investigate the deployment of relay-assisted systems to extend transmission ranges. In the second part of this thesis, we propose the use of single-photon avalanche diode (SPAD) for vehicular VLC systems. With their higher sensitivity in comparison to conventional photodetectors, SPADs can be efficiently used to detect weak signals. Under the assumption of on-off keying (OOK), we present the error rate performance of vehicular VLC systems. Since the SPAD output is modelled by Poisson statistics, BER takes the form of a semi-infinite summation. Based on Anscombe transformation, we approximate Poisson noise as Gaussian and derive a closed-form BER expression. Using this expression, we obtain a closed-form expression for maximum achievable link distance to ensure a targeted BER. We further investigate the deployment of relay-assisted systems to extend transmission ranges. In the third part of this thesis, we propose the use of SPAD array receiver for vehicular VLC systems. We obtain the closed-form expression for maximum achievable distance to ensure targeted BER. We present an extensive numerical study to validate our derivations and demonstrate the performance of vehicular VLC systems under different weather conditions.
Visible light communication techniques for future generation underwater networks
The demand for underwater communication systems has increased due to the increasing expansion of human activities in underwater environments such as underwater scientific data collection, environmental monitoring, offshore oil field exploration, maritime archaeology, port security and tactical surveillance. Wire-line systems (especially optical fiber systems) can be used to provide real-time communication, but their lack of flexibility, high cost and operational disadvantages become restrictive for most practical scenarios. This increases the demand for underwater wireless communications. The typical choice for wireless transmission underwater is acoustic signaling, which can support transmission ranges in the order of kilometers. It, however, falls short with its low data rate (in the order of tens of Kilobits Per Second (Kb/s)) for high-bandwidth underwater applications such as image and real-time video transmission. For short and medium ranges, optical communication has emerged as a high-capacity alternative. Since water is relatively transparent to blue or green light (450 nm – 550 nm), Laser Diodes (LDs) or Light Emitting Diodes (LEDs) can be used as transmitters for underwater wireless connectivity with data rates in the orders of tens of Megabits Per Second (Mb/s). Despite the increasing attention on Underwater Visible Light Communication (UVLC) systems, there is still a need for investigating the performance limit of UVLC, investigating the benefits of link adaptation, accurate modeling of Underwater Optical Turbulence (UOT), mitigating the turbulence effect and developing the physical layer algorithms. Motivated by these, we firstly investigate the performance limit of UVLC system in the absence of turbulence and pointing errors, which can be considered as an ideal benchmark. Particularly, we investigate the maximum achievable link distance to achieve a specified targeted Bit Error Rate (BER) in the pure sea, clear ocean, coastal water and harbor water based on the recently introduced closed-form path loss ratio expression. We further consider the deployment of relay-assisted transmission and determine the maximum achievable distance for a given number of hops to satisfy a targeted end-to-end BER. Investigating the performance limit of UVLC systems requires also studying it with the required data speed for high-bandwidth underwater applications. Since UVLC aim to achieve high data rates sufficiently high for real-time image and video transmission, the transmission may experience frequency-selectivity. This necessitates the use of efficient multi-carrier techniques such as Orthogonal Frequency Division Multiplexing (OFDM). Therefore, we consider an adaptive DC-biased optical-OFDM (DCO-OFDM) and investigate the benefits of link adaptation in the context of OFDM-based UVLC systems in the absence of turbulence and pointing errors. In the above points, the effect of UOT on the performance of UVLC systems is excluded. However, the performance of UVLC is severely affected by UOT. UOT introduces instantaneous fluctuations over the average received power as a result of the fluctuation in the refractive index of seawater. This is mainly caused by temperature and salinity fluctuations. Therefore, we develop an accurate spatial power spectrum model of turbulent fluctuations of the sea-water refraction index that accurately describe how the fluctuation in both temperature and salinity contribute to the fluctuation in the refractive index. Additionally, since both temperature and salinity of the seawater are not constant but change with depth and hence vertical links may experience variation in turbulence strength through the transmission range, we characterize the vertical UVLC optical turbulence channels. Utilizing our spectrum model and turbulence channels, we investigate the performance of UVLC systems. Particularly, we derive closed-form expressions for the BER performance of UVLC system based on the cascaded structure of the fading coefficient considering both weak and moderate/strong turbulence conditions. Then, we analyze the asymptotic behavior of BER and determine the diversity orders. Additionally, we derive closed-form expressions for the average Ergodic capacity of UVLC systems. Since the performance of UVLC is severely affected by UOT, we investigate the benefit of using different fading mitigation techniques on the performance of UVLC systems. Particularly, we consider the deployment of multiple transmitters and/or receivers to extract the spatial diversity. As an alternative, we consider the deployment of Transmit Laser Selection (TLS) that can extract full diversity while significantly reducing the complexity of the front-end. We propose the concept of Incremental Diversity Order (IDO) to quantify diversity gains of communication systems over Lognormal (LN) fading channels, which is defined as the incremental change in the decreasing rate of BER in log-log scale and determines how fast the diversity order changes with Signal-to-Noise Ratio (SNR). In the last part of this dissertation, we consider a practical implementation of Underwater Sensor Networks (USNs) that requires the design of multiple access systems for supporting several sensor nodes. We first introduce the use of adaptive Orthogonal Frequency Division Multiple Access (OFDMA) in the context of UVLC systems. The adaptive system is formulated such that the overall data rate is maximized subject to two constraints: identical user's data rate and BER of each user not exceeding a pre-defined maximum acceptable BER. We further consider Non-Orthogonal Multiple Access (NOMA) in the context of USN and investigate the achievable NOMA capacity.
Adaptive multiple-input multiple-output (MIMO) techniques for visible light communications
Visible Light Communication (VLC) technology is a promising solution of wireless communication systems in beyond 5G communication networks, which provides low implementation cost, high energy efficiency, and high-speed data transmission. The VLC systems can be considered as a complementary or alternative for the existing Radio Frequency (RF) based wireless communication systems to satisfy the demands of increasing high capacity. In the VLC systems, Light-Emitting Diodes (LEDs) are used as transmitters, and Photodetectors (PDs) are used as receivers. Multiple LEDs are installed to offer sufficient illumination for indoor environments. This feature can be utilized to implement Multiple-Input Multiple-Output (MIMO) communications systems in order to achieve high data rates by improving spectral efficiency. However, LEDs can only convey unipolar signals. Furthermore, the nature of the VLC channel is frequency-selective, which causes significant degradation in the performance of the VLC system due to critical Inter-Symbol Interference (ISI). Several schemes based on pulse modulation and Orthogonal Frequency Division Multiplexing (OFDM) have been proposed to overcome the ISI of the VLC system. A new Optical Orthogonal Frequency Division Multiplexing (O-OFDM), referred to as a Unipolar Orthogonal Frequency Division Multiplexing (U-OFDM), appears as an attractive solution for emerging VLC systems. In the future of 5G and beyond 5G communication networks, the spectral efficiency needs further improvement through MIMO with adaptive transmission technique. Motivated by these, we focus on improving the spectral efficiency and Bit Error Rate (BER) performance for the MIMO VLC systems in both the single-carrier and multi-carrier. Firstly, we propose a novel Adaptive Spatial Modulation (ASM) scheme, referred to as Flexible Generalized Spatial Modulation (FGSM), for single-carrier MIMO VLC systems to achieve better average Symbol Error Rate (SER) and higher spectral efficiency with a fixed overall number of LEDs compared to existing ASM and Generalized Spatial Modulation (GSM) schemes. The proposed system varies the modulation sizes over the available LEDs as well as the number of active LEDs. The selection of the modulation sizes is based on an optimization problem for the average SER under a predefined value of the spectral efficiency. We derive a closed-form expression of approximate average SER for the proposed system and evaluate its decoding complexity. We show that the FGSM scheme can be a potential candidate for future MIMO-VLC systems. Secondly, we firstly propose adaptive transmission technique for the MIMO VLC system in conjunction with U-OFDM to exploit the U-OFDM benefits. The proposed adaptive MIMO U-OFDM VLC system is implemented to support three different MIMO modes that enable a set of different modulation sizes. The considered MIMO modes are Repetition Coding (RC), Spatial Multiplexing (SMUX) and Spatial Modulation (SM). In the RC, the same signal is transmitted simultaneously from all LEDs, while independent signals are transmitted simultaneously from all LEDs in the SMUX. In the SM technique, a single LED is activated and the index of the activated LED carries information in addition to modulated signals. Depending on the received Signal-to-Noise Ratio (SNR) and target BER, the proposed adaptive transmission system switches between the available MIMO modes and adjusts its modulation size to achieve higher spectral efficiency. The proposed U-OFDM system is applied over different VLC MIMO setups with realistic channel models for 8×8, 4×4 and 2×2 MIMO systems. Recently, Generalized LED Index Modulation (GLIM) appears as an attractive MIMO OFDM scheme for emerging VLC systems. Therefore, in the last part of this thesis, we propose a Magnitude and Wrap-Phase OFDM (MW-OFDM) scheme for the MIMO VLC systems. The proposed MW-OFDM scheme relies on the conversion of complex signals into polar form with the magnitudes and wrap-phases to decrease the restriction on the number of LEDs compared to the conventional GLIM-OFDM. Moreover, the Maximum Likelihood (ML) estimator for the proposed scheme is derived. The proposed MW-OFDM scheme improves the average bit error rate and provides a significant reduction in the decoding complexity, compared to the conventional GLIM-OFDM. Moreover, a half number of LEDs are required for the proposed scheme to deliver the same spectral efficiency in a comparison with the conventional GLIM-OFDM.
Görünür ışık haberleşme ağları için hareketlilik yönetimi ve kaynak atama stratejileri
There has been a growing interest in Visible Light Communications (VLC) in recent years. The existing works in the literature on point-to-point VLC systems are well established, and especially the studies on physical layer aspects demonstrate the potential of VLC. However, to transform VLC into a multi-user, multi-cell, scalable, and fully networked wireless technology, significant further efforts for upper layer issues are needed. This dissertation focuses on mobility management and resource allocation techniques for indoor and vehicular VLC systems. In the first part of this thesis, we consider a centralized Direct Current biased Optical Orthogonal Frequency Division Multiplexing (DCO-OFDM) based VLC network and formulate a subcarrier resource allocation problem to maximize the user satisfaction index. We have used the Particle Swarm Optimization (PSO) method for the solution to the resulting problem. The superiority of the proposed algorithm is demonstrated through an extensive simulation study. In the second part of the thesis, we propose an energy-efficient dynamic resource management framework for indoor OFDM-based VLC networks. Specifically, we formulate and investigate a stochastic cross-layer optimization problem to optimize the network resources under the constraints of queue stability and power to maximize the average system throughput. In the last part of the thesis, we propose handover algorithms for indoor and outdoor VLC networks. First, we propose a handover algorithm for indoor VLC networks with Coordinated Multipoint (CoMP) transmission and evaluate its performance in physical channel settings based on ray tracing. Then, we present a novel dynamic handover algorithm for outdoor vehicular VLC networks. The proposed algorithm dynamically revises the handover parameters by considering the vehicle speed without requiring additional feedback information.