**Özet Metin**

We investigate the performance of receiver diversity techniques by considering energy detector based optimal spectrum sensing in cognitive radio networks over Rayleigh fading channel for both non-cooperative and cooperative communication systems. The analysis is first focused on the non-cooperative spectrum sensing part, including the analyses of selection combining (SC), equal gain combining (EGC), and maximal ratio combining (MRC) receiver diversity systems. We derive exact closed-form expressions for the optimal detection thresholds for each diversity scheme. By using these expressions, we present their detection performance compared to the no-diversity case. Then in the second part of this paper, the performance analysis of the proposed system is investigated for amplify-and-forward relaying spectrum sensing with SC, EGC, and MRC diversity techniques in Rayleigh fading channels. Particularly, average detection probability of end-to-end signal-to-noise ratio for SC and EGC schemes and the exact closed-form expressions of the optimal detection threshold for SC, EGC, and MRC schemes are derived. We also present detection performance for the cooperative system compared to the non-cooperative one. The analyses are validated by simulated receiver operating characteristic curves.

**Anahtar Kelimeler**

Cognitive radio networks, Spectrum sensing, AF relaying, Energy detector, Optimal detection threshol

**Özet Metin**

In this paper, optimum transmission distances are obtained for different relay nodes in the serial and parallel decode-and-forward relaying schemes by using differential evolution algorithm in free-space optical communication systems. The transmission distances are investigated by optimizing the place of the relay nodes at a target outage probability of 10^−6. In this study, the numerical results reveal that the optimum transmission distances are increased for both the whole power margins and different number of relay nodes with the help of the proposed optimization technique.

**Anahtar Kelimeler**

Free-space optical communication, Relay-assisted transmission, Atmospheric turbulence fading, Outage

**Özet Metin**

In this comparative study we evaluate the performance of Differential Evolution (DE) and Particle Swarm Optimization (PSO) algorithms in free space optical communications systems. In particular we obtain the optimal transmission distances for different relay nodes in the parallel decode-and-forward relaying schemes by optimizing the relay placements. We analyze that the cost functions are suitably minimized. Then we investigate the execution time and the stability of the DE and PSO algorithms for decode-and-forward relay-assisted free space optical links. The simulation results demonstrate that the PSO algorithm outperforms DE algorithm in terms of the minimization of the cost function. Furthermore the results indicate that we get the similar performance for the execution-time and optimization results in the DE and PSO algorithms.

**Anahtar Kelimeler**

communication systems, cooperative systems, distance measurement, evolutionary computation, particle

**Özet Metin**

The authors propose novel spectrum sensing and utilisation schemes for cognitive radio networks using the chirp-z transform. To improve the spectral efficiency, a dispersed chirp-z transform is introduced with the energy detection method. The dispersed chirp-z transform enables one to analyse the dispersed frequency spectrum of any frequency range of interest. The analysis is first focused on the sensing part, including the derivations of closed-form expressions for the optimal detection thresholds minimising the total error rate over additive white Gaussian noise (AWGN), Rayleigh, Rician and lognormally distributed fading channels. Then, the performance analysis of the proposed system is investigated for efficient spectrum utilisation over AWGN and fading channels by presenting the receiver operating characteristics. Conventional and segmented chirp-z transform-based spectrum utilisation techniques are also introduced for performance comparison purposes. Finally, the theoretical framework and the optimal threshold derivations through simulations are verified. The analyses reveal that the proposed schemes have a considerable potential to improve the non-cooperative spectrum sensing and utilisation performance.

**Anahtar Kelimeler**

**Özet Metin**

**Anahtar Kelimeler**

**Özet Metin**

In this study, the performance analysis of the spectrum sensing in cognitive radio networks is investigated with the energy detector method over Rician fading channels with the selective sampling and Otsu algorithm. Here, a new method based on noise reduction using the wavelet transform is proposed. Performance analysis of the proposed system model over Rician fading channels is presented by receiver operating characteristic curves for different signal to noise ratio values. The results show that the spectrum sensing performance increases when the selective sampling and Otsu algorithm are used at the receiver.

**Anahtar Kelimeler**

cognitive radio wavelet transform, energy detector, selective sampling, Otsu algorithm, spectrum sen

**Özet Metin**

This paper proposes the use of chirp-z transform (CZT) in spectrum sensing to achieve higher frequency resolution in spectrum monitoring. The CZT method can efficiently utilize the dispersed frequency spectrum of any desired band of interest. The performance of the proposed system over additive white Gaussian noise (AWGN) and Rayleigh channels is analyzed and presented through ROC curves. In order to maximize the total error rate performance of the proposed system, the optimal detection threshold expressions are derived in the case of AWGN and Rayleigh faded channels. Afterwards, the optimal detection threshold expressions are validated by simulation and numerical results.

**Anahtar Kelimeler**

chirp-z transform, Cognitive radio, spectrum sensing, energy detection, optimal detection threshold

**Özet Metin**

Bilişsel radyo, birincil kullanıcıların lisanslı spektrumlarında kullanılmayan kanalları fırsatçı bir şekilde kullanarak, kısıtlı spektrum kaynaklarının verimliliğini arttırmaya çalışan bir teknolojidir. Lisansı olmayan ikincil kullanıcıların fırsatçı erişimi ile birincil kullanıcıları ve spektrum boşluklarını tespit eden spektrum algılama, bilişsel radyo teknolojisinin önemli bir fonksiyondur. Enerji Dedektörü metodu, lisanslı kullanıcılar hakkında hiçbir bilgiye sahip olunmadığı durumda, en uygun algılama yöntemi olarak literatürde yer almaktadır. Öte yandan enerji dedektörü yöntemi, gürültüden etkilenmektedir. Bu çalışmada, enerji dedektörü temelli spektrum algılama performansını iyileştirmek amacıyla, dalgacık dönüşümü kullanılarak, gürültüden arındırmaya dayalı yeni bir yöntem önerilmiştir. Alıcıya gelen işareti gürültüden arındırmak amacıyla, ek olarak Otsu algoritması kullanılmıştır. Alıcı işletim karakteristik eğrisi benzetim sonucu spektrum algılama performansı için sunulmuştur.

**Anahtar Kelimeler**

Bilişsel radyo, dalgacık dönüşümü, enerji dedektörü, otsu eşikleme algoritması, spektrum algılama

**Özet Metin**

In this study the performance analysis of energy detector method for the spectrum sensing model in cognitive radio networks over Rayleigh fading channel with selection combining of the receiver diversity technique is examined. Here, energy detector model for non-cooperative spectrum sensing is investigated in which Chirp-z transform is used. The Chirp-z transform is proposed as an efficient algorithm to increase the dispersed frequency spectrum utilization, provides a flexibility to work only on the frequency spectrum of interest. The general closed form mathematical equation of the optimal detection threshold for selection combining receiver diversity technique is derived over Rayleigh fading channel. The performance analysis of the proposed system model is investigated with the optimal detection threshold value and presented through ROC curve for different SNR values. The achieved results validate the spectrum sensing performance increase in selection combining of the receiver antenna diversity technique, compared to the no diversity case.

**Anahtar Kelimeler**

cognitive radio, Spectrum sensing, energy detector, selection combining, Chirp-z transform

**Özet Metin**

In this study, the performance analysis of Fast Fourier Transform and Chirp Z transform based OFDM systems for cognitive radio networks with energy detector model for the spectrum sensing method is realized. Here, energy detection model for noncooperative spectrum sensing is investigated. On the other hand, the analysis is implemented by segmented Chirp z transform. In this study Chirp z transform based spectrum sensing is proposed for the increase of dispersed spectrum utilization and in this context firstly used in literature. Also with the system model, using inverse fast Fourier transform in transmitter and Chirp z transform in receiver rather than fast Fourier transform, the computational complexity and the processing time are reduced, and performance increase is observed for high signal to noise ratio values.

**Anahtar Kelimeler**

**Özet Metin**

**Anahtar Kelimeler**

**Özet Metin**

**Anahtar Kelimeler**

**Özet Metin**

Bu çalışmada, Ku bandında tanımlı uydu haberleşme kanalı için, dalgacık paket dönüşümü temelli OFDM (Orthogonal Frequency Divison Multiplexing) metodunun performans analizi üzerinde durulmuştur. OFDM, kablosuz haberleşme sistemlerinde kullanılan çok taşıyıcılı bir modülasyon yöntemidir. Yüksek hızda veri iletimini ve spektral verimliliği ön plana çıkarması, OFDM’in günümüz uydu haberleşme sistemlerinde uygulamasını artırmıştır. Bu çalışmada, GEO (Geostationary Earth Orbit) uydu haberleşme sistemi için bant verimliliğini artırmak amacıyla dalgacık tabanlı OFDM yöntemi, Ku bandı için önerilmiştir. SNR (Signal to Noise Ratio) değerine bağlı olarak BER’in (Bit Error Rate) hesap edilmesi ile performans benzetimi gerçeklenmiştir. Uydu haberleşme kanalı, AWGN (Additive White Gaussian Noise) Rayleigh ve Rician dağılım karakteristiği ile modellenmiştir. Her bir uydu kanal modeli için, birbirinden farklı dikgen dalgacıklar üzerinde detaylı çalışılarak, performans karakteristikleri çıkarılmıştır. AWGN, Rayleigh ve Rician uydu kanal modellerinde maksimum SNR minimum BER ile desteklenen sistem modeli için en uygun dalgacık ailesinin tespit edilmesi sağlanmıştır. Dalgacıkların kanal modellemesinde ve performans analizindeki etkileri detaylı olarak incelenmiştir.

**Anahtar Kelimeler**

**Özet Metin**

In this chapter, we evaluate the outage performance of decode-and-forward relaying in cognitive radio networks over Rayleigh fading channels, subject to the relay location for a secondary user. In particular, we obtain the optimal relay location in wireless communications systems for the cognitive radio networks, using differential evolution optimization algorithm. Then, we investigate the optimal transmission rate of the econdary user. We present the numerical results to validate the proposed theoretical analysis and to show the effects of the Rayleigh fading channel parameters for the whole system performance.

**Anahtar Kelimeler**

cognitive radio networks, decode-and-forward relaying, differential evolution optimization algorithm

**Özet Metin**

In this study, we evaluate the performance of differential evolution (DE) and particle swarm optimization (PSO) algorithms in free-space optical (FSO) and mobile radio communications systems. In particular, we obtain the optimal transmission distances for multiple-relay nodes in FSO communication systems and optimal relay locations in mobile radio communications systems for the cooperative-diversity networks, using both algorithms. We investigate the performance comparison of DE and PSO algorithms for the parallel decode-and-forward (DF) relaying. Then, we analyze the cost functions. Furthermore, we present the execution time and the stability of the DE and PSO algorithms.

**Anahtar Kelimeler**

free-space optical communications, cooperative-diversity networks, optimal distance, differential e

**Özet Metin**

In cognitive radio (CR) systems, due to the inefficiency in spectrum usage, some approaches are available proposing the usage of frequency band dynamically instead of allocating static carriers. They all take an important part in wireless communication standards developed in the recent years. Using dynamic spectrum access methods is a new generation radio communication technology. The CR system monitors the whole frequency spectrum and is aware about the spectrum usage based on the specific time, location and frequency and it also defines intelligent policies to exploit the existing spectrum resources. In a major part of this thesis, the performance analysis of CR networks over additive white Gaussian noise (AWGN) and fading channels is studied. Besides, the system models are investigated to increase the optimal and more efficient usage of the resources in the radio frequency spectrum. In this thesis non-cooperative and cooperative communication systems are analysed. In addition, it is indicated that the system performance can be increased using the receiver antenna diversity techniques both in non-cooperative and cooperative models. In this context, the thesis consists of two main parts. In the first part of the thesis, the performance of the detection probability in the dispersed frequency spectrum over AWGN and Rayleigh, Rician, Log-Normally distributed fading channels for the non-cooperative communication systems using energy detector (ED) method in CR networks is investigated. The increase in the utilization of the dispersed frequency spectrum and in the detection probability for non-cooperative spectrum sensing is realized with the chirp-z transform (CzT) based algorithms. Besides, it is denoted that, the ED based spectrum sensing performance is increased in CR networks over Rayleigh fading channels with multiple antennas in single-input multi-output (SIMO) system model using selection combining (SC), equal gain combining (EGC), and maximal ratio combining (MRC) techniques in the receiver side. Then general closed form expressions for the optimal detection threshold values, minimizing the total error rate and increasing the system performance over AWGN and Rayleigh, Rician, Log-Normally distributed fading channels are derived. In the simulations, it is shown that the proposed dispersed chirp-z transform (DCzT) algorithm has considerably better performance than the conventional FFT-based approach especially for low signal-to-noise ratio (SNR) values. Then, the simulation results show that the MRC scheme has considerably better performance than the others. The accuracy of the results based on the theoretical analysis is verified by means of the simulation results. As a last item, the proposed system model with DCzT algorithm may be associated with the 4th generation (4G) wireless communication standard called long term evolution (LTE) is presented in the first part of this thesis. The investigation of the spectrum utilization effects in the cooperative communication systems constitutes the second part of this thesis. In the cooperative sytem model, using SC and MRC receiver diversity, the general closed form expressions for the optimal detection threshold values over Rayleigh fading channels are derived. In the proposed single-relay cooperative communication scheme, the theoretically derived expressions for the probability density function (pdf) and the detection probability (Pd) in SC method are presented. In the simulations, it is shown that the spectrum sensing performance for cooperative communication is much better than the non-cooperative one for different SNR values. Detection performance is increased while SC, EGC, and MRC diversity techniques are used in the receiver side as compared to the no diversity case. The simulation results are presented for different SNR values using the optimal detection thresholds. Finally, we verify our theoretical analysis by means of simulation results. Our analysis reveals that the theoretical results confirm a good agreement between the simulation results, validating the accuracy of the derived analytical derivations.

**Anahtar Kelimeler**

Cognitive radio, non-cooperative/cooperative spectrum sensing, dispersed spectrum utilization, energ