الفهرس 
Chapter 1 IntroductionOnly 14 pages are availabe for public view
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Abstract
Nowadays, wireless communication systems are essential for modern societies. Moreover, number of wireless devices, applications and services are rabidly increased to keep pace with technological advances. All of these wireless systems need a wide spectrum band to operate, so it makes the entire radio spectrum more crowded with the licensed systems. In accordance with that the traditional static spectrum allocation does not be acclimating with requirements of the rapidly communication demands. where large portions of the spectrum are largely unoccupied most of the time. As described by the Federal Communication Commission (FCC) report which indicates that the licensed communication systems allocated the available radio spectrums and these systems utilize only from 15% to 85% of their spectrum bands at maximum usage. This leads us to think of proposed techniques to efficiently utilize the allocated spectrum band.
from this perspective, with seeking of efficient spectrum utilization, theCognitive Radio Networks (CRNs) must be mentioned. It is the key technology and promising technique proposed to efficiently managing and exploiting the radio spectrum band. CRNs provide the users of a secondary network (unlicensed network) with the ability to communicate concurrently with the users of a primary network (licensed network) and share all the spectrum allocated to the licensed systems after sensing the available vacant bands in that spectrum to prevent harmful interference and assuring the required Quality of Service (QoS). In fact, CRNs have three modes of operation such as: Overlay, Underlay and Interweaved modes. Firstly, in Overlay mode (Opportunistic Spectrum Access), the Secondary Users (SUs) have the ability to sense the presence of Primary Users (PUs) and opportunistically access the PUs authorized spectrum when it doesn’t use it, while ensuring the required QoS. Secondly, in Underlay mode (Spectrum Sharing), the SUs are allowed to share all the spectrum band with the PUs at the same time provided that the caused interference to PUs does not exceed a certain threshold.Finally, in Interweaved mode (Sensing-Based Spectrum sharing), SUs can access the licensed spectrum when PUs are not exist and controls its transmitted power to protect PUs transmission.
This thesis, studies different power allocation strategies for maximizing QoS of the secondary system through increasing their total throughput in downlink CRNs. These power allocation strategies are based on using several Multicarrier Modulation (MCM) techniques and different optimization algorithms in CRNs. This QoS maximization will occurred under some constraints such as maximum transmit power of the secondary network and the threshold of interference level at each PU receiver.
The first topic in this thesis, aims to enhance the Quality of Service (QoS) of the secondary system. This is achieved by maximizing its throughput using the Pattern Search (PS) algorithm. It is an optimization technique for the OFDMmulticarrier system in the underlay downlink CR network mode. Further, the effect of varying some parameters such as: number of SUs, number of PUs and number of Subcarriers on the system performance will be investigated.
The second topic, proposing an optimal power allocation strategy to maximize the total throughput of the secondary system using the Pattern Search (PS) algorithm for the Generalized Frequency Division Multiplexing (GFDM) system. Moreover, the proposed GFDM system helps to solve some of the OFDM system problems such as high Out-Of-Band (OOB) radiation, high Peak to Average Power Ratio (PAPR), it is sensitive to carrier frequency offset and the insertion of cyclic prefix can reduce the overall efficiency of the system.
The last topic, proposing a hybrid optimization technique called Particle Swarm Optimization and Pattern Search (PSO-PS) algorithm to maximize the total throughput for the secondary users using different multicarrier modulationtechniques like OFDM and GFDM systems in the downlink underlay CR network mode. Moreover, the effect of varying some parameters such as: SUs number, PUs number and Subcarriers number will be investigated.