الفهرس 
Orthogonal Frequency Division MultiplexingOnly 14 pages are availabe for public view
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Abstract
High data rate wireless access is demanded by many applications. Usually, more bandwidth is required for higher data rate transmission in any of the system. With promising technology and ever-increasing wireless devices, the spectrum is becoming scarcer day by day which is considering as a problem. In this case, Cognitive Radio (CR) was proposed to solve this problem. OFDM can be used in CR. OFDM is a bandwidth efficient multicarrier modulation where the available spectrum is divided into subcarriers, with each subcarrier containing a low rate data stream. OFDM has gained a tremendous interest in recent years because of its robustness in the presence of severe multipath channel conditions with simple equalization, robustness against Inter Symbol Interference (ISI), multipath fading, in addition to its high spectral efficiency. However, the Peak-to-Average Power Ratio (PAPR) is a major drawback of OFDM system. In our research, we focused on PAPR problem and proposed a technique to make reduction of PAPR in OFDM system that is Spread Spectrum Roll-off filter. Comparisons between our proposed method and the system that uses other techniques such as clipping technique and interleaving technique will be made. Cognitive radio (CR) has been proposed to solve the problem of spectrum scarcely. To use the unused spectrum, spectrum sensing is needed to know if the primary user (licensed user) exists or not where the spectrum sensing problem has gained new aspects with cognitive radio and opportunistic spectrum access concepts. It is one of the most VI important operations of cognitive radio, that determines the efficiency with which the secondary users or cognitive users (CU) can use the primary spectrum, without causing appreciable interference to the primary users. Several factors like fading and shadowing affect the ability of the cognitive radio to detect the primary user. In this research, the performance of cooperation based spectrum sensing scheme will be studied for based on cognitive users using the energy detector. Some parameters will be studied in OFDM cognitive radio system such as channel gain and the effect of fusion rules under using AWGN. Moreover, an adaptive OFDM system, with a precoded cyclic prefix (PCP), will be proposed to address the recent need of robust and flexible transmission technique in Cognitive Radio (CR) communications which is not clarified until now. Moreover, different combining methods and diversity protocols are compared. To combine the incoming signals the channel quality should be estimated. Amplify and Forward (AAF) will be applied in this system. Also, the effect of the positions of the relay will be studied. Finally, we propose a cognitive transmission scheme with the best relay selection in a multiple-relay cognitive radio network to improve the secondary transmission performance while guaranteeing the quality-ofservice (QoS) of the primary transmissions.