الفهرس 
Chapter 1 IntroductionOnly 14 pages are availabe for public view
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Abstract
The importance of the broadband wireless communications can be explained by a
large demand on high frequency utilization as well as high data rate access under high
mobility environments. All these requirements, without doubt, call for intensive research
efforts on how to cope with problems faced by current broadband wireless
communications. The primary challenges in broadband wireless communications are the
limited availability of radio frequency spectrum. With the increasing broadband
communication demands, the shortage of spectrum is becoming an outstanding problem.
The conflict between spectrum supply and demand is increasing. where the spectrum is
considered the backbone of wireless communications and its allocation issues are
becoming a problematic issue that is inadvisable to have a choice of its bandwidth without
careful analysis of information theoretic limits.
The utilization of licensed spectrums is very low under the present spectrum management
mode. Under the above background, the Cognitive Radio (CR) technology is proposed in
order to promote the efficient use of the spectrum and revive the lack of spectral resource.
The main aim of the research is to study some crucial issues of the CR and how to optimize
it’s performance:
Firstly, the hard and softened one bit and two bits hard combination scheme for
cooperative spectrum sensing is analyzed by taking into account the error effect on the
decision reporting. Also, Amplify and Forward (AAF) relaying mechanism to reduce the
performance loss is proposed. Then, three bits softened decision scheme in cooperative
spectrum sensing among Cognitive Radio Networks (CRN) is proposed. Moreover, Space
Time Block Coding (STBC) with Alamouti code to reduce the performance loss from
imperfect reporting channels is considered.
Secondly, the physical layer security in CRNs will be considered. The relay selection
scheme is proposed for security constrained CRNs with single eavesDROPper, multiple
eavesDROPpers and multiple primary users.
Thirdly, the Peak to Average Power Ratio (PAPR) in Orthogonal Frequency Division
Multiplexing (OFDM) based CR is considered. Some enhanced techniques to reduce the
PAPR in Non-Contiguous Orthogonal Frequency Division Multiplexing (NC-OFDM)