الفهرس 
Background and MotivationOnly 14 pages are availabe for public view
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Abstract
The cognitive radio (CR) concept has been proposed as a solution to the growing demand of high data rate multimedia applications and the underutilization of the radio spectrum. To improve the radio spectrum utilization, cognitive radio network (CRN) technology allows the coexistence of licensed and unlicensed systems over the same spectrum. In underlay spectrum sharing based CRN, secondary users (SUs) transmit simultaneously with the primary users (PUs) in the same frequency band given that the interference caused by the SU to the PU remains below a tolerable interference limit. Besides the transmission power limitation, there are a lot of challenges which face the SUs on accessing and sharing the spectrum band of the PUs.
Through this thesis, two proposed schemes have been investigated to manage the spectrum for a centralized underlay multi-user orthogonal frequency division multiplexing (MU-OFDM) based CRN considering the rate loss and the interference constraints those the PU can tolerate to efficiently distribute the available subcarriers among the SUs while maintaining the QoS of primary users.
The first proposed spectrum allocation algorithm is based on allocating the subcarrier to the best SU that has the minimum interference channel gain, and hence this SU can transmit with the highest allowable power at this subcarrier interference power constraint (IPC) and achieve the maximum throughput over this subcarrier. The proposed scheme provides a significantly higher throughput for the CRN over the conventional schemes especially as the number of SUs increases.
The target of the second proposed subcarrier and power allocation scheme is to enhance both the fairness among SUs and the CRN throughput while maintaining the QoS of primary users. In the first phase, the proposed subcarrier allocation algorithm allocates the subcarrier to the best SU that has the minimum interference channel gain under the proposed fairness constraint to enhance both the throughput and fairness performance of CRN. In the second phase, the proposed power allocation algorithm efficiently distributes the power budget of each SU over its allocated subcarriers to enhance the SU’s throughput. The proposed scheme achieves a significantly higher CRN throughput than that of the conventional schemes and provides a significantly enhanced fairness performance. Both of the proposed schemes have demonstrated their robustness and efficiency on managing the spectrum in simulated real environments under imperfect channel state information (CSI).