الفهرس | Only 14 pages are availabe for public view |
Abstract In the last few decades, the spectrum of wireless communication systems has been subjected to the scarcity problem. This is due to the exponential increasing demand of wireless resources in order to fulfill the growth of multiple wireless communication applications which is characterized by an orthogonal assignment policy. Cognitive Radio (CR) was proposed as one of promising techniques to exploit the scarcity in the spectrum underutilization. To improve the spectrum utilization, CR networks (CRNs) technology permit the coexistence of licensed and unlicensed networks over the same spectrum. In an underlay spectrum sharing CRNs, secondary users (SUs) (i.e., unlicensed users) transmit simultaneously with the primary users (PUs) (i.e., licensed users) in the same resource block (time/frequency/code) where the interference caused by the SU to the PU remains below an acceptable level or limit. However, besides the transmission power limitation, the secondary network is subject to numerous restrictions. Some of those main problems are impairments of the communications’ channels such as fading channels and interference from the PUs’ transmissions. Furthermore, due to the broadcasting nature of CR nodes, CRNs are vulnerable to threating or eavesdropping challenges. To increase the transmission reliability within CR nodes, we jointly study cooperative CR and non-orthogonal multiple access (NOMA) techniques. Particularly, NOMA has been recently proposed for 3GPP Long Term Evolution (LTE) and envisioned to be a promising technique for the future 5th generation (5G) mobile networks. The key idea behind NOMA is to serve multiple users by the same resource block (i.e., time/frequency/code) but with different power levels. Thus, NOMA technique introduces superior spectral efficiency (SE) for future networks. This thesis is divided into two parts: The aims of first part of this thesis is to analyze the performance of a practical cooperative relaying CRN model. Owing to their CR characteristics, the underlay CRN is considered regarding the spectrum sharing and security constraints. Distinct relay selection (RS) and SU transmit power policies are proposed by developing exact mathematical methods under various mutual inter-network interference constraints (i.e., the worst-case scenarios), secrecy outage constraints and achievable rates for both delay-sensitive and delay-tolerant applications. Furthermore, a new promising transmission framework based on enhancing the physical layer security is introduced. Describing the interplay between the CRN nodes, new closed-form expressions are derived for the achievable rate, outage probability, non-zero secrecy capacity, secrecy outage probability and diversity order which reveal an insightful system design. The aims of second part of this thesis is to apply the basic principles of NOMA technique in CRNs by integrating NOMA with the CR nodes (i.e., the CR elements) in order to increase the transmission reliability, achieve superior performance, enhance the spectral efficiency and achieve better utilization of the resources. This approach provides several advantages over the existing members of multiple access techniques (i.e., conventional orthogonal multiple access (OMA)). Specifically, the performance of the relay-sharing based NOMA system within CR nodes is investigated for both directional and bi-directional communications. Secondly, cooperative multiuser selection schemes inspired NOMA system are proposed to overcome the problem of deep-fading environment and achieve higher reliability for CRNs. Finally, the issue of joint user fairness and the best choice of power allocation is addressed here which play an important role in boosting the system performance while simulation results are conducted to emphasize the superior performance and verify the correctness of our analysis. |