الفهرس 
Chapter 1: IntroductionOnly 14 pages are availabe for public view
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Abstract
Cognitive Radio Network (CRN) allows an unlicensed users known as secondary users (SUs) to use the unused licensed spectrum by the primary user (PU) with special conditions. The SUs have to vacate the spectrum band when the PU reclaims its band. This process called spectrum handoff and it may happen multiple times in highly dynamic environments. This may cause discontinuity in the secondary user transmission and performance degradation due to the selection of suitable target channel to resume its transmission. The time delay until it finds the suitable target channel is another problem that will affect on the transmission quality.
In this thesis, different spectrum handoff policies have been applied and the performance of the queuing delay point has been studied for each policy. In change policy, the SU has to leave the channel when the PU appears. The always stay policy with resuming transmission allows to the SU to stay in the same channel and after the PU leaves, it can resume its transmission. In always stay policy with restarting transmission, the SU stays in the same channel and after leaving of the PU, the SU restarts its transmission.
Simulation results indicate the difference in queuing delay between the three policies. The change policy has the lowest values of delay, but the target channel selection problem appears. Every policy from the three policies is suitable for a specific application.
To solve the problem of target channel selection for the change policy, two backup channels are reserved for the SU that vacated the channel for the PU. To select the best channel that has the shortest queue length consequently; the shortest delay, an approach has been proposed to decrease the handoff delay as much as we can. The SU takes the handoff decision due to the arrival of PU to prevent the interference with the PU. The SU transmit its signal with power above PU interference level, this technique called spectrum overlay technique. Whereas the spectrum underlay technique, the SUs transmit its signal with power below PU noise level which means that the PU and the SU can use the same channel at the same time. …………………………………
Abstract
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In case of the total interference to the PU exceed a certain threshold; the handoff decision will be the only selection for the SU. Spectrum handoff underlay technique is proposed by using Fuzzy Logic Control (FLC) that enables one to make effective spectrum handoff decisions using new information for handoff operation. The parameters: data rate, the received power at the SU from the PU and the probability of PU’s occupancy are considered as inputs of the proposed fuzzy based system. SU makes handoff only if it is not able to modify its transmitted power within the tolerable interference limits or if the quality of service received by SU is not satisfactory. Therefore, it reduces the probability of spectrum handoff. Consequently; the performance of SU’s communication will be improved.