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In designing cognitive radio systems, one of the most critical issues is handling the channel handover process (CHP). The CHP consists of spectrum sensing, spectrum decision, negotiation on the common control channel, and adjustment of frequency and modulation settings, and such, it can be a time-consuming process. Consequently, initiating the CHP after each detected user activity (UA) can decrease the aggregate spectrum utilization. To alleviate this problem, we introduce a novel handover strategy to find the optimal trade-off between the durations of the CHP and UAs. With the use this model, secondary users (SUs) track only local information on their current data channel to make the decision to initiate the CHP or to wait for the termination of the ongoing UA. The system adapts to the dynamic conditions of the data channels and reduces the frequency of handovers to increase throughput and decrease access delay. We give analytical utilization bounds for SUs and also compare the performance of our model to those of other channel handover strategies by using extensive simulations. Our results for channels with heterogeneous loads and dynamic environments show that this model can clearly decrease the frequency of handover and consequently increase the aggregate SU utilization.

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Cognitive Radios (CRs) are the most promising solution to underutilization of spectrum and spectrum scarcity problems. However, designing suitable and reliable CRs have many crucial challenges. CRs are required to utilize the spectrum holes, share the spectrum resources with other users and bound harm on primary (licensed) transmission. To overcome these problems an accurate mathematical model for the spectrum access mechanism of the CR networks is mandatory. In this paper, we propose an analytical model for CR networks, which is based on the Enhanced Distributed Channel Access (EDCA) function of the 802.11e standard. We divide the users into two different access categories (ACs), namely, primary AC (pAC) and low-priority secondary AC (sAC) users. Here, we benefit from well-known EDCA parameters ( , , and ) to provide this prioritization. In addition, we determine optimal values of the EDCA parameter set for CRs in order to balance the trade-off between the protection of primary users and the throughput of secondary users. We analyze a single channel environment with saturation performance of two ACs. To verify our analytical model, we also perform simulations. Results obtained from model and simulations show that the proposed channel access protocol for CRs can be reliably used in 802.11e channels without any modification in the existing Wi-Fi systems.

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