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fact shows that the fixed spectrum allocation policy conveys to the low
spectrum utilization. The cognitive radio technique promises to improve the low
efficiency. This paper proposes an optimized access strategy combining overlay
scheme and underlay scheme for the cognitive radio. We model the service state
of the system as a continuous-time Markov model. Based on the service state,
the overlay manner or/and the underlay manner is/are used by the secondary
users. When the primary user is not transmitting and only one secondary user
has the requirement to transmit, the secondary system adopts the overlay
scheme. When the primary user is transmitting and the secondary users want to
transmit simultaneously, an underlay scheme with an access probability is
adopted. We obtain the optimal access probability in a closed form which
maximizes the overall system throughput.
In this paper, the performance of multichannel transmission in cognitive radio is studied. Both QoS constraints and interference limitations are considered. The activities of the primary users (PU)s are initially detected by cognitive users (CU)s who perform sensing process over multiple channels. They transmit in a single channel at variable power and rates depending on the channel sensing decisions and the fading environment. The cognitive operation is modeled as a state transition model in which all possible scenarios are studied. The QoS constraint of the cognitive users is investigated through statistical analysis. Analytical form for the effective capacity of the cognitive radio channel is found. Optimal power allocation and optimal channel selection criterion are obtained. Impact of several parameters on the transmission performance, as channel sensing parameters, number of available channels, fading and other, are identified through numerical example.