Abstract-This paper addresses the spectrum-sharing for wireless communication where a cognitive or secondary user shares a spectrum with an existing primary user (and interferes with it). We propose two lower bounds, for the primary user mean rate, depending on the channel state information available for the secondary-user power control and the type of constraint for spectrum access. Several power control policies are investigated and the achieved primary-user mean rates are compared with these lower bounds. Specially, assuming all pairs of transmitter-receiver are achieving real-time delay-sensitive applications, we propose a novel secondary-user power control policy to ensure for both users, at a given occurrence, predefined minimum instantaneous rates. This power control uses only the secondary-user direct links gains estimations (secondary-to-secondary link and secondary-to-primary link).Index Terms-Channel state information, cognitive radio, interference channels, interference constraints, power control, radio spectrum management, Rayleigh channels.

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The reliability of usual Markov chain models for Rayleigh fading depends strongly on the channel variation regime. In this letter, a flat Rayleigh fading channel and Rake receiver output model for all fading variation speeds is proposed. The complex Gaussian fading and the Rake output processes are bandlimited. A variable-order Markov chain provides the desirable discrete process with suitable correlation properties at a frequency that respects the sampling theorem. An interpolator provides samples at the transmission rate. This model is available for all fading variation speeds, and the Markov chain is built only once. High soft-output reliability with important simulation time reduction is obtained.Index Terms-Markov chain, multipath channel, rake receiver.

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