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Spectrum sharing between license holders (primary users) and unlicensed secondary users has been proposed as a solution to frequency wastage. Anyhow, the coexistence among these two types of users should not affect the quality of service of primary users. In this paper, we propose an interference control approach when one primary and multiple secondary users are simultaneously transmitting on the same frequency band. We first evaluate the Signal to Interference-plus-Noise Ratio (SINR) of the primary user in the presence of multiple secondary transmissions. Assuming a constraint on the primary SINR, we explicitly compute the probability that this SINR exceeds a certain threshold in the case of cohabitation. These precise probabilities are afterwards used in the transitions between different states of the Continuous Time Markov Chain that we proposed to model the system. A new model with additional outage probabilities is then built, leading to do a cross layer design of dynamic spectrum access, by considering both the traffic queuing problem and the channel outage in physical layer. Finally, some performance metrics of the system are analyzed, namely the primary throughput, the mean number of secondary users accessing the spectrum, and the secondary deprivation rate.