Résumé : In this paper we present a new approach for cognitive radio. In the usual approach the secondary network is in charge of monitoring the channel to determine whether or not the primary network is active in the area. If not, the secondary network is allowed to use the spectrum. In the new scheme we propose, the primary network encompasses the techniques which allow it to capture the bandwidth even if the secondary network is transmitting in the area. The access scheme of the primary network is preemptive towards the secondary network. In this paper we present a scheme which is preemptive over the IEEE 802.11 decentralized access scheme. This protocol is a generalized Carrier Sense Multiple Access scheme using active signaling. Instead of only sensing the carrier, this algorithm also transmits bursts of signal which may be sensed by the other nodes. If so, they give up the selection process. We show that this scheme is preemptive over the IEEE 802.11 decentralized access scheme if the bursts transmitted by the node in the primary network are built with special sequences which alternate bursts of signal and periods of sensing. These sequences called $(d, k)$ sequences~\cite{jasz06} encompass a maximum number of zeros during which the node senses the channel to find other possible concurrent transmissions. In practice we use $d=0$ and $k$ depends on the duration on the IEEE 802.11 interframe and the duration of a signaling burst. We compute the number of $(0, k)$ sequences with respect to the length $n$ of the sequence. We also show that $(d, k)$ sequences (with $2d > k$) can used if by mistake during the the signaling phase one burst is not detected. We evaluate the number of such sequences. In a second part of this paper we propose a simple analytical model to compute the mean delay for the primary users versus the load of the primary and secondary users. We show that we have hierarchical independence between the primary and secondary users.