**4. System model**

In this section we consider two models for coordinated spectrum access. In the first one, secondary users are accepted or rejected according to an admission policy that only considers the impact on the blocking probability for primary users. In this first model there is a trade-off between the blocking probability of licensed and unlicensed users. The second model includes a spectrum bidding procedure, in which secondary users offer a price, within a finite countable set of prices for mathematical tractability, for the use of a channel. In the second model the trade-off appears between the the blocking probability of licensed users and the expected benefit obtained from spectrum rental.
