  The future deployment of cognitive radios is critically dependent on the fact
that the incumbent primary user system must remain as oblivious as possible to
their presence. This in turn heavily relies on the fluctuations of the
interfering cognitive radio signals. In this letter we compute the level
crossing rates of the cumulative interference created by the cognitive radios.
We derive analytical formulae for the level crossing rates in Rayleigh and
Rician fast fading conditions. We approximate Rayleigh and Rician level
crossing rates using fluctuation rates of gamma and scaled noncentral $\chi^2$
processes respectively. The analytical results and the approximations used in
their derivations are verified by Monte Carlo simulations and the analysis is
applied to a particular CR allocation strategy.
