  Cognitive radio technology is an innovative radio design concept which aims
to increase spectrum utilization by exploiting unused spectrum in dynamically
changing environments. By extending previous results, we investigate the
capacity gains achievable with this dynamic spectrum approach in asymmetric
fading channels. More specifically, we allow the secondary-to-primary and
secondary-to-secondary user channels to undergo Rayleigh or Rician fading, with
arbitrary link power. In order to compute the capacity, we derive the
distributions of ratios of Rayleigh and Rician variables. Compared to the
symmetric fading scenario, our results indicate several interesting features of
the capacity behaviour under both average and peak received power constraints.
Finally, the impact of multiple primary users on the capacity under asymmetric
fading has also been studied.
