Spectrum availability model of secondary users in cognitive radio networks

Cognitive Radio systems are a promising solution to the spectrum scarcity problem, but precise modeling of spectrum utilization is vital to the analysis and design of such systems. In this paper, we mathematically derive the Probability Mass Function (PMF) of the fraction of available subcarriers for the secondary users in an OFDMA based cognitive radio system for different requesting distributions. The PMF of the total number of requested subcarriers from all primary users is derived and the effect of the requesting distribution on the asymptotic spectrum availability for secondary users is studied. It is shown that if a requesting distribution is undesired, providing more subcarriers will not increase the spectrum availability for the secondary network.