However, it has been shown that the performance of CR systems can be very sensitive to any slight change in user densities, interference threshold, and transmission behaviors of the licensed system.
where we notice that the spectral efficiency decreases quickly with the increase in the number of primary receivers.
However, the spectral efficiency can be improved by either relaxing the interference threshold of the primary system or considering only the CR users who have short distances to the secondary BS.
In, hybrid CR networks have been proposed for adoption in cellular networks to explore additional bands
and expand the capacity.