The explosion in demand for wireless services experienced over the past 20 years
has put significant pressure on system designers to increase the capacity of the
systems being deployed. While the spectral resource is very scarce and practically
exhausted, the biggest possibilities are predicted to be in the areas of spectral reuse
by unlicensed users or in exploiting the spatial dimension of the wireless channels.
The former approach is now under intense development and is known as the cogni-
tive radio approach (Haykin 2005).
