  The growing demand for wireless applications and services on the one hand,
and limited available radio spectrum on the other hand have made cognitive
radio (CR) a promising solution for future mobile networks. It has attracted
considerable attention by academia and industry since its introduction in 1999
and several relevant standards have been developed within the last decade.
Cognitive radio is based on four main functions, spanning across more than one
layer of OSI model. Therefore, solutions based on cognitive radio technology
require cross layer (CL) designs for optimum performance. This article briefly
reviews the basics of cognitive radio technology as an introduction and
highlights the need for cross layer design in systems deploying CR technology.
Then some of the published standards with CL characteristics are outlined in a
later section, and in the final section some research examples of cross layer
design ideas based on the existing CR standards conclude this article.
