Information Theoretic Analysis of Cognitive Radio Systems

Cognitive radios have recently emerged as a prime candidates for exploiting the increasingly flexible licensing of wireless spectrum. Regulatory bodies have come to realize that most of the time, large portions of certain licensed frequency bands remain empty [17]. To remedy this, legislators are easing the way frequency bands are licensed and used. In particular, new regulations would allow for devices which are able to sense and adapt to their spectral environment, such as cognitive radios, to become secondary users. 4 Such users are wireless devices that opportunistically employ the spectrum already licensed to primary users. Primary users generally associate with the primary spectral license holder, and thus have a higher priority right to the spectrum. The intuitive goal behind secondary spectrum licensing is to increase the spectral efficiency of the network, while, depending on the type of licensing, not affecting higher priority users. The exact regulations governing secondary spectrum licensing are still being formulated [18], but it is clear that networks consisting of heterogeneous devices, both in terms of physical capabilities and in the right to the spectrum, will emerge. Among the many questions that remain to be answered about cognitive networks, is that of the fundamental limits of possible communication. Although this may be defined in various ways, information theory is an ideal tool and approach from which to explore the underlying, implementationindependent limits of such heterogeneous networks. In this chapter, we will