Spectrum is a limited resource whose allocation is done by several regulatory bodies across the globe. Licence is provided to the prospective service provider and a certain frequency range is fixed for a service. This process of fixed frequency allotment is however not efficient. To understand the problem, consider the two frequency bands licensed for GSM service at 900 MHz and 1800 MHz band respectively. At an instant, the GSM 900 band may get crowded with customers such that all resources are allocated and any new customer will have to wait in queue to get service. While, at the same instant, in the GSM 1800 band, there may be few customers latched and the unused spectrum gets wasted. This problem can be mitigated if spectrum allocation to the end user is done only on the basis of requirement. Cognitive Radio technology helps to transform this idea into reality.

Cognitive Radio concept was first presented by Joseph Mitola III. It can be simply understood as an intelligent wireless communication system which can sense and adapt to the changing RF environment in order to facilitate efficient utilization of the spectrum and ensure reliable communication. In the year 2003, IEEE Committee on Communication and Information Policy recommended FCC to use Cognitive Radio as a solution for efficient spectrum allocation. These intelligent radios can be classified into different types on the basis of some criteria as illustrated in Fig. C.1. The Cognitive Radio terminal can have different capabilities.