Future mobile terminals will be able to communicate with various heterogeneous systems which are different by means of the algorithms used to implement baseband processing and channel coding.This represents many challenges in designing flexible and energy efficient architectures. Using the sensing phase, the mobile can sense its environment and detect the spectrum holes and use them to communicate. Current research are investigating different techniques of using cognitive radio to reuse locally unused spectrum to increase the total system capacity. They aim also to develop efficient algorithm able to maximize the quality of service (QoS) for the secondary (unlicensed) users while minimizing the interference to the primary (licensed)users. However, there are many challenges across all layers of a cognitive radio system design, from its application to its implementation.