Radio spectrum is becoming exhausted because of the growing demands for wireless broadband communications. To cope
this problem, the paradigm shift from the conventional exclusive use of frequency resources to the flexible frequency
utilization is indispensable for future wireless networks. Cogni…
Radio spectrum is becoming exhausted because of the growing demands for wireless broadband communications. To cope this problem, the paradigm shift from the conventional exclusive use of frequency resources to the flexible frequency utilization is indispensable for future wireless networks. Cognitive radio and heterogeneous wireless networks are key technologies that bring us an emerging paradigm of flexible frequency spectrum usage. By cognition of the radio frequency environment, cognitive radio terminals dynamically select the optimal frequency and transmission media with cooperation of networks. Technologies for heterogeneous wireless networks also realize the seamless system roaming among wireless systems.