1.1.1        COGNITIVE CONCEPT

             The term CR was coined by Joseph Mitola and by its definition it enables CRAHN to access radio spectrum dynamically by its capability of sensing the environment, adapting to the changes by learning the interactions and reconfiguring the operating parameters to enhance the diverse QOS parameters. The spectrum access capability and dynamic programming through reconfigurability are considered as two main characteristics of cognitive radio.

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1.1.2         ARCHITECTURAL OVERVIEW

                        In the context of standard OSI (Open System Interconnection) model, the architecture of CR network is viewed as illustrated in Fig.1.

Fig.1 Architecture of CR in layered OSI model

 

                               Spectrum sensing plays a vital role in discovering spectrum hole as well as preventing PU from harmful interference in both physical and link layers. Physical layer sensing is done by various signal detection methods and Medium access control (MAC) sensing improves the detection performance of PU by the process of co-operative sensing. The function of network layer is to opportunistically route in a multi-hop environment such that the best route is discovered through cross layer interactions taking into account hop-by-hop spectrum availability. Protocol design is carried out in transport layer for improving traditional routing protocols or to completely create a new protocol to improve the overall spectrum availability. Spectrum access rights are transferred between PU and SU by mechanisms such as spectrum auction and leasing. This process of usage right transfer is termed as spectrum trading which is the function of application layer. Therefore, to evaluate the performance of CR, quality of service (QOS) awareness and security are considered as the vital functions for efficient spectrum utilization.