1.1.1 SPECTRUM MANAGEMENT FUNCTIONS
In order to adapt to DSA environment, CRAHN necessitate spectrum aware operation termed as cognitive cycle. Since it is a infrastructure less network, cross layer protocols are in need for incorporating these functionalities. A brief description about the functions :
Spectrum sensing: identifies the spectrum hole by observing the RF environment without causing harmful interference against PU.
Spectrum decision: Recognize appropriate frequency bands according to diverse QOS requirements.
Spectrum sharing : Sharing of available spectrum resource among multiple CR users.
Spectrum mobility: Process of vacating the spectrum band and to switch to another if PU is in the band of interest.
1.1.2 ISSUES AND CHALLENGES IN CRAHN
The coexistence of CR user with PUs impose some of the challenges through different functions of the cognitive cycle. They are optimization and coordination of sensing, joint detection of spectrum and reconfiguration, PU activity modelling, spectrum decision model, distributing topology information, flexible spectrum handoff, efficient design of routing algorithms.
1.2 COGNITIVE RADIO: A ROUTING PERSPECTIVE
1.2.1 MOTIVATION FOR CROSS LAYER DESIGN
Most of the research work in CRN has its focus on single hop wireless access networks. The researchers later realized CR as a paradigm shift for multi-hop networks to exploit the unexplored services with wide network coverage. To fully harvest the advantage of multi-hop CRN, it is necessary to study routing by considering the unique properties of CRN. Existing research works on CR routing mainly focuses on PHY and MAC layer issues but in some cases due to link failure, there is a need for re-routing at the network layer leading to performance degradation of MAC protocols. Thereby, to deliver reliable traffic in DSA system all communication protocols are closely related. In addition, multi-hop CRAHN requires cross layer interactions to develop routing protocols incorporating the best spectrum management functions. However, the major breakthrough of this research lies in utilizing the diversity gains in all dimensions namely time, frequency and space of the scarce spectrum to enhance transmissions among CR components.