  Cognitive Network is a technique which is used to improve the spectrum
utilization. Current network scenario is experiencing the huge spectrum
scarcity problem due to the fixed assignment policy so in this method great
amount of spectrum remain unused. To overcome this limitation the spectrum
allocation must be in dynamic manner. In this paper the spectrum allocation is
discussed thoroughly. Interference is the most important factor that needs to
be considered. It is caused by the environment (noise) or by other radio users.
It limits the possibility of spectrum reuse. Channel assignment is one of the
techniques used to control interference in the network. There exist a trade-off
between network capacity and level of contention. In cognitive radio networks
spectrum assignment or spectrum allocation or frequency assignment is used to
avoid interference. It is the process of simultaneous selection of operating
central frequency and bandwidth. In doing so, the process of sensing the
spectrum becomes very crucial; it must be reliable, accurate and efficient. The
accuracy of sensing affects the overall operation of cognitive networks.
Accurate results not only lead to higher utilization of the spectrum but also
preserve the privacy of primary user. Accuracy of sensing is highly affected by
the natural causes like noise, shadowing, fading etc. There are many other
challenges as well, like, hardware requirements, hidden node problem, security,
sensing frequency and duration, decision fusion etc.
