The spectrum sensing problem has gained new

aspects with cognitive radio and opportunistic spectrum access

concepts. It is one of the most challenging issues in cognitive

radio systems. In this paper, a survey of spectrum sensing

methodologies for cognitive radio is presented. Various aspects

of spectrum sensing problem are studied from a cognitive radio

perspective and multi-dimensional spectrum sensing concept is

introduced. Challenges associated with spectrum sensing are

given and enabling spectrum sensing methods are reviewed.

The paper explains the cooperative sensing concept and its

various forms. External sensing algorithms and other alternative

sensing methods are discussed. Furthermore, statistical modeling

of network traffic and utilization of these models for prediction

of primary user behavior is studied. Finally, sensing features of

some current wireless standards are given.