Advances in cognitive radio networks: A survey <s> B. Spectrum Sensing <s> Trends in technology are enhancing the future capability of devices to access the electromagnetic spectrum using the full range of dimensions associated with the spectrum. This increased capability to access the full spectrum “hyperspace” not only improves the ability of systems to use the spectrum but can also reduce interference and the adverse impact to system performance. This paper presents technical research into advanced adaptive techniques and identifies those parameters or characteristics where regulation and policy need to be reviewed. Fundamental to these new adaptive techniques is a behaviorbased approach to design and, possibly, policy. Consequently, this paper also presents a technical case study of one behavior-based approach, Dynamic Frequency Selection, that provides insight into the benefits and possible limitations of such approaches to cognitive radio design and policy. <s> BIB001 </s> Advances in cognitive radio networks: A survey <s> B. Spectrum Sensing <s> 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. <s> BIB002
Spectrum sensing enables the capability of a CR to measure, learn, and be aware of the radio's operating environment, such as the spectrum availability and interference status. When a certain frequency band is detected as not being used by the primary licensed user of the band at a particular time in a particular position, secondary users can utilize the spectrum, i.e., there exists a spectrum opportunity. Therefore, spectrum sensing can be performed in the time, frequency, and spatial domains. With the recent development of beamforming technology, multiple users can utilize the same channel/frequency at the same time in the same geographical location. Thus, if a primary user does not transmit in all the directions, extra spectrum opportunities can be created for secondary users in the directions where the primary user is not operating, and spectrum sensing needs also to take the angle of arrivals into account BIB002 . Primary users can also use their assigned bands by means of spread spectrum or frequency hopping, and then secondary users can transmit in the same band simultaneously without severely interfering with primary users as long as they adopt an orthogonal code with respect to the codes adopted by primary users BIB001 . This creates spectrum opportunities in code domain, but meanwhile requires detection of the codes used by primary users as well as multipath parameters. A wealth of literature on spectrum sensing focuses on primary transmitter detection based on the local measurements of secondary users, since detecting the primary users that are receiving data is in general very difficult. According to the a priori information they require and the resulting complexity and accuracy, spectrum sensing techniques can be categorized in the following types, which are summarized in Fig. 5 .