Spectrum Sensing Methodologies for Cognitive Radio Systems: A Review <s> C. Detecting Spread Spectrum Primary Users <s> In this paper we present an experimental study that comprehensively evaluates the performance of three different detection methods proposed for sensing of primary user signals in cognitive radios. For pilot and energy detection, our measurement results confirmed the theoretical expectations on sensing time performance. However, a physical implementation of these detectors in the presence of real noise uncertainties, analog impairments and interference allowed us to establish practical bounds on the detectable signal levels. In the case of collaborative detection, our analysis of experimental data collected in indoor environments identified the design parameters that can significantly improve the sensing gain: adaptive threshold, spatial separation and multiple antennas. <s> BIB001 </s> Spectrum Sensing Methodologies for Cognitive Radio Systems: A Review <s> C. Detecting Spread Spectrum Primary Users <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
There are two main types of technologies for detecting commercially available devices, they are: fixed frequency and spread spectrum. There are two main spread spectrum technologies available. The frequency hopping spread spectrum (FHSS) and direct sequence spread spectrum (DSSS). Fixed frequency devices operate at a single frequency or channel BIB002 . An example of this kind of system is the IEEE 802.11 a/g based WLAN. FHSS devices have the ability to change their operational frequencies dynamically into multiple narrowband channels. This technique is known as hopping and is implemented according to a sequence that is known to both transmitter and receiver. However, they use a single band to spread their energy. Primary users that use spread spectrum signaling are difficult to detect as the power of the primary user is distributed over a wide frequency range even though the actual information bandwidth is much narrower BIB001 . This problem can however be avoided partially if the hopping pattern is known and perfect synchronization to the signal can be achieved.