Spectrum Sensing Methodologies for Cognitive Radio Systems: A Review <s> D. Sensing Duration and Frequency <s> In November/2004, we witnessed the formation of the first worldwide effort to define a novel wireless air interface standard based on cognitive radios (CRs): the IEEE 802.22 working group (WG). The IEEE 802.22 WG is chartered with the development of a CR-based wireless regional area network (WRAN) physical (PHY) and medium access control (MAC) layers for use by license-exempt devices in the spectrum that is currently allocated to the television (TV) service. Since 802.22 is required to reuse the fallow TV spectrum without causing any harmful interference to incumbents (i.e., the TV receivers), cognitive radio techniques are of primary importance in order to sense and measure the spectrum and detect the presence/absence of incumbent signals. On top of that, other advanced techniques that facilitate coexistence such as dynamic spectrum management and radio environment characterization could be designed. In this paper, we provide a detailed overview of the 802.22 architecture, its requirements, applications, and coexistence considerations that not only form the basis for the definition of this groundbreaking wireless air interface standard, but that will also serve as foundation for future research in the promising area of CRs <s> BIB001 </s> Spectrum Sensing Methodologies for Cognitive Radio Systems: A Review <s> D. Sensing Duration and Frequency <s> Dual site spectrum measurements have been made in the public safety band in Howard County, Maryland, USA. The public safety band is of interest because of its obvious importance and the increasing need to determine whether improved spectral utilization would accommodate increased usage for public safety. Two receiver suites were synchronized and used to measure concurrent spectra at separations between a few meters up to a few kilometers. These measurements prove useful in examining issues of spectrum sensing for dynamic spectrum access, including receiver sensitivity, primary user signal detection, adjacent channel interference, and policy performance with local and cooperative sensing. We analyze the collected data to characterize the usage of this public safety network and provide insights into how white space can be identified and utilized. We propose a class of opportunistic access strategies and demonstrate how collected data can be used to evaluate the performance of such schemes. <s> BIB002 </s> Spectrum Sensing Methodologies for Cognitive Radio Systems: A Review <s> D. Sensing Duration and Frequency <s> In cognitive radio (CR) systems, secondary users (SUs) have to detect the channel periodically during their data transmission to decide whether the channel is idle in order to avoid unacceptable interferences to primary users (PUs). The duration of the detection process in a cycle affects the channel efficiency of the SU. Also, the detection time needs to be adaptively adjusted due to path loss variations caused by movements of PUs and SUs. In this paper, we explore the issue on the detection time selection that aims at improving the channel efficiency. We formulate this issue as an optimization problem, but it is extremely difficult to get the explicit solution. A numerical optimization algorithm is proposed to obtain the numerical solution to the problem. Numerical results show that the highest channel efficiency can be achieved by the optimization of the detection time. <s> BIB003 </s> Spectrum Sensing Methodologies for Cognitive Radio Systems: A Review <s> D. Sensing Duration and Frequency <s> Spectrum sensing is crucial for dynamic spectrum management systems. In this paper, we propose a scheme that utilizes the guard interval of OFDM symbol at the transmitter for spectrum sensing. The cyclic prefix is not inserted in the guard interval at the transmitter, whereas the circulant convolution is secured at the OFDM receiver through the proposed mechanism. Simulation results show that the scheme can be implemented with no impact on the BER under various channel conditions, and detection of incumbent DTV signal is possible in the OFDM guard interval. In addition, we develop enhancements to the circulant convolution preserving mechanism for handling the transceiver imperfections in practice. <s> BIB004 </s> Spectrum Sensing Methodologies for Cognitive Radio Systems: A Review <s> D. Sensing Duration and Frequency <s> One of the key challenges of the emerging cognitive radio-based IEEE 802.22 wireless regional area networks (WRANs) is to address two apparently conflicting requirements: ensuring QoS satisfaction for WRAN services while providing reliable spectrum sensing for guaranteeing licensed user protection. To perform reliable sensing, in the basic operation mode on a single frequency band (non-hopping mode), one must allocate quiet times, that is, periodically interrupt data transmission that could impair the QoS of WRAN. This critical issue can be addressed by an alternative operation mode proposed in 802.22 called dynamic frequency hopping (DFH), where WRAN data transmission is performed in parallel with spectrum sensing without interruptions. DFH community, as described in this article, is a mechanism that coordinates multiple WRAN cells operating in the DFH mode, such that efficient frequency usage and reliable channel sensing are achieved. The key idea of DFH community is that neighboring WRAN cells form cooperating communities that coordinate their DFH operations <s> BIB005 </s> Spectrum Sensing Methodologies for Cognitive Radio Systems: A Review <s> D. Sensing Duration and Frequency <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> BIB006
In order to prevent interference to and from primary license owner, cognitive radio should be able to identify the presence of primary users as quickly as possible and should exit the band immediately. Therefore, sensing methods should be able to identify the presence of primary users within certain duration. This requirement poses a limit on the performance of sensing algorithms and creates challenges for cognitive radio design. Selection of sensing parameters brings about tradeoff between the speeds (sensing time) and reliability of sensing. Sensing frequency is a design parameter that needs to be chosen carefully BIB002 . In a case when the status of primary users is known to change slowly, sensing frequency requirements can be relaxed. In addition to sensing frequency, the channel detection time, channel move time and some other timing related parameters are also defined in BIB001 . Another factor that can affect the sensing frequency is the interference tolerance of primary license owners. An example is when a cognitive radio is using opportunities in public safety bands, sensing should be done as frequently as possible in order to prevent any interference. The effect of sensing time on the performance of secondary users is investigated in  .The aim is to maximize the average throughput of secondary users while guiding primary users from interference BIB006 . Similarly, detection time is obtained using numerical optimization in BIB003 . Channel efficiency is maximized for a given detection probability. Sensing time can be decreased by sensing only changing parts of the spectrum instead of the entire target. A channel that is being used by secondary users cannot be used for sensing. Hence, secondary users have to stop data transmission for spectrum sensing BIB004 . This however, decreases the spectrum efficiency of the overall system BIB002 . To solve this problem, a method known as dynamic frequency hopping (DFH) is proposed. The DFH method is based on the assumption of having more than a single channel. This was proposed in BIB005 .