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Top1. Introduction
The great development of wireless communication and dramatic growth in the number of wireless devices beside the static management of the radio spectrum have produced a scarcity of available radio spectrum. With this static allocation, the spectrum is heavily used in some portions while in some others is not or rarely used. The scarcity of the radio spectrum is one of the most crucial issues at the forefront of future network research (Y. Arjoune and N. Kaabouch, 2019). Cognitive radio CR offer a solution to the spectral overcrowding problem by introducing opportunistic usage of the frequency bands that are not heavily occupied by licensed users (primary users PU) (T. Yucek and H. Arslan, 2009).
Spectrum sensing SS is the main component for the establishment of cognitive radio. It is generally defined as the amount of radio frequency energy over the spectrum; however, however, in cognitive radio system more characteristics of spectrum usage is considered like time, space, frequency, and code (T. Yucek and H. Arslan, 2009).
IEEE 802.22 standard is the first international CR standard to specify wireless regional area network (WRAN) systems operating in TV white spaces. The sensing requirements are summarized in terms of four parameters: sensing receiver sensitivity, sensing duration, probability of detection, and probability of false alarm. All CR in the 802.22 network will detect PU' signals using an omnidirectional antenna with a gain bigger than 0dBi. The antenna must be outdoors, clear of obstacles as much as possible, and at a minimum height of 10m above ground level. The sensing receiver reference sensitivity is specified for this 0 dBi antenna gain, and after all losses between the antenna and the input to the receiver is included. For digital TV, the sensing receiver sensitivity is –116dBm. For analog TV the sensitivity is –94dBm, while for wireless microphones the sensitivity is –107dBm. The sensing time is less than 2 sec. The probability of detection is 0.9, while the probability of false alarm is 0.1 for all signal types (Carl R. Stevenson, Gerald Chouinard, Zhongding Lei, Wendong Hu, Stephen J. Shellhammer, Winston Caldwell, 2009).
Many SS techniques have been proposed in literature, such as energy detection(ED), Matched filter (MF), eigenvalues based detection, and cylostationary detection technique (CSD). ED is a non-coherent detection method where the SU receiver does not need any prior knowledge on the primary user (PU) s' signals. It is also with low implementation complexity, but it is not effective at low SNR (Al-Hmood, 2015). Eigenvalues detection is also non-coherent and it is better than ED method for many reasons. Firstly, it overcomes the noise uncertainty problem and it has better performance when the PU's signals are highly correlated, but these methods have high computational complexity and low detection performance when PU's signals are not correlated (A. Kortun, T. Ratnarajah, M. Sellathurai, C. Zhong, and C. B. Papadias, 2011). In Matched filter technique, the secondary user (SU) needs to know the perfect information about the PU signals such as the pilot, preamble and training sequence that are used for channel estimation and synchronization. In addition to its easy implementation, this method has the shortest sensing time to obtain a good detection performance. However, the complexity of MF is very high, because the SU needs receivers for different PU signals. Another drawback of MF is the estimation error for the PU signal becomes high when SNR is low (E. Axell, G. Leus, E. G. Larsson, and H. Poor, 2012). CSD can detect the signals with low SNR; however, the detection requires long observation time and higher computational complexity. In addition, CSD needs the prior knowledge of the primary users (T. Yucek and H. Arslan, 2009).
Because of the limitation of traditional SS techniques and for efficient sensing IEEE 802.22 standard, two stage SS was proposed in several literature (F. Wasonga, T. Olwal, and A. M. Abu-Mahfouz, June 2018). It was considered to obtain the trade-offs of advantages and disadvantages between single stage SS technique. Two stage SS techniques offer some benefits compared to its single stage counterpart; enhancement of probability of detection, less detection time and promising performance in low SNR conditions.