In wireless Communication system, Radio Spectrum is the most valuable and limited resource. Due to traditional fixed spectrum allocation policy, there is a problem of spectrum under-utilization in licensed band whereas an unlicensed band is getting overcrowded. A new device called Cognitive Radio (CR) is introduced which allows unlicensed users to access licensed spectrum, without interfering with the operation of licensed user. Cognitive Radio follows Dynamic Spectrum Access Policy which makes it more vulnerable to the various attacks compared to traditional policy. An attacker can disrupt the basic functions of a Cognitive Radio Network (CRN) and cause harmful interference to the licensed user. Therefore security of CR is also a major concern. This paper provides a brief overview on operation, principles, architecture and security of Cognitive Radio. Finally, persistently unsolved challenges are highlighted.

1. FCC, ET Docket No 03-222, "Notice of Proposed Rule Making and Order", December 2003.
2. FCC, ET Docket No 02-135, "Spectrum Policy Task Force Report", November 2002.
3. I. F. Akyildiz, W. -Y. Lee, M. C. Vuran, and S. Mohanty, "Next Generation/ Dynamic Spectrum Access/ Cognitive Radio Wireless Networks: A Survey," Computer Networks, vol. 50, pp. 2127–2159, May 2006
4. I. F. Akyildiz, W. Lee, M. C. Vuran, S. Mohanty, "A survey on spectrum Management in Cognitive Radio Networks," IEEE Communications Magazine, April 2008.
5. A. M. Wyglinski, M. Nekovee, and T. Hou (eds), "Cognitive Radio Communications and Networks: Principles and Practice", Academic Press, 2009.
6. R. Thomas, L. DaSilva, and A. Mackenzie, "Cognitive networks," in Proc. IEEE DySPAN 2005, November 2005, pp. 352–360.
7. J. Mitola III, "Cognitive radio: an integrated agent architecture for software defined radio," Ph. D. dissertation, KTH Royal Institute of Technology, 2000.
8. F. Jondral, "Software?defined radio?basic and evolution to cognitive radio," EURASIP Journal on Wireless Communication and Networking, 2005.
9. J. Mitola III, "Cognitive radio for ?exible mobile multimedia communications," in Proceedings of the IEEE International Workshop on Mobile Multimedia Communications, San Diego, CA, USA, vol. 1, pp. 3–10, Nov. 1999. .
10. J. Mitola and G. Maguire, "Cognitive radio: Making software radios more personal,"IEEE Communication Magazine, vol. 6, no. 4, pp. 13–18, Aug. 1999.
11. Haykin, Simon. "Cognitive radio: brain-empowered wireless communications. "Selected Areas in Communications, IEEE Journal on 23. 2 (2005): 201-220.
12. Wassim El-Hajj; Haider Safa; Mohsen Guizani, "Survey of Security issues in Cognitive Radio Network," journal of internet technology, volume 12 2011.
13. T. C. Clancy and N. Goergen, "Security in cognitive radio networks: Threats and mitigation," Proc. , Intl. Conf. on Cognitive Radio Oriented Wireless Networks and Commum. (Crown-Com'2008), May 2008
14. Liu, KJ Ray, and Beibei Wang. "Cognitive radio networking and security. " A Game-Theoretic View, ISBN (2011): 978-0.
15. Chen, Shaxun, Kai Zeng, and Prasant Mohapatra. "Hearing is believing: Detecting mobile primary user emulation attack in white space. " INFOCOM, 2011 Proceedings IEEE. IEEE, 2011.
16. Chen, Ruiliang, Jung-Min Park, and Jeffrey H. Reed. "Defense against primary user emulation attacks in cognitive radio networks. " Selected Areas in Communications, IEEE Journal on 26. 1 (2008): 25-37.
17. Yu, F. Richard, et al. "Defense against spectrum sensing data falsification attacks in mobile ad hoc networks with cognitive radios. " Military Communications Conference, 2009. MILCOM 2009. IEEE. IEEE, 2009.
18. Yuan, Zhou, et al. "Routing-Toward-Primary-User Attack and Belief Propagation-Based Defense in Cognitive Radio Networks. " Mobile Computing, IEEE Transactions on 12. 9 (2013): 1750-1760.

Cognitive Radio Network Functions Spectrum Sensing Attack Open Challenges