Efficient utilisation and sharing of limited spectrum resources in an autonomous fashion is one of the primary goals of cognitive radio. However, decentralised spectrum sharing can lead to interference scenarios that must be detected and characterised to help achieve the other goal of cognitive radio—reliable service for the end user. Interference events can be treated as unusual and therefore anomaly detection algorithms can be applied for their detection. Two complementary algorithms based on information theoretic measures of statistical distribution divergence and information content are proposed. The first method is applicable to signals with periodic structures and is based on the analysis of Kullback-Leibler divergence. The second utilises information content analysis to detect unusual events. Results from software and hardware implementations show that the proposed algorithms are effective, simple, and capable of processing high-speed signals in real time. Additionally, neither of the algorithms require demodulation of the signal. 1. Introduction Cognitive radio (CR) is the term used to describe smart, reconfigurable wireless communications devices that are capable of automatically adjusting their operating characteristics in order to adapt to changes in the radio environment. The purpose of such a system is to enable efficient use of the available radio spectrum and provide reliable service to the end user [2]. The motivation for efficient spectrum utilisation arises from the fact that it is a very limited resource. Although the electromagnetic spectrum is (for all intents and purposes) infinite, only a small fraction of it is usable for personal wireless communications as we know it today. Furthermore, while the spectrum available remains fixed, the number of wide-band wireless systems contending for access keeps growing—further compounding the spectrum scarcity problem. Traditionally, the radio spectrum has been divided into a number of usable bands by regulatory bodies such as the Federal Communications Commission (FCC) in the USA and the Office of Communications (Ofcom) in the UK. Each of the bands is then assigned for exclusive access by a particular operator or service. A notable exception is of course the set of bands known as the industrial, scientific and medical (ISM) bands where emission from unlicensed consumer electronic devices is tolerated. While this restrictive approach to sharing the radio spectrum succeeds at providing a certain degree of interference protection, it is an inefficient use of the available resources since it is
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