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This paper presents a spectrum monitoring algorithm for Orthogonal FrequencyDivision Multiplexing (OFDM) based cognitive radios by which the primary userreappearance can be detected during the secondary user transmission. Theproposed technique reduces the frequency with which spectrum sensing must beperformed and greatly decreases the elapsed time between the start of a primarytransmission and its detection by the secondary network. This is done bysensing the change in signal strength over a number of reserved OFDMsub-carriers so that the reappearance of the primary user is quickly detected.Moreover, the OFDM impairments such as power leakage, Narrow Band Interference(NBI), and Inter-Carrier Interference (ICI) are investigated and their impacton the proposed technique is studied. Both analysis and simulation show thatthe \emph{energy ratio} algorithm can effectively and accurately detect theappearance of the primary user. Furthermore, our method achieves high immunityto frequency-selective fading channels for both single and multiple receiveantenna systems, with a complexity that is approximately twice that of aconventional energy detector.

Spectrum Monitoring Using Energy Ratio Algorithm For OFDM-Based Cognitive Radio Networks