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Cognitive radio has emerged as an efficient approach to address the problem of spectrum under-utilization. OFDM has been proposed as a potential candidate for the physical layer of cognitive radios due to its inherent characteristics of spectrum shaping, spectrum analysis and robustness to frequency selective fading. One of the major problems of OFDM is high Out Of Band Radiation (OOBR). This is an undesired phenomenon especially if the nearby band is occupied by primary (or licensed) user. Among the proposed techniques to overcome this problem, Active Interference Cancellation (AIC) is considered to be one of the efficient techniques. In this paper, we evaluate the performance of this technique for single and multiple primary bands in OFDM based cognitive radios. For the case of single primary band, it is shown through simulations that narrower primary bands and use of more number of cancellation carriers increases the OOBR reduction in the desired band. For the case of multiple primary bands, the AIC technique is shown to provide better OOBR reduction for larger inter primary band spacing, lesser number of primary bands and more number of cancellation carriers. We also evaluate the bandwidth efficiency for single and multiple primary bands. The bandwidth efficiency shows a linear decrease with the increase in number of cancellation carriers. Multiple primary band system has more bandwidth efficiency degradation than the single primary band system.

Performance Evaluation of AIC Technique for Single and Multiple Primary Bands in OFDM based Cognitive Radios