Multicarrier Communication Techniques for Spectrum Sensing and Communication in Cognitive Radios
In this tutorial article, we review different multicarrier communication methods for cognitive radio (CR) systems. There, the secondary users (SUs) need to (a) dynamically and reliably determine spectral holes and (b) transmit data in these resources without interfering with other parts of the frequency band. To satisfy (a), each SU has to be equipped with a spectrum analyzer. To satisfy (b), it is widely accepted that a multicarrier modulation technique should be adopted. Moreover, to maximize efficiency, it has been recognized that the side-lobes of each subcarrier band must be minimized. Much of the attention in the present literature emphasizes on the use of conventional orthogonal frequency division multiplexing (OFDM) – exploiting the fact that the fast Fourier transform (FFT) as part of the OFDM modulator can also be used for channel sensing. Herein, we discuss the performance of OFDM and also introduce filterbanks for multicarrier communication and spectral analysis in a CR setting. Moreover, the multitaper method (MTM) has been proposed as an effective method for spectrum analysis. Our article provides an insight into the pros and cons of these technologies.
The demand for ubiquitous wireless services has been on the rise in the past and is expected to remain the same in the future. Unfortunately, the vast majority of the available spectral resources have already been licensed. It thus appears that there is little or no room to add any new services, unless some of the existing licenses are discontinued. On the other hand, studies have shown that vast portions of the licensed spectra are rarely used [12]. This has initiated the idea of cognitive radio (CR), where secondary (i.e., unlicensed) users are allowed to transmit and receive data over portions of spectra when primary (i.e., licensed) users are inactive; demanding that the secondary users (SUs) be invisible to the primary users (PUs). To fulfill the invisibility requirement, SUs need to sense the spectrum, and this involves some sort of spectral analysis. In cases when the nature of the licensed signal is known, such analysis can be performed through feature (e.g., pilot ) detection. This is the case in research 802.22 which is currently being designed to operate in the TV bands [7]. However, in the general case where such knowledge is unavailable, spectral analysis has to rely on energy detection, with likely higher requirements for sensitivity and frequency
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