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.

Abstract-Random Packet CDMA, a novel packet-based multiple access scheme for connectionless, uncoordinated random channel access is proposed. Random Packet CDMA, or RP-CDMA, utilizes a novel packet format which consists of a short header and a data portion. Each header is spread with a unique spreading code which is identical for all users and packets, while the data portion of each packet is spread by a randomly chosen spreading sequence. The receiver operates in two stages: header detection and data detection. For header detection a conventional spread spectrum receiver is sufficient. Headers are spread with a large enough processing gain to allow detection even in severe interference. The data portion is decoded with a sophisticated receiver, such as a multiuser detector, which allows for successful decoding of overlapping active packets. It is shown that the RP-CDMA system is detector capability limited and that it can significantly outperform Spread ALOHA systems whose performance is limited by the channel collision mechanism. RP-CDMA also experiences a much smaller packet retransmission rate than conventional or Spread ALOHA, and provides better spectral efficiencies.Index Terms-Random Packet Networks, RP-CDMA, ALOHA, Spread ALOHA, CDMA, multiuser detection.

Recent works in cognitive radio (CR) have suggested multicarrier communication based on orthogonal frequency-division multiplexing (OFDM) for CR networks. In this paper, we propose two alternative multicarrier techniques. The first one, filterbank-based OFDM-OQAM (OFDM-offset quadrature amplitude modulation), permits to maximize the secondary users' spectral efficiency by eliminating the need for guard bands. Also, OFDM-OQAM's filterbank spectral estimator has greater dynamic range than the conventional fast fourier transform in OFDM, which further reduces the probability of undesirable collisions between the secondary users (SU) and primary users (PU). However, OFDM-OQAM still requires an SU base station for distributed sensing. The number and the frequency of required sensing messages for uninterrupted PU operation depends on the randomness of the PU processes. We establish the cost of the CR methodology in the form of signaling bits per bit of payload data and relate our simulation results to the entropy of our PU processes by modeling the behavior of the PUs as finite state Markov chains.For cases where sensing is not available, when sensing information cannot be conveyed, or when the entropy of the PU processes results in unacceptable sensing-to-payload overhead, we suggest another multicarrier technique, namely multicarrier code division multiple access (MC-CDMA). While MC-CDMA does not achieve comparably high spectral efficiency as OFDM-OQAM, the spreading gain of MC-CDMA results in graceful degradation in the case of collisions between PUs and SUs. Furthermore, compared to direct sequence (DS) CDMA, MC-CDMA can exclude narrowband PU interferers locally at the SU receiver, hence improving SU performance. We show that MC-CDMA makes it possible to operate the SU network without distributed sensing and hence without a base station infrastructure and/or a signaling channel.

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