## 1 Introduction  
The spectrum requirements for wireless multimedia applications have increased exponentially, leading to spectrum scarcity. However, recent studies show that most of the licensed spectrum is underutilized. Cognitive radio (CR) has been proposed to bridge the gap between spectrum scarcity and underutilization of the available spectrum [1]. The concept of cognitive radio enables coexistence of the legacy systems and new users, which are called primary users (PU) and secondary users (SU) through dynamic spectrum access [2,3]. CR is an intelligent radio that has two important characteristics namely, observation and adaptability. The observation characteristic means that the CR can observe and learn from the radio environment through spectrum sensing, channel estimation, and modulation type identification. The adaptability characteristic shows that the CR is able to change its communication protocols and parameters through channel allocation, power allocation, modulation and coding scheme selection, and waveform adaptation, based on its observation results [4]. There are three major types of cognitive radio systems: interweave, underlay and overlay CRs. In the interweave type cognitive radio systems, the secondary devices detect the presence of primary user's signal through spectrum sensing [5] and transmit opportunistically only when there is no primary user transmission. In the underlay cognitive radio systems, secondary users are allowed to communicate along with primary users, as long as the interference created to the primary system is below some predefined threshold. The secondary users detect the presence of primary users and control their transmission power accordingly, in the underlay CR systems. Beamforming, MIMO and spread spectrum (SS) techniques are generally used to control the interference to primary users. Finally, in the overlay systems the secondary users transmit concurrently with the primary users. The key enable of the overlay system is that, the secondary user can split its power for secondary communication part and the remainder of the power to assist primary transmissions by relay operation. The power split is done in such a way that, the increase in a primary user's signal to noise ratio (SNR) due to the relay operation of the secondary user, is exactly offset by the decrease in the primary user's SNR due to the interference caused by secondary user's transmit power that is used for secondary communication. Single carrier modulation, multicarrier modulation and spread spectrum modulation are the most common types of modulation schemes used in wireless communication systems. In single carrier modulation, one radio frequency carrier is used to carry the information. In multicarrier modulation, multiple carriers at different frequencies are used to carry the information, sending a group of bits on each sub-channel. Multicarrier modulation is similar to Frequency Division Multiplexing (FDM) wherein all of the sub-channels are dedicated to a single data source. There are various advantages of multicarrier modulation schemes over single carrier modulation such as bandwidth efficiency and simplicity in the design of equalizers. In wireless channels, multipath propagation of signals leads to fading effects. Fading results in intersymbol interference (ISI), which limits high data rate wireless communications. The multicarrier modulation schemes (MCM) are used to overcome ISI by converting high-rate data stream into multiple low-rate data streams. Thus, MCM techniques are suitable for high data rate CR applications, in multipath fading channel conditions. MCM schemes such as Orthogonal Frequency Division Multiplexing (OFDM) and Filter Bank Multicarrier Modulation (FBMC) have been recognized as potential candidates for the physical layer of interweave cognitive radio systems [6]. Adaptive transceiver architecture based on MCM provides flexibility andreconfigurability to cognitive radio systems.