## 3 Multicarrier CDMA Based Modulation Schemes
### [PERSON] _et al_. [45] proposed Multi Carrier Code Division Multiple Access (MC-CDMA) as a promising technique for cognitive radio systems by exploring the reconfigurability options in MC-CDMA. MC-CDMA system is able to mitigate the ISI and the multiuser interference (MUI) in the frequency selective multipath fading channels by combining OFDM and CDMA techniques. The MC-CDMAtransmitter spreads the original data stream over different subcarriers in frequency domain, using a spreading code [46]. Multiple secondary users are assigned with orthogonal spreading codes to guarantee their separability at the receiver. The reconfigurability options in MC-CDMA systems include spreading code type and length, FFT size, number of active subcarriers, modulation method, bandwidth, data rate, channel coding, forward error correction and number of simultaneous users. A reconfigurable MC-CDMA transmitter schematic is shown in Figure 5.  
In a CR network, interference between MC-CDMA based secondary users and primary users can be avoided by using the subcarrier deactivation technique, similar to Non-Contiguous OFDM (NC-OFDM) and such schemes are called Non-Contiguous MC-CDMA (NC-MC-CDMA). [PERSON] _et al_. [47] investigated the performance of NC-OFDM and NC-MC-CDMA techniques for CR transceivers and made the following conclusions. With respect to PAPR, NC-OFDM exhibits the same problem as the conventional OFDM, while in MC-CDMA PAPR is reduced through frequency domain spreading. With respect to the nulling of subcarriers, NC-OFDM exhibits better error robustness compared to NC-MC-CDMA. In [48], space time block coding and modified interference cancellation techniques are considered for achieving antenna diversity and suppressing interference due to subcarrier deactivation in MC-CDMA based CR systems.  
[PERSON] _et al_. [49] proposed a cognitive MC-CDMA scheme which can adaptively change its transmission parameters, like bandwidth and power of sub-bands, according to the interference level in each sub-band. Compared to fixed-bandwidth sub-band deactivation method, the proposed method increases the spectral efficiency by excluding only the portion of the sub-band affected by the interference. In [50], the authors showed that, MC-CDMA is more suitable for underlay cognitive radio transmission. In MC-CDMA based underlay Cognitive Radio systems, the secondary user can spread its signal across the entire band, including that occupied by the primary user. The secondary user is allowed to use the spectrum of the primary user only when the interference received from the secondary user is less than the interference level that is tolerable by the primary receiver. Distributed sensing is not required in such wide-band multicarrier systems and hence, the DSA network can operate without a base station infrastructure and/or a signaling channel. The BER performance of MC-CDMA system over Rayleigh fading channel is shown in Figure 6 with spreading factor as  
Figure 5: Reconfigurable MC-CDMA transmitter.  
SF = 32. As shown in Figure 6, when the number of subcarriers deactivated (\(U_{d}\)) to avoid causing interference to primary users increases, the BER increases due to interference among different users. Thus, interference cancellation techniques are used in MC-CDMA systems [48] to suppress the interference caused due to subcarrier deactivation.