## 3 Multicarrier CDMA Based Modulation Schemes  
There are various spectrum sharing approaches proposed in the literatures for CDMA based cognitive radio systems. In [33; 34], the authors have proposed an interference-minimizing code assignment strategy for the secondary users in the CDMA underlay cognitive radio networks. The interference caused from the existing primary and secondary users is minimized by minimizing the mean square cross-correlations between the candidate codes and the received signal. The authors demonstrated that the proposed code-assignment strategy exhibits significant performance gains over random code-assignment strategy. In [35], the authors investigated the relationship among the interference threshold settings, the transmission rate of secondary links and the transmission power of the primary links in a CDMA based underlay cognitive wireless network. Given the interference threshold, the maximum achievable rate for the secondary links is found by formulating an optimization problem. In [36], the authors proposed a two-phase channel and power allocation scheme that improves the total number of subscribers that can be simultaneously served by the cognitive radio base stations. The channels and power are allocated to base stations such that the total coverage is maximized while maintaining the interference caused to each primary user below a prescribed limit. The authors of [37] proposed the adaptive spectrum sharing schemes namely intrusive and non-intrusive spectrum sharing, for CDMA based cognitive medium access control (MAC) in the uplink communications over the cognitive radio networks. The secondary users can adaptively switchbetween the schemes based on the channel utilization, traffic load and interference power constraints of primary users. The secondary users are allowed to transmit either in underlay mode or interweave mode based on the availability of the primary users. In [38], the authors proposed an iterative least square based primary user identification algorithm for detecting the signature codes utilized by the primary users in a CDMA network. Based on the availability of code channels, the secondary users are allocated with the channels and power simultaneously. The advantage of the proposed method is that, it does not require any prior knowledge about the primary CDMA network. In [39], the authors investigated the design of channel-aware access control (CAAC) algorithms to protect the primary user transmission, in cognitive radio networks. In their proposed algorithm, the access probabilities of secondary users are adjusted according to the channel-state information and the measured interference temperature. The authors have demonstrated the convergence and effectiveness of the distributed CAAC algorithms, in the work. In [40], the authors investigated the joint power and rate allocation strategies for CDMA based underlay cognitive radio networks with heterogeneous traffic. In their contribution, a combination of streaming and elastic traffic is considered and a cross-layer resource allocation is proposed. Max-min and proportional fairness (PF) approaches are compared for the optimization problem and it was demonstrated that, PF outperforms max-min approach in both homogenous and heterogeneous traffic conditions. In [41], the authors have investigated two joint admission and power control (JAPC) algorithms namely, minimal revenue efficiency removal (JAPC-MRER) and Minimal signal strength removal algorithm (JAPC-MSSRA) in the presence and absence of log-normal shadowing. The authors studied the combined effect of path loss and log-normal shadowing on total secondary revenue and blocking probability and also compared the performance of the two algorithms for shadowing and non-shadowing cases. In [42], the authors proposed a new cognitive radio network model based on CDMA and evaluated the performances of secondary users and primary users in terms of outage probability, blocking probability, BER and average data rate of secondary users.