## 2 Related Works
Once the users are assigned onto certain CCs by CC selection schemes, the assignment of resource blocks in each CC is carried out. This is the RB assignment phase. For RB assignment schemes, throughput and fairness are considered as target objectives. The Proportional Fair (PF) scheme is selected as benchmark due to its simplicity and good performance [18, 22, 23]. Since PF is aware of the channel condition for each user, it can exploit the multi-user diversity. In the long term, this scheme can achieve fairness among active users in each CC [23]. But it has no consideration of different SA capability of terminals and does not exhibit good delay performance [24]. In order to improve the fairness between LTE and LTE-Advanced users, Cross-CC PF is proposed in [18, 22]. In Cross-CC PF, exchange of information on the past user throughput on each CC is required. Taking the past user throughput over all CCs makes the scheduling metric of LTE-Advanced users smaller compared with that of PF. Cross-CC PF can improve the average user throughputs for LTE users with no degradation in the average cell throughput. Cross-CC PF is the optimal scheduling scheme for a given CC selection [25]. There are also other RB assignment schemes. Round Robin (RR) [14, 16, 26, 27] is the basic scheduling scheme. In order to achieve better fairness among users, a user grouping PF algorithm (UG-PF) is proposed in [28]. With UG-PF, the users in poor channel conditions can obtain throughput improvement and better fairness among users is achieved compared with the PF scheme [29]. However, the aforementioned schemes do not consider that the spectrum aggregation may be carried out in CR networks, and the aforementioned scheduler structures and resource scheduling schemes become questionable and are not suitable for SA in CR networks. It is worth noting that there have been several alternative schemes for spectrum aggregation in the CR-based SA system. In [30], a spatial spectrum sharing-based carrier aggregation (SSS-CA) is considered from a game theory perspective. In SSS-CA, a network operator can not only transmit on its own licensed spectrum but it can also access and aggregate the licensed spectrum of other operators on payment of a certain price. In [31], a CR-based dynamic spectrum access scheme is proposed for an LTE-Advanced supporting carrier aggregation functionality. The scheme is referred to as DSA-CA. DSA-CA can activate and deactivate dynamically assigned CCs at an eNB depending on the traffic load of its cells. A spectrum aggregation algorithm and two spectrum allocation algorithms are proposed in [32]; the proposed spectrum aggregation algorithm can significantly increase the available bandwidth that secondary users (SUs) can access, and the proposed spectrum allocation algorithms can markedly improve the spectrum utilization efficiency. The authors in [33] have proposed a SAGCA algorithm based on the capacity of CR equipment. SAGCA can satisfy users' bandwidth requirements and utilize the available spectrum more efficiently. In [34], a spectrum aggregation algorithm called CCASA is proposed, which takes spectrum aggregation and channel state information into consideration. CCASA enables SUs to achieve a higher transmission rate and greater total throughput. A spectrum allocation algorithm aware spectrum aggregation is proposed in [35], and the spectrum utilization is improved by aggregating different idle spectrum fragments and assigning them to SUs opportunistically. In [36], a positional proportional fairness (PPF) model based on spectrum aggregation is proposed; its purpose is to solve the problem of unfairness resources aggregation caused by disadvantage positions, and the fairness and bandwidth jitter of PPF are better than the traditional proportional fairness in shadow. The authors in [37] have proposed a spectrum-aggregation-based cooperative routing protocol (SACRP) for CR Ad-Hoc Networks.