## 2 Related Works  
For resource scheduling in SA based systems, there are two different scheduler structures [14, 15]: the joint queue scheduler (JQS) and the disjoint queue scheduler (DQS). In the structure of the JQS, each user has only one queue for all CCs, and the CCs share the joint queue of each user. The shared scheduler maps the users' traffic load to the resource blocks (RBs) on all CCs. Although one user's data may only be transmitted on some of the CCs, the JQS requires each user to be able to receive a signal from all CCs simultaneously and continuously [15]. It largely increases the signal processing complexity and the power consumption of the UEs [14, 16]. Considering that the bandwidth of the SA based system is wide, the JQS implementation leads to high complexity [17]. DQS has a two-step scheduling structure. In the structure of the DQS, each user has one traffic queue on each CC. Through two-layer scheduling, the traffic loads of users are assigned onto certain CCs and the assignment of RBs in each CC is then carried out. When comparing the performance of two-scheduler structures, the performance of the DQS is inferior to that of the JQS in terms of spectral efficiency and resource utilization [15]. JQS is considered the optimal scheduler for the LTE-Advanced system with SA, at the cost of high complexity. The aggregated spectrum allocation for SA can be decomposed into CC selection and RB assignment phases. CC selection, to assign multiple CCs to users, is the new RRM functionality introduced in LTE-Advanced [15]. As methods for balancing the load across CCs will affect the system performance, the following three most notable CC selection methods to address load balancing have been the main focus of research in the literature: random selection (RS), circular selection (CS), and least load (LL). In RS scheme [18, 19], CCs for each UE are chosen randomly from the available CC set by the eNB. From the long-term point of view, it can provide balanced load across CCs. However, at each instant, the load across CCs may not be balanced and the system may suffer from reduced spectrum utilization. The CS scheme [16, 17] selects CCs circularly for the traffic data. Compared to RS, CS offers higher throughput and better coverage performance due to better balancing of traffic load over multiple CCs [16]. However, when the users traffic packet sizes are significantly different, the efficiency of CS can be decreased. The LL scheme [14, 17, 18] allocates the users' packets to each CC according to the current traffic load of CCs. Since the packets are always allocated to the CC with the lowest traffic load, LL provides better performance than other schemes (e.g., RS and CS) that do not consider the system state information [18]. And there are also other CC selection schemes for the inter-band aggregation. In order to optimize the performance of inter-band SA, the CC selection scheme should not only take the traffic load, but also the radio channel characteristics into consideration as proposed in: inter-band carrier switch method, RSRP based method, and G-factor based method. The inter-band carrier switch method [20] can allocate CCs at lower frequencies _i.e._, 2 GHz first, and then after checking the load on CCs, moves the UEs with the highest channel quality indicator (CQI) in 2 GHz to the 5 GHz band, but this method has increased complexity due to switching and does not consider UEs using CCs in different bands at the same time. The reference signal received power (RSRP) based CC selection scheme [1] allocates the better CC to the UE whose average data rate is relatively small, it is particularly effective for the real-time traffic. The G-factor based carrier selection algorithm [21] makes use of identities of cell-edge UEs based on geometry (G-) factor and is particularly effective in scenarios with large differences in G-factor distributions between CCs. These methods have the good performance for CC selection of SA in the LTE-Advanced system.