## 2 Related Works  
The differentiated characteristics of M2M traffic have been identified in [6], and are evaluated by field measurements in a large scale network in [17]. The most prominent features of M2M traffic are massive number of devices and uplink small data transmission. Therefore, the prime challenge faced with LTE/LTE-A networks is RACH congestion due to bursts of M2M access requests. Comprehensive surveys of load control mechanisms have been made in [12; 13; 14; 15]. Third generation partnership project (3 GPP) classifies these proposals into two main categories, namely push-based and pull-based methods [11]. As two of the push-based methods, ACB scheme aims at restricting the contending levels of services with different priorities, and dynamic resource allocation scheme aims at providing sufficient access opportunities according to traffic load. In ACB, BS firstly broadcasts an access barring factor \(p\) (\(0\leq p\leq 1\)). Then each device generates a random number and proceeds with the following random access procedure if the number is not larger than \(p\), otherwise the device has to wait for the next available access opportunity. Thus the determination of optimal ACB factor is the main concern. In [22], both fixed and dynamic preamble allocation schemes are proposed. The ACB factor is adjusted based on load estimation without prior information. In [23], in order to maximize success probability and to satisfy the delay requirement, a Markov chain is exploited to jointly optimize the extended access barring factor and RACH resource allocation. In [24], the throughput is maximized by dynamically partitioning preambles for delay-sensitive and delay tolerant classes. In order to reduce the collision probability, the authors in [25] propose to reserve multiple uplink channels for Msg3 for a single detected preamble. Another solution for congestion avoidance is to reuse the available preambles and uplink resources. In [26], the devices are grouped according to their spatial locations. Then for each group, a single root can generate more preamble sequences due to the reduced radius. Moreover, if the distance between two groups is larger than a threshold, the devices from different groups are allowed to reuse the same uplink channel for Msg3 [27]. The BS can decode the overlapped information through successive interference cancelation. As a pull-based overload control method, the performance of group paging is analyzed by an iterative method in [28]. Compared to uniform distributed traffic, the devices in group paging are usually activated simultaneously in the first access slot, which results in an extremely high system load. Due to the similar reason, surge of beta distributed M2M traffic can also cause severe performance reduction [11; 29; 30]. Therefore, backoff based load balance schemes for synchronized M2M traffic have been widely adopted. The authors in [31] adjust the backoff indicator dynamically to ensure the delayed devices would not enter into the next paging cycle. In [32; 33], uniform pre-backoff scheme has been applied to group paging. The authors also investigate the impact of backoff window size on the system performance. Further in [34], based on the stability analysis, an optimal pre-backoff scheme has been proposed for group paging to reduce energy consumption. In [35], the UPBO mechanism is also used for beta distributed traffic. The above works mainly focus on the M2M traffic issues. However, the H2H traffic is still taking an important role in current cellular networks. Then how to accommodate both H2H and M2M traffics at the same time becomes an urgent issue. One of the possible solutions is resources splitting. The authors in [36] analyze the impact of M2M traffic on H2H traffic. Simulation results show that H2H traffic has a better performance with RACH resource separation scheme while the M2M traffic prefers ACB scheme. A preamble set partition scheme is proposed in [37]. The impact of backoff indicator and retransmissions has been investigated by simulations.