# Dynamic RACH Partition for Massive Access of Differentiated M2M Services
## 1 Introduction  
With the rapid development of cellular networks, the M2M communications, known as an indispensable part of Internet of Things (IoT) [1; 2], have received more and more research attention. Generally, M2M communications provide a flexible solution for many scenarios in IoT. On the one hand, M2M in LTE, relying on cellular networks as the infrastructure, provides the long-distance access option for IoT, such that better coverage can be achieved. On the other hand, M2M communications may also offer the possibility for short-range applications. In general, a M2M communication system includes a large number of machine-type communication (MTC) devices that can communicate without human intervention [3; 4; 5; 6; 7; 8; 9]. However, when the number of MTC devices trying to transmit data to the eNB (eNodeB) is considerably large within a very short period of time, the radio access network (RAN) overload issue will arise accordingly. In such a condition, the network congestion [4; 5] inevitably increases delays, causes packet loss and even leads to service interruption. To alleviate network congestion caused by RAN overload, several schemes [6] were proposed and studied, among which the access class barring (ACB) [7] scheme is currently regarded as a simple and popular solution in M2M networks. The key idea of the ACBscheme can be summarized as follows. Under the ACB scheme, the eNB broadcasts two parameters to all MTC devices. Based on the two parameters, each MTC device performs random access barring for itself. Particularly, each MTC generates a random number and compares the number with the threshold broadcast by the eNB. If the number is smaller than the threshold, it proceeds with the access attempt to the network; otherwise, it will back off for a random time period before attempting to access again. The ACB scheme is simple to implement, and thus, it receives wide attention. Generally speaking, the ACB scheme has been shown to be beneficial when MTC devices can tolerate long access delays due to frequent collisions during random access. However, when there also exist delay-sensitive services, the ACB scheme might not work efficiently. In the literature, the extended access barring (EAB) scheme [8] takes the delay-sensitive devices into account. The basic idea of EAB can be summarized as: as long as EAB is activated in the case of network congestion, the delay-sensitive devices are enabled to send access requests while the delay-tolerant ones are disabled from doing so. This approach actually sacrifices the performance for delay-tolerant services. However, it is worth noting that in realistic M2M networks, the number of delay-sensitive services is far less than that of delay-tolerant ones. Clearly, benefiting delay-sensitive services too much would severely degrade the performances of delay-tolerant services and use the resource in a highly-inefficient way. To address the aforementioned problems, this paper proposes an efficient scheme that performs dynamic allocation of the random access channel (RACH) resources to clustered MTC devices with differentiated delay requirements. Specifically, the proposed scheme can adjust the preamble partition ratio between the two given clusters. The original ACB scheme is further applied to each cluster for access attempts. The novelty and contribution of our work lie in the optimized partition solution, as well as the theoretical analyses. Simulation results show that compared to the baseline ACB scheme, the proposed scheme obtains significant improvement in access success probability and also achieves performance improvement in reducing access delay. The remainder of this paper is organized as follows. Section 2 reviews the related work on existing research efforts and candidate solutions on M2M congestion control. Section 3 introduces the system framework and M2M traffic model. Section 4 proposes the dynamic RACH partition scheme in detail. Section 5 analyzes the performance for our proposed scheme.