# Dynamic RACH Partition for Massive Access of Differentiated M2M Services
## 3 System Model
### Random Access Procedures  
For MTC devices, we consider the typical random access procedures defined in LTE protocols [35], where all MTC devices compete for the available wireless resources. It is worth noting that in M2M networks, the contention-based random access procedure usually applies for delay-tolerant access requests. However, contention-free opportunistic access is also supported by pre-allocating particular resources, which are for delay-constrained requests. Because the amount of devices sending request with strict delay demands is quite small compared to delay-tolerant ones and the total number of such MTC devices is often huge, here we focus on the typical contention-based random access procedure, which consists of four steps [35] between MTC devices and the eNB. Moreover, the resource pre-allocation approach is very inflexible, and thus, not able to provide the requests with fine-grained delay requirements. Next, we elaborate on the detailed random access procedures as follows. When an MTC device attempts to access the network, it needs to send out an access request over the random access channel (RACH), which is comprised of several random access slots, which are used for the transmission of access requests. The length of the RA slot depends on the value of the configuration index. Relying on the existing protocols, the configuration index is valued as six, which means that in the RACH, there is an access opportunity every five milliseconds. In other words, the RACH finishes configuration every five milliseconds. Furthermore, there are in total 64 orthogonal available preambles. Only 54 of them are available for contention-based access, while the remaining 10 preambles are reserved for contention-free access. An access request is completed only if the four steps [31] are successfully finished, as shown in Figure 2 and Algorithm 1. However, note that the massive access attempts by sending the preambles occur in Step 1, which cause the majority of collisions. Thus, in this paper, we mainly concentrate on Step 1 to design the access control schemes. ```
1:Step 1:Preamble transmission Once an MTC device launches an access request to the RACH, it firstly selects a preamble of the RACH to transmit an access request. Under this condition, if two or more devices select the same preamble during the same slot, such that eNB is unable to decode any of the preambles, a collision occurs. 2:Step 2:Random access response (RAR) For each successfully-decoded preamble, the eNB computes an identifier and then transmits an RAR to the UEdevices. 3:Step 3:Connection request The M2M device transmits a connection request message with a UE identifier to the eNB. 4:Step 4:Contention completion Upon reception of a connection request in Step 3, the eNB transmits a contention resolution message as an answer to Step 3. Therefore, a device that does not receive Step 4 indicates a failure in the contention completion and requests a new access attempt. Figure 2: Random access procedures between the M2M device and eNodeB (eNB) in LTE. Before proceeding further, we need to explain the fundamental mechanism of the ACB mechanism and to define the corresponding parameters used in this paper. The ACB scheme [11] was proposed for random access control of MTC devices. Particularly, the eNB broadcasts two parameters: an access barring factor, denoted by ac_barringfactor, and an access barring time, denoted by ac_barringtime. Each MTC device attempting to access the network generates a uniformly-distributed random variable \(q\), \(0\leqslant q\leqslant 1\). If \(q\leqslant\) ac_barringfactor, the MTC device continues with the random access procedures. Otherwise, it is barred for a random time duration based on ac_barringtime by using Equations (24) and (25) to retry the access.