through a randomly selected PRACH. For each preamble, it has an equal probability ${\frac{1}{\xi}}$ to be chosen by each IoT device. Similarly, each PRACH has an equal probability $\frac{1}{F_s}$ to be selected. Thus, the average number of IoT devices in mIoT slice $s \in \mathcal{S}^I$ choosing the same PRACH and the same preamble is $\frac{\lambda_s^I}{\xi F_s}$. Notably, a greater $\xi F_s$ may significantly reduce signal interference experienced at each RRH.

Then, the following question should be tackled: *how many PRBs should be reserved for mIoT transmission?* To improve the resource utilization, the resource allocated to mIoT should be determined according to the requirements of mIoT and other coexistence services. It motivates us to optimize the resources orchestrated for the mIoT service except for analyzing the RACH procedure of IoT devices. The optimization procedure will be discussed in detail in the next section.

![The frame and minislot structure. ’R’ and ’D’ denote the resource block reserved for preamble and IoT data transmission. PBCH, PSS and SSS represent the PRBs for physical broadcast channel, primary synchronization signal and secondary synchronization signal transmission, respectively.[]{data-label="fig:fig_frame_minislot_structure"}](frame_slot_structure.eps){width="2.9in"}

### Access control scheme

In a mIoT network slice, as the slotted-ALOHA protocol allows all active IoT devices to 