# On the Evaluation of the NB-IoT Random Access Procedure in Monitoring Infrastructures
## 2 State of the Art
### Current Evaluations of the Random Access Procedure  
In a typical NB-IoT monitoring system, uplink transmissions prevail among the rest of exchanged messages. Consequently, the random access procedure represents a key relationship between devices and the base station. However, severe performance degradation occurs in the network when a multitude of simultaneous transmissions takes place since the random access procedure is built on top of a contention-based mechanism. Therefore, analytical models and system-level simulation tools able to investigate the behavior of the aforementioned random access procedure are extremely important for smart monitoring infrastructures. Current scientific literature offers some interesting analytical models for estimating the random access success probability. Nevertheless, depending on both the significant effort to provide a tractable analysis framework and the specific focus of each work, available studies still lack various features. In particular, [PERSON] et al. [16] applied a Markov chain to model retransmission number in a steady-state system with a Poisson arrival process. Instead, the authors of [17] derived the success probability under time correlated interference by considering different outage conditions. Nonetheless, both these systems are considerably distant from 3 GPP specifications, hence resulting in inadequacy for veridical performance assessment. In [18], the main 3 GPP parameters associated with the random access procedure are investigated to determine how they influence the design of network capacity, the average delay, and normalized flow rate. However, the authors wholly neglected multiple coverage classes. The same happens in [19], where the authors tried to optimally trade repetitions and retransmission values to achieve a target successful probability. Besides, [PERSON] et al. [20] and [PERSON] et al. [21] focused on the preamble transmission to investigate the performance in terms of detection probability, taking into account either the standard preamble design or a new detection and synchronization algorithm, respectively. However, the random access procedure is not considered as a whole, since only the first message is analyzed. Finally, [PERSON] et al. [22] presented a detailed model for the random access procedure, thereafter employed to perform a joint optimization technique to maximize the access success probability. Nevertheless, it still exposes different flaws. Differently from the standard [47], it only considers RAOs of different classes with the same starting time, hence constraining the total number of subcarriers. Besides, it only considers users requests generated according to the Poisson statistics and it has not been validated through reference tools of recognized importance. In addition, the authors extended a multiband multichannel slotted ALOHA framework, where the collision probability is only approximated through the bins and balls problem. The discussion above demonstrates clearly that simulation tools could be beneficial for addressing several open issues and driving future research activities. Such a simulation platform would be useful to validate the available analytical frameworks and to conduct performance assessments of entirely new studies. Nonetheless, with respect to recent simulation tools, only preliminary developments of NB-IoT are proposed. [PERSON] et al. [23] presented an ns-3 implementation that is largely incomplete, encompassing solely the signaling protocol. In addition, [PERSON] et al. [24] only proposed a link-level simulator, which is not freely available for the research community. Moreover, the NB-IoT open-source simulation presented in [25] does not offer a complete and standard-compliant implementation of the random access procedure. It is reasonable to conclude that state-of-the-art frameworks aiming to model contention-based access in NB-IoT systems are extremely important. However, from this review, it is evident that a 3 GPP-compliant NB-IoT simulation platform is still missing in the current literature.