## 1 Introduction  
The first phase of standardization for the future 5 th generation of mobile networks (5G) was concluded in the Release 15 of the 3 GPP Evolved Universal Terrestrial Radio Access (E-UTRA) specifications [1]. Previous mobile generations, 3G and 4G, have merely focused on providing a faster and better mobile broadband service. Instead, 5G relies on three main use cases: massive machine-type communication (mMTC), enhanced mobile broadband (eMBB), and ultra-reliable low-latency communication (URLLC). The support for mMTC and URLLC are the main distinctive characteristics of 5G, and are expected to enable a great number of novel applications. These include, among others, the Internet-of-Things (IoT), where every daily-life object is connected to the Internet to empower a wide range of functionalities. Nevertheless, the Release 15 of the 3 GPP specifications introduced few advances in terms of mMTC support when compared to Release 14. One of the most critical requirements to efficiently support mMTC in 5G was to introduce enhancements to the random access procedure (RAP) of 4G. The RAP is the method used by user equipments (UEs) to switch from idle to connected mode. In other words, it is the method used by UEs for initial access to the cellular network. Several studies have demonstrated the inefficacy of the RAP, as defined by the 3 GPP [1], to support mMTC in 4G [2; 3; 4; 5]. Specifically, severe congestion can occur when a bulk of UE access attempts occur in a highly synchronized manner. The latter, which is a typical behavior in mMTC applications, is likely to exceed the signaling capacity of the cellular system. Since the RAP remains the same in 5G, the same limitations and problems persist. Enhancements are expected to be introduced to the RAP in the following standardization phases of 5G. At the present time, it remains uncertain whether these will be sufficient to support mMTC applications efficiently. On the other hand, access control schemes present an appealing solution to congestion. Because of this, a wide range of access control schemes have been proposed in the literature [6; 7; 8; 9; 10; 11]. One of the most promising access control schemes is the access class barring (ACB); hence, it has been included in the LTE-A Radio Resource Control (RRC) specifications [1]. The ACB scheme redistributes the UE access attempts through time. For this, each UE may delay the beginning of its RAP according to the barring parameters: barring rate and mean barring time. If the barring parameters are correctly configured, sporadic periods of congestion can be relieved in exchange for a longer access delay. This is true even if barring parameters are selected beforehand and remain fixed throughout the congestion period [5; 10; 12; 13]. However, congestion is a transitory phenomenon, so an adequate performance under several conditions can only be obtained by continuously adapting the barring parameters to the intensity of access attempts. Doing otherwise would increase the access delay of UEs during intervals in which no congestion occurs, but also fail to relieve congestion episodes that are more severe than expected. Needless to say, correctly configuring the barring parameters in real-time is a complicated task. Numerous methods to continuously adapt the barring parameters have been proposed in the literature [9; 14; 15; 16; 17; 18]; we refer to these methods simply as ACB configuration (ACBC) schemes. In other words, we use the term ACBC scheme to refer to any method or approach to select the barring parameters. In theory, some of the above-mentioned schemes can lead to a near-optimal performance in terms of success probability (i.e., probability of successfully completing the RAP) and access delay. However, most of these ACBC schemes were not designed for 3 GPP cellular networks. Specifically, such a high performance is normally achieved by assuming an idealized ACB scheme and by setting extremely precise barring parameters, for which complex processes and bold assumptions are oftentimes needed.