# mIoT Slice for 5G Systems: Design and Performance Evaluation
## 3 Gap Analysis for 4G System
### RAN Consideration  
The RAN segment offers radio resources to devices to transmit their data. When coming to mIoT, the RAN has an important role from a CP point of view as devices perform the random access (RA) procedure to establish the bearer to carry data. While mIoT UP traffic requires a small amount of radio resources due to the transmission of small and infrequent packets, the RA might represent the bottleneck and for this reason it has been object of several enhancements to improve its capacity and efficiency. Referring to the current RA as in LTE, usually the amount of resources (i.e., a pre-defined set of pseudo-orthogonal preambles) for the RA is composed of 54 preambles available every 5 ms, which brings to a capacity of 10,800 RA resources per second. Obviously, collisions limit this capacity but it is worth noticing that collisions happen only when two or more devices select the same preamble in the same RA opportunity and this is obtained only for very high load scenario. For instance, by considering a RA periodicity of 5 ms, a heavy load of \(10^{3}\) devices transmitting in 1 s (as for instance generated through a beta distribution as discussed in Section 2.2) brings to five devices per RA opportunity in LTE with a collision probability lower than 10%. In the case of huger load, congestion can be handled by means of the access class barring (ACB) [20] where a backoff mechanism is exploited before the preamble transmission to avoid a high number of devices performing the RA in the same RA opportunity. To further increase the capacity of the RA, [PERSON] et al. [21] proposed the code-expanded approach where the RA is performed by transmitting a code-word composed of two  
Figure 1: 4G System Reference Architecture. HLR: Home Location Register; MME: Mobility Management Entity; eNB: evolved NodeB; SGW: Serving GateWay; PGW: Packet GateWay; PCRF: policy and charging rules function; DN: Data Networks; UE: User Equipment. The terminologies used here are all the 4G standard terminologies. or more preambles instead of a single one, thus drastically reducing the probability for two devices to select the same code-word. A more detailed survey on the works on RAN can be found in [22; 23] with additional solutions to reduce the congestion in the RAN. Such works testify the interest of research community in increasing the capacity on the RAN and showing the feasibility of different solutions over LTE to handle mIoT traffic. An equivalent effort as surveyed in [22; 23] and references therein does not have its counterpart in terms of analysis on the CP signaling, and this further motivates the novelty of the work carried out in this paper. A further aspect testifying the effectiveness of the enhancements on the access of 4G systems is that the RA in 5G systems (as well as in IoT-tailored technologies such as NB-IoT (Narrow Band IoT) and LTE-M (LTE for machine-type communications) is strongly based on the solutions developed for LTE. This underlines that the access is not considered as a segment requiring major enhancements towards the support of mIoT use cases in 5G. Indeed, the main issues in effectively handling the mIoT slice comes from the design of E2E solutions that need to consider the reduction of the CP signaling to support scalability. From this point of view, this work fills this gap proposing an effective solution to limit the CP signaling in the CN. It is worth underling another aspect related to the role of the RAN (and in particular of the RA procedure) in controlling the CP/UP load towards the CN in case of overload. Radio overload means that there are not enough RA resources to handle all devices trying to connect to the network. Thus, a large portion of these devices will not be able to perform the RA procedure due to collisions. Consequently, CP/UP traffic from the RAN towards the CN will be reduced (this aspect will be analyzed in Sec. V.B of this paper). This means that, currently, the CN design takes advantage of this aspect as the CP signaling is expected to be low bounded by the RAN capacity.