# Optimal User Association Strategy for Large-Scale IoT Sensor Networks with Mobility on Cloud RANs
## 1 Introduction  
Utilizing short-range, low-power base stations (LPBSs) for wireless communication has been considered as a breakthrough in the information and communication technology (ICT) field. Although the conventional cell towers or macro base stations (macro BSs or MBSs) have advantages such as large coverage, they are associated with high operating expenses (OPEX) and capital expenditure (CAPEX) for their maintenance and installation, respectively, as well as low cell-edge user throughput. Therefore, LPBSs have been considered as an alternative solution to solve these issues. Their key advantage is the short distance between the transmitter and receiver that guarantees the high channel gain. Moreover, the short coverage increases the spatial reuse [1]. This technology was widely discussed in relation to third generation partnership program (3 GPP) long-term evolution (LTE) cellular networking systems in the form of pico-cell and femto-cell, or, more generally, small-cells [2; 3]. Consequently, in the era of next generation 5G networking systems, LPBSs have become one of the essential components. One of the most evident transformations from the current 4G to the next generation 5G cellular network is the network architecture called cloud-based radio access network (C-RAN). C-RAN [4; 5; 6; 7], in general, has a three-layer structure [8]: the remote radio head (RRH), the fronthaul network, and thebaseband processing unit (BBU) as shown in Figure 1. As can be seen in the figure, the general C-RAN architecture utilizes short-rage RRHs to provide the connection and data service to users. As previously mentioned, the short distance between communicating parties yields a reduced power consumption. Owing to collective powerful computing resources provided by the BBU pool, C-RAN is capable of handling a large number of users processing much larger volume of traffic than the 4G configuration with MBSs. To be specific, compared to the current 4G technology, the 5G technology is expected to achieve 1000 times larger mobile data throughput, to handle 10-100 times more user devices, to yield 10 times less power consumption, to enable 1 ms latency, and to have other desirable characteristics [9; 10]. The interested readers can refer to [4] for an in-depth technical introduction to the 5G wireless systems. Such enhancements will lead to further development of emerging applications including a large-scale network of Internet of Things (IoT) [9]. IoT applications are expected to be widely used in many applications related to the concept of smart home/city/factory, opening up wide business opportunities [11]. For example, a corresponding increase in the annual economic impact of USD $2.7-$6.2 trillion is projected by 2025 [11]. However, the gigantic market opportunity is associated with the large amount of burden to the underlying networks in terms of the heavy traffic load and a large number of IoT devices to be connected to the network. It is expected that in the very near future 212 billion IoT devices are to be connected, and the aggregated traffic from these devices is to account for nearly half of the entire traffic over the Internet [11]. Although the 5G wireless system is a promising platform for the future IoT technologies [4], combined with the C-RAN structure, the projected IoT device influx may result in many challenges. In this study, we particularly consider the following two: the unbalanced traffic load among RRHs and the overflow of control messages. These two challenges arise from the fact that RRHs are short-ranged, and the mobility of end user devices is inevitable [4]. To provide the coverage, RRHs have to be densely deployed [7], letting many of them be overlapped. Therefore, many users could have access to multiple RRHs to associate with. Considering the fact that IoT traffic is not the only load required to be handled by RRHs, it is possible that if a small number of RRHs happen to be over-populated by IoT devices they will easily become congested.