# Optimal User Association Strategy for Large-Scale IoT Sensor Networks with Mobility on Cloud RANs
## 2 Related Work  
Optimal user association for networks with short-range BSs have been widely studied in the literature. [PERSON] et al. [3] proposed an optimal user-BS association scheme for LTE with small-cell BSs. They proposed a tractable and distributed solution which minimizes the power consumption from BSs while satisfying users service demand (i.e., traffic requirement). However, this study considers neither the task of load balancing among small-cell BSs nor possible frequent handover problems. In the paper [8], the authors proposed an optimal resource scheduling method for heterogeneous C-RANs. Considering the uncertainties in users' mobility and their traffic demand, the authors proposed a multi-stage stochastic programming based optimal solution that minimizes the overall power consumption from BSs while maximizing the profit to network service operators. However, the proposed solution does not consider the need for load balancing or preventing control message flooding due to frequent handovers. Load-aware or load-balancing association schemes have also been studied in previous research works. [PERSON] et al. [12] proposed an optimal user association and power adjustment scheme for ultra-dense wireless networks. Along with maximizing the network throughput-energy efficiency, the proposed scheme considers the traffic load handled by BSs by means of the interference level. However, the proposed scheme does not consider the possible performance deterioration caused by frequent handovers. The authors of [13] proposed an optimal user association scheme for load balancing in heterogeneous cellular networks. After forming a network-wide weighted utility maximization problem, the authors transformed it into a distributed algorithm of a low complexity. However, there is no discussion on the issue of frequent handovers, which should not be overlooked in dense networks such as C-RAN. The importance of load-balancing mechanisms in wireless networks has been discussed in detail in [14]. As it is an effective way of enhancing the resource utilization, fairness, delay performance, throughput, and other important aspects of mobile networks, there have been many efforts made to investigate this topic, including the study discussed in [15]. It has an initial approach similar to our proposed scheme; however, their solution for wireless multi-hop networks cannot be applied to C-RAN which is of our interest in this work. [PERSON] et al. [16] proposed a load-aware user-cell association method for load balancing in small cell assisted heterogeneous cellular networks. By using the decomposition technique, the authors proposed a distributed algorithm of low complexity to be applicable in real-time applications. However, the proposed scheme does not consider the effect of frequent handovers, which can cause severe network congestion in dense networks such as C-RAN. [PERSON] et al. [17] proposed a low-complexity algorithm to maximize the network energy efficiency with a fairness constraint for a dense heterogeneous network. To achieve this goal, the authors designed a joint small cell on/off control and load-balancing method aimed to optimize each task separately. Despite of the proposed scheme's practicality in heterogeneous networks, it is not applicable to the C-RAN network considered in this research. This is due to the fact that coverage holes may occur in CRANs if some RRHs become unavailable [18]. In addition, there have been studies trying to reduce the number of handovers using an advanced transmission coordination scheme such as coordinated multi-point transmission, or transforming a generic C-RAN into a heterogeneous C-RAN [19]. However, such approach can significantly increase the computational complexity of the user association procedure. A signaling overhead problem caused by handovers is discussed in detail in [20] focusing on heterogeneous networks with small cell BS.