# Joint AP Association and Bandwidth Allocation Optimization Algorithm in High-Dense WLANs
## 1 Introduction  
In recent years, the characteristics of IEEE 802.11 Wireless Local Area Networks (WLANs), such as its easy deployment, flexibility, and low cost, have made it a popular connectivity method to access the Internet. The IEEE 802.11 standards use the Received Signal Strength Indicator (RSSI) as the access point (AP) association scheme, which may lead the stations (STAs) to make associations with congested APs, while leaving adjacent APs to carry very light load or even to be idle. This kind of load unbalancing could result in network performance declining. Therefore, AP association strategy is crucial for load balance. However, various adopted metrics of AP association such as the traffic of clients [1; 2], the number of associated clients [3], and the channel busy ratio [4] still cannot refrain AP performance degradation. In addition, the MAC protocol of 802.11 based on DCF (Distributed Coordination Function) provides equal long-term transmission opportunities to all clients associated with the same AP [5]. In multi-rate WLANs, due to this kind of throughput-based fairness scheme, clients with lower rates occupy the channel for a longer time than those with higher rates do, which leads to the unfairness of channel access among clients and suppresses the aggregated throughput. Besides, the types of application services are diversified in various wireless terminals, and the different types of clients have different requirements for Quality of Service (QoS) [6], such as bandwidth and delay. Nevertheless, almost all proposed AP bandwidth allocation strategies are based on the assumption that any client has an enormous bandwidth demand; therefore, they cannot be satisfied no matter how the bandwidth of APs is allocated to them. The flexible AP bandwidthallocation strategy based on their demand and business priority can provide more diverse service and thus perhaps bring the user a better experience.  
In this paper, AP association and bandwidth allocation are jointly considered as the constraints of channel access for achieving load balance and proportional fairness in multi-rate WLANs scenario. We firstly present a Fair Bandwidth Allocation strategy based on clients' Business Priority (FBA-BP), in which the clients' bandwidth is calculated based on their demand and business priority. In addition, then the aggregated transmission time demanded by all the clients will be regarded as the metric of AP load. Consequently, we propose a Categorized AP Association algorithm Based on the Demands of clients (CAA-BD), which chooses the optimal associating AP according to its category and the aggregated transmission time demanded by all the associating clients. Eventually, these two joint algorithms can realize the proportional fairness of channel access and resolve the performance anomaly, thus bringing significant throughput improvement, especially in high-dense scenarios. The basic idea of AP associating algorithm was first published in CSOC2017 [7]. However, our new proposed CAA-BD is considering not only clients' data rates but also their business demand as the associating metric of the AP, and we also present a more detailed analytical model in the new scheme. Finally, the joint AP association and bandwidth allocation can conduct a more delicate AP selection and channel allocation, and the evaluation experiments also demonstrate that the more significant performance can be obtained in terms of AP utilization and channel fairness index.  
The remainder of the paper is organized as follows. We give some related works in Section 2. Section 3 provides a motivation model and theoretical analysis. Section 4 presents the details of FBA-BP and CAA-BD. We give the simulation results in Section 5 and conclude this paper in Section 6.