# An Admission Control Mechanism for 5G LWA
## 3 System Model
### The Proposed LWA Algorithm  
We consider the LWA architecture shown in Figure 2 using radio resource management (RRM), for which details were provided by [PERSON] et al. [28]. The RRM entity in eNB is responsible for admittingor rejecting new UE requests for practicing LWA procedure. RRM functionalities can be divided into four different parts, i.e., database, interface, cell control, and UE control. The RRM database stores UE records such as current state, bearer configurations, ongoing procedure, LWA related configuration information from APs, and WLAN measurement reports. The interface entity allows interaction between radio resource control (RRC) and mobility management entity (MME) layers. The cell control helps to control cellular configurations for interference and broadcasting messages. The UE control section controls UE admission. In this UE control section, the LWA entity has been introduced to handle the WLAN measurement reports and to control the downlink data path by executing LWA activation, deactivation, and mobility. There is a load balancing (LB) entity which mitigates uneven distribution of the traffic load over LTE and WLAN. Similarly, the radio admission control (RAC) entity interacts with the RRM (responsible for LTE and WLAN radio resource management) to decide whether new radio bearers should be established according to available resources in LTE/WLAN and their QoS requirements. The radio bearer control (RBC) establishes the data radio bearers (DRBs) based on their radio bearer configurations after they are admitted by the RAC. Our algorithm resides on the LB entity which executes whenever a new bearer is created by RAC and distributes the data through the LTE and WLAN by modifying radio bearers at the RBC entity. There is an additional connection mobility control (CMC) entity which helps for mobility management and handover between the APs. The proposed LWA activation algorithm is summarized in Algorithm 1 and is described as follows. The APs are operated by the same cellular network providers. All the APs' parameters, as well as their connected UEs' information, are accessible to the eNB through backhaul networks, such as optical links. In the RRM database, UEs under each LWA-supported WLAN AP are listed with their LTE and WLAN throughputs. Among those UEs, LWA-preferred UEs are listed with normalized throughput gain. The normalized throughput gain is the achievable throughput gain in the network when the UEs' data are offloaded to the WLAN via LWA activation. Likewise, in order to find needy LTE UEs that can maximize network performance under both standards, UEs are sorted in decreasing order of achievable throughput gain. By doing this, LTE UEs that suffer from the lowest throughput and have the capacity to gain the highest throughput under LWA will be selected first from the list for LWA activation. This makes the most of system throughput performance, as well as UEs' throughput in both networks. Figure 2: Functional block diagram of LWA system. Furthermore, the maximum number of LTE UEs, \(m_{max}\), that the LTE network can admit, can be calculated by  
\[\frac{R_{w}(N\ +\ m_{max})}{N\ +\ m_{max}}\geq r_{w}^{th} \tag{6}\]  
where \(r_{w}^{th}\) is the throughput threshold of LWA UEs. This equation guarantees the LTE UEs (\(m_{max}\)) with the minimum throughput of \(r_{w}^{th}\) in the unlicensed band. This threshold can be dynamically set by eNB based on the type of data and the network conditions. To consider the total system throughput gain from the LWA network, we define the throughput gain function from WLAN and LTE network perspectives. Throughput gain in a WLAN network is due to the additional LWA UEs accessing the WLAN network. Similarly, in an LTE network, it is due to offloaded LWA UEs relinquishing bandwidth for the remaining LTE UEs. We define function _f(n)_ as follows:  
\[f(n)=(R_{w}(N+n)-R_{w}(N))+\ \sum_{i=1}^{M-n}G_{i} \tag{7}\]  
\[n=0,1,2,3\ \ldots m_{max}.\]  
where \(G_{i}\) is the throughput gain for the remaining LTE UEs.