# An Admission Control Mechanism for 5G LWA
## 1 Introduction  
The unprecedented escalation in demand for more capacity in cellular networks [1] has imposed significant challenges due to the limited licensed spectrum [2]. Any effort to achieve capacity growth through network densification faces the challenge of severe inter-cell interference. The solution to this issue is to make the best use of all spectrum types through matured technology. Therefore, the free and abundant amount of underutilized spectrum below the 6 GHz band motivates operators to combine long term evolution (LTE) with wireless local area network (WLAN) technology in fifth-generation (5G) networks [3; 4; 5]. LTE-WLAN aggregation at the link layer, popularly known as LWA [6], is considered to be one of the latest groundbreaking innovations with the third generation partnership project (3 GPP) release 13, providing high performance and a better user experience under a unified network. Motivated by the existence of a significant number of WLAN access points (APs) and indoor coverage deployed by a mobile network operator (MNO), LWA takes advantage of better usage of both WLAN and LTE access technology, eliminating their individual challenges. A WLAN benefits from large downlink bandwidth, but suffers from inefficient uplink bandwidth caused by contention, whereas LTE benefits from efficient scheduling on uplink but suffers from bandwidth scarcity on downlink. Hence, LWA redirects uplink traffic from WLAN to LTE and avoids resource waste due to WLAN contention on uplink. Since a radio channel is fully under the control of the scheduler (i.e., downlink is controlled by the WLAN AP, and uplink is controlled by the LTE scheduler), delays and rates canbe guaranteed. The LWA radio protocol architecture supports two kinds of data bearers: Split and switched bearers. As the names suggest, packets belonging to a switched LWA bearer are always scheduled over a WLAN by evolved node B (eNB), whereas packets belonging to a split LWA bearer can be scheduled over either a WLAN or LTE. In both cases, the packets received through both interfaces are reordered at the link layer and delivered to the higher layer in order. At present, the majority of traffic offloading studies, such as licensed assisted access (LAA) [7], LTE in the unlicensed band (LTE-U) [8], and LWA, focus on the use of unlicensed spectrum to leverage an overloaded LTE network with its free and abundant bands. However, the unlicensed bands below 6 GHz are already crowded by WLAN user equipment units (UEs) [9]. When other coexisting technologies along with LWA come into action, it will be more congested [10; 11]. For incumbent WLAN UEs, the network will be highly congested and will waste most of its time in contending for the channel. On the other hand, LWA UEs face high latency due to bottlenecks at WLAN APs. Eventually, these conditions lead to performance degradation for both standards, hurting system-level quality of service (QoS). Therefore, LWA should wisely consider steering the UEs' traffic, scrutinizing network loads, and improving WLAN utilization when it is available and not congested. Moreover, LWA should contribute to increasing the intra-cellular fairness between UEs. This means that the LWA opportunities must be given to UEs that make the best use of it (i.e., proportional fairness). Inspired by the above challenges with LWA, we present a framework of mode selection for LWA-preferred UEs, taking into account the QoS of both WLAN and LTE UEs. Our proposed algorithm smartly assigns LWA service to needy UEs, and guarantees an enhanced user experience for LWA UEs without degrading the QoS of existing WLAN UEs. Additionally, this algorithm contributes to maximizing network throughput and performance. The rest of the paper is organized as follows. In Section 2, the related work on LWA is presented along with the main contributions of our work. In Section 3, the system model is described, and the problem formulated. LWA admission control algorithms are also presented.