In LTE-Advanced (LTE-A), system enhancements have been studied in the 3GPP (3rd Generation Partnership Project) for standardizing integrated wireless communications using multiple radio access technologies (RATs) toward 5th generation (5G) mobile communications. One of study topics is LTE-WLAN (Wireless LAN) aggregation, aiming at enabling simultaneous usage of LTE radio access and WLAN radio access. An important technical challenge is the existing layer 2 structure. Specifically, the layer 2 structure for LTE-WLAN aggregation should be designed so that the Quality of Service (QoS) level of Evolved Packet System (EPS) bearers is kept even when each EPS bearer is sent over the WLAN radio. Requirements should include: (1) the layer 2 structure must be backward compatible to existing LTE-A specifications, and (2) the layer 2 structure must not have any impacts to existing WLAN specifications. In order to meet these requirements, in this paper, three layer 2 structures are proposed. The characteristics of proposed layer 2 structures are clarified, the pros and cons are discussed, and then one of proposed layer 2 structures is selected as a desirable structure to be standardized in 3GPP for LTE-WLAN aggregation.
3rd Generation Partnership Project (3GPP)  standardizes cellular mobile communications systems referred to as LTE- Advanced (LTE-A) [2-4]. One of major discussion topics in LTE-A Release 13 is the system enhancements for integrated radio communications using multiple radio access technologies (RATs). The target is to enable simultaneous usage of LTE-A radio access and another radio access technology in order to improve the user experience and deal with explosive amount of data traffic as part of the motivation for the 5th generation (5G) mobile communications. In particular, the rapid spread of Wireless LAN (WLAN), for example, it is widely deployed and used in home, office, and smartphones, has motivated the standardization of radio level integration and aggregation of LTE-A and WLAN.
There are some relevant existing and standardized methods for increasing data throughput in current 3GPP specifications. In LTE Release-8, network level traffic offloading from LTE to WLAN was standardized [5-6]. Until LTE-A Release-12, the method was enhanced so that the traffic offloading is carried out by taking the channel utilization rate of WLAN and the user preference of the traffic offloading into account . In LTE-A Release-12, Dual Connectivity (DC) was standardized. It allows Carrier Aggregation (CA) among base stations (eNBs: evolved NodeBs). In LTE-A Release-13, a study item called Licensed Assisted Access (LAA) has been newly established, targeting CA between licensed carriers and unlicensed carriers within one eNB . DC and LAA can be categorized as inter- eNB CA and intra-eNB CA, respectively.
The radio level integration and aggregation of LTE-A and WLAN is another technology direction, for which a new work item called LTE-WLAN aggregation was established in LTE-A Release-13 [10-11]. It has couple of benefits compared to the above techniques. Firstly, the above mentioned network level traffic offloading is not able to take radio aspects (e.g. wireless channel quality) into account since the core network does not have any information on LTE-A radio due to the functional separation between the core network and the access network. In addition, control signaling needs to be exchanged between the core network and the mobile terminal for handling the traffic offloading, which increases signaling overhead. Secondly, the above mentioned LAA can overcome the drawbacks of the network level traffic offloading, but inter-eNB CA cannot be performed. LTE-WLAN aggregation can overcome these drawbacks: radio aspects can be taken into account, control signaling to the core network is not needed, and inter-eNB CA feature enables CA with already deployed WLAN Access Points (APs) and Access Controllers (ACs).
The standardization of LTE-WLAN aggregation is ongoing and the layer 2 structure is still under discussions. One of key challenges is traffic classification when incoming data traffic to LTE-A is offloaded to WLAN since traffic classification methods are different. Specifically, data traffic in LTE-A is classified into bearer based on QoS level defined in LTE-A, while traffic in WLAN is classified into access category based on QoS level defined in WLAN. Therefore, it is important that the QoS level of traffic is transparent between LTE-A and WLAN, so that the QoS level in LTE-A can be guaranteed in WLAN. Key requirements of LTE-WLAN aggregation should include that (1) it must be backward compatible to existing 3GPP specifications to minimize specification impacts, and (2) it must not have any impacts to existing IEEE 802.11x specifications to enable the aggregation between LTE-A and already deployed APs/ACs. There is related work in . It is proposed that GRE (Generic Routing Encapsulation) protocol is used for the traffic offloading. However, the proposal does not consider the case when traffic with different QoS levels in LTE is offloaded to WLAN. In addition, implementing GRE on top of LTE layer 2 increases the GRE header overhead.
The objective of this paper is to tackle the challenge: layer 2 structure that enables the traffic classification. Three layer 2 structures are proposed. The characteristics of each structure are clarified, the pros and cons are discussed, and then one of f1 match Packet filtering by TFT (for UL) Start no f2 no no TFT f1 match Packet filtering by TFT (for DL) Start no f2 no no TFT proposed layer 2 structures is suggested as a desirable layer 2 structure to be standardized for LTE-WLAN aggregation.
II. IP FLOW TRANSMISSION IN LTE-A NETWORK
This section describes the system architecture of LTE-A (II-A) and the layer 2 structure (II-B) of LTE-A.
A. System Architecture of LTE-A
Fig. 1 depicts the overall system architecture of LTE-A. There are two gateways that IP flows must go through. In addition, there is one control node.
• The Packet Data Network Gateway (PGW) is one of the gateways that is located between external IP networks and the LTE-A network. For the downlink, IP flows are delivered to each destination node, which is referred to as Use Equipment (UE).
• The Serving Gateway (SGW) is another gateway that is located between PGW and eNB. SGW and PGW are connected via S5 interface. In addition, SGW and eNB are connected via S1-U interface. Neighbor eNBs are connected with each other via X2 interface.
• The Mobility Management Entity (MME) is a control node which is responsible for the overall control of UE, for example, the configuration of Evolved Packet System (EPS) bearer, security, and mobility. MME and SGW are connected via S11 interface. In addition, MME and eNB are connected via S1-MME interface.
An IP flow entering the LTE-A network is delivered from the PGW to the UE as an EPS bearer. It is uniquely mapped to the corresponding Radio Bearer (RB) when transmitted over LTE-A radio. The maximum number of EPS bearers is eleven.