 illustrated in Figs. 8.31 and 8.32. The LWA architecture introduced a new interface Xw which was defined for communica- tion between eNB and Wi-Fi AP, which is similar to X2, where user data is delivered through GTP tunnel, while control messages are delivered as Xw-AP - messages over SCTP. Upon arriving at eNB, downlink user traffic is split in PDCP sublayer and is for- warded over LTE and Wi-Fi. Some PDCP packets are delivered via data radio bearer over the LTE radio link and other packets are delivered to Wi-Fi AP by eNB, which adds a DRB ID to the packets to indicate which DRB they belong to and delivers the LWA PDUs to Wi-Fi AP through the IP tunnel established over Xw [5]. Wi-Fi AP then sets the packet type to PDCP and forwards the LWA PDU to LWA UE over IEEE 802.11 interface. Upon receiving IEEE 802.11 frames, the LWA UE forwards the frames to LTE PDCP sublayer, if the packet type is set to PDCP. The PDCP sublayer then collects PDCP packets received from LTE and Wi-Fi that belong to the same LWA bearer by checking their DRB IDs and aggregates them through reordering. The LWA adaptation protocol supports LWA operation as shown in Fig. 8.32 [5]. 8.3 Physical Layer Aspects To support a flexible LAA operation, a new type 3 frame structure was introduced in LTE Rel-13, in which UE considers each subframe as empty unless downlink transmission is detected in that subframe. For LAA, the LBT procedure can be completed at any time and downlink transmission may not start/end at the subframe boundary. Frame structure type 3 896 Chapter 8 is applicable to LAA unlicensed carrier exclusively with normal cyclic prefix. Each radio frame is 10 ms long and consists of 10 subframes of length 1 ms. Any of these 10 subframes can be used for uplink/downlink transmission or can be empty. LAA transmission can start and end at any subframe and can consist of one or more consecutive subframes in the burst. Partial subframes are introduced by 3GPP, as part of frame structure type 3, to support LBT and for efficient use of unlicensed