tegrated into an eNB. This type of interworking enables timely accounting of radio aspects in the offloading decisions and transparent integration of the Wi-Fi access with a single node connection to the cellular core network (in this case, the eNB), which not only reduces control signaling but simplifies network management operations. In this section, we focus on 3GPP RAN-level features, for example, LTE-Wi-Fi aggregation (LWA) and LTE- Wi-Fi radio-level integration with IP security tunnel (LWIP) introduced by 3GPP in Rel-13 and extended in Rel-14 [16,22]. In both features, the aggregated LTE link (licensed Operation in Unlicensed and Shared Spectrum LTE/NR Network LTE/NR (KH)) (HH)) ((a)) LTE/NR Licensed LTE/ Network Network NR RCell NB/eNB Wi-FiAP NB/eNB Licensed LTE/ Unlicensed NR SCell Licensed LTE/ Wi-Fi (SCell) Licensed LTE/ LTE/NR SCell Licensed LTE/ NR PCell NR PCell Licensed LTE/ NR SCell NR PCell LTE/NR Carrier Aggregation using Licensed Bands (Link Licensed LTE/NR + Unlicensed Wi-Fi Carrier Aggregation Licensed LTE/NR + Unlicensed LTE/NR Carrier Aggregation Level at MAC Sub-layer) (Link Levelat PDCP Sub-layer) (Link Level at MAC Sub-layer) Figure 8.10 Comparison of various solutions for cellular operation in unlicensed spectrum [18]. operation) provides robust mobility and coverage, whereas the aggregated Wi-Fi link (unli- censed operation) allows routing of high data rate traffic, providing higher user throughput and more efficient use of both types of radio access. Despite their commonalities, LWA and LWIP have significant differences, mainly in the protocol layer at which the aggregation occurs and in the adopted user-plane security mechanisms. Fig. 8.10 summarizes various unlicensed access mechanisms that have been studied in 3GPP. In the following sections, we discuss in more details various schemes that have been used to enable LTE and NR access and operation in unlicensed spectrum. 8.1.4.1 License-Assisted Access Licensed assisted access was introduced in LTE Rel-13. It uses carrier aggregation