## 2 Millimeter-Wave Integrated Cellular Network
### LTE-WLAN Interconnection and Aggregation  
The Discovery and Selection Function of the Access Network (ANDSF) is a Core Network (CN) function that controls UP data offload between the 3 GPP RAT and the non-3 GPP RAT. The ANDSF is located in the Evolved Packet Core (EPC), which is part of the 3 GPP CN. The ANDSF conveys UE context-based assistance information that is required for the discovery and selection of the RAT to the UE. The UE selects an appropriate RAT based on the delivered policy to connect to the network. Figure 2a shows an ANDSF-based LTE-WLAN interconnection architecture. In the LTE network, UE is connected to a base station (BS) through an LTE air interface. Wherein, the LTE air interface is referred to as the Uu interface, and the base station is referred to as an evolved Node B (eNB). When processed from the external packet data network (PDN) by the LTE-Uu interface, the traffic is transmitted to the UE through PDN Gateway (PDN GW), Service Gateway (S-GW), and eNB. There are two options  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**MCS Index** & **PHY Pattern** & **Modulation** & **Coding Rate** & **PHY Data Rate (Mbps)** \\ \hline
1 & SC & \(\pi\)/2-BPSK & 1/2 & 385 \\
6 & SC & \(\pi\)/2-QPSK & 1/2 & 1540 \\
9 & SC & \(\pi\)/2-QPSK & 13/16 & 2502.5 \\
10 & SC & \(\pi\)/2-16 QAM & 1/2 & 3080 \\
12 & SC & \(\pi\)/2-16 QAM & 5/8 & 4620 \\
13 & OFDM & SQPSK & 1/2 & 693.00 \\
15 & OFDM & QPSK & 1/2 & 1386.00 \\
18 & OFDM & 16 QAM & 1/2 & 2772.00 \\
22 & OFDM & 64 QAM & 5/8 & 5197.50 \\
24 & OFDM & 64 QAM & 13/16 & 6756.75 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Example of modulation and coding scheme for IEEE 802.11 ad. BPSK: binary phase shift keying, MCS: Modulation and Coding Scheme, OFDM: orthogonal frequency division multiplexing, PHY: physical, QAM: quadrature amplitude modulation, QPSK: quadrature phase shift keying, SC: single carrier, SQPSK: represents extended QPSK.  
available when traffic is offloaded to WLAN: (1) non-seamless WLAN offloading (NSWO); and (2) seamless WLAN offloading.  
ANDSF is an efficient and scalable mechanism for controlling UE discovery and the selection of RATs in the network. However, ANDSF cannot control UP offloading in an ultra-dense HetNet.  
Figure 2b shows RAN-level LTE-WLAN integration architecture. In this architecture, UP and CP, LTE eNB, and the WLAN AP are all directly connected through a cooperative interface that transmits real-time traffic and radio information. The mobile anchor point resides in eNB, minimizing the interruption time during the handover process. The architecture supports CP/UP segmentation between LTE and WLAN.  
The LTE-WLAN integration architecture was proposed and studied in the process of 3 GPP(3 rd Generation Partnership Project) standardization. Thus, two types of LTE-WLAN integration architectures are defined in Release 13: LTE-WLAN Integration (LWA) and LTE-WLAN Radio Integration (LWIP) using IPsec tunnel. Figure 3 provides the protocol stack for these architectures.  
The main difference between the two architectures lies in the forwarding packet layer. In LWA, eNB forwards Packet Data Convergence Protocol (PDCP) Data Units (PDUs) to the WLAN AP. The PDCP PDU encapsulated in the LWA Adaptation Protocol (LWAAP) can be transmitted over the WLAN. In LWIP, eNB forwards PDCP Service Data Units (SDUs), i.e., IP packets, to WLAN AP. The forwarded IP packets are encapsulated in the Generic Routing Encapsulation (GRE) protocol and are protected by IP Security (IPsec). Therefore, the IWIP (Light Weight IP Protocol) bearer has a more complicated setting than LWA.  
Figure 2: LTE (Long Term Evolution)-WLAN (wireless local area networks) system and integrated architecture. ANDSF: Discovery and Selection Function of the Access Network.