cture Colby Harper and Sasha Sirotkin Introduction 123 4.1.1 Monolithic gNB Architecture 124 4.1.2 Common Public Radio Interface (CPRI) 4.1.3 Antenna Interface 4.1.3.1 Before 5G: Where We Have Been 4.1.3.2 New 5G Era: Where We Are 4.1.3.3 Release-17 and Beyond: Where We Are Going 4.1.4 gNB Functional Split(s) 4.1.5 Conclusions 4.1.6 Further Reading References 138 High-Level gNB-CU/DU Split 4.2.1 Key Ideas 4.2.2 Market Drivers 4.2.3 Functional Description 4.2.3.1 F1 Control-Plane Protocol 4.2.3.2 User-Plane Protocol 4.2.3.3 OAM Aspects 4.2.4 Further Reading References Contents Multi-Radio Dual Connectivity Sergio Parolari 4.3.1 Key Ideas 4.3.2 MR-DC Options 4.3.3 Market Drivers 4.3.4 Functional Description 4.3.4.1 Control Plane 4.3.4.2 User Plane 4.3.4.3 Procedures 4.3.5 Further Reading References 175 Control-User Plane Separation Feng Yang 4.4.1 Key Ideas 4.4.2 Market Drivers 4.4.3 Functional Description 4.4.3.1 Control Plane 4.4.3.2 OAM Aspects 4.4.3.3 Relation to SDN 4.4.3.4 Relation to 5GC 4.4.4 Further Reading References Lower-Layer Split 4.5.1 Key Ideas 4.5.2 Market Drivers 4.5.3 Functional Split 4.5.3.1 Fronthaul Bandwidth Requirements 4.5.3.2 Low-Level Functional Split Details 4.5.3.3 Latency Management 4.5.4 Fronthaul Interface 4.5.4.1 Messages 201 4.5.4.2 Scheduling Procedure 4.5.4.3 Beamforming Methods 4.5.5 Fronthaul Timing Synchronization 4.5.6 Operation, Administration and Maintenance (OAM) 4.5.7 Further Reading References 212 Small Cells Clare Somerville 4.6.1 Key Ideas 4.6.2 Market Drivers 4.6.3 Barriers and Solutions 4.6.3.1 Site Locations 4.6.3.2 Scaling Up Deployment 4.6.3.3 Backhaul 216 Contents 4.6.3.4 Edge Compute 4.6.4 Small Cell Variants 4.6.4.1 Disaggregation Architectures 4.6.4.2 Platform Architectures 4.6.4.3 Operating Frequency Impacts on Architecture 4.6.4.4 Operational Models 4.6.5 Key Interfaces for Small Cells 4.6.5.1 FAPI 222 4.6.5.2 nFAPI 226 4.6.5.3 Management Plane 4.6.6 Worked Examples 4.6.6.1 Indoor Enterprise Example 4.6.6.2 Outdoor Urban Example 4.6.6.3 Private Network Examp