indoor operation, just like Wi-Fi. Spectrum in the 6-7 GHz band is an obvious candidate for NR-U deployment. This spec- trum was first opened in the United States but is opening in other areas of the world as well. Traditional base station architectures At a high level, these systems include the following functional components that are described in more detail elsewhere in this document and are shown in Fig. 1-9. User plane PDCP and transport Transport functions include IP/IPSec and GTP tunneling to carry the user IP packet between the RAN and the core network. PDCP key functions include air interface ciphering, packet (re-)ordering and retransmission, header compression. User plane MAC/RLC RLC functions include packet segmentation and retransmission. MAC layer functions include wireless air interface scheduling, packet concatenation and multiplexing, and HARQ retransmission. High-physical layer Functions include packet encoding/decoding including cyclic redundancy check. Scrambling, rate adaptation, and modulation. Frequency-domain grid (resource element) mapping. Low-physical layer 3GPP functions include time frequency conversion (OFDM modulation) and optionally beamforming. Non-3GPP-defined functions are digital front end (DFE) processing such as up/down conversion and filtering, crest factor reduction, and digital pre-distortion. Chapter 1 Open radio access network overview Converts the digital baseband IQ signal into a high-power radio frequency signal that is transmitted over the air (and vice versa). Control plane. All-in-one base station Built and deployed in the 1980s and 1990s, 1G and 2G systems were architected as a system- in-a-box where all components described above were integrated into a single unit, with coax cables toward the RU that is mounted on top of the RF tower-conceptualized in Fig. 1 10. The base station was located at the bottom of the cell tower, together with power supplies, air-conditioning, backhaul transmission equipment, and other components. These cell sites are bulky in nature an