s Architecture and Design 10Gbps Elec. or Opt. 2-4 ant 25Gbps 100MHz To <7 More RUs Compute + Router To <3 more FHGWs FIGURE 1-33 Connectivity picture for example indoor CU/DU/RU system. CU, central unit; DU, distributed unit; RU, radio unit. reception. This makes each set of RUs connected to a single GW look to the DU as a single "logical cell." Scaling out to multiple logical cells is a matter of instantiating more links from DU to FHGWs, for example, using 25 Gbps optical fiber carrying uncompressed IQ SO to off- load eCPRI compression/decompression from the DU processing chain, making it more lightweight on implementation. The overall system can support up to 4 (FHGW) X 8 RUs = 32 RUs, satisfying the request for small-to-medium enterprise deployments. We can draw the following diagram for a system proposal as shown in Fig. 1-33. Additional system specifications include a peak throughput (single user) of 100 MHz X 2-4 antennas or <2.5 Gbps MAC/PHY throughput. This book aims to cover systems like this as well as smaller and larger designs from ISC up to data center scale centralized O-RAN systems. We cover connectivity and bandwidth calculations; component choices in CU, DU, and RU; and dimensioning of each component for software, IO, and hardware processing load. Summary and conclusions We showed how the O-RAN standard evolved from various other initiatives with similar goals around extending SDN from the wireline space into the wireless domain. The first enabler of 5G is newly available spectrum being made available around the world, both in the traditional licensed operator space and private spectrum for industrial and enterprise This nontraditional 5G spectrum is driving new deployment options, ranging from ISCs through mmWave and massive MIMO, which pushes for implementations that can scale from the very small to the very large. This drives for an ecosystem of vendors and standard- ized interfaces between various components in the RAN domain. We show how O-RAN Alliance and standards including 3GPP, IEEE, a