LC Layer Layer Transport FIGURE 1-15 Distributed antenna system architecture. 20 Open Radio Access Network (O-RAN) Systems Architecture and Design between base station and (now, more complex) radio head is maintained as shown in Fig. -16. Challenges to make massive MIMO work include the following: Raw compute requirements and compute precision, especially in the radio to enable signal processing from a large amount of antennas. System scalability-cost targets dictate that each antenna path needs to be built from relatively low-cost components so that the antenna scaling factor does not make the system prohibitively expensive. Power consumption associated with the above. Time and frequency synchronization between all the antenna elements. C-(as in: centralized) radio access network The concepts of RAN centralization are not new, as we have shown earlier. In its simplest form, a C-RAN system can be implemented with the same/similar components as a tradi- tional macro base station, just by increasing the amount of processing power dramatically, targeting deployments in dense urban environments where there is not enough space to deploy individual macro cells as shown in Fig. 1 1-17. Benefits of the C-RAN architecture have been discussed earlier SO we will not repeat them again. The intention here is to highlight the scale of deployment form factors in 5G, from femtocell to large-scale C-RAN. Functional splits The previous discussions on form factors open the discussion on "functional splits"-wha is the correct functional partitioning between different functional units (products), what is the Massive MIMO Base Station Line Card Network IF Card Front Low Physical High Physical PDCP/ Ethernet DFE/RF MAC/RLC Layer Layer Transport Massive MIMO Base Station Line Card Front Low Physical High Physical Ethernet DFE/RF MAC/RLC Layer Layer Massive MIMO Base Station Line Card Front Low Physical High Physical DFE/RF MAC/RLC Layer Layer FIGURE 1-16 Massive MIMO architecture. MIMO, multiple-input, multiple-output. Chapter 1 Open ra