AC/RLC Layer Layer Transport Low Physical High Physical DFE/RF MAC/RLC Layer Layer FIGURE 1-14 Pico/micro cell architecture. Chapter 1 Open radio access network overview Distributed antenna systems Distributed antenna systems (DAS) as shown in Fig. 1-15 - are a different physical implemen- tation but targeting the same goal as ISC or pico/micro cells and that is coverage and capac- ity enhancements. Rather than implementing the complete RAN functionality in a single- cost/power-optimized box, these systems rely on a central base station (potentially reused from a macro base station system design) connecting to multiple/many (low-power, low- cost) RUs that provide RF coverage over a wider geographical range such as a mall or a sports arena: The DAS design reuse from traditional macro base stations implies that the solutions are inherently compatible with macro systems and benefit from the same hardware/software enhancements over time. This makes DAS systems an interesting value proposition to the system vendors that can now leverage existing engineering work to a large extent. Ericsson Radio Dot and Huawei Lampsite are commercially successful examples of DAS systems. Massive multiple-input, multiple-output Given that below 6-7 GHz, there is only limited amounts of spectrum available, and that deploying physical sites is expensive with regards to install and maintenance, it makes sense to extract highest potential value from existing (macro) network sites. Massive MIMO is one of the techniques to enable this. Use of MIMO for capacity extension to move from single antenna deployments to two, four, or eight antennas is nothing new: both 3GPP and Wi-Fi systems employ these techni- ques since many years. What massive MIMO does is to increase the antenna count to much higher numbers (e.g., 32 or 64), to enable a step function in potential throughput of the sys- tem, without changing the system architecture-the traditional "macro base station" split Base Station Proprietary Low Physical High Physical PDCP/ Ethernet MAC/R