adio Access Network Evolution To understand the 5G deployment requirements, you must start with a high-level understanding of the evolution of the RAN architecture and how the 5G RAN architecture is fundamentally designed to adopt not only CUPS, but the disaggregation and virtualization of the base station. It is not helpful that there are different sets of nomenclature used when describing the different generations of the 3GPP standards, but unfortunately, the situation for the RAN is much worse. Depending on which stand- ards documents you consult and what standards organization the RAN architecture is based upon, each com- ponent of the RAN can have one of three or four different names. To reduce the likely confusion that would result from using different terms depending on how a topic is discussed, this book tends heavily toward the 4G/5G 3GPP and O-RAN terminology, even if some organizations use completely different terms. Mobile networks are continuously evolving to support much higher frequency bands because that allows much higher data rates. On the downside, the propagation properties of the radio waves suf- fer in the higher frequency bands; signals are easily absorbed/attenuated, SO the coverage area falls away very quickly with increasing frequency. This means that for the operators to retain good COV- erage with higher frequency bands, many additional BS sites must be built-a process called densi- fication. Operators adopt either of the following two approaches to increase the coverage-the decision of which to use may depend on several factors such as population density: Densification by continuing to increase the number of macro sites. Densification with a layered approach where many low-powered small cells are deployed under the coverage areas of the macro layer. These sites tend to be small, indoor or under cover, and located in high-traffic areas. In both cases, densification causes interference on both the current channel and adjacent channels, resulting in deterioration in the signal quality an