door coverage in offices and shopping malls, but also, as way to offload data from a macro cell in traffic hotspots. However, although there is this history of small cells stretching back over a decade, it is 5G that is the first generation which is likely to truly embrace them. The prominence of small cells in 5G comes from the ever-increasing data consumption by users, as data demand continues to grow eventually the only way to support this growth will be by building a denser network with more cells, SO there will be more base stations but each transmitting at a lower power and over a shorter distance, creating an ultra-dense network of small cells. In addition, most data are generated indoors where coverage is often poor, and small cells are the best way to improve indoor mobile coverage. Low transmission power is the key constraint of a small cell, with 3GPP specifying a local area base station class with maximum transmission power of 24 dBm, whereas for their wide area base station (macro cell) there is no upper power limit (3GPP TS 38.104, Section 6.2.1). Beyond this 3GPP power definition there are no other specific constraints on what a small cell is, resulting in small cells being used in many variants across a large range of use cases. This section highlights popular use cases, different variants of small cells, the NG-RAN architecture used for them, and interfaces defined to enable the small cell ecosystem. 4.6.1 Key Ideas In 5G small cells are mainstream, and the different 5G disaggregation options and frequency choices apply equally to both small cell and macro cells. Barriers to small cell deployment are predominately business related, not technical. The trend to virtualization and centralization in 5G also applies to small cells. Small cells will carry a significant portion of the data in a mobile network. The largest market for small cells is indoor, while the biggest growth area for small cells is outdoor. Private networks and unlicensed networks are more specialized network options uniquely suite