 Column-4 2 TXRUs 2 TXRUs 2 TXRUs 2 TXRUs Figure 11.6 Schematic of four columns of a conventional passive antenna array, each column connect- ing to one transceiver per polarization via a radio distribution network (RDN) deployment scenario provides a good indication of the gains achievable by 3D beamforming (compared to 2D). The geometrical environment as depicted in Figure 11.8 occurs in ultra-dense urban macro deployments. The base-stations are typically installed above rooftops, the users are mostly in NLOS conditions and distributed at the street level and within a few floors of the buildings to a certain extent. The cell sizes vary from ~150 to over 500 m on average inter-site dis- tances (ISD). Simulated system performance in Figure 11.9 shows that the gains due to 3D beamforming compared to 2D beamforming are significantly higher in smaller cell sizes in such environments and the reasoning follows naturally from the geometrical considerations above. Figure 11.10 shows that due to the better separability of users in the elevation domain in a denser deployment, multi-user transmission is more prevalent - this directly improves the gains due to 3D beamforming. Therefore, ultra-dense macro scenarios with above rooftop deployments and small to medium cell sizes are attractive for Massive-MIMO installations. Another deployment scenario naturally fit for Massive-MIMO antenna arrays is an urban macro deployment with high-rise buildings - in this case an antenna array installed outdoors can utilize both up-tilting and down-tilting to serve users located inside high-rise buildings [5]. This is especially relevant in situations where alternative indoor solutions such as small cells or distributed antenna systems are logistically expensive. In certain cases due to constraints on antenna array installation sizes as well as due to the expense of replacing existing antenna systems Massive-MIMO deploy- ment opportunities in the macro cells can be limited. In such cases, however, Massive-MIMO LTE Macro Cell Evolution Colu