mn-1 Column-2 Column-3 Column-4 16 TXRUs 16 TXRUs 16 TXRUs 16 TXRUs Figure 11.7 Schematic of four columns of an AAS where each antenna element is integrated with a radio and a transceiver arrays deployed below rooftops or mounted on walls targeting capacity hot-spots like train stations can be the solution of choice [11]. These can be small cells targeted for capacity and a compact power-efficient AAS installation fits the bill. In such instances, Massive-MIMO systems with directive patch antennas operating at higher frequencies (like 3.5 GHz band) can be attractive. The discussion above considers the performance gains due to 3D beamforming ability of AAS that is achieved by precisely controlling the gains and phases of the antenna elements dynamically at baseband. In addition, the ability to steer beams in the elevation dimension in a relatively slow fashion and the ability to form multiple beams in the elevation dimension can also be applied for different deployment needs. Such functionalities do not require baseband control and can be achieved by adapting the RF configuration of the AAS. Examples of such Figure 11.8 Users closer to the base-station allows better angular separability in the elevation dimension LTE Small Cell Optimization Deployment ISD = 500 m Deployment ISD = 200 m Cell average Cell edge Figure 11.9 System performance of a column of 16 TXRUs (as in Figure 11.7) compared to a column of 2 TXRUs (as in Figure 11.6) - in both cases 16 antenna elements are used. This shows that the gains due to 3D beamforming compared to 2D beamforming are higher for denser deployments 500 m ISD 200 m ISD Number of paired users Figure 11.10 The gains shown in Figure 11.9 are in part due to the ability of the AAS base-station to transmit to multiple users (MU-MIMO) more easily in the denser scenario. This figure shows more users are paired for MU-MIMO transmission in the denser scenario as also expected from Figure 11.8 LTE Macro Cell Evolution Figure 11.11 Application of AAS systems capable of steering multiple ele