ber of transceivers could be smaller than the number of antenna elements with each transceiver driving a subset of antenna elements [4]. Figures 11.4 and 11.5 show two typical AAS architectures where the left-slanted bars correspond to elements with -45° polarization and the right-slanted bars correspond to elements with +45° polarization. A conventional passive antenna system for macro cells would typically have a single transceiver feeding eight antenna elements per polarization arranged vertically in a column of cross-polarized antennas (as in Figure 11.6). As shown in Figure 11.7, an AAS can potentially allow two, four or even eight distributed transceivers feeding into the eight vertical antenna elements per polarization. The significance of a transceiver is that it allows precise baseband control of the amplitude and phase of the signals feeding into the antenna elements. Without delving into the details of AAS architectures we can simply equate the number of transceivers to the number of antenna ports in LTE terminology. Therefore, a 64-element passive antenna array comprised of 4 columns with a total of 8 transceivers each driving a set of 8 co- polarized antenna elements (vertically) can represent a typical 8 Tx macro configuration (Figure 11.6). This is comparable to an AAS offering 64 transceivers with a similar form factor thereby transitioning an 8 Tx conventional base-station to a 64 Tx Massive-MIMO base-station (Figure 11.7). The 'Massive' in Massive-MIMO, therefore, does not correspond to physical size but to the increased number of transceivers with baseband control ability. LTE Small Cell Optimization Figure 11.4 An example AAS architecture where a TXRU is connected to a sub-array of co-polarized antenna elements contained in a single column [4] It is worth noting that conventional tower-top passive antenna systems as well as RRHs can provide a limited ability to steer beams in the elevation dimension using a remote electrical tilt (RET) feeder network. An AAS, without the need of a separate RET