e being addressed in Release 13 [1,2] as covered later in this chapter. 11.3 Evolution of Antenna Array Technology Antenna array technology for cellular communications is continuously focusing on reducing the hardware footprint and improving the energy efficiency and cost-effectiveness of the base- station installations. Conventional base-station architecture can be viewed as a set of tower-top passive antenna panels served with coaxial cables from a BTS unit at the ground level housing the radio and transceivers (TXRUs). The radio and the transceiver unit includes up/down converters, power amplifiers, sample rate converters, power conditioning circuitry, and SO on. This model is gradually evolving to a remote-radio-head (RRH) solution where the radio and the transceivers are positioned much closer to the set of passive antenna panels and could be inside the antenna casing as in the case of antenna-integrated radio (AIR) or integrated antenna system (IAS) [3]. This solution is both compact and more efficient in terms of power transfer due to reduced cable losses. An RRH installation is connected to a BBU via a high-speed link such as a CPRI or OBSAI. An active antenna system (AAS) defines the next evolution of technology where the transceiver and the radio are integrated and distributed with the antenna elements and this system connects with the BBU via a high-speed link. Physically, this provides an attractive solution with a reduced and clean footprint due to the elimination of tower-top hardware components (such as cables, connectors, mast-head amplifiers and RET network). It also eliminates cable losses and can deliver power more efficiently to the antenna elements. The most distinguishing feature of an AAS is the ability to provide electronic beam steering in the elevation dimension due to the distributed nature of the transceivers. In one architecture example, each antenna element can be connected to a transceiver providing baseband amplitude and phase control to every antenna element. Alternatively the num