baseband. The evolutionary steps in the base station site show that the antenna is a key ele- ment in the future base station site because most functionalities will be integrated with the antenna. Figure 8.9 describes the typical steps in antenna evolution. The traditional passive antenna approach has two separate cross-polarized (x-polarized) antennas: one antenna covering low bands below 1 GHz and another antenna covering the high band around 2 GHz. The second phase shows dual x-polarized antennas, which has become a common solution lately, especially at the 1800-2600 MHz bands. It allows four transmit- ters and four receivers (4T4Rs), which improves coverage and data rates. The peak data 8 Deployment Aspects Two passive 4T4R at least mMIMO for Integrate mMIMO for antennas high band 3.5 GHz active+passive Integrate xx-pol Inter- low + high leaved mMIMO mMIMO 3.5 GHz mMIMO mMIMO mMIMO ~2 GHz mMIMO <1 GHz (4T4R) LTE 4x4 5G devices at Sites with 5G to FDD + devices 3.5 GHz single antenna all active Figure 8.9 Base station antenna evolution. rate can be doubled by using 4x4 MIMO, which is supported by high-end LTE devices at the 1800-2600 MHz bands. 4T4R is sometimes also used at low bands even if devices do not support 4x4, because 4R uplink can improve coverage. The coverage improve- ment is beneficial especially for Voice over LTE (VoLTE) in order to match the coverage of CS voice service. The next step in the evolution is the addition of massive Multiple Input Multiple Output (MIMO) antennas for 5G at 3.5 GHz. The first massive MIMO antenna is typically a separate antenna. Many base station sites have limitations in terms of the number of antennas. It may be beneficial to integrate all the passive antennas into a single antenna to make room for the new massive MIMO antenna. A further evolution- ary step integrates all the passive antennas with 3.5 GHz massive MIMO antenna, which enables supporting all bands with just a single antenna: 700-2600 MHz with a passive antenna and 3.5 GHz with a massive MIMO antenna. 