 signals (CRSs) that cannot be used for beamforming. There are no legacy device limitations in 5G since beamforming is included in 5G from the first specifications. LTE beamforming must be based on uplink Sounding Refer- ence Signal (SRS) measurements since legacy devices do not support beamforming feedback. More transmission branches are supported in 5G. 5G supports initially feedback for 64TX, while LTE supports 4TX in Release 8, 8TX in Release 10, 16TX in Release 13, and 32TX in Release 14. Massive MIMO active antennas combine an antenna and a large number of small RF units into a single package. The traditional solution has been to separate the passive antenna and the RF unit. An active antenna enables practical beamforming implemen- tation when the phasing of the small power amplifiers can be controlled with digital processing. The typical number of RF units can be 16, 32, 64, or 128 inside the active antenna. The active antenna also enables a simpler practical installation since there are no cables between the antenna and the radio frequency (RF). The power efficiency can also be enhanced since there are no losses in RF cables and connectors. A Nokia active antenna with 64 TRXs is shown in Figure 3.8. See Chapter 6 for more details about mMIMO signaling and Chapter 10 for mMIMO antenna structures and performance. 3.4 Flexible Physical Layer and Protocols 5G radio design requires enhanced flexibility in the physical layer and in the protocols. The main new solution areas are highlighted in this section. The details can be found in Chapters 6 and 7. 3.4.1 Flexible Numerology 5G radio must have the flexibility to support all the different spectrum options. The solution is flexible numerology as shown in Table 3.3. 5G is designed to support several 3 Technology Components NOKIA AirScale Massive MIMO Adaptive Antenna Figure 3.8 Massive MIMO antenna for 3.5 GHz band with 64 TRXs. Table 3.3 5G numerology is designed for all spectrum options. Subcarrier spacing 15 kHz 30 kHz 60 kHz 120 kHz 240 kHz Symbol duration 6