aring models are of course not directly linked to a particular technology: using all three approaches with LTE as well as 5G is likely in the future (Table 17.2). 17.5 Key 5G Radio Technologies Different research organizations and public research programs such as the EU-funded METIS and 5G-PPP are conducting radio-related research on multitude of different new technological components that could become 5G building blocks. The well-proven OFDMA with good MIMO compatibility as well as the DFT-spread single-carrier FDMA known from the LTE uplink are the benchmarks in the signal waveform. In the other domain, massive MIMO, native CoMP support, dynamic TDD, full-duplex TDD and solutions like in-band self-backhauling are being looked into. Further Outlook for LTE Evolution and 5G Ant # 1 RF Tx # 1 stream # 1 Ant # 2 RF Tx # 2 stream # 2 Digital Analog Ant # 3 RF Tx # 3 stream # 3 former former Ant # kxn RF Tx # n stream # n Figure 17.3 A block diagram of a hybrid digital/analog beamformer for massive MIMO Massive MIMO, where, for example, 64, 128 or 256 antenna elements are used, becomes more commercially attractive when the frequencies increase. The size of one antenna element becomes smaller when frequencies increase, and it becomes possible to integrate an antenna array directly in the chip itself. The large number of MIMO antennas is capable of directing transmitted energy towards the intended user, and thus obtaining higher link efficiency as well as reducing interference. Massive MIMO arrays and high spatial selectivity of the narrower beams are also convenient for multiuser MIMO operation. Fully digital beamforming, for example, for a 256-antenna transmitter would require 256 power amplifiers, one per antenna. This would lead to high energy consumption and high cost. Thus a hybrid digital/analog beamforming is likely more attractive with Massive MIMO (Figure 17.3). A dynamic TDD structure allows for better overall spectrum utilization than FDD or static UL/DL TDD time allocation. With dynamic TDD the data part o