sion characteristics are big concerns. Moreover, studies also have shown mmWave signals have high attenuation due to atmospheric gaseous, rain, concrete structure, glasses, even foliage. The real-world deployment of such nmWave systems needs to be carefully studied and planned. Secondly, from the transceiver design perspective, higher-frequency signals impose challenges in circuit design, materials, and heating issues. Nyquist theorem sets the lower boundary for sampling rate in communication systems. With wide bandwidth in mm Wave spectrum, sampling rate can reach up to 10 Gbit/s level, and high-speed circuit design becomes very difficult. It is also reported that the energy efficiency for components (power amplifier, analog-to-digital converter, digital-to-analog converter) in high frequency is low, only around 10%. One of the major concerns from network operators is that power consumption will hike due to 5G. Furthermore, the low efficiency in these components also brings thermal issues in hand-held devices, degrad- ing user experiences. Thirdly, with mmWave band, performance gain largely comes from large-scale antenna array, current design can integrate hundreds of antenna elements in a small area (due to small wavelength of mmWave signals). Even though this can facilitate 1.2 Enabling Technologies the beamforming, which generates narrow but stronger signals toward desired direction, the overhead for channel estimation, precoding, and beam tracking is too large. Fourthly, in UDN networks, since the transmitter density is high, signals can cause higher interferences with each other. The problem will be more severe with high-density users in the same area. Challenges in mobility management, interference management, and heterogeneity nature of devices are severe. Lastly, it is expected to support intelligent applications in beyond 5G systems. For example, conventional communication systems are transparent of mes- sage (i.e. they are only responsible for transmitting bits but do not know any further info). Semant