width limited, and solutions are needed for inter-cell interference minimiza- tion. Millimeter wave cases are typically noise limited, and solutions are needed for cell range maximization. The millimeter wave can take better advantage of beamform- ing with a large number of antenna elements because the size of the antenna becomes smaller at high frequencies. Signal attenuates faster as a function of distance at higher frequencies. That is true in the line-of-sight (LOS) case and even more true in the non-line-of-sight (NLOS) case. Additionally, the penetration loss through walls typically increases as a function of Table 4.1 New spectrum and new challenges. Below 6 GHz Above 20 GHz Bandwidth Limited spectrum. Typically Large spectrum resources. Up <200 MHz per operator to 800 MHz per operator Interference Inter-cell interference main Coverage (noise) main limitation limitation Cell size Large > km Small <1km Antenna elements Less than 256 at 3-5 GHz Even more than 1024 Less than 128 at <2 GHz Base station maximum Up to 200 W per band Less than 10 W power Main challenge Inter-cell interference control Link performance for maximizing capacity with improvement with limited spectrum beamforming antennas for maximizing coverage area 4 Spectrum Challenges Opportunities Figure 4.6 Main factors impacting propagation at millimeter wave. Higher path loss Higher antenna in line of sight Higher indoor penetration loss Lower output power Path loss constant [dB] 2 GHz 18 GHz 28 GHz 38 GHz Figure 4.7 Measurement-based path loss constant for line-of-sight conditions [1]. the frequency. We also need to consider that the base station output power is generally lower at millimeter wave than at low bands. When all these factors are considered, the cell range at millimeter wave tends to be very small. We illustrate the impact of high frequency on propagation in this section. On the other hand, the antenna gain can be increased at millimeter wave because the wavelength becomes smaller, which allows a higher-gain antenna in a smaller fo