ents, both within traditionally licensed bands and in combination with other models of spectrum licensing. 10.4.1 5G Frame Structure-Scalable OFDM Numerology 4G LTE supports carrier bandwidth up to 20 MHz with "fixed" 15-kHz spacing between sub-carri- ers, and these values were fixed for LTE. However, to support diverse spectrum bands and various heterogeneous deployment models, 5G introduces flexible spacing for sub-carriers known as scal- able numerology or multi-numerology. An OFDM numerology set is a set of values consisting of the number of sub-carriers, the sub-carrier spacing, slot duration, and CP time duration values used in a specific deployment. The preceding section on 4G shows that the CP is utilized to avoid ISI and ICI in OFDM radio de- ployments. OFDM defines spectrum efficiency by the number of sub-carriers that can be carried in a specific frequency range; the more sub-carriers, the more data that the device can transmit or re- ceive. With the sub-6-GHz bands, a narrow sub-carrier spacing (for example, 15 kHz) is used with longer OFDM symbols, which allows a larger CP to be used. With a larger CP, there is less ISI, and the signal has more tolerance to fading. When using high-frequency bands such as mmWave, however, the carrier frequency gets increas- ingly sensitive to any frequency drift and phase misalignment. A wider sub-carrier spacing (for ex- ample, 120 kHz) can be used to reduce the interference and phase issues, but this wider sub-carrier spacing reduces the available space for the CP. Hence, with NR, the sub-carrier spacing is no longer fixed but is adjusted to different values for dif- ferent frequency bands. There is a numerology factor, u, that is used to scale the size of the sub-car- rier spacing based on the frequency band (it uses 2u X 15kHz). As shown in Table 10-14, as the numerology, or value of , increases, the number of slots in a subframe increases, which increases the number of symbols sent in a given time, resulting in more bandwidth. Table 10-14 5G Numerology Structures