igned to a higher-tier user and may also operate opportunistically on unused Priority Access channels. The process of assigning spectrum is automated, with several Spectrum Allocation Servers (SAS) coordinating the scheme nationwide. Each base station must report its position. On the basis of the propagation data, SAS estimates the impact of the new transmitter on other nearby small cells. Additional outdoor radio measurement receivers are also used to assess background levels. If the RF power density is less than -80 dBm, then the SAS authorizes spectrum use. CBRS rules are shown in detail in [4]. 4.7 3GPP Frequency Variants 3GPP has defined frequency variants for all relevant bands where 5G likely will be deployed. The work for each frequency variant is initiated by mobile operators and completed by equipment vendors. Frequency variants are released independently: vendors can take a frequency variant from the Release 16 specifications but still use Release 15 functionality. More frequency variants are defined during the evolution of 3GPP specifications. Table 4.2 shows the list of frequency variants in September 2019. 4.8 Summary 5G is the first mobile radio technology that can utilize any spectrum from below 1 GHz up to 90 GHz, both FDD and TDD, and both licensed and unlicensed bands. The mil- limeter wave frequencies above 20 GHz provide extremely high local data rates with bandwidths up to 800 MHz initially. Mid-band spectrum at 2.5-5.0 GHz offers high capacity and data rates in urban areas with existing cell sites. Low-band spectrum is beneficial for providing wide and reliable coverage for low-latency critical communica- tion. Unlicensed bands are useful for local networks and for boosting the capacity of the licensed bands. The target of 5G is to combine the benefits of different spectrum blocks together for optimized performance. References 1 Sun, S., Thomas, T., Rapparport, T. et al. (2015). Path loss, shadow fading, and line-of-sight probability models for 5G urban macro-cellular scenarios. IEEE Globe-