annels Three-year term by census tract Tier-3 (GAA) General authorized access (GAA) At least 80 MHz of commercial use Figure 8.2 CBRS three-tier shared spectrum licensing structure [19]. PAL licenses within the 3550-3650 MHz portion of the band are assigned based on spec- trum auctions. Each PAL license covers a 10 MHz channel for a single census tract (i.e., a small, relatively permanent statistical subdivision of a county) for a 3-year term. For any given census tract, up to seven total PALs may be assigned. With over 50,000 census tracts in the United States, each PAL spectrum license is expected to cost much less and encour- age participation from a variety of participants. It should be noted that a PAL frequency range may change over time based on incumbent activity. The lowest-tier GAA users are permitted to use any portion of the 3.5 GHz band not assigned to higher-tier users. With an open-access rule, GAA provides free access to the spectrum similar to unlicensed spectrum. Because PAL licenses are limited to a maximum of 70 MHz in any given census tract, a minimum of 80 MHz bandwidth is available for GAA use when not in use by the incumbent users. While GAA operation does not require a costly license, GAA operators must coordi- nate their use of the spectrum through the dynamic spectrum sharing system [19]. As shown in Fig. 8.3, a key element of the CBRS spectrum sharing architecture is the spec- trum access system (SAS). A SAS maintains a database of all CBRS base stations, also known as CBRS devices (CBSDs), including their tier status, geographical location, and other pertinent information to coordinate channel assignments and manage potential inter- ferences. To mitigate possible interference to tier-1 military radar systems, environmental sensors known as environmental sensing capability (ESC) will be deployed in strategic loca- tions near naval stations, mostly along coastal regions, to detect incumbent activities. When incumbent use is detected, the ESC alerts the SAS, which then directs CBSDs util