izing impacted CBRS channels in that area to move to other channels. The cloud-based SAS enforces the three-tier spectrum sharing mechanism based on FCC rules via centralized, dynamic coordination of spectrum channel assignments across all CBRS base stations in a region. The CBRS rule-making defines two classes of base stations: class-A and class-B. A Operation in Unlicensed and Shared Spectrum 853 Environmental sensing Spectrum access system capability ((IIII) (())) (())) ((a)) ((< HI) CBSD Class A (indoor small cell) CBSD Class B (outdoor station) Figure 8.3 CBRS network architecture [19]. class-A base station can be an indoor or low-power outdoor small cell with a maximum conducted power of 24 dBm (per 10 MHz) and a maximum EIRP of 30 dBm. This type of small cell is similar to commercial enterprise-class small cells with 250 mW transmit power and a typical 2 dBi omnidirectional antenna or up to 6 dBi directional antenna. The class-B base station is meant for outdoor use with a maximum EIRP of 47 dBm. The highly direc- tional antenna makes the outdoor CBRS base stations more suitable for fixed wireless use cases. While indoor and outdoor base stations can be assigned to either GAA or PAL, more indoor GAA deployments are expected until ESC certification and PAL auctions are con- cluded [19]. 8.1.2 Use Cases and Deployment Models The introduction of carrier aggregation in LTE-advanced required the distinction between a primary cell (PCell) and SCell. The PCell is the main cell with which a UE communicates and maintains its connection with the network. One or more SCells can be allocated to and activated for the UEs supporting carrier aggregation for bandwidth extension. Because the unlicensed carrier is shared by multiple systems, it can never match the licensed carrier in terms of mobility, reliability, and quality of service (QoS). Hence, in LAA, the unlicensed carrier is considered only as a supplemental downlink or uplink SCell assisted by a licensed PCell via carrier aggregation. LAA deployment scenarios enc