## 5 DSS Techniques and Generalized Finite State Machine  
In this section, we discuss the DSS techniques that have been implemented and evaluated in selected use cases. We first introduce a generalized finite state machine (FSM) that works as the core of the DSS techniques, followed by a brief description of the two DSS techniques: (1) DSS1--collaboration protocol (CP); and (2) DSS2--decentralized spectrum sensing. When designing DSS techniques, we have not focused solely on the two selected use cases and satellite operation in Ka-band. The DSS techniques we designed are generic and can be applied to other scenarios and in different satellite bands, such as S, C, X, Ku, etc. More specifically, they can be applied in situations where: (1) interference management is an issue due to the coexistence of other satellites/operators in the same spectral band; (2) improving spectrum utilization is required; and (3) distributed interference management is required. As the proposed DSS techniques are generic and agnostic to physical layer (PHY) technology (spectrum band, modulation, and coding techniques) and topology/constellations, they can be employed for solving the foreseen interference management and spectrum utilization problems in various use cases. Employing proposed DSS techniques for satellites operating in different bands would not require any modification of the DSS techniques, and we envisage that it would not lead to any performance degradation. Figure 2 shows the generalized FSM and the generic entities, including gateway (GW) for the different systems considered in this study: GSO, NGSO, and terrestrial networks. Depending on the DSS technique, the operation of FSM at different entities can be mandatory or optional. Each entity that runs an FSM can collect local information such as spectrum occupation data (collected via spectrum monitoring), node position, antenna characteristics, etc., intra-system information from other entities operating within the same system, and inter-system information from entities operating in other systems. This information will be shared through established intra-system or inter-system CCs, depending on whether the entities communicate within or across systems. For instance, if the FSM operates on a GSO-ES, then it requires information from other GSO (satellite, ES), NGSO (satellite, ES), and terrestrial systems through their respective GSs/GWs and CCs. The FSM includes the following building blocks: steady state, spectrum decision module, spectrum allocation module, spectrum release module, interference management techniques, and local DSS enablers. The starting point of the FSM is the steady state. While the system is in the steady state, as the dynamicity increases, the following events could happen: (1) low traffic demand; (2) high traffic demand; and (3) interference detection. In case of low traffic demand from the application side, the spectrum release module will be executed which will release some spectrum resources so that those resources become available for other entities, and the system will resume its operation in the steady state. In case of high traffic demand from the application side, the spectrum decision and spectrum allocation modules will be executed, which will allocate new spectrum resources for meeting the traffic demand. The job of the spectrum decision module is to find out the most suitable slots/channels to be used by GSO/NGSO satellites and ESIMs. It is the receiver of a communication link that proposes suitable slots/channels to the sender by utilizing:  
Figure 2: (a) DSS finite state machine (FSM) running on satellite/terrestrial entities. (b) CCs between system entities (GSO systems, NGSO systems, terrestrial networks) with FSM. (1) local DSS techniques such as spectrum monitoring; and (2) transmission characteristics received from other entities that might interfere.