## 2 Related Work  
First, we are going to present the main spectrum sharing techniques used today in terrestrial networks. Similarly, as in SATCOM, sharing of the spectrum between terrestrial networks in licensed bands is characterized by static and fixed spectrum assignment. Licensed bands are being assigned for exclusive use by specific wireless operators or technologies. This approach leads to severe overprovisioning and underutilization both in time and geographically. In an attempt to alleviate this issue in licensed bands, licensed shared access (LSA) is introduced. LSA is a regulatory approach to allow any primary user to share its licensed spectrum with secondary users following a predefined sharing framework [11]. However, this approach is quite restrictive and will limit the quality of service that can be provided by secondary users. Furthermore, LSA access is applicable most of the time for a known set of two technologies that will coexist, hence it is not a generic solution that enables the transparent coexistence of multiple technologies. In unlicensed bands, for multiple technologies to share the same spectrum, listen before talk (LBT) schemes are used. LBT schemes are not scalable, and additionally, with large distances and propagation delays in SATCOM, they do not represent a suitable solution. Citizens broadband radio service (CBRS) [12] is a shared wireless spectrum from 3.5 GHz to 3.7 GHz. Devices that want to use the CBRS band requests from a spectrum access system (SAS) database to reserve unused channels in a particular geographic area. In the TV whitespace (TVWS) approach [13] in the 470-698 MHz spectrum range, secondary users are allowed to opportunistically access the spectrum unused by the primary TV services. In TVWS, access to the shared bands is managed by the centralized SAS database. Downsides of solutions based on databases are explained in detail later in the paper. The recent work on coexistence in SATCOM scenarios can be categorized into two main groups: satellite-terrestrial coexistence and dual satellite coexistence. In satellite-terrestrial coexistence, the spectrum is shared between a satellite network and a terrestrial network, whereas in dual satellite coexistence two satellite networks operate simultaneously in the same spectrum band. Literature analysis shows that major research efforts on cognitive SATCOM sharing and interference mitigation are based on the construction and maintenance of centralized databases, where activity rules and main characteristics of primary users are stored. The key idea of a centralized database is that, before assigning a channel to a secondary user, the vacancy of the channel must be verified from the database for the duration of the targeted transmission period [14]. In the remainder of this section, we first present studies that investigated coexistence scenarios in satellite-terrestrial systems. Next, contributions in dual satellite coexistence scenarios are addressed. Finally, we present a couple of coexistence studies in both satellite-terrestrial and dual satellite systems that are based on database technique. In [15], the authors proposed a joint power and subchannel allocation algorithm for the uplink scenario of the cognitive satellite-terrestrial network, where the cognitive GSO reuses the frequency band of incumbent terrestrial cellular networks in the S-band on 2.5-2.6 GHz. In [16] cooperative and cognitive radio (CR) techniques are applied to the satellite-terrestrial network operating in downlink and uplink in various frequency bands. The satellite segment consists of three GSOs, while the terrestrial segment is a 3G/4G heterogeneous network. Coexistence is achieved by cooperative spectrum sensing, where each earth station (ES) detects primary users' transmitted signal and reports it to the satellite. Then, the satellite processes the received information from all the cooperative ESs and makes the final decision on the state of the spectrum, which is further broadcasted back to all the ESs.