## 1 Introduction  
Wireless sensor networks (WSNs) have been widely used in many fields, such as agriculture, monitoring, and geographical routing [1; 2; 3; 4]. The nodes of wireless sensor networks can share the effective information with each other, and transmit the information to the remote monitoring hosts. For the obvious superiority in high data rate services and providing coverage in the remote areas with various users, satellite-based sensor network systems have gradually drawn the attention of researchers and become a new direction of sensor networks [5; 6; 7; 8]. Meanwhile, a fundamental challenge for satellite communications (SatCom) is the spectrum exploitation for the continuous growth of broadband applications and multimedia services [9; 10; 11; 12]. To improve the spectrum efficiency, cognitive radio (CR) has exploited the spectrum sharing in terrestrial networks for more than ten years as a mainly approach, which is also a very promising candidate in SatCo [13; 14; 15]. In such a network, CR can be used not only in satellite networks, but also in the integrated satellite-terrestrial networks. In CR networks, the legitimate licensed users are the primary users (PUs), and the unlicensed users are referred to as secondary users (SUs). Generally, the approaches of CR are to permit the SUs to access the licensed bands assigned to the PUs while the PUs are inactive or the SUs can coexist with the PUs in a non-interfering manner [16; 17; 18; 19]. The above operation paradigms for cognitive radio are known as opportunistic spectrum access and spectrum sharing [20; 21]. In [22], the robust max-min fairness resource allocation in opportunistic spectrum access and sensing-based spectrum sharing is studied. For the easy implementation, the underlay paradigm is widely employed in spectrum sharing, and the precondition is that the SUs control the transmit power to avoid deterioration of the communication quality of PUs. Consequently, many authors focus on the efficient power allocation approaches for the satellite users in the uplink. In [23], the novel power and rate allocation approach for multi-users is proposed for the cognitive satellite uplink, where satellite users reuse the channels of fixed-service terrestrial microwave systems. However, this approach cannot be applied in fading channels. Considering the fading channel condition, optimal power control approaches are presented in [24; 25]. An interference-based constraint to the transmit power control of the satellite users is introduced in [24], which ensures an acceptable threshold for the interference-to-signal of the terrestrial link. In [26], delay-limited capacity and outage capacity are proposed for the real-time application in cognitive satellite links. However, the aim of the above-mentioned research is to share the channels by taking a strictly power control approach and does not consider the higher capacity performance and wide coverage of satellite networks, which are the main characteristics of satellite communication systems. According to the data presented in [27], many of the prized spectrum channels that have been assigned to the terrestrial users lie idle. In this context, the SUs can access the channel based on its own benefit to get higher capacities if the primary users are idle, which is just the idea of opportunistic spectrum access. However, the premise is that the SUs have to accurately identify the work state of the PUs in time. The authors in [28] make an overview of spectrum sensing approaches according to the different detection methods. The issue of opportunistic spectrum access for multiple SUs is addressed for hybrid satellite-terrestrial networks in [29]. The optimal spectrum access based on energy harvesting is investigated in [30], which designs the variable optimistic sensing intervals considering the PU traffic statistics. However, it is obvious that the SUs cannot access the channel if there is no idle spectrum, thus the approach is not ideal for real-time video transmission, such as living streaming or video conferencing.