## 1 Introduction  
Currently, mobile satellite systems (MSSs), adopting Geostationary (GEO) and Non-Geostationary (NGEO) orbits such as Low-altitude Earth Orbits (LEO) and Medium-altitude Earth Orbits (MEO), have attained a renewed R&D interest and market opportunities with extensive application in providing communication services to fixed or mobile users for a vast range of sectors (e.g., land-mobile, aeronautical, maritime, military and disaster relief, etc. ), especially in the scenario of terrestrial communication infrastructure congested or gusty paralysis. Each orbit has some desirable properties as well as a set of drawbacks for efficient service delivery. Due to the concerns regarding the ease of use of large-sized terminals equipped with large antennas and power supplies as handsets, as well as the need for direct radio communications between the user located anywhere around the world, it leads to the idea of using low earth orbit (LEO) satellites at a height between 500 km and 2000 km of altitude rather than the geostationary orbit. Besides, in the case of delay sensitive application, LEO-based MSSs have a typical end-to-end propagation delay of 20-25 ms, which is comparable to the delay of a terrestrial link. On the other hand, considering the mobility issue and the number of LEO satellites required for global coverage, network management becomes more complex and more costly. According to the orbit types, [PERSON] summarized some basic features of the operational GEO based MSSs and non-GEO based MSSs (i.e., the GEO-based systems of Immarsat BGAN (Broadband Global Area Network), Thuraya, ACeS (Asia Cellular System), Eutelsat & SES Astra (Solaris Mobile, Pairs, France), Hispasat, MSV (Mobile Satellite Ventures) and TerreStar (Reston, VA, USA); the LEO-based systems of Iridium, Globalstar and Orbcom; MEO-based systems of ICO (Intermediate Circular Orbit)). Details of the satellite orbit, user link frequency bands, physical layer, multiple access, satellite features and ISL can be found in [1; 2; 3]. However, these systems are spectrum limited due to the static spectrum allocation policies at present. Frequency bands for MSSs are assigned at the World Radiocommunication Conferences (WRCs), which is organized by International Telecommunication Union-Radiocommunication sector (ITU-R), periodically. Although the rapid growth demands for the increasing wide band and data services have promoted the application of Ku/Ka band for MSSs [4], almost all of the operational MSSs regard L/S band as the preferential frequency resources since they permit small onboard antennas due to better foliage penetration and smaller impact of atmospheric affects [5; 6]. According to the final documentation of WRCs held in Geneva, Switzerland, 2012 (WRC-12) [7], the allocation of L/S band for mobile satellite services promulgated by ITU Radio Regulation (ITU-RR) is described in Table 1. In order to reuse frequency and alleviate interference, ITU-RR divides the earth into three regions: Regions I, II and III. The statistical results of Table 1 used by the incumbent systems shows that the scarcity of L/S frequency resources makes it impossible for another planned global or even regional mobile satellite system to seek for a completely pure L/S band, which has attained pervasive attention in those countries who are not in possession of such independently controllable system. In fact, satellite frequency of interest may be not used at a particular geographic location or at a particular time. Thus, exploring efficient spectrum sharing techniques to enhance spectral efficiency in future MSSs without interfering to the operational services has become an important research challenge. Cognitive radio (CR) proposed by [PERSON] is identified as the promising technology to improve the utilization efficiency of the available spectrum [8; 9]. As the core function of CR, spectrum sensing enables the secondary user (SU) to adapt to the detected underutilization spectrum without causing interference to primary user (PU) [10; 11].