## 1 Introduction  
Recently, the low earth orbit (LEO) satellite constellation has received much attention for its low transmission delay, small path loss, large bandwidth, and global seamless coverage ability [1, 2]. However, the available spectrum for the emerging LEO satellite constellation is scarce. The declared frequency bands for the typical mega-constellation satellites, such as Starlink, OneWeb, and Telesat, are mainly located in Ku-band, Ka-band, and Q/V-band [3]. Moreover, the same frequency bands available for LEO satellites will also be used by the existing geostationary earth orbit (GEO) satellite systems and terrestrial mobile systems. Thus, severe intra-system and inter-system co-channel interference will occur when the LEO satellite constellations are deployed, and interference mitigation techniques are essential to guarantee the performance of these systems. The existing interference suppression schemes can be divided into several types, including spatial isolation (SI), cognitive radio (CR), adaptive power control (APC), beam hopping (BH), beam pointing optimization (BPC), etc. SI refers to setting the exclusion zone for the interfering system. Then, the interfering system operating outside the exclusion zone can avoid harmful interference [4]. The fixed exclusive angle is designed for the GEO satellite earth station to reduce the in-line interference from the LEO system [5, 6]. However, the fixed exclusion zone is not suitable for the non-geostationary earth orbit (NGEO) satellite with high mobility. Thus, a dynamic protection area is defined to mitigate the harmful interference between GEO-NGEO [7] or NGEO-NGEO satellite systems [8]. However, the disadvantage of SI is that the protection area for the interfered satellite is the non-communication zone for the interfering satellite. CR is also an effective technique to mitigate co-sharing spectrum interference [9]. It can sense the available frequency band in the spectrum for use, which can improve spectrum efficiency and reduce the mutual interference between systems [10; 11; 12]. [PERSON] et al. have proposed a spectrum strategy to distinguish the GEO signal from the interfering NGEO and noise [13], and the GEO satellite can use the appropriate power to protect its normal communication. In [14], a spectrum-sensing and power allocation-aided spectrum-sharing method is proposed to ensure the LEO system can work simultaneously with GEO systems in the interference region. However, CR cannot guarantee the long-term performance of the secondary systems. APC has received great attention as a solution to reduce interference between satellite systems. In [15], an adaptive power control technique is proposed to mitigate the in-line interference for the GEO-NGEO coexistence scenario. In [16], a dynamic beam power adjustment strategy is proposed to avoid the in-line interference between the GEO-LEO satellite systems. APC can adapt the transmit power of the interfering system to satisfy the desired quality of service while protecting the interfered system. However, the co-channel interference is severe and dynamic for the LEO mega-constellation, and the power balance among many satellites is difficult. BH can realize full frequency reuse over a certain beam hopping pattern and suppress the co-channel interference. In [17], a beam hopping strategy is proposed to match uneven traffic demands and avoid interference to GEO ground stations. Moreover, the multi-satellite interference avoidance problem for NGEO satellite communication systems is solved by designing BH patterns with spatial isolation characteristics [18]. Thus, a suitable BH pattern is designed for dynamically allocating resources and resisting interference. However, the complexity of BH pattern design will rapidly increase as the number of LEO satellites increases. BPO can be used to adjust the antenna beam pointing direction to mitigate interference. In [8], the harmful interference can be mitigated by turning off the beam or adjusting the satellite beam pointing.