Due to the increasing demand for low latency real-time systems and broadband data,people pay more attention to how to deploy more Low Earth Orbit(LEO) and Medium Earth Orbit(MEO) satellites in several available frequency bands. When the number of available Non-Geostationary Earth Orbit(NGEO,i.e.,LEO/MEO) satellites increases,It has become a must for the spectrum coexistence between the NGEO satellites with the Geostationary Orbit(GEO) satellites in orbit. In this context,it is crucial to explore interference mitigation techniques between GEO and NGEO systems in order to allow their spectral coexistence. More specifically,in the coexistence scenario of GEO and NGEO satellite networks,in-line interference may be a serious problem,especially in the equatorial region. In this paper,we provide several frequency sharing studies in the context of the coexistence of an NGEO satellite link with another NGEO/GEO satellite link. Moreover,we suggest several cognitive solutions for mitigating the in-line interference and provide future research issues.%由于对低延时实时系统和宽带数据的需求日益增加，人们越来越关注如何在几个可用的频带中部署更多的近地轨道（LEO）和中地轨道（MEO）卫星。当可用的非静止轨道（NGEO，即LEO/MEO）卫星数目增多时，其与在轨的地球静止轨道（GEO）卫星频谱共存已经成为了一种必须。在这种情况下，为了使它们的频谱可以共用，探索一项技术来抑制GEO和NGEO系统间的干扰是非常关键的。更具体地说，在GEO和NGEO卫星网络共存的情况下，共线干扰可能是一个严重的问题，特别是在赤道上空。本文提供了几个关于如何在NGEO卫星链路和GEO卫星链路共存情况下频率共用的研究。除此之外，本文提出了几种认知方法来解决共线干扰，也提出了未来研究方向。
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