## 1 Introduction  
In response to the increasing connectivity demands across all terrains, weather conditions, and scenarios, satellite communication systems are expected to play a crucial role in future 6G architectures [1]. Based on their operational orbital altitudes, satellites can be categorized into geostationary Earth orbit (GEO) satellites and non-geostationary Earth orbit (NGEO) satellites. Due to their lower transmission delay and reduced manufacturing costs, NGEO satellites, including low Earth orbit (LEO) and medium Earth orbit (MEO) satellites, are poised to offer more versatile services like mobile communications and the Internet of Things (IoT) [2; 3; 4]. To achieve superior wireless connectivity and increased transmission speeds, novel access technologies [5; 6] have been meticulously designed in numerous specialized scenarios. Recently, decoupling uplink and downlink access (DUDA) has garnered considerable attention in terrestrial heterogeneous systems (THS) [7; 8; 9]. Such systems encompass various types of base stations (BS), including Femto BS and Macro BS. Concretely, Macro BSs generally exhibit lower deployment density due to their greater transmission power and broader cell coverage. Often, the user terminal (UT) is located closer to Femto BSs, while the downlink reference signal's signal-to-interference-plus-noise ratio (SINR) from MBS is higher. Evidently, downlink and uplink coupled access (DUCA), wherein a UT typically connects to the same BS for both links, is not conducive for optimizing THS performance. In [10], the authors examined the probability of coverage and rate coverage probability for multi-antenna THS by employing DUDA. In [11], different user distributions based on stochastic geometry were analyzed alongside DUDA's energy and spectral efficiency. Morerecently, in [12], DUDA was shown to be effective in UAV heterogeneous systems, with UAVs positioned in cuboid space. In [13], UAV-aided THS was considered, utilizing DUDA to mitigate interference between terrestrial UTs and UAVs. Current DUDA methodologies for THS or UAV-aided THS, however, are not applicable to NGEO multi-layer satellite systems (MSS). MSS is also a heterogeneous system due to the different communication abilities of satellites. Geometrically, the BSs in THS follow a two-dimensional planar distribution and the UAVs are distributed in a three-dimensional cube; however, the satellites in MSS are positioned at a spherical surface, which presents a significant challenge for depicting the distribution of satellites. Secondly, the antenna employed in MSS is distinct from those of other systems; thus, the decoupled criterion necessitates redesign. Moreover, as depicted in Figure 1, spectrum coexistence between the NGEO MSS and the GEO system is a unique problem should be considered. Specifically, satellite communication priority is determined by filing date, necessitating that the planned NGEO MSS must avoid interfering with the satellites which have higher communication priority. In-line interference, which occurs when an NGEO satellite traverses the line-of-sight path between a GEO satellite and its served UT, presents a significant challenge. To address this, we investigate DUDA to enhance the transmission performance of NGEO MSS in the presence of in-line interference. To mitigate in-line interference, ITU endorses three conventional methodologies encompassing equivalent power flux density (EPFD), protection area, and exclusion angle strategies [14]. The EPFD of the proposed satellite system should comply with the threshold to guarantee minimal interference. The exclusion angle and protection area, also predicated on the EPFD threshold, are configured for the fixed satellite service to safeguard the UT of high priority. In [15], the authors minimized the downlink power of the NGEO satellite under the interference threshold and carrier-noise radio constraints, then the transmission rate of uplink is maximized by controlling the transmit power.