# Dynamic Divide Grouping Non-Orthogonal Multiple Access in Terrestrial-Satellite Integrated Network
## 2 Related Work  
In the terrestrial networks, the authors of [13; 14] studied the influence of user pairing of NOMA system on system performance, and indicated that the larger the difference between the channel conditions for paired users, the higher the capacity of the NOMA system, which verified the influence of NOMA users' selection strategy on system performance. Combining NOMA technology with a millimeter wave (mmWave) communication model, the authors of [15; 16] proposed a user pairing approach based on the mean clustering aiming at maximizing the channel difference. For the sake of the system sum rate, the maximization of the sum rate of a 2-user mmWave NOMA system was investigated in [17], where the authors proposed a NOMA method based on the spatial distribution of users and analyzed the influence of user location on outage probability and throughput. To reduce the computational complexity of user pairing, in [18], the authors proposed a greedy user grouping algorithm to pair users with the largest difference in current channel conditions. However, as the algorithm is local optimal, it can hardly guarantee the channel difference of remaining users. To maximize the difference in channel gains of paired users, a non-orthogonal multiple access algorithm based on dynamic clusters is proposed [19], in which the sum-throughput maximization problem is formulated to optimize user clustering and power allocations in NOMA clusters. The work in [20] presented the throughput performance of NOMA vehicle-to-everything (V2X) with the fixed power allocation factors to further evaluate the performance of a group containing two vehicles. To maximize the sum-capacity and total energy efficiency for small-cell network, the authors of [21] proposed a joint NOMA-enabled optimization framework by utilizing the concepts of the multi-objective problem. Some relatively recent studies also investigate the inventive mechanism in cooperative NOMA and cognitive radio (CR) networks [22; 23]. To further improve the performance, a partial relay selection (PRS) technique is adopted to improve the diversity gains and to reduce the overhead of CR-NOMA networks [24]. In the satellite networks, the authors of [25] performed a detailed analysis of coexistence scenarios for 5G and fixed satellite, it showed that 5G can satisfy interference protection criteria of the FSS while allowing simultaneous transmissions from at least several thousands of sectors and tens of thousands of UEs under various LoS and NLoS channel conditions and with various sets of parameters for the fixed satellite service. However, in TSIN, the dynamic of LEO satellite need to be considered for high-speed LEO satellite, a coded random access is employed to realize a high-capacity interface for 5G users, while the slotted Aloha protocol is adopted at the space uplink [26]. However, at the space uplink, the use of slotted Aloha protocol means that it is inevitable that one of the scarce bandwidth resources is solely occupied by this user, despite its poor channel conditions. In such a situation, the use of NOMA ensures not only that the user with poor channel conditions is served, but also that users with better channel conditions can concurrently utilize the same bandwidth resources as the weak user. More recently, the application of NOMA in satellite system have also been investigated. For example, according to the influence of rain attenuation on the link fading, the authors of [27] proposed a heuristic algorithm to solve the optimal power allocation problem of UAV access to satellite. Furthermore, to improve the energy efficiency of satellite, a beamforming algorithm based on iterative penalty function is proposed to maximize the sum capacity of NOMA users [28]. To ensure the channel conditions difference for paired users, a NOMA algorithm based on channel correlation coefficient is proposed [29], where the alternate direction method is used to obtain the suboptimal power allocation.