# Dynamic Divide Grouping Non-Orthogonal Multiple Access in Terrestrial-Satellite Integrated Network
## 1 Introduction  
In the past few decades, the rapid development of terrestrial wireless communications has triggered increasing demand for high broadband and massive access applications, such as augmented reality, virtual reality and high definition video, which in turn raised emergent requirements on achieving massive connectivity and high capacity in future communication systems [1, 2]. However, limited by the constrained power storage or insufficient base station deployment, the network coverage in rural or harsh areas is quite low. Due to the wide coverage, broad operating spectrum, and ultra-dense topology, Low Earth Orbit (LEO) satellite networks have been identified as the most cost-affordable technology to meet the the terrestrial coverage requirements [3]. Recently, Terrestrial-Satellite Integrated Network (TSIN) is developed as a combination of satellite networks and terrestrial networks, which have the advantage of providing ubiquitous network services with full frequency reuse to network users [4, 5, 6]. However, with the exponential growth of wireless connection equipment, the fixed multiple access techniques such as time division multiple access (TDMA), code division multiple access (CDMA) and frequency division multiple access (FDMA), which guarantee orthogonality in time, code and frequency, respectively, will no longer be able to sustain this growing demand for wireless access. To address this challenge in the communicationsystems, several new techniques for multiple access have recently emerged based on the concept of non-orthogonal multiple access (NOMA) as discussed in [7; 8], which can achieve better spectrum utilization and connection density than conventional Orthogonal Multiple Access (OMA) under limited resources. In power-domain NOMA, multiple users can share each subcarrier and the diversity on that subcarrier is obtained by allocating different power levels to the users. The basic principle of NOMA is to exploit the difference in channel gains among users in order to offer multiplexing gains, which can improve the spectral efficiency and massive connectivity. Despite the proven benefits of NOMA in TSIN [9; 10], several practical challenges must be addressed before NOMA can be effectively deployed. First, the base station (BS) and satellite system share the entire bandwidth resources can lead to excessive in-band interference, which will reduce the communication quality of user access. Therefore, to reduce inter-cell interference, it is necessary to consider how to achieve reasonable band selection and power allocation for ground users and satellite users. Second, due to the high dynamic characteristics of satellite, the terrestrial-satellite communication link is time-varying. The user pairing method based on the channel fading model [11] or distance attenuation model [12] will not guarantee a stable pairing relationship. Similarly, by using the antenna gain and quality of service to pair users, the channel difference of users cannot be guaranteed, which restricts the flexibility of satellite user pairing. Therefore, to improve the system throughput, it is necessary to design a NOMA user pairing method for TSIN according to the channel characteristics of terrestrial-satellite link. Motivated by the advantages of the TSINs, in this paper, aiming at satisfying the high requirements of information capacity, we propose a dynamic divide grouping NOMA method in terrestrial-satellite cooperation system. We consider that, the satellite and the BSs could serve the ground users under coverage cooperatively based on the requirements of them. Moreover, to improve the users throughput, a NOMA problem is modeled as an optimization problem of joint user grouping, power control and resource allocation by using stochastic geometry theory.