# High Reliable Uplink Transmission Methods in GEO-LEO Heterogeneous Satellite Network
### Literature Review  
Taking advantage of both GEO and non-GEO SATCOM networks, the heterogeneous satellite network was proposed, which integrates GEO, MEO, and LEO satellites with a unified protocol [11]. To fully develop the onboard resource, non-orthogonal multiple access (NOMA) has been widely investigated. The authors in [12] studied a two-timeslot NOMA-aided transmission method in a satellite-terrestrial integrated network. In the scenario, the user with a better channel condition is able to decode the weak user's signal and puts forward the signal in the additional timeslot. Then, the signal-to-noise ratio (SNR) can be enhanced. For the multi-user scenario, [13] discussed a joint beamforming method at the satellite and base station (BS) sides. Specifically, two users with great channel condition difference are selected as one NOMA group. Then, beamforming optimization is applied to support multi-user usage. In [14], the authors proposed a coordinated NOMA scheme in LEO SATCOM. By exploiting the in-born advantage of LEO constellation, NOMA was proposed to be carried out with two satellites. The outage probability was discussed to confirm the advancement in coordinated NOMA compared with orthogonal multiple access (OMA). Inspired by NOMA, our previous work in [15] proposed a co-carrier transmission method that enables two users' signals to be allocated in one downlink carrier. Further, Ref. [16] extended the idea to multi-user cases. The results showed that the co-carried method can provide a promising ergodic sum rate with marginal bit error rate (BER) degradation. The existing works were mainly focused on downlink NOMA. For uplink NOMA, the authors in [17] discussed a two-user uplink NOMA where the expression of outage probability was derived for performance evaluation. The authors in [18] studied the ergodic sum rate for uplink satellite NOMA, where users are assumed to be randomly deployed. Near and far users can be classified based on the distances compared with the beam center. Expressions of the ergodic capacity for both NOMA and OMA methods are derived and deeply analyzed. Under the background of multi-antenna, the authors in [19] investigated a multi-user multi-antenna random-beam-based NOMA scheme. Equipped with a uniform planar array, the satellite is able to generate random beams. Further, with the help of channel state information, the ergodic sum rate can be improved by designing user selection and power allocation algorithms. In a heterogeneous satellite network, users can connect to LEO satellites or be served by the GEO satellite. The authors in [20] focused on one user circumstance where the user can be served by the GEO and LEO satellitessimultaneously. In detail, the ground user transmits a superposed signal to both satellites by applying NOMA, where the power allocated to the GEO signal is much higher. To measure the performance gain of NOMA in a heterogeneous satellite network, the authors provided analytical expression of the ergodic sum rate. The simulation results showed that NOMA is able to achieve a remarkable ergodic sum rate compared with OMA under different channel conditions. Extending the idea to multiple GEO and LEO users cases, the authors in [21] proposed a multi-subcarrier NOMA transmission scheme. In that work, users equipped with larger-size antennas connect the GEO satellite while LEO satellites serve small-size users. To alleviate the inter-user interference, users are separated into several groups, where each group takes an isolated frequency band. Each group consists of one GEO user and multiple LEO users who share the same frequency resource. Also, the authors focused on the ergodic sum rate and tried to maximize the performance by designing user pairing and power allocation schemes. Simulation curves were presented under different optimization methods, which showed that the proposed approach can provide a higher ergodic sum rate, while fairness is also guaranteed at the same time.