# High Reliable Uplink Transmission Methods in GEO-LEO Heterogeneous Satellite Network
###### Abstract  
As a significant component of non-terrestrial networks, satellite communication has gradually evolved into a heterogeneous topology where the network nodes are classified as low Earth orbit (LEO) and geostationary Earth orbit (GEO) nodes. Considering the advantage of low transmission delay of LEO satellites in a heterogeneous network, ground users are arranged to access LEO satellites in the first priority. By fully understanding the equivalent channel gain difference in the beam-edge and beam-center users, we develop non-orthogonal multiple access (NOMA) in an LEO network where different users are allocated in the same resource block. As a complementary strategy, the beam-edge user has the ability to connect the GEO satellite if the LEO links are not available. That is, the two users are served cooperatively by LEO and GEO satellites in an orthogonal manner. In this paper, we provide deep insights for the uplink transmission performance of NOMA and cooperative orthogonal multiple access methods. As a general metric, the analytical expressions of outage probability are derived and the diversity orders are also provided. The simulation results show that NOMA is capable of providing remarkable performance in low signal-to-noise ratio regions and is more promising when the channels are assumed to be ordered.  
heterogeneous satellite network; non-orthogonal multiple access; cooperative orthogonal multiple access; outage probability +
Footnote †: journal: Applied Sciences  
space, named Starlink, which is well-known as a huge satellite-based internet [6]. Starlink is a next-generation LEO SATCOM network that extends the conventional SATCOM into a communication, navigation, and internet integrated network [7].  
As mentioned above, a GEO SATCOM network is able to provide wide area coverage, and the network topology is stable. As one of the most well-known GEO SATCOM networks, Inmarsat satellite system, which works in the L-band, has developed to the fourth generation and can provide low-rate voice service and high-rate IP data service [8]. However, due to the long travel distance between ground users and GEO satellites, signals may experience great path loss. In this case, we could not expect high transmission rates for small-size terminals. Fortunately, an MEO SATCOM network can attain a much higher achievable rate while the available satellite resources are relatively limited. The O3b constellation consists of more than 20 satellites whose orbit height is approximately 8063 km [9]. O3b works in Ka-band and applies spot beam technology on board. For the LEO SATCOM network, the transmission rate is able to be further improved compared with that of the MEO SATCOM network. The available resources can be guaranteed by planting a large number of LEO satellites in space. As one of the main drawbacks of LEO SATCOM, the network topology keeps changing when users transmit information. StarLink was initiated in 2015 and has launched more than 4000 satellites into space [10]. All the satellites work in Ku/Ka-band, while the orbit height is about 550 km. Thanks to the short transmission distance, the data rate can attain 500 Mbps at terminals and is able to allow more than 1 million users to access the system.