# A B5G Non-Terrestrial-Network (NTN) and Hybrid Constellation Based Data Collection System (DCS)
## 3 Topology Design  
As described in the B5G roadmap 2022, B5G NTN system contains multiple space orbits with different altitudes. For IoT applications, orbit design must balance global coverage and communication costs. Based on the link analysis, LEO has the advantages of low latency, fast scanning, and low transmission loss. In addition, a symmetrical constellation topology can achieve global coverage. Therefore, a LEO constellation is suitable for the application.  
There are two different symmetry constellation topologies, namely the Walker constellation and polar constellation. The former topology contains several vertical circle orbits. The inclination of the polar axis is about 90 degrees, as shown in Figure 1a. The later topology contains several inclined circle orbits, evenly distributed on the equator. The inclination of the Walker is about 65 degrees, as shown in Figure 1b.  
The Polar constellation can achieve global coverage but suffers from reverse seams and traffic hotspots. The Walker constellation obtains better coverage performance in the middle and low latitude areas. However, it suffers from no coverage area in the polar regions.  
As we all know, the traffic of ground sensors relates to population distribution. The traffic is higher in the low latitude regions. NTN-mMTC needs to satisfy traffic hotspots in low-latitude regions while achieving global coverage. Therefore, a hybrid constellation consisting of the Walker constellation and Polar constellation is suitable.  
A hybrid constellation is shown in of Figure 1c, two different constellations work together. The design of hybrid constellation needs to take into account not only global coverage, connectivity but also safety. For a multi-satellite constellation, the most important  
Figure 1: Symmetry constellation overview.  
criterion is collision avoidance. This safety mechanism is mainly through orbit design and prediction. In the design phase, the distance between any two satellites must be greater than the minimum safe distance (MSD). For this optimization goal, some design item in Table 3 are iterated and simulated by commercial software STK. RAAN means the right ascension of the ascending node.  
The specific hybrid constellation consists of 42 CubeSats. 36 CubeSats belong to the Walker topology, and 6 CubeSats belong to the Polar topology. Due to the narrowband requirement of DCS, this small-scale constellation is more efficient than a mega constellation like StarLink.  
As shown in Figure 2, 42 CubeSats scatter around the world. They periodically scan terrestrial sensors. The half power band width (HPBW) of each DCS antenna is not less than 50 degrees. For an orbital altitude of 500 km, the receiving range of the DCS equipment exceeds 1000 km.  
In order to achieve higher resource reuse efficiency, multi-beam antennas and MF-TDMA can also be applied to DCS equipment.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Item** & **Walker** & **Polar** \\ \hline Orbit plane number & 6 & 3 \\ Total satellite number & 36 & 6 \\ Orbit latitude & 500 km & 550 km \\ Orbit inclination & 55\({}^{\circ}\) & 89\({}^{\circ}\) \\ RAAN & 60\({}^{\circ}\) & 60\({}^{\circ}\) \\ Phase delta & 4 & 8 \\ \hline \hline \end{tabular}
\end{table}
Table 3: DCS constellation design.  
Figure 2: Hybrid satellite constellation design.