# Performance Analysis of NB-IoT Uplink in Low Earth Orbit Non-Terrestrial Networks +
## 6 Numerical Results
### Link Budget Simulation
Therefore, in LEO satellite communication, the performance of one orbit is not good. Figure 15 presents the result of six orbits. At six orbits, the number of satellites is up to 162. The simulation is performed in increments of 12 total satellites (two satellites per orbit). First of all, when \(N_{dmrs}=1\), the minimum \(P_{OTR}\) is 1.87% and more orbits are needed to satisfy 0% \(P_{OTR}\). On the other hand, at \(N_{dmrs}=2,3\), the minimum \(P_{OTR}\) is 0%, which shows good performance. The proposed method satisfies \(P_{OTR}\) of 1% or less when using at least 102 satellites (17 satellites per orbit). The proposed \(DMRS\) symbol addition shows lower \(P_{OTR}\) using fewer satellites. Figure 16 shows the maximum persistent service time (\(MPST\)) for the different numbers of \(DMRS\) symbols per slot. The number of \(DMRS\) symbols has little effect on the \(MPST\) performance of the fixed-spacing. However, it affects the \(MPST\) of the variable-spacing. In Figure 16, the result of one orbit, the maximum \(MPST\) is 2.5 h for \(N_{dmrs}=1\) and the maximum \(MPST\) for \(N_{dmrs}=2,3\) are more than 3 h. In one orbit, there is little effect of adding \(DMRS\) symbols. Figure 16 shows the result of six orbits, which offers a large difference from one orbit. The maximum \(MPST\) is 12 h for \(N_{dmrs}=1\) and 24 h for \(N_{dmrs}=2,3\). In addition, the maximum \(MPST\) for \(N_{dmrs}=2,3\) can be obtained using 114 satellites (19 satellites per orbit). On the other hand, the \(MPST\) for \(N_{dmrs}=1\) cannot exceed 12 h, even for 162 satellites. As a result, the maximum \(MPST\) is obtained using fewer satellites. Figure 15: The per\(-\)day outage time ratio for different numbers of \(DMRS\) symbols per slot. (**a**) one orbit, (**b**) six orbits. The _MPST_ for \(N_{dntrs}=2\), 3 increases as the number of satellites increases, and 114 satellites in operation achieve the maximum _MPST_. However, if the number of satellites increases by more than 114, _MPST_ will be reduced. Satellite intervals narrow when the number of satellites increases, and if the spacing is too narrow, the performance decreases, depending on the communication situation. Therefore, research on LEO satellite orbit is required for more stable communication. Figure 17 shows the maximum persistent outage time (_MPOT_). The _MPOT_ analysis is based on variable-spacing because the performance is better when applying variable-spacing than fixed-spacing. In Figure 17a, the results of one orbit, the minimum _MPOT_ is 10.12 h for \(N_{dntrs}=1\), and the minimum _MPOT_ for \(N_{dntrs}=2,3\) are less than 9.9 h. As a result, in one orbit, it is difficult to see the performance difference of _MPOT_ according to the proposed method. Figure 17b shows the results in six orbits. Compared to Figure 17a, _MPOT_ shows better results. The minimum _MPOT_ is 0.1 h for \(N_{dntrs}=1\) and the minimum _MPOT_ for \(N_{dntrs}=2,3\) are zero h. Therefore, the proposed method obtains the minimum (zero h) _MPOT_. There is also a difference between fixed-spacing and variable-spacing. The variable-spacing allows using fewer satellites to achieve good performance. For example, the number of satellites to meet less than 0.1 h is 90 \(N_{dntrs}=2,3\) and 162 in \(N_{dntrs}=1\). Although the performance difference is not significant in the _MPOT_, it is possible to achieve similar performance using a smaller number of satellites. Figure 18 presents the \(T_{S}\) result. When applying variable-spacing, not fixed-spacing, one can use fewer satellites. For example, in Figure 18a, when \(N_{dntrs}=2\), the number of satellites that satisfy \(T_{S}>5\) h is 12 in variable-spacing and 18 in fixed-spacing. Variable-spacing can save about six satellites. Therefore, the performance according to \(N_{dntrs}\) is analyzed in variable-spacing. In Figure 18a, the results of one orbit, the maximum \(T_{S}\) is 5 h for \(N_{dntrs}=1\) and the maximum \(T_{S}\) for \(N_{dntrs}=2,3\) are more than 6.5 h. Using the method suggested, one can communicate for 1.5 h longer.