# Onboard Centralized ISL-Building Planning for LEO Satellite Constellation Networks
## 5 Performance Evaluation
### Performance in Small-Scale Constellation  
Next, we evaluated and analyzed the onboard centralized network ISL-building planning in the Walker Delta 32/4/1 constellation. Under the small-scale constellation configuration, we simulated the onboard centralized network ISL-building planning scheme. The results indicated that each satellite in the constellation can act as the dynamic primary satellite for onboard centralized ISL-building  
Figure 5: ISL and topology duration with different value of time segment lengths. planning, which can complete ISLs terminal status collection, visibility calculation, link-building planning, and the link-building planning table distribution. The non-primary satellites can send their terminal status, retrieve the link-building planning table from the primary satellite, and control the ISL terminal to rotate and build the ISLs, according to the link-building planning table. The link-building planning process is the core of the onboard centralized network ISL-building planning scheme. Therefore, in the following, we mainly analyze and compare the link-building planning results. Figure 7 shows the link-building planning results for the Walker Delta 32/4/1 constellation using the LPTSO proposed in this paper. The figure shows four topology snapshots, and each one indicates the ISL-building status. The figure is also a graphic display of the planning table that are generated and distributed by the primary satellite. We can see, from the figure, that the pattern is similar to the pattern of MSN, in which the outcomes are regular, making it simple to manage, maintain, and debug onboard. It is essential to increase the satellite network's simplicity of use and improve the reliability of the constellation. On the other hand, different from MSN, the topology generated by LPTSO is not a static mesh-like topology, and it presents a limited dynamic pattern. Every sub-node has a small-size optional pair set, and every option in the set is high-quality one, such that the network topology can improve the network topology stability on the premise of reducing network delay. Table 3 provides the ISL duration, topology duration, average delay, maximum delay, and other performance indicators of the network topology in a period, consistent with the analysis in the previous subsection. As stated in Section 1, ILP, heuristic algorithms, and graph matching algorithms are unsuitable for onboard centralized ISL-building planning. In this subsection, we compare the results of the link-building planning method proposed in this paper with those of MSN and LCTS. Figure 8 shows a comparison of the average ISL duration under different strategies. The average ISL duration of LPTSO was similar to that of MSN and much higher than that of LCTS. This is because, unlike LPTSO and MSN, LCTS builds some ISLs with shorter visible time segments, resulting in a shorter average duration of ISLs and a larger standard deviation of duration. Figures 9 and 10 illustrate the topology switching and topology duration in a period when using three methods. The duration length of each topology snapshot of LPTSO was the same (i.e., 300 s). The average duration of MSN was shorter, while the standard deviation was larger, partly because MSN does not use equal-length time segments. On the other hand, the average duration of MSN is also shorter than that of LPTSO-1s, as a given antenna competes with other antennas to build an ISL with LPTSO-1s. Thus, breaking an ISL often occurs at the same time as building another ISL, which decreases the network topology switching frequency. However, with MSN, once an ISL is visible, it is built immediately, producing many fragmented topology snapshots. In addition, the average  
Figure 6: Network average delay and maximum delay with different time segment lengths. duration of LCTS is short, while its standard deviation is large. This is because LCTS builds ISLs with different lengths of visible time segments. Although the total number of ISLs is high, frequent network topology switching occurs.