# Forward Link Optimization for the Design of VHTS Satellite Networks
## 5 Case Studies
### Scenario 1  
Following the proposed methodology, the first step is the calculation of the system KPIs in clear sky conditions (95% availability) for the system set and constraints. Figure 6a shows the average \(CINR_{b}\) depending on the number of beams and the user \(G/T\). It is shown that when the required user \(G/T\) increases, the optimal number of beams decreases and the average \(CINR_{b}\) increases. The same idea applies to the maximum beam capacity (Figure 6b); if \(G/T\) increases, the optimal number of beams decreases and the Gbps per beam increases. There is a trade-off because the number of beams has a strong impact on factors such as the total capacity of the satellite, the satellite mass (14), the total satellite cost (15) and therefore the cost of Gbps in orbit. Figure 7 shows the cost per Gbps in orbit that would be obtained for the optimal number of beams with different user \(G/T\) for scenario 1. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameters** & **Scenario 1** & **Scenario 2** \\ \hline Service Area & CONUS, Mexico and Cuba & Europa and selected sites \\  & & in Northern Africa \\ Satellite Position & 109 W & 9 E \\ Bandwidth per gateway (Q/V band) & 2.5 GHz & 2.5 GHz \\ Gateway antenna diameter & 9.1 m & 9.1 m \\ Bandwidth per beam (Ka band) & 250 MHz & 250 MHz \\ Frequency plan & 4 colours & 4 colours \\ Maximum investment cost & 500 M€ & 360 M€ \\ Maximum satellite mass & 6700 Kg & 6700 Kg \\ Maximum satellite antenna diameter & 2 m & 2 m \\ (user link) & 99.9\% & 99.9\% \\ Minimum satellite capacity (Shannon) & 98\% & 98\% \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Definition of scenarios. Figure 6: Analysis in clear sky conditions in scenario 1. **(a)** Variation of the \(CINR_{b}\) depending on the numbers of beams and the user \(G/T\); **(b)** variation of the \(C_{b}\) depending on the numbers of beams and the user \(G/T\). The decision factors are shown in Table 1, showing the optimal number of beams for a set of user \(G/T\) along with KPIs (satellite capacity, satellite mass, system cost, and cost per Gbps in orbit). Table 2 shows that when the user \(G/T\) decreases, the optimal number of beams increases. However, the \(CINR_{b}\) and beam capacity decrease due to cochannel interference, and the number of gateways and total cost increase. On the other hand, the total system capacity increases and therefore the cost per Gbps in orbit decreases. The minimum cost per Gbps in orbit is obtained for a \(G/T\) of 15 dB/K and an optimal number of beams of 180. To achieve these conditions, the maximum cost of the system is 41 Mc higher than required (see Table 1). For this reason, the lowest possible cost per Gbps in orbit fully compliant with requirements of Table 1 is obtained when the user \(G/T\) is 17 dB/K and the number of beams is 159. This minimum cost is 2.5869 Mc per Gbps in orbit. In the second step of the methodology, the service area percentage for a limited set of DVB-S2X ModCods has been calculated. Figure 8 shows the results using CCM for a user G/T of 17 dB/K, 159 beams, and different link availabilities. The use of 64 APSK 11/15 is not feasible, and with 32 APSK 2/3-L or 16 APSK 7/9, the maximum link availability is 99.7%. Thus, in order to meet the 99.9% requirement, 16 APSK 3/5 shall be used.