# Forward Link Optimization for the Design of VHTS Satellite Networks
## 7 Conclusions  
This contribution presents a methodology for the evaluation of VHTS satellite networks, taking into account technical and commercial requirements. The method is based on the use of an analytical expression of the \(C\)\(INR_{b}\) as a function of the user \(G\)/\(T\), the number of beams, and link availability. The methodology helps to evaluate the trade-off resulting from the different combinations of user \(G\)/\(T\), the optimal number of beams, and the system cost. In addition, the cost per Gbps in orbit for clear sky conditions and the actual channel capacity using CCM or VCM for different link availabilities are obtained.  
Figure 14: Performance comparison of methodology on Very High Throughput Satellite (VHTS) affordability for throughput according to [3]. Note: the launch cost and the ground segment cost have been added.  
The method includes the contribution of the ground segment cost to the total system cost and evaluates the number of required gateways. The results considering the Q/V band in the feeder link have been presented. In the scenarios under study, up to two redundant gateways would be required to ensure an availability above 99.9% in case the Q/V band is used in the feeder link by using the model in [29]. A redundant gateway is added to the network when a nominal gateway is in outage due to a heavy rain attenuation, therefore, all traffic from nominal gateway impaired is rerouted to redundant gateway assigned, ensuring high availability on the network.  
The proposed methodology is flexible, since the system designer can modify input values ad-hoc, such as the service area, technical and commercial requirements, and system constraints, to evaluate the impact on the KPIs and thus select the most appropriate solution.  
The methodology has been validated in two service areas with different characteristics (North America and Europe) to compare the results of the cost per Gbps in orbit. The results show realistic KPI figures and their variation with the number of beams and the user \(G\)/\(T\).  
As future work, the methodology will be extended to include the return link, adding new requirements, constraints, cost models, and KPIs to consider ACM (Adaptive Coding and Modulation) and the use of flexible payloads and irregular beams.  
Conceptualization, F.G.O.-G. and R.M.; methodology, F.G.O.-G. and R.M.; software, F.G.O.-G.; validation, R.M., M.A.S.-N. and S.L.-A.; formal analysis, F.G.O.-G. and R.M.; investigation, F.G.O.-G. and A.C.; writing--original draft preparation, F.G.O.-G.; writing--review and editing, R.M. and M.A.S.-N.; supervision, R.M., M.A.S.-N. and S.L.-A.; project administration, R.M.; funding acquisition, R.M. All authors have read and agreed to the published version of the manuscript.  
This work has been supported by the Spanish Government, Ministerio de Economia, Industria y Competitividad through the National Program of Research, Development and Innovation within the FUTURE-RADIO project under Grant TEC2017-85529-C3-1-R and a PhD scholarship provided by Universidad Politecnica de Madrid. This paper was made possible thanks to the Spanish Government and the Universidad Politecnica de Madrid for their financial support.  
The authors declare no conflict of interest.