# Forward Link Optimization for the Design of VHTS Satellite Networks
## 1 Introduction  
Traffic demand is increasing significantly in satellite communication systems. Very High Throughput Satellites (VHTS) are usually geostationary satellite systems that will be able to meet traffic demand in the upcoming years [1; 2; 3]. In this sense, VHTS systems exceed the capacity of traditional FSS and MSS systems (Fixed and Mobile Satellite Services, respectively) with contoured regional footprints and aspire to achieve 1 Terabit/s per satellite in the near future [4]. Indeed, VHTS has been claimed either by users, satellite operators, or manufacturers to play a key role in future 5G networks as a complement to terrestrial networks [5; 6]. VHTS systems are based on the following technical considerations to achieve expected performance and overcome atmospheric attenuations in Ka-band satellite links: multibeam footprint in the service area, the reuse of frequency and polarization [4; 7], the use of highly spectral efficient transmission techniques (e.g., DVB-S2X) and, advanced link adaptation strategies, such as Variable and Adaptive Coding and Modulation (VCM and ACM, respectively) [8]. From a system design perspective, the use of link adaptation techniques is motivated by the need to select the most appropriate modulation and coding scheme to meet the link budget conditions and availability requirements of each user terminalin the service area. A worst-case scenario satellite-link-based system design involves an inefficient approach and the expenditure of valuable communication resources [9]. One of the main problems in dimensioning and optimizing the VHTS user link is the evaluation of co-channel interference between user beams of the same color (frequency and polarization pair) [10]. This impairment is characterized by the Carrier-to-Interference-plus-Noise Ratio (CINR), which limits the capacity of the satellite link. In addition, in a multi-beam satellite system, the \(\mathit{CINR}\) depends on the number of colors, the angular separation and number of co-channel beams (frequency channeling plan), the carrier power, the operation point of interfering transponders, and the radiation pattern of the user beams in terms of sidelobe levels. Therefore, the characterization of the \(\mathit{CINR}\) is essential and must be considered in the VHTS dimensioning process [11]. Due to the limited spectrum available in the Ka band (30/20 GHz), the use of Q/V bands (50/40 GHz) in the feeder link is suggested to increase satellite capacity [12]. When the feeder link is moved to the Q/V bands, there is a larger bandwidth available in Ka for the user link and therefore a higher number of user beams can be assigned per gateway, resulting in a reduced number of gateways. However, the use of Q/V bands in the feeder link imposes the need to introduce fade mitigation techniques using gateway diversity to overcome the attenuation caused by rain, gases, and clouds [13; 14; 15]. Either using Ka or Q/V in the feeder link, the cost of the ground segment shall be considered as part of the system cost, as it is proposed in the cost model introduced in this paper. The design of VHTS systems must take into account various technical and commercial requirements and constraints. In the literature, the formal optimization and dimensioning methodology for multibeam satellite networks has been presented in pioneering contributions in [16; 17], where analytical expressions of some key parameters of satellite link design are presented. However, the cost of the ground segment is not included, and neither the user station G/T nor link availability are part of the methodology. The authors of [3] provide an overview of VHTS satellites, including technical and business considerations, and examine important key metrics and trade-offs for these satellites, including a cost per bps decision tree and cost-performance mapping. However, they leave out important aspects, such as the ground segment.