# Quantification and Analysis of Carrier-to-Interference Ratio in High-Throughput Satellite Systems
## 1 Introduction  
Satellite communication services are available to any user within the service area of the satellite communication system, regardless of the user's location. This unique feature makes satellite communication systems attractive, especially in remote areas where ground network deployment is difficult or even impossible. In addition, satellite communication can provide various services such as television, broadcasting, telephone, and Internet services, making it indispensable to our hyperconnected society. At present, multimedia application demand and data exchange are experiencing explosive growth [1], and multi-beam high throughput satellite (HTS) systems have proven to be one of the most promising solutions [2]. The antenna gain of multi-beam HTS systems is significantly higher when compared to the ones which use a single large beam to cover the entire service area. Since the first HTS was launched, the number of HTSs in geosynchronous orbit has increased year by year. The first generation of HTSs, represented by IPSTAR and AMC-15/16 satellites, employed spot beams with system throughput ranging from 2 to 5 Gbps. The second generation began to utilize narrow spot beam and frequency reuse technology with throughput ranging from 5 to 10 Gbps, represented by WildBlue-1 (35 spot beams) and Spaceway-3 (24 downlink beams) satellites. The third generation largely used narrow spot beams and frequency reuse technology, with a throughput of up to about 100 Gbps, represented by KA-SAT (82 spot beams) and ViaSat-1 (72 spot beams)satellites. The fourth generation, most of which are currently under research, use large mesh deployable antennas that are capable of forming hundreds of narrower spot beams through beamforming technology, leading to a throughput of more than 300 Gbps, even up to 1 Tbps. ViaSat-2 (more than 100 spot beams), ViaSat-3, and SES17 (about 200 spot beams) are representative satellites. It can be seen that the development of HTS has led to a significant increase in system capacity, accompanied by substantial growth in the number of beams. Currently, mainstream satellite manufacturers in Europe and the United States have nearly 100 high-throughput satellites in orbit, and major satellite operators around the world are actively promoting the deployment of HTS satellites, with more than 1000 orders for HTS satellites around the world [3]. Figure 1 shows the difference between multi-beam and single-beam coverage. Since HTS multibeams are spatially separated in space, enabling the same frequencies to be reused, multibeam coverage is an efficient way to exploit the system bandwidth, which is a very valuable yet expensive resource in any communication system. On the one hand, the frequency reuse technology greatly improves the total bandwidth of the system and then increases the system's capacity. Nevertheless, on the other hand, the side-lobe effect of multi-beam antennas will inevitably lead to co-channel interference between beams using the same frequency band [4; 5], which occupies a more dominant position in the whole system interference compared with cross-polarization interference, adjacent channel interference, and other interferences. In reference [6], the authors claim that spot beam isolation and beam-to-beam interference become increasingly important as the number of beams is increased to tessellate a given geographic area, and they have to be carefully attended to. However, this paper does not focus on the quantification of the carrier-to-interference ratio (CIR). Therefore, it is necessary to research a method for quantifying the CIR (mainly co-channel interference) in multi-beam HTS systems, and based on this, the system capacity-oriented frequency allocation method is explored with a focus on the system's CIR optimization. References [7; 8; 9; 10; 11] mention the role of the CIR in the link budget and give typical values of the CIR to explain or simulate relevant problems in satellite systems, but the typical values given by them are not uniform.