# Quantification and Analysis of Carrier-to-Interference Ratio in High-Throughput Satellite Systems
## 1 Introduction
In reference [18], the authors discuss the uplink transmission in the DtS-IoT whilst disregarding interference from other sources and focusing on the impact of terrestrial network interference on a satellite uplink transmission. As mentioned in this reference, it is quite possible to utilize shared frequency bands in areas where the terrestrial interference level is low enough. However, the terrestrial deployment over populated regions could drastically increase the uplink channel interference, which has to be addressed well. In reference [19], the authors focus on the random-access protocol based on Aloha for the MAC layer in DtS-IoT networks and introduce an intelligent traffic load distribution strategy with low implementation complexity, wherein the interference issue is addressed by applying successive interference cancellation (SIC) at the receiver for removing all copies once one of the messages is successfully decoded. The authors of [19] propose another traffic load distribution strategy based on successive convex approximation (SCA) in reference [20] and subsequently handle the interference between users in the same way. In reference [21], considering the intermittent link availability between the end devices and the gateways,the authors propose a scheduling algorithm to ensure reliable communication and avoid packet drops/collisions. As discussed in [21], the communication between the end devices and the gateways is based on a spread spectrum modulation at the physical layer, and the importance of the interference issue lies in affecting the selection of the spreading factor, which suffers from a trade-off between the resulting bit rate, the transmission range, and the energy required to transmit. In reference [22], the authors focused on designing sparse constellations for the DtS-IoT with as few in-orbit DtS-IoT satellites as possible while guaranteeing the services. As mentioned, users' access to the radio channel is scheduled with a trade-off between mitigating interference and signaling traffic, which is consistent with the above references. It is worth noting that more and more studies are being carried out to deal with co-channel interference. For example, satellite communication coding optimization design based on large frequency multiplexing factor and precoding technology can obviously eliminate the co-channel interference [23], which is especially suitable for full frequency multiplexing in the future. In addition, new communication schemes, such as the coded-beam high-throughput satellite system [24], are being investigated to avoid spectrum reuses and co-channel interference. However, the quantification of the CIR in current high-throughput satellite systems is still of great significance. Aimed at the problem of dominant co-channel interference in multi-beam systems, this paper proposes a quantification method of the CIR for user uplink and downlink in multi-beam HTS systems, and the closed-form expressions are given. Furthermore, the influencing factors and selection strategies of the CIR are expounded. Finally, the feasibility of the proposed quantification method is verified through simulation. The contribution of this paper is two-fold: first, a quantification method of the CIR for user uplink and downlink in multi-beam HTS systems is proposed; second, through numerical simulation, the effects of the frequency plan and PSD on the CIR are presented. The remainder of the paper is organized as follows: Section 2 shows the model of the multi-beam HTS systems. In Section 3, a quantification method for the CIR is proposed with closed-form expressions. The influencing factors and selection strategies of the CIR are expounded in Section 4. In Sections 5 and 6, the simulation results and discussion are presented. Finally, Section 7 concludes the paper.