# Quantification and Analysis of Carrier-to-Interference Ratio in High-Throughput Satellite Systems
## 6 Analysis of the CIR under Different Power Spectral Density (PSD)  
In this section, we conduct a simulation analysis of the CIR under different PSD. The number of beams, the beam layout, the \(-3\) dB beamwidth of all the beams, and the gain function of the onboard user antenna are the same as those in Section 4. In addition, the frequency plan adopts 4-color regular mapping, as shown in Figure 10a. What is particular about this simulation setup is that the output power of downlink TNTs for all the beams are different. In detail, in order to cover more potential cases concerning how different PSD levels are distributed among the co-channel beams, the output power of downlink TNTs are set to 80 W, 100 W, 80 W, 60 W, 80 W, 100 W, 80 W, 100 W, 100 W, 80 W, 100 W, 100 W, 100 W, 100 W, 100 W, 80 W, 80 W, and 100 W for beams 1\(\sim\)19, respectively. The PSD is obtained as a mean value for each TWT/transponder, i.e., power/bandwidth. It can be concluded that the PSD levels of all the beams are not the same due to their equal bandwidth and unequal transmit power. The locations of the target user and interfering users in their respective areas are randomly generated and repeated 1000 times to obtain the cumulative distribution function of the user link CIR in 19 beams. Taking beam 1 as an example (the other beams are similar), we illustrate the cumulative distribution function of the CIR in Figure 13, represented by a solid line. In order to present the comparative effect concerning different or uniform PSD, we also put the previous simulation result in Figure 13, as shown as the dotted line, which is achieved in Section 4 when assuming all of the downlink TNTs as 100 W.  
Figure 13: _Cont._  
It can be seen from Figure 13a that the CIR levels are almost the same for the user uplink of beam 1, regardless of different or uniform PSD. The reason for this is that we set the different PSD by adjusting the output power of the downlink TMTs, while keeping the output power of user terminals unchanged. In other words, only the downlink's PSD is varied. As shown in Figure 13b, for the user downlink of beam 1, the CIR level under different PSD is evidently lower than that under uniform PSD. The reason is as follows. The user downlink PSD of beam 1, 80 W/(20,200-19,575) MHz, suffers from a significant decrease compared to that under uniform PSD, 100 W/(20,200-19,575) MHz. Meanwhile, the user downlink PSD of beam 10/13/16/19, which shares the same frequency band as beam 1, remains unchanged. In other words, the carrier signal power of beam 1 decreases while the interference power remains constant. In order to comprehensively conduct an analysis of all 19 beams, the CIR values satisfying the requirements of 100% coverage and 85% coverage are taken as the reference. Moreover, the CIR levels are almost the same for the user uplink regardless of different or uniform PSD, which is inferred from beam 1 to all the beams. Therefore, not paying useless attention to the user uplink, we only simulate the user downlink CIR of all the beams under both different and uniform PSD, as shown in Table 3. In order to show the comparison effect under different and uniform PSD more clearly, bar charts illustrating the user downlink CIR of all beams with the 85% coverage requirement (the other coverage requirements are similar) are formed, as shown in Figure 14.  
It can be seen from Table 3 and Figure 14 that the size relation of user downlink CIR values under different PSD and uniform PSD is uncertain among the 19 beams. In particular, for co-channel beam group 1 (beams 1, 10, 13, 16, and 19), the user downlink CIR level of beam 1 under different PSD is lower than that under uniform PSD, which has been explained before; the user downlink CIR level of beams 10/13/16/19 under different PSD is higher than that under uniform PSD because their downlink PSD levels remain unchanged while the interference power coming from beam 1 decreases evidently.