## 2 CCFD Self-Interference Suppression Performance
### Digital-Domain Self-Interference Suppression  
After suppression in the RF domain, the residual self-interference signal needs to be further suppressed in the digital domain to enable the identification and demodulation of the desired signal. In the CCFD system, the near-end transmitted signal is considered known, while the characteristics of the self-interference channel are unknown. Therefore, digital-domain self-interference suppression requires the use of the known transmitted self-interference signal to estimate and compensate for the self-interference channel, achieving the goal of self-interference suppression [55].  
Current research on digital-domain self-interference suppression techniques primarily includes self-interference channel reconstruction suppression [56], adaptive filtering-based self-interference suppression [57], and nonlinear self-interference suppression [58]. Table 4 presents the status of digital-domain self-interference suppression. With the introduction of advanced technologies such as artificial intelligence and deep learning, the best suppression performance in the digital domain can exceed 45 dB.  
[PERSON] et al. conducted a study on a spread-spectrum telemetry system with a 20 MHz bandwidth, utilizing multi-tap RF cancellation and digital nonlinear self-interference suppression methods. Through simulations and experimental validation, they achieved self-interference suppression levels of 41.5 dB in the RF domain and 38 dB in the digital domain, resulting in a combined suppression of approximately 81 dB [64]. This research provides a significant reference value for the CCFD communication and measurement systems in BDS-3 ISLs. Based on the current research, it is evident that for a CCFD spread-spectrum measurement and communication system using Ka-band array antennas with an RF reception bandwidth of 20 MHz, a total self-interference suppression level of approximately 180 dB can be achieved. This sets a performance benchmark for the feasibility demonstration of CCFD technology in the following discussion.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Reference** & **Year** & **Bandwidth/MHz** & **Suppression Method** & **Best Suppression Effect/dB** \\ \hline
[59] & 2021 & 10 & IVSSLMS adaptive filtering algorithm & 48 \\
[60] & 2022 & 20 & Digital cancellation of PA nonlinearity and & 72 \\
[61] & 2023 & 100 & Self-interference channel reconstruction & 37.78 \\
[62] & 2023 & 750 & Bi-LSTM-based self-interference channel estimation & 47.17 \\
[63] & 2024 & 2000 & Deep learning-based self-interference signal & 60.62 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Status of digital-domain self-interference suppression.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Reference** & **Year** & **Central Frequency** & **Bandwidth** & **Suppression Method** & **Best Suppression Effect/dB** \\ \hline
[51] & 2015 & 25 & 10 & Mach–Zehnder modulator design & 27 \\
[52] & 2019 & 28 & 1024 & Rat-Race coupler design & 40 \\
[53] & 2020 & 28 & \(\backslash\) & Hybrid transformer design & 28.4 \\
[54] & 2021 & 28 & 400 & 80-tap RF cancellation & 30 \\
[45] & 2022 & 26–27 & 100 & 16-tap channel reconstruction & 37 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Status of RF-domain self-interference suppression.