## 6 Conclusions  
WiFi, 4G LTE, and other signals are widely used in passive radar. The rise of 5G has led scholars to study passive radar based on 5G. In addition to the classic processing methods of passive radar, some scholars have studied signal processing methods based on certain signals within 5G, such as signal processing methods based on SSB signals. This paper focuses on whether other signals in 5G can be used for target detection and proposes a method of using CSI-RS signals of 5G for channel estimation and extracting Doppler frequency offset from channel responses. Meanwhile, compared to other studies using passive radars based on 5G for vehicle detection or personnel localization, the method in this paper focuses more on detecting weak and small targets at short distances. In order to verify the correctness of the method proposed in this paper, the rotating target was chosen as the detection object, as the actual motion state of the rotating target, such as velocity, can be more easily obtained. In this paper, a rotating target echo model is established, including bilateral rotating targets and unilateral rotating targets. Additionally, the frequency offset curve characteristics of different types of rotating targets are analyzed. The impact of interference signals and receiver phase noise in actual measurements is also analyzed. At the same time, a rotating target model for actual measurement is established. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Theoretical Speed (rps)** & **Measured Speed (rps)** & **Error** \\ \hline
0.125 & 0.1221 & 2.3\% \\
0.25 & 0.2563 & 2.5\% \\
0.5 & 0.5005 & 0.1\% \\
0.625 & 0.6226 & 0.38\% \\
0.75 & 0.7568 & 0.9\% \\
0.875 & 0.8789 & 0.45\% \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Results of unilateral targets with a radius of 0.15 m. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Step** & **Number of Floating Point Addition** & **Number of Floating Point** \\  & **Multiplication** & **Description** \\ \hline Random phase noise suppression & \(2N_{1}\times M_{1}+2M_{1}\) & \(4M_{1}\) & Add up the channel responses obtained from \(N_{1}\) subcarriers; the channel response has \(M_{1}\) complex sampling points. Signal has \(M_{1}\) complex sampling points; Signal has \(M_{1}\) complex sampling points; the order of the HPF filter is \(N_{HPF}=48\). \\ Time-frequency analysis & \(2J\times I\times\log_{2}I\) & \(4J\times\frac{I}{2}\times\log_{2}I\) & \(I\) is length of FFT; \(J\) is the number of sliding windows. \\ Doppler frequency offset curve extraction & \(J\times 2(I-2)\) & Peak search \(J\) times, with \(I\) data searched each time. \\ Smooth filtering & \(N_{LPF}\times J\) & \((N_{LPF}+1)\times J\) & Signal has \\  & & & \\  & & & \\ FFT & \(2N_{FFT}\times\log_{2}N_{FFT}\) & \(4\times\frac{N_{FFT}}{2}\times\log_{2}N_{FFT}\) & \(N_{FFT}\) is length of FFT and \\  & \(+2\frac{N_{FFT}}{2}\times\log_{2}N_{FFT}\) & \(4\times\frac{N_{FFT}}{2}\times\log_{2}N_{FFT}\) & \(N_{FFT}=4096\). \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Analysis of computational complexity for each processing step. This paper conducted measurement experiments in different environments and parameter configurations, and compared the measurement results with theoretical values, providing data for research on passive radar based on 5G. According to the experiment results, the speed estimation error does not exceed 5% within the measurable range. The results have verified the feasibility and correctness of the detection method for rotating target detection and velocity estimation using CSI-RS signals. This paper can provide a foundation and direction for further research and application of passive radar based on 5G. The limitations of this method and its application in other scenarios can be further studied in the future. Conceptualization, P.C. ; investigation, P.C. ; methodology, L.T. and Y.B. ; project administration, P.C. and J.W. ; supervision, P.C. and J.W. ; validation, L.T. and Y.B. ; writing--original draft, L.T. ; writing--review and editing, P.C.