## 6 Conclusions  
This paper applies spread-spectrum technology to the detection of the QD's low beacon optical-power position and communication. By increasing the bandwidth of the transmitted beacon communication data signal via spread-spectrum technology, gains are achieved in the SNR after despreading. This enhances the position-detection precision achieved by the QD and the sensitivity of the system. The experimental results demonstrate that the beacon spread-spectrum modulation scheme can effectively detect light-spot positions, with a detection range and linear region similar to those of direct detection. When the received optical power at the QD is \(-37\) dBm, the RMSE values for spreading depths of 1023, 127, and 31 are 0.983 \(\upmu\)m, 2.876 \(\upmu\)m, and 7.275 \(\upmu\)m, respectively. This represents improvements of 14.96 dB, 10.29 dB, and 6.26 dB compared to the position-detection precision of the direct method, which is 30.811 mm. Additionally, under an identical signal bandwidth,  
Figure 10: Graph depicting the relationship between the BER and the received optical power at the QD.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{\begin{tabular}{c} **Pseudocode** \\ **Sequence Depth** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Received Optical** \\ **Power (dBm)** \\ \end{tabular} } & \multicolumn{1}{c}{\
\begin{tabular}{c} **Comparison of** \\ **Detection Sensitivity** \\ **Improvement with** \\ **Direct Detection (dB)** \\ \end{tabular} } & \multicolumn{1}{c}{
\begin{tabular}{c} **Theoretical** \\ **Sensitivity** \\ **Improvement** \\ **(dB)** \\ \end{tabular} } \\ \hline \(1\) & \(-39.6\) & \(0\) & \(0\) \\ \(31\) & \(-46\) & \(6.4\) & \(7.45\) \\ \(127\) & \(-49.7\) & \(10.1\) & \(10.51\) \\ \(1023\) & \(-54.2\) & \(14.6\) & \(15.04\) \\ \hline \hline \end{tabular}
\end{table}
Table 5: Sensitivity of QD system at different spreading gain levels.  
the sensitivity improves by 14.6 dB, 10.1 dB, and 6.4 dB, respectively. Therefore, spread-spectrum technology is able to overcome the hardware limitations associated with reception sensitivity and the precision of position detection to some extent. This comes at the expense of bandwidth, but it leads to higher sensitivity and precision in position detection. Thus, this method could be applied in practical scenarios requiring high-precision detection and long-distance space laser communication using quadrant detectors.  
Conceptualization, S.C. and X.Y.; methodology, X.Y. and S.C.; software, S.C., P.L. and J.Y.; validation, T.W. and Z.Z.; formal analysis, L.X.; investigation, S.C.; resources, S.C. and Z.Z.; data curation, S.C., X.Y., P.L. and J.Y.; writing--original draft preparation, S.C. and L.X.; writing--review and editing, S.C., X.Y. and P.L.; visualization, S.C., X.Y., P.L., J.Y. and T.W.; supervision, X.Y. and P.L.; project administration, L.X. and T.W.; funding acquisition, X.Y. and S.C. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Key R&D Program of China (2022 YFB3902500, 2022 YFB2903402, 2021 YFA0718804).  
Not applicable.  
The data presented in this study are available in the article.  
The authors declare no conflicts of interest.