## 5 Conclusions  
In this paper, we theoretically proposed the pilot alternately assisted scheme of orthogonal dual-polarization joint time and frequency division multiplexing for the LLO CV-QKD system. By analyzing the influence mechanism of polarization perturbation, time-domain diffusion, and frequency-domain modulation residual, the orthogonal dual-polarization theoretical model was established to analyze the influence of the polarization extinction ratio (PER) on the key parameters of the CV-QKD system quantitatively. To investigate the performance of the proposed orthogonal dual-polarization scheme, a series of simulations were conducted to verify this theoretical model. Simulation results indicate that as the transmission distance increases, the required PER increases correspondingly to guarantee the performance of quantum key distribution. The overall isolation degree between pilot tones and quantum signals achieves 85.8 dB-90.4 dB, which means the noise due to crosstalk between pilot tones and quantum signals was suppressed efficiently. Then,  
Figure 8: Secure key rate versus transmission distance curves corresponding to different polarization extinction ratios under optimal parameters. (The repetition frequency is 292.5 MHz, s = 0.01, and the pulse width is 1 ns. The modulation variance VA = 1.6.) by optimizing the modulation variance, the required PER reduces accordingly. In the 25 km transmission distance, when the pulse width is fixed to 1 nanosecond, the scale of optimal pulse repetition frequency rate is 292.5-302.5 MHz during the PER\(\geq\)18 dB scale. Compared with the previous literature, the SKR with 0.0197 bit/pulse@25 km is achieved in the proposed dual-polarization scheme based on multi-dimensional multiplexing, which is significantly higher than 0.0078 bit/pulse@25 km in ref. [26]. Furthermore, we analyzed the SKR under a relatively low PER. The results show that more than 1.02 Mbps SKR can still be achieved even if the PER deteriorates to 17 dB. Our work greatly extends the application scenarios of the orthogonal dual-polarization multiplexing CV-QKD system. Consequently, the proposed pilot alternately assisted scheme of dual-polarization joint time and frequency division multiplexing has high potential for improving the performance of LLO CV-QKD. With the research deepening (i.e., larger block size, narrower pulse width, higher pulse isolation degree, and higher repetition frequency), we believe the proposed dual-polarization LLO CV-QKD scheme can be widely applied in practical confidential communication scenarios in the future. Conceptualization, Y.L. ; methodology, S.Z., Y.L. and H.W. ; software, S.Z. ; validation, Y.L., H.W. and Y.P. ; formal analysis, S.Z. and Y.S. ; investigation, S.Z. ; resources, Y.L., W.H. and B.X. ; data curation, S.Z. and Y.L. ; writing--original draft preparation, S.Z. ; writing--review and editing, Y.L., W.H. and B.X. ; visualization, S.Z. ; supervision, Y.L. and B.X. ; project administration, H.W., T.Z. and Y.P. ; funding acquisition, Y.P. and T.Z. All authors have read and agreed to the published version of the manuscript. This work was supported in part by the National Key Research and Development Program of China (Grant No. 2020 YFA0309704), the National Natural Science Foundation of China (Grant Nos. U19A2076, 62101516, 62171418, and 62201530), the Sichuan Science and Technology Program (Grant Nos. 2023 JDRC0017, 2023 YFG0143, 2022 ZDZX0009, 2021 YJ0313, and 2023 ZYD0131), the Natural Science Foundation of Sichuan Province (Grant Nos. 2023 NSFSC1387 and 2023 NSFSC0449), the Basic Research Program of China (Grant No. JCKY2021210B059), the Equipment Advance Research Field Foundation (Grant No. 315067206), the Chengdu Major Science and Technology Innovation Program (Grant No. 2021-YFB08-00040-GX), the Chengdu Key Research and Development Support Program (Grant No. 2021-YF09-00116-GX), and the Foundation of Science and Technology on Communication Security Laboratory (Grant No. 61421030402012111, 6142103042201). Not applicable.