## 1 Introduction  
In recent decades, quantum key distribution (QKD) technology has developed rapidly for its information-theoretical security based on the laws of quantum physics, which have potentially wide application in cryptography [1]. The QKD technology can guarantee the information-theoretic secure key exchange between two distant participants, [PERSON] and [PERSON] [2; 3]. Due to its high performance in metropolitan areas and compatibility with traditional optical communication systems, the continuous-variable quantum key distribution (CV-QKD) has attracted great interest [4; 5; 6; 7; 8; 9; 10; 11]. In the CV-QKD system, the secure key rate (SKR) is severely limited by the excess noise and the channel transmittance. The channel transmittance is influenced by the fiber's quality and the transmission distance. However, the excess noise is influenced by many factors, among which, time-domain diffusion, frequency-domain modulation residual, and polarization perturbation are the research focuses of this article. In fiber-based CV-QKD systems, when the quantum signals are transmitted, many kinds of influence, including polarization perturbation, time-domain diffusion, as well as imperfect modulation, will influence the extremely weak quantum signals and introduce excess noise [12; 13]. To suppress the photon leakage from the pilot light to the weak quantum light, time division multiplexing (TDM) [12; 13; 14], polarization division multiplexing (PDM) [12; 13; 14; 15; 16], as well as frequency division multiplexing (FDM) techniques [12; 17] are used to isolate the quantum signal from the pilot light [12; 14; 15; 16; 17; 18; 19]. In the previous polarization multiplexing LLO CV-QKD scheme, the pilot light and quantum light are modulated on perpendicular polarization modes, respectively [15]. Therefore, the fiber utilization rate is limited. To further improve the performance of the CV-QKD system, the orthogonal dual-polarization scheme has been proposed [16; 18]. In this framework, quantum signals are modulated in two polarization directions, and it is expected to double the SKR compared with the single-polarization method. However, the crosstalk between the strong pilot lights and weak signal lights increases dramatically and deteriorates the performance of the CV-QKD system. There is still a lack of research on the quantitative analysis of the crosstalk in the dual-polarization LLO CV-QKD scheme. Therefore, it is of great significance to conduct theoretical research on the orthogonal dual-polarization LLO CV-QKD scheme. In this paper, we propose a pilot alternately assisted scheme of orthogonal dual-polarization joint time and frequency division multiplexing for the LLO CV-QKD system. Compared with the earlier works (shown in Figure 1) [20], we further optimize the pulse distribution by applying the pilot alternately assisted scheme (shown in Figure 2) to suppress the crosstalk between signal pulses. By the application of polarization multiplexing and time and frequency division multiplexing techniques, the simulation results show that the overall isolation degree can achieve higher than 85.8 dB\(\sim\)90.4 dB, and the crosstalk between pilot tones and quantum signals can be suppressed to a very small range. By optimizing the system parameters (e.g., modulation variance and repetition frequency), the SKR with 12.801 Mbps@25 km is achieved under the infinite polarization extinction ratio (PER) and 30 dB residual ratio of the frequency modulation in the nanosecond-level pulse width. Further, even when the polarization extinction ratio (PER) deteriorates to 17 dB approximately under the nanosecond-level pulse width condition, the SKR achieves higher than 1.02 Mbps@25 km. In addition, we also studied the LLO CV-QKD system for 50 km transmission distance. The results show that the requirement for PER should be above 23 dB, and greater than 0.331 Mbps SKR can still be achieved even if the PER deteriorates to 24 dB.