# Optical Link Design for Quantum Key Distribution-Integrated Optical Access Networks
## 4 Integration of CV-QKD Protocol in OANs
### CV-QKD Implementation Technique
#### 4.2.1 LO or Pilot  
Each symbol of a CV-QKD signal is in a coherent state and requires an LO for detection. To compensate for phase noise in the laser generating the QKD signal, an interference scheme using the same laser at the transmitter is employed, known as the TLO scheme [40]. Since the signal-to-noise ratio (SNR) of the received signal is proportional to the LO power, a high-power TLO must be transmitted. However, TLO experiences attenuation during fiber transmission, potentially necessitating an additional optical amplifier at the receiver [39]. While an erbium-doped fiber amplifier (EDFA) can reduce excess noise compared to a semiconductor optical amplifier (SOA), it is expensive and occupies significant space, making it less suitable for OANs. Moreover, the TLO induces a security loophole, as Eve  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Authors** & **Laser** & **Modulation** & \begin{tabular}{c} **LO/Pilot** \\ **(Multiplexing)** \\ \end{tabular} & \begin{tabular}{c} **Distance** \\ **(km)** \\ \end{tabular} & \
\begin{tabular}{c} **Polarization** \\ **Tracking** \\ \end{tabular} &
\begin{tabular}{c} **Optical** \\ **Receiver** \\ \end{tabular} & **SKR** \\ \hline [PERSON] & Pulse & Gaussian & TLO & & & & \\ et al. [40] & (350 kHz) & (AM+PM) & (TDM+PDM) & 25 & MPC & Homodyne & 20 kbps \\ [PERSON] et al. & Pulse & Gaussian & TLO & & & & \\ et al. [44] & (1 MHz) & (AM+PM) & (TDM+PDM) & 80 & DPC & Homodyne & 0.2 kbps \\ [PERSON] et al. & Pulse & Gaussian & TLO & & & & \\ et al. [45] & (10 MHz) & (AM+PM) & (TDM+PDM) & 25 & DPC & Homodyne & 1 Mbps \\ [PERSON] et al. & Pulse & Gaussian & TLO & & & & \\ et al. [39] & (5 MHz) & (AM+PM) & (TDM+PDM) & 202.81 & DPC & Homodyne & 6.214 bps \\ [PERSON] et al. & Pulse & Gaussian & Pseudo LLO & & & & \\ et al. [43] & (500 MHz) & (AM+PM) & (TDM) & 15 & MPC & Heterodyne & 26.9 Mbps \\ [PERSON] et al. & CW & QPSK & LLO & & & & \\ et al. [46] & CW & (IQM) & (TDM) & 25 & DPC & Heterodyne & 52.48 Mbps \\ [PERSON] et al. & & PCS- & & & & & \\ et al. [47] & CW & 128 APSK & LLO & 40 & DSP & Heterodyne & 1.2 Mbps \\ [PERSON] et al. & & & (IQM) & & & & \\ et al. [48] & CW & Gaussian & LLO & & & & \\ et al. [48] & & (IQM) & (FDM) & 20 & DSP & Heterodyne & 4.71 Mbps \\ \hline \hline \end{tabular}
\end{table}
Table 2: CV-QKD implementation.  
Figure 7: Configuration of DS CV-QKD-integrated PON.  
can manipulate the LO [43]. Alternatively, using an independent laser at the receiver as the LO is known as the LLO scheme. In this case, the optical signal transmitted from the source laser for phase noise tracking is called the pilot. Unlike TLO, which requires high power, the pilot signal is transmitted with just enough power for phase estimation and compensation.  
Both CV-QKD signals and LO (or pilot) must be transmitted simultaneously, employing various multiplexing techniques such as TDM, polarization-division multiplexing (PDM), and frequency-division multiplexing (FDM) [39; 40; 43; 44; 45; 46; 47; 48]. Typically, TDM is used with the TLO scheme, where QKD signals and TLO are placed in orthogonal polarizations to prevent interference-induced beat components. In contrast, when LLO is used, the QKD signals and the pilot are typically transmitted via FDM, as they do not require direct interference for detection.