# Key Distribution Scheme for Optical Fiber Channel Based on SNR Feature Measurement
## 4 Analysis of Simulation Results
### Consistency Analysis  
Key consistency comes from precise channel reciprocity, so [PERSON] and [PERSON] share a highly related key source. According to the physical characteristics of the reciprocity of the channel, a consensus key is generated. The reciprocity of the channel mainly improves the consistency of the key. The metric characteristic of the channel generated by the key is the SNR. As shown in Figure 6a, the KGR of the system first increases and then decreases. With the increase in the SNR sampling interval, the calculation of BER requires more data bits, leading to the decrease in the KGR. When the sampling time is 2 us, the maximum KGR is 25 kbps. The amount of data obtained at each sampling point is relatively insignificant when the sampling interval is very small, leading to a relatively severe miscalculation of SNR. Therefore, the SNR values calculated by [PERSON] and [PERSON] are inconsistent, which affects the KGR. When the sampling interval is large, the SNR value calculated within the same amount of time is relatively small, leading to a relatively low KGR rate. Therefore, \begin{table}
\begin{tabular}{c c} \hline \hline
**Equipment** & **Parameter Configuration** \\ \hline AWG & Transmitting Rate: 10 Gb/s \\ Light source & Wavelength: 1550 nm \\ EDFA & Launch Power: 1 mW \\ Ultra-low loss fiber & Power: 12 dBm \\ OSC & 200 km, 0.2 dBm/km \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Simulation parameter list. Figure 5: (a) Key NIST test results, and (b) 0/1 distribution probability of the key. it is necessary to set a reasonable sampling interval to obtain a relatively high KGR. It is obvious that faster fluctuation of channels corresponds to a smaller sampling interval, thus extracting more channel feature and generating more secret keys. In Figure 6b, red represents the sampling value measured by [PERSON], and green characterizes the sampling value measured by [PERSON]. It is evident that the change curves of [PERSON] and [PERSON] almost coincide, and the curve consistency is significant since they simultaneously measure the same fiber. As a result, we quantize the sample value to one and zero when it is greater and less than the mean, respectively. In addition, if the sample value is near the mean, we discard the data. Furthermore, the KGR changes when varying the transmission power, sampling interval, and system's laser power. The signal transmission rate is altered to extract more channel features, and the KGR increases when the transmission rate is relatively low. However, the KGR does not increase and tends to be stable when the transmission rate increases to a certain extent. The sampling interval is varied to find the maximum KGR from different sampling intervals. At this time, the change in the KGR should be non-linear to determine the suitable KGR. As shown in Figure 7, the relationship between the length of the selected data and the consistency is direct. The smaller the data block length is, the worse the key consistency is. By quantifying the SNR, the consistency of the initial keys obtained by [PERSON] and [PERSON] is as above. The SNR interval gradually increases with key consistency, and the obtained key consistency rate reaches 98% when the SNR interval is greater than 1.4 us. As the SNR measurement interval increases, the error curve is smoother, and the fluctuation reduces, so the key consistency quantized by [PERSON] and [PERSON] is enhanced. The extracted keys can be subjected to off-line DSP to eliminate the influence of the inconsistency between the two key sequences of [PERSON] and [PERSON]. More extensive data are needed to calculate the SNR required to increase the KCR at both ends, because the longer the data, the more stable the SNR and the higher the KCR. If the selected data are too long, the KGR for generating the key decreases, in that the calculated SNR drops for extensive data. Therefore, a reasonable SNR measurement interval should be selected to strike a balance between the KCR and the KGR.