# Combined Optical Fiber Transmission System Based on QNSC and BER-LM
## 4 Results and Analysis  
First, we studied the encrypted transmission performance of a PSK/QNSC system. Figure 7 shows the signal Q-factor at different transmission distances with and without PSK/QNSC. As the transmission distance increased from 10 km to 200 km, the Q-factor with and without QNSC decreased gradually. When the transmission distance increased to 200 km, the Q factor decreased to the 7% FEC limit. At the same time, it was found that the transmission performance with PSK/QNSC was lower than that without QNSC. When the transmission distance was 100 km, the Q-factor with QNSC was about 1.7 dB lower than that without QNSC. This shows that QNSC gained system security at the cost of losing part of the transmission performance. When the transmission distance was less than 200 km, this cost was worthwhile because correct data could be recovered through FEC. Figure 7 compares three QNSC schemes with modulation formats of 1024 \(\times\) 1024 PSK, 512 \(\times\) 512 PSK, and 256 \(\times\) 256 PSK. With the increase in OSNR, the detection error probability (DFP) of Eve decreased. DFP refers to the probability that [PERSON] could obtain the correct data bit by analyzing the stolen optical signal. This is because, with the increase in OSNR, the masking effect of the noise signal decreased, which made it easier for [PERSON] to obtain the correct data bits. When the OSNR was 30 dB, the DFPs corresponding to the three QNSC schemes were 0.999, 0.998, and 0.994, respectively. This indicates that the higher the modulation order of the PSK signal, the higher the security of the data. This is because with the increase in the modulation order of the PSK signal, the distance between the adjacent constellation points was closer, which made it easier to be masked by noise. Figure 6: (**a**) System device diagram of loopback optical fiber channel. (**b**) Received PSK/QNSC signal and decrypted QPSK signal. Next, we analyzed the key distribution system based on BER-LM. Figure 8 shows the loopback channel BER samples measured by [PERSON], [PERSON], and [PERSON] using negotiation bits. It was found that the BER measured by [PERSON] and [PERSON] ranged from 0.25 to 0.3, and their trends were very consistent. However, the BER measured by [PERSON] was concentrated between 0.4 and 0.45, and the BER change trend was different from that of [PERSON] and [PERSON]. This was because the measurement position of [PERSON] was different from that of [PERSON] and [PERSON]. [PERSON] and [PERSON] could measure the BER change of the entire optical fiber channel. However, [PERSON] could only obtain signals from optical fiber lines through light splitting, so the BER of the intercepted signals was high, and the change trend was different from that of the legitimate sender and receiver. Thus, the security of key distribution was guaranteed. Finally, we studied the performance of the QNSC and key joint distribution system. As shown in Figure 9, the Q-factor decreased with an increase in the negotiation bit position. This was because the closer the negotiation data were to the raw data, the more serious the impact on the raw data, resulting in a decline in transmission performance. As shown in Figure 9, as the negotiated bit position increased, the loopback channel BER measured according to the negotiation data gradually decreased. This was because the higher the negotiation bit, the greater the signal amplitude, and the lower the impact of noise, and the lower the BER. As the negotiated bit position decreases, the BER measured by [PERSON] and [PERSON] tends to 0.5, which reduces the security of key distribution. Therefore, in order to ensure that the Q-factor is high enough, the negotiation bit position should be reduced. In order to increase the security of the negotiation key, the negotiation bit position should be improved. We finally chose a negotiated bit position of seven, which can not only ensure that the Q-factor meets the 7% FET limit but also ensures the security of key distribution.