# A Novel 64 QAM-OFDM Optical Access System Based on Bit Reconstruction
## 3 Simulation Setup and Results
(**a**) After the NOT-gate in the pre-encoder; (**b**) after the heterodyne gate in the pre-encoder; (**c**) after the LS; (**d**) after the delay. are combined into one signal, thus halving the signal source. Due to the presence of the partition mapping stage, the amplitude signal with positive and negative values needs to be converted to a positive signal at this point, as shown in Figure 4d. Figure 5 shows the bit error rate (BER) performance of the 64 QAM-OFDM and conventional OFDM methods at 10 GBaud for a 10 km/20 km transmission. It can be seen from Figure 5 that the proposed scheme (red line) and the general OFDM system (yellow line) start to diverge when the optical signal-to-noise ratio approaches 18 dB at a transmission distance of 10 km. The BER performance of the red line decreases rapidly and stays below the purple line. This indicates that the proposed scheme has a better performance. Setting a soft decision threshold at 0.02, we compare the performance of these two schemes. The gain of the proposed scheme is about 1.3 dB compared to the conventional OFDM system, while the performance of the proposed scheme (blue line) has a gain of 0.51 dB over the general OFDM scheme (green line) at the SD-FEC threshold for a transmission of 20 km. The reason for this is that information bits are introduced into the transmitter where we set bit reconstruction. Even after the OFDM and fibre channel, the signal still retains a large degree of correlation that we can easily obtain accurate information from. Figure 5a,b represents the proposed 64 QAM signal at an OSNR of 19 dB and 30 dB, respectively. Figure 6 shows the BER performance and algorithm complexity of the proposed scheme compared with other OFDM systems. To further evaluate the performance of the bit reconstruction-based 64 QAM-OFDM scheme, we tested the BER performance of a 10 GBaud signal transmitted over 10 km on a coherent communication system. As can be seen from Figure 6a, both the proposed scheme (yellow line) and the adaptive algorithm-based OFDM (green line) reach the SD-FEC threshold first at a lower SNR than the conventional OFDM system. The adaptive algorithm-based OFDM scheme has a gain of about 0.92 dB compared to the conventional OFDM system. The bit reconstruction scheme still has a gain of about 0.38 dB compared to the adaptive algorithm-based OFDM scheme. This suggests a definite superiority of the proposed theory. To understand the computational complexity of the proposed theory, we calculated the CPU runtime required  
Figure 4: The change in the signal amplitude during bit reconstruction. (**a**) After the delay; (**b**) after the attenuator; (**c**) after the adder; (**d**) the final output of the logic code circuit. by the scheme and compared it with the other two systems, as shown in Figure 6b. The CPU runtime test was performed on a personal computer with a Core i5-9300H processor at 2.40 GHz. The proposed 64 QAM-OFDM scheme had a better BER performance, but its complexity was slightly higher than that of the adaptive algorithm-based OFDM scheme. Moreover, compared with the standard OFDM signal, our scheme does not deteriorate the PAPR performance, and the two curves are fitted well as shown in Figure 7.  
At the same time, we collected and calculated the occurrence probability of each signal constellation at the receiver. It can be seen from Table 2 that the distribution of constellation mapping points in our scheme is not uniform and the probability of the preset high-power region is significantly lower than that of the low-power region. Figure 5: The BER performance of the OFDM\(-\)based and the proposed scheme. (**a**) The proposed 64 QAM signal at 19 dB; (**b**) the proposed 64 QAM signal at 30 dB. Figure 6: The BER performance and algorithm complexity of the proposed scheme compared with other OFDM systems. (**a**) BER performance of the three systems; (**b**) comparison of algorithm complexity.