# A Novel 64 QAM-OFDM Optical Access System Based on Bit Reconstruction
## 3 Simulation Setup and Results  
In this section, simulation experiments are designed to verify the performance of the proposed bit reconstruction scheme. The left half of Figure 2 represents the OLT data transmission process and the right half represents the ONU data reception process. Short-distance transmission of a 10 Gbaud 64 QAM signal over SMF in a coherent optical OFDM access system is established. Firstly, the pseudo-random binary sequence (PRBS) with a length of 60,000 is converted into six channels of information flow at the TX. Then the proposed bit reconstruction process is performed. After encoding mapping, the symbol sequence enters OFDM modulation. The optical signal source uses a 1550 nm light wave and splits into two mutually orthogonal light waves. By interacting on the MZM to produce a modulated optical signal, the two light waves are combined. Before being passed through the optical fibre link, the output optical signal passes through the VOA to reduce the signal power before transmission. The transmission link is composed of SMF, whose dispersion is 16.75 ps/nm/km and attenuation is 0.2 dB/km. At the RX, the optical signals are converted to electrical signals by a coherent receiver. The length of the CP is 16. The signal is captured through the coherent receiver. We remove the CP and convert the data into a parallel signal. Removing bit reconstruction occurs after FFT and QAM demodulation. The detailed parameters of the simulation system are shown in Table 1.  
Figure 2: Simulation of the 64 QAM-OFDM scheme. Figures 3 and 4 indicate that the signal amplitude changes during the bit reconstruction process. The horizontal coordinate represents the signal length and the vertical coordinate represents the signal amplitude. The red circles show the specific amplitude of the signal during each process. The signal enters the bit reconstruction section by first performing a series-parallel transformation and grouping two by two, where the signal amplitudes are 0 and 1. Figure 3a depicts the amplitude of the signal after the NOT-gate in the pre-encoder stage. The original signal has amplitudes of 0 and 1, which are flipped after the NOT-gates. Figure 3b shows the amplitude change in the signal after the heterodyne gate in the pre-encoder stage. The previously mentioned reference bits are shown here. Figure 3c shows the amplitude after going through the level shifter. At this point, the signal amplitude changes from the previous unipolarity to bipolarity. Similarly, the reference bits are shown here too. Figures 3d and 4a show the change in amplitude after the delayers. The bipolar signal amplitudes \((1,-1)\) may produce amplitudes of \((0,\pm 1,\pm 2)\) with the delayers. After the attenuator, the signal amplitude becomes half of the original data as shown in Figure 4b. In Figure 4c, it can be seen that after the adder the original signal becomes a bipolar signal symmetrical about zero with a signal amplitude of \((0,\pm 1,\pm 2,\pm 3)\). Figure 4d shows the final output signal of the logic coding circuit. Depending on the structure of the logic coding circuit, the input raw signal varies with the structure of the circuit. Starting from an initial amplitude of \((0,1)\), the amplitude is continuously varied. In particular, it can be seen in Figure 4b that the signal at this point is a two-level signal symmetrical about the origin. The reason for this is that the logic coding circuit consists of multiple levels of branches, where the signals of each two-level branch \begin{table}
\begin{tabular}{l c} \hline \hline
**Parameters** & **Value** \\ \hline Switching Bias Voltage & 4 V \\ Switching RF Voltage & 4 V \\ Bias Voltage 1 & 2 V \\ Bias Voltage 2 & \(-2\) V \\ Responsivity of PD & 1 A/W \\ Dark Current of PD & 10 nA \\ Fibre & SMF \\ OSNR & 0 dB–32 dB \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The detailed parameters of the simulation system. Figure 3: The change in the signal amplitude during bit reconstruction.