## 6 Relationship Between RF Input Power and EVM for Different Modulation Formats  
The correlation between RF input power and EVM is crucial in optical communications, as it impacts signal quality, system performance, and overall efficiency. Increasing the RF input power can enhance the quality of the signal until a certain threshold, beyond which high power might cause nonlinearities. In general, in scenarios in which the power levels are low, the signal may be weak and susceptible to noise, which can result in high EVM values. The signal-to-noise ratio (SNR) can be improved by increasing the input power, which can first increase the signal quality by overcoming noise and improving the output signal, leading to reduced EVM values and improvement in the demodulation performance. Various modulation schemes have varied criteria for the quality of the signal and the required power level. Understanding the relationship between RF input power and EVM can help optimize the power levels and achieve the desired signal quality without consuming any additional power. Examining EVM across various RF input power levels can help determine the performance limits of every modulation scheme. Figure 9 presents the EVM performance as a function of RF input power for the different modulation formats and the relevant maximum fiber lengths. For QPSK (100 km), the EVM is high at lower RF input power levels (3 dBm to 9 dBm), and the same applies for 16-QAM and 64-QAM. This indicates poor signal quality due to the significant effect of ISI \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Performance Indicator** & **LMS** & **CMA** & **DFE** & **MLD** & **RDE** & **AMF** \\ \hline
**Error Vector Magnitude (EVM)** & Moderate & Moderate & Low & Low &
\begin{tabular}{c} Very low (with restrictions \\ in our system) \\ \end{tabular} & Moderate \\ \hline
**Convergence Rate** & Slow & Moderate & Moderate & Slow & Fast & Fast \\ \hline
**Time Complexity** & Low & Moderate & High & Moderate & Low \\ \hline
**Space Complexity** & Low & Low & Moderate & High & Moderate & Low \\ \hline
**Adaptability** & Moderate & Moderate & Moderate & High & Moderate \\ \hline
**Memory Usage** & Low & Low & Moderate & High & Moderate & Low \\ \hline
**Convergence Time** & Moderate & Moderate & Fast & Slow & Moderate & Fast \\ \hline
**Number of Taps** & Low & Moderate & High & High & Moderate & Low \\ \hline
**Computational Complexity** & Low & Low & Moderate & Moderate & Low & Low \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Comparisons for the different performance indicators of different equalization algorithms. and noise that are not adequately compensated for. For RF input power levels from 9 to 11 dBm, there is a substantial decrease in EVM, which suggests that increasing power helps to improve signal quality by overcoming, to some extent, the impairments. Beyond 11 dBm, further increases in RF input power continue to reduce EVM, but at a slower rate, indicating marginal benefits. Also, EVM, in all cases, does not fall below the corresponding 3 GPP thresholds except for QPSK, which at 11.5 dBm input power is just below the threshold. On the contrary, Figure 10 presents the EVM performance as a function of RF input power after equalization for the different modulation formats, indicating that although the trend of the curves is similar to that above, the system can perform efficiently with less input power; for example, for QPSK and 100 km fiber length, when the DFE algorithm is used, a 9 dBm input is sufficient, while in the case of MLD, the corresponding value is 9.5 dBm. Figure 10: EVM results for QPSK, 16 QAM, and 64-QAM modulation at the corresponding maximum fiber length vs. RF input power levels with DFE and MLD equalization. Figure 9: EVM results for QPSK, 16 QAM, and 64-QAM modulation at the corresponding maximum fiber length vs. RF input power levels without equalization.