# Experimental Evaluation of Hybrid Fibre-Wireless System for 5G Networks
## 5 Experimental Results  
The experimental findings of the tests deployed as described in the preceding section are explained. One way of measuring the accuracy of coefficient estimates for various designs is the mean square error (MSE). The MSE is 27 dB when no DPD is employed, whereas CNN reduces it to 20 dB. The standardized performance metric in 3 GPP is the error vector magnitude (_EVM_), which gives the optimum constellation location for each received symbol. The _EVM_ value of the device is the root mean square (RMS) of all error vector magnitudes between the received symbol positions and their nearest ideal constellation sites. \[EVM\left(\%\right)=\sqrt{\frac{\frac{1}{M}\sum_{m=1}^{M}\left|S_{m}-S_{0,m} \right|^{2}}{\frac{1}{M}\sum_{m=1}^{M}\left|S_{m}\right|^{2}}} \tag{4}\]  
where \(M\) gives constellation symbols, \(S_{m}\) represents the constellation's real symbol associated with the symbol \"\(m\)\" and \(S_{0,m}\) denotes the real symbol linked with \(S_{m}\). 3 GPP has set the _EVM_ standard limit for the 256 QAM modulation rate to be 3.5% [62, 63]. The _EVM_ is displayed in Figure 3(a) for flexible waveforms without DPD, with the GMP DPD method followed by the CNN DPD method. For all flexible waveform designs, it is notable that the CNN DPD approach achieves a better reduction than GMP. \begin{table}
\begin{tabular}{c c} \hline
**Framework** & **Parameters** \\ \hline Optimizer & ADAM \\ Activation function & ReLu \\ Type of output layer & Softmax \\ Loss type & Mean Square Error (MSE) \\ Hidden Layers \(N\) & 20 \\ Neurons per layer \(K\) & 15 \\ \hline Regularization Type & L1 \\ Regularization factor & 0.01 \\ Learning rate & 0.01 \\ Batch size & 16, 32, 64, 128, 256 \\ Validation split & 0.3 \\ Training specimens & 500,000 \\ Testing specimens & 200,000 \\ \hline Complexity & \\ \((4+N)\)\(K+(N-1)\)\(K^{2}+6\) & 5986 \\ \hline \end{tabular}
\end{table} 
Table 3: CNN architecture values. In Figure 4b, we compare a case called back-to-back (B2B) where a laser and a photodiode are connected via a patch cord of less than 1 m SMF. It can be seen that B2B has 2% EVM, which signifies that this is the least EVM obtained if we remove the presence of fibre or wireless integration into the system. Similarly, we have added the results of a generic RoF case from our previous study [18] and compared it with the proposed hybrid FiWi link. It is noticeable that the hybrid FiWi system has a 5% EVM. The performance of the system is further enhanced using CNN-based DPD, bringing the EVM value to 3.5% Additionally, we show EVM results for the received optical power for the F-OFDM and 5G NR waveforms in Figure 5. The received optical power for the 5G NR waveform is not meeting the 3 GPP criteria, which means that the dynamic range is affected. Therefore, it becomes fundamentally important to improve the performance via DPD. DPD ensures that EVM is within the limits from \(-8\) to \(-6\) dBm. If we consider the F-OFDM waveform, the dynamic range in the 3 GPP limit is 2 dB, whereas F-OFDM with DPD has extended the dynamic range to 7 dB from \(-12\) dBm to \(-6\) dBm within 3 GPP limits. Figure 4: Comparison of EVM for 5G NR and F-OFDM waveforms: (**a**) with GMP, CNN and without DPD for 0 dBm RF input power, while (**b**) with DPD and No DPD for B2B, RoF and FiWi. In addition to EVM, the performance is also measured in terms of power spectral density (PSD). The PSD measures the ratio of power in the useful band to the power in the adjacent bands. It is also known as adjacent channel leakage or power ratio (ACLR/ACPR). The trends in Figure 6 show that the PSD for a case without DPD has a \(-25\) dBc value. The 3 GPP has set these limits to be \(-35\) dBc. To improve the performance, DPD with GMP and CNN is utilized and it is signified that CNN improvement is better than GMP, which has been discussed in [18]. In Table 4, the EVM and PSD at \(0\) dBm are expressed for the DPD utilized. Figure 5: EVM performance comparison for 5G NR and F OFDM waveforms with and without DPD for the received optical power.