# Neural Network DPD for Agramizing SM-VCSEL-SSMF-Based Radio over Fiber Link Performance
## 5 Experimental Setup  
The experimental setup is presented in Figure 6, similar to the one that we presented in [17]; however, the DPD-NN-based method is an upgradation of this bench. An RoF link comprising an SM-VCSEL laser with an 850 nm wavelength is connected to a Standard Single Mode Fiber (SSMF) of 1 km. The optical signal is detected by a photodiode with a 3.4 GHz of bandwidth. The biasing current is set as 5 mA so that the power consumption is at an acceptable level, while the threshold current, is set to be 2 mA so that it is not very close to the threshold level. We use an inhouse software realized on MATLAB that emulates 20 MHz LTE with 256 QAM modulation baseband signal oversampled at 122.8 MSa/s. In  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Specifications** & **Values** \\ \hline Optimizer & ADAM \\ Loss Function & Mean Square Error \\ Hidden Layers & 6 \\ Neurons per layer & 25 \\ Hidden Layer Type & ReLu \\ Regularization & L1 \\ \hline \hline \end{tabular}
\end{table}
Table 1: NN specifications utilized.  
the first phase, this signal is upconverted by a Vector Signal Generator (VSG) at a carrier frequency of 2.4 GHz. This is passed through an RoF link, and performance evaluation is carried out at the receiver with the Rohde and Schwarz FSW signal and spectrum analyzer used as a vector signal analyzer (VSA). In this step, the performance evaluation is recorded without the use of DPD block, i.e., No DPD.  
In the second phase, the baseband signal oversampled at 122.8 MSa/s is fed into the DPD block and is upconverted at 2.4 GHz via VSG which is then fed to the optical link. The received signal at VSA is fed to the DPD training phase. Here, initially, the primary synchronization sequence and secondary synchronization sequence available in the LTE reference frames are engaged for synchronization between input and output (Figure 2). This block is responsible for the synchronization by finding the cross correlation by employing the M-part correlation method [34]. This operation is followed by employing the photodiode (PD) model to obtain the predistorter coefficients (Figure 2).  
For the DPD validation phase, the switches move to the opposite direction, as shown in Figure 2. The evaluation is accomplished for the general LTE frames, which includes sampling, predistortion and passing them to the VSG. The nonlinear properties of the RoF link fluctuate slowly due to thermal effects and ageing of the components, which makes us assume that real-time processing in the adaptation is not required. Table 2 summarizes the parameters utilized in this work.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Link Components** & **Values** \\ \hline SM-VCSEL & \\ Wavelength & 850 nm \\ \(l_{bias}\) & 5 mA \\ \(l_{threshold}\) & 2 mA \\ RIN & \(-130\) dB/Hz \\ \hline SSMF & \\ Length & 1 km \\ Attenuation & 3 dB/km \\ \hline PD & \\ Responsivity & 0.71 A/W \\ Bandwidth & 2.5 GHz \\ \hline \hline \end{tabular}
\end{table}
Table 2: Experimental setup parameter values.  
Figure 6: Block diagram of experimental testbed. DPD implemented for general LTE frames. DPD-NN or DPD-MP/GMP can be selected from the choice present in the simulator.