# Neural Network DPD for Agramizing SM-VCSEL-SSMF-Based Radio over Fiber Link Performance
## 7 Real Time Realization of the NN DPD Method  
Bringing the feedback signal from the BS to RAU is among the main challenging task in the adaptive recompense of the RoF link. This is due to possible nonlinearity of the feedback link; in fact, it can be as nonlinear as the RoF link, which is compensated for. The present work is based on the fact that the predistorter identifies only the nonidealities that it needs to compensate. Since it is assumed that the nonlinear feedback connection is uncompensated, it would destroy the performance of the compensation. In other words, this means that an approach is utilized where the RoF link is first compensated for using a postdistorter, and the known training signal from the RAU is used here. After that, the already compensated for downlink RoF link can be used as a feedback connection for the compensation [3].  
However, as a possible feedback scenario, Figure 11 represents a realistic implementation of an adaptive DPD that shifts complex processing of the signals at the Central Office / Base Transmit Station (CO-BTS). In the figure, switches SW1, SW2 and SW3 are in configuration (1), and switches SW4 and SW5 are in configuration (2), while SW6, SW7 are not influent.  
Figure 11: Possible realization of an adaptive predistortion scheme.  
Figure 10: Experimental results for Error Vector Magnitude (EVM) received signal without and with linearization (DPD-NN and DPD-MP/GMP).  
This correlates to the \"normal\" operation of the bidirectional link. Indeed, the (DPD) is trained and performs a requisite processing of the digitized baseband signal, which is eventually brought back to analog form and modulated at radiofrequency (RF). After having passed through the Optical DL, the signal enters a circulator (Ci) that allows the transmission of the signal through a remote antenna unit (RAU).  
Conversely, the signal received by the RAU passes through (Ci) and is transmitted to the Optical UL up to the CO/BS. Then, the signal is brought back to BB and processed by the CO-BTS (operation not shown). When switches SW4, SW6 and SW7 are set to configuration (3-A), and SW5 is connected to the ADC, a postdistortion operation is accomplished with reference to the UL. An RF test signal is sent through the Optical UL and arrives at the CO-BTS.  
The characteristic parameters of the DPD methods (generalized/Memory Polynomials/DNN) are fed to the PD Controller. The predistorter controller block is able to augment the sampled version of the baseband signal that was received at the RAU similarly to if the operation of demodulation and sampling were performed at the RAU. It is anticipated that the nonlinear properties of the UL are stable across time, and so this process is seldom performed at the same frequency at which the DPD training is acquired.  
The training of the digital predistorter is accomplished when switches SW1, SW2, SW3, SW4 and SW6 are set to configuration (3-B), while SW5 is connected to the ADC (SW7 not influent). This is the configuration described in the submitted work, where the digital predistortion block compares the transmitted and received sampled versions of the transmitted BB signal. It can be additionally discerned that a slight modification in the architecture shown in Figure 11 will utilize the Up-Link postdistortion mechanism for a reliable DPD feedback system.