# Neural Network DPD for Agramizing SM-VCSEL-SSMF-Based Radio over Fiber Link Performance
###### Abstract  
This paper demonstrates an unprecedented novel neural network (NN)-based digital predistortion (DPD) solution to overcome the signal impairments and nonlinearities in Analog Optical fronthauls using radio over fiber (RoF) systems. DPD is realized with Volterra-based procedures that utilize indirect learning architecture (ILA) and direct learning architecture (DLA) that becomes quite complex. The proposed method using NNs evades issues associated with ILA and utilizes an NN to first model the RoF link and then trains an NN-based predistorter by backpropagating through the RoF NN model. Furthermore, the experimental evaluation is carried out for Long Term Evolution 20 MHz 256 quadraturre amplitude modulation (QAM) modulation signal using an 850 nm Single Mode VCSEL and Standard Single Mode Fiber to establish a comparison between the NN-based RoF link and Volterra-based Memory Polynomial and Generalized Memory Polynomial using ILA. The efficacy of the DPD is examined by reporting the Adjacent Channel Power Ratio and Error Vector Magnitude. The experimental findings imply that NN-DPD convincingly learns the RoF nonlinearities which may not suit a Volterra-based model, and hence may offer a favorable trade-off in terms of computational overhead and DPD performance. digital predistortion; radio over fiber; neural network; error vector magnitude; adjacent channel power ratio 2021891  
fiber (D-RoF) [8; 9] and Sigma Delta radio over fiber (S-DRoF) [10; 11; 12] systems. However, A-RoF suffers from nonlinearities, still due to the complexity of the other realizations and advantages in A-RoF that include simplicity, cost effectiveness and already widely spread legacy infrastructure, making it a better choice as compared to more efficient and costly solutions, such as D-RoF and S-DRoF [12]. Analog fronthaul applications are limited due to the nonlinearities that arise due to microwave and optical parts. The amelioration of these nonlinearities has gained immense importance to increase the optical system capacity. Many nonlinearity mitigating techniques have been proposed in recent years to subdue these nonlinearities and improve the performance of such cost-effective systems [3; 5; 6; 7; 12; 13; 14; 15; 16; 17; 18; 19]. However, these solutions have been questioned due to a limited bandwidth and need of feedbacking the output, which is an overlap process [5]. Similarly, for 5G networks, it would be strenuous to perform dynamic tracking while compensating the nonlinear channel response with broadband time varying data traffic from different RATs. When we consider short range A-RoF links, the impact of impairments that arise due to concoction of fiber chromatic dispersion and laser frequency chirp is usually negligible [4; 5]. However, still, the impact of signal impairments owing to the laser diode and photodiode is significant. Similarly, these nonlinearities result in a rise of near channel interference and affect the quality of transmission. As explained in [6], similar to fifth generation (5G) and Long-Term Evolution (LTE) signals, Orthogonal Frequency Division Modulated (OFDM) signals are susceptible to distortions attributed to a high peak to average power ratio (PAPR) in their signal envelope [7]. These systems generate undesired spurious terms due to relatively stable causes such as nonlinear characteristic curves of lasers and possibly photodiodes. It is, therefore, necessary to reduce these spurious terms, to investigate optimal DPD techniques. Moreover, in fiber-wireless integrated systems, the nonlinearities become serious when the analog fiber and wireless links are cascaded together [20]. They may arise due to the nonlinear laser transfer function [21; 22], combined effect of laser chirp and fiber chromatic dispersion [23], fiber kerr nonlinearity [24] and various nonlinearities attributed to the microwave photonic link. The generation of inadmissible specious terms is due to nonlinear characteristics of lasers and perhaps photodiodes. Therefore, reducing these undesired terms can enhance performance.