# Mitigation of Nonlinear Distortions for a 100 Gb/s Radio-Over-Fiber-Based WDM Network
## 2 Proposed Model Layout  
The main objective of this paper is to present a distortion-tolerant WDM-based RoF link, employing advanced modulation formats such as 16 and 64-QAM OFDM for different transmission distances (1 km to 10 km length of fiber), high receiver sensitivity and low input power requirements. This is accomplished by management of NLIs, LDs and PAPR at different stages to ensure optimized performance for the proposed layout. The internal structure of the presented framework is explained in this section. The detailed framework of the proposed RoF system using WDM system is explained in Figure 1a, declaring how RF signals are propagated over the OFTL. The main feature of the WDM-based RoF transmission system is the low cost system by avoiding the need of high frequency LO for upconversion of baseband signal to the desired RF frequency, analogue to digital conversion (ADC) at the central unit or digital to analogue conversion (DAC) at the remote antenna unit. The simulation model is built in Optisystem software which is a well-known simulation tool for analysis of optical fiber communication systems. Optisystem provides a considerable amount of library elements to model the RF and optical components according to the realistic values and characteristics. The RF signals are generated at the central unit in both OFDM and non-OFDM formats to investigate the performance of the two schemes. The signals is amplified and a directly modulated laser (DML) is installed at the output pin of the amplifier, aiming to convert the electrical signal into optical domain [25]. The DML module helps in specifying different parameters for the laser like linewidth, adiabatic and transient chirp, side mode, suppression ratio and relative intensity noise (RIN) [26]. Moreover, RIN and chirp characteristics of the DML model are taken from a commercial low-cost DFB laser to provide a realistic approach. The RIN is set to be \(-130\) dB/ Hz and chirp parameters, linewidth enhancement factor (\(\alpha\)) and adiabatic chirp coefficient (k) are set to be 2.0 and \(25\times 10^{12}\) Ws\({}^{-1}\), respectively. For the WDM system, a number of such optical signals from different users are multiplexed with the help of a WDM-MUX module, where all channels are transported over a single mode fiber (SMF) link and the propagation loss and insertion loss of WDM module is compensated by using an erbium doped fiber amplifier (EDFA). At the RAU, a WDM-deMUX is used in the form of a fiber Bragg grating (FBG). FBG also helps to eliminated some portion of the amplified spontaneous emission (ASE) due to the filtering operation. To perform the O/E procedure, a PIN photodiode is installed at the output of the FBG which directly converts the optical signal to RF frequency due to the RoF transport. As a result, the RF signal received at the Rx is recovered at the desired radio frequency, where then it can be transmitted in to the free space. To further remove the out-of-band noise, a bandpass filter (or low pass filter with large bandwidth) is implemented before passing the RF signal to the QAM sequence decoder. The FDE block is connected at output of QAM, aiming to mitigate LDs and NLIs further. The internal parameters of FDE are presented in Figure 1b. However, FDE alone is not sufficient to compensate the NLIs and PAPR. FDE outcomes are further purified from NLIs NLIs and PAPR using full field digital back propagation (DBP), decision directed least mean square (DD-LMS) and carrier phase estimation (CPE). Afterwards, to analyze and measure the quality of the received signals, the Optisystem software offers a BER analyzer tool which takes re-amplified, reshaped and re-timed form of the signal as an input from 3R-regenerator module. The custom design of the equalizer is explained in analytical modeling section to show that the proposed system provides high tolerance to the nonlinear effects and distortions.