# Overcoming Challenges in Large-Core SI-POF-Based System-Level Modeling and Simulation
## 6 System Level Simulation Example: PAM-4 Transmission over Large-Core Plastic Optical Fiber  
In this section, we describe an example implemented in the ModeSYS simulation framework based on the component models and system-level modeling techniques discussed above. This example is motivated by real world layouts though it does not attempt to implement the details of any real POF link. The goal is to show an instance that demonstrates the utility of the models for large-core SI-POFs and modeling techniques implemented in the simulation package rather than to create an optimal link design based on the assumed link scenarios and parameters.  
Particularly, the example demonstrates how PAM-4 modulation with DSP equalization at the receiver can help overcome the inherent bandwidth limitations of POF at Gb/s data rates for short-distance data interconnects [6; 63]. Based partially on work presented in [63; 64], Figure 8 illustrates a topology that implements end-to-end 1-Gb/s PAM-4 transmission over 1-mm diameter SI-POF. ModeSYS is used to simulate the fiber using the computationally efficient model based on the Gloge power-flow equation, while MATLAB co-simulation models the transmitter and receiver DSP using functions from the OptSim DSP Library for MATLAB. The transmitter generates an electrical PAM-4 signal with Nyquist pulse shaping and pre-emphasis filtering [65], and the receiver DSP uses a least mean square (LMS) equalization algorithm to recover the transmitted symbols and analytically estimate the BER. A Fiber Injection block is used to model the impact of the strong initial power diffusion at the fiber input [23; 29]. The large-core SI-POF parameters used in the simulation are based on extracted model parameters for a 1 mm diameter ESKA-PREMIER GH4001 fiber from Mitsubishi [23]. The fiber injection uses parameters from [30] for the power diffusion, as well as the same angle-dependent loss as that used in the fiber. We would expect that the fiber injection, which tends to spread the power to higher angles, should result in a slower overall system response and reduced system bandwidth. Figure 9a shows the intensity in W/sr (not normalized) as a function of internal angle. Figure 9b shows the normalized version, illustrating the strong power diffusion. Figure 10 depicts the optical eyes at the POF system input and output, showing eye closure due to intermodal dispersion and mode coupling. Figure 11a illustrates the recovered eye diagram after the receiver's analog-to-digital conversion (ADC) and equalization, which has compensated for the eye closure, while Figure 11b depicts the analytically estimated BER in comparison with an ideal PAM4 reference curve. The BER of \(4.6\times 10^{-4}\) is below an FEC threshold of \(3.7\times 10^{-3}\)[66; 67]. As described in [63], the supported bit rate for this typical system can be much higher than the 1 Gb/s used in this example. Close to the absolute bandwidth limit, we can also employ a detector coupler to eliminate the higher angle modes at the receiver, thus increasing the bandwidth; however, the resulting power loss in combination with the receiver sensitivity may force other system adjustments to meet the BER specifications.  
Figure 8: Topology for simulating PAM4 transmission over SI-POF.  
Figure 9: Injection model intensity profile at input and output: not normalized (**a**) and normalized (**b**), showing the strong initial power diffusion from lower to higher angles.