RSoft Enewsletter

April 2016

Modeling of PAM-4 Transmission Over Large-Core Plastic Optical Fiber Using ModeSYS and OptSim DSP Library for MATLAB

Data center and automotive fiber-optic links demand cost-effective solutions for carrying large amounts of data. Due to its relaxed alignment tolerances and low cost, step-index (SI) plastic optical fiber (POF) is an attractive technology for short-distance data interconnects. Furthermore, PAM-4 modulation with digital signal processing (DSP) equalization at the receiver can help overcome the inherent bandwidth limitations of POF at Gb/s data rates. 

Schematic of PAM-4 transmission over SI-POF

Figure 1: Schematic of PAM-4 transmission over SI-POF

Figure 1 illustrates an RSoft™ ModeSYS™ topology that implements end-to-end 1 Gb/s PAM-4 transmission over 1 mm diameter SI-POF. ModeSYS simulates the fiber using a computationally efficient model based on the Gloge power-flow equation, while MATLAB co-simulation models the transmitter and receiver DSP using a mix of functions from the RSoft OptSim™ DSP Library for MATLAB and custom routines. The transmitter generates an electrical PAM-4 signal with Nyquist pulse shaping and pre-emphasis filtering, and the receiver DSP uses an LMS equalization algorithm to recover the transmitted symbols and analytically estimate the BER. 

POF input and output eyes

Figure 2: POF input (left) and output (right) eyes, showing the effects of intermodal dispersion and mode coupling

Figure 2 depicts the optical eyes at the POF input and output, showing eye closure due to intermodal dispersion and mode coupling. Figure 3 illustrates the recovered eye diagram after the receiver’s analog-to-digital conversion and equalization, which has compensated for the eye closure. 

Equalized eye at the receiver

Figure 3: Equalized eye at the receiver demonstrating compensation of the eye closure

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