Performance comparison of 112 Gb/s DMT, Nyquist PAM4 and partial-response PAM4 for future 5G ethernet-based fronthaul architecture

For a future 5G Ethernet-based fronthaul architecture, 100G trunk lines of a transmission distance up to 10 km over a standard single-mode fiber (SSMF) in combination with cheap gray optics to daisy chain cell site network interfaces are a promising cost- and power-efficient solution. For such a scenario, different intensity modulation and direct detect formats at a data rate of 112 Gb/s, namely Nyquist four-level pulse amplitude modulation (PAM4), discrete multitone transmission (DMT), and partial-response (PR) PAM4, are experimentally investigated, using a low-cost electroabsorption modulated laser, a 25G driver, and current state-of-the-art high-speed 84-GS/s CMOS digital-to-analog converter and analog-to-digital converter test chips. Each modulation format is optimized independently for the desired scenario, and their digital signal processing requirements are investigated. The performance of Nyquist PAM4 and PR PAM4 depends very much on the efficiency of pre- and postequalization. We show the necessity for at least 11 feedforward equalizer (FFE) taps for pre-emphasis and up to 41 FFE coefficients at the receiver side. In addition, PR PAM4 requires a maximum likelihood sequence estimation with four states to decode the signal back to a PAM4 signal. On the contrary, bit loading and power loading are crucial for DMT, and an FFT length of at least 512 is necessary. With optimized parameters, all modulation formats result in a very similar performances, demonstrating a transmission distance of up to 10 km over an SSMF with bit error rates below an FEC threshold of 4.4E-3, allowing error-free transmission.