Higher-Order Modulation vs Faster-Than-Nyquist PAM-4 for Datacenter IM-DD Optics: An AIR Comparison Under Practical Bandwidth Limits

The 4-ary pulse amplitude modulation (PAM-4) has been widely deployed to both electrical and optical interfaces in datacenter applications. Based on 100-GBd PAM-4, the 200G per lane optical interface is being actively developed in both industrial interoperability groups and standard bodies. Going beyond 200G, it is questionable if PAM-4 could support an even faster interface with higher baud-rate, considering the bandwidth growth for both electronics and optical components has started lagging the ever-increasing capacity demand. Under limited bandwidth, one option is to improve the spectral efficiency by higher order formats like PAM-6 and PAM-8, but this induces backward incompatibility as well as potential modulation mismatch between electrical and optical interfaces that requires additional energy for gearboxing. This revives the study of faster than Nyquist (FTN) signaling to pursue higher PAM-4 baud-rate breaking the bandwidth limit. In ECOC 2021, we demonstrated a reduced-complexity maximum a posteriori probability (MAP) decoder in intensity-modulation (IM) direct-detection (DD) systems with superior FTN performance. In this paper, we extend our work by adding more algorithm details and experimental results. Moreover, we perform the achievable information rate (AIR) comparison between the FTN PAM-4 and the higher-order modulation within the Nyquist limit in different channel conditions. Remarkably, in a practical IM-DD channel with gradually decayed frequency response, FTN shows AIR gain over the Nyquist approach even at a low FTN decoding complexity. The finding suggests FTN PAM could be a promising candidate at post 200G era to offer both AIR and implementation advantages.