Modulation Format and Digital Signal Processing for IM-DD Optics at Post-200G Era

200-Gb/s per lane intensity-modulation (IM) direct-detection (DD) optics are being commercialized to support 800G and 1.6T applications inside datacenters. Though IM-DD remains its cost and power consumption advantages over Coherent at 1.6T for short-reach interconnect below 10 km, its roadmap is not clear towards the next capacity doubling, considering it becomes more challenging to scale the components bandwidth linearly with the capacity demand. This makes both advanced modulation formats and digital signal processing (DSP) indispensable for an IM-DD system aiming at higher speed. From a system perspective, this article reviews candidate modulation formats and DSP for IM-DD optics at post 200G (per lane) era. By taking into account generic constraints for future IM-DD systems like bandwidth limit, peak power constraint, transceiver nonlinearity, fiber dispersion, and so on, it discusses a wide range of techniques including probabilistic constellation shaping (PCS), high symbol rate pulse amplitude modulation (PAM), faster than Nyquist (FTN) signaling, nonlinear equalizations, and multicarrier modulations. Different from prior IM-DD review literature, we mainly focus on if it is meaningful to exploit a DSP technique with respect to constraints in practical systems, rather than just the technique itself. The study is backed with rich simulation and experiment results.