## 1 Introduction
To the best of their knowledge, experimental verification yields the maximum net data rates of 142.3/143/134.6/102.8/73.7/61.4 Gb/s across 0/5/10/20/40/60 km SSMF for C-band DML-based IM-DD links without requiring chirp and dispersion management. In [26], an experimental demonstration of a high-speed 131.4 Gb/s IM/DD optical link is presented operating at the 2-micron waveband using an offline DSP at the receiver. A 3 rd-order VNLE is utilized to address both linear and nonlinear impairments. Following this, a simple 2-tap noise whitening filter (NWF) working at 1 sample per symbol (sps) is employed to mitigate the equalization-enhanced colored noise, resulting in a 118.2 Gb/s probabilistically constellation-shaped (PCS)-PAM-8 signal and a 131.4 Gb/s PCS-PAM-16 signal transmission over 100 m solid core fiber (SCF), respectively, at a threshold BER of 4 \(\times\) 10\({}^{-2}\), by considering a 25% overhead of soft-decision forward error correction (SD-FEC). In [27], the authors conducted an experimental comparison of uniform PAM8 and cap-shaped Maxwell-Boltzmann-distributed PAM-8, utilizing various Gaussian orders processed with a VNLE to attain a net data rate of 200 Gb/s. A memory length of 311 taps was employed for linear equalization to reduce the impact of reflections from the discrete component setup, copper cables, and connectors. Alongside linear taps, second- and third-order kernels of VNLE with a memory length of 11 were employed to address nonlinearities caused by an arbitrary waveform generator (AWG), an electro-absorption modulated laser (EML), a semiconductor optical amplifier (SOA), an electrical amplifier (EA), and square law detection. In [28], experimental demonstrations are carried out with a 40 Gb/s PAM-4 signal transmission over a 20 km (30 km) SMF with a link power budget of 38 dB (30.7 dB) and a pre-forward error correction (FEC) BER of \(10^{-3}\) using a 17-tap tunable adaptive NLE with noise reduction to compensate for various linear and nonlinear effects from the elements and the SMF. In [29], the performance of several DSP techniques for short-reach high-speed IM/DD systems using different schemes of PAM, such as PAM-4, PAM-6, and PAM-8, are experimentally compared at net data rates of 180 Gb/s to 300 Gb/s. Results showed that PAM-4 duobinary (DB) VNLE + Viterbi equalization and PAM-6 with VNLE + 2-tap post-filter + Viterbi equalization are ideal for a system that uses commercially accessible components, has a 3 dB bandwidth of 20 GHz, and achieves a net rate of 200 Gb/s per lane while minimizing chromatic dispersion. In [30], the authors proposed a fair comparison and thorough study of the complexity of a 56 Gb/s multi-band carrier-less amplitude and phase (CAP) and discrete multi-tone (DMT) across 80 km dispersion compensation fiber links based on IM/DD for data center interconnects. The results show that the matched finite impulse response filters and the inverse fast Fourier transform (IFFT) and fast Fourier transform (FFT) account for the majority of the complexity in multiband CAP and DMT, respectively. The selection of the multi-band CAP sub-band count and the DMT IFFT/FFT size significantly affects system complexity and performance, necessitating careful consideration of trade-offs. The multi-band CAP exhibits significant complexity sensitivity to the number of sub-bands, indicating a preference for a limited sub-band count. The DMT requires a moderate IFFT/FFT size to optimize the balance between complexity and OSNR performance. Moreover, in [31; 32], the least mean square algorithm (LMS), the constant modulus algorithm (CMA), and the adaptive median algorithm (AMF) are implemented for equalization in a converged OFDM 5G mm-wave A-RoF system at 60 GHz. Quadrature phase shift keying (QPSK) and 16-QAM are used as modulation formats in the OFDM subcarriers. The maximum achieved transmission distance is 100 km after equalization when QPSK is used, while a transmission distance of 50 km is achieved after equalization for the 16-QAM case.