## 1 Introduction
The authors in [16] demonstrate the feasibility of transmitting and receiving a 100 Gbit/s data rate link at 28 GHz. The performance of three modulation formats (16-PSK, 16-QAM, and 64-QAM) is tested for optical fiber lengths ranging from 5 km to 35 km using two detection systems: coherent and direct detection with a DSP block implemented in simulation software. In [17], the authors introduce an iterative block (IB) decision feedback equalization (DFE) method for an intensity modulation and direct-detection (IM/DD) based optical code division multiplexing (OCDM) system for a 50 km SSMF link. This method effectively reduces signal degradation caused by chromatic dispersion (CD) in the fiber. In [18], an optical phase-locked loop (PLL) technique to generate mm-waves is proposed, and a pilot-assisted radio frequency (RF) method isused for phase offset equalization at the receiver side for 16-QAM modulation. In [19], the authors present and demonstrate convolutional neural network (CNN) and binary convolutional neural network (BCNN) decision schemes that effectively address both linear and nonlinear impairments resulting from signal modulation, transmission, and detection. The proposed schemes are very computationally efficient, and the CNN and BCNN decision methods are showcased in a 5 Gb/s 60 GHz RoF system, covering a fiber reach of up to 20 km. In [20], the authors propose a Volterra Nonlinear Equalizer (VNLE) for directly modulated laser (DML)-based IM/DD systems and a simulation of Pulse Amplitude Modulation (PAM-4) signals for high baud rate transmission over 10 km SSFM. In [21], the authors are experimentally transmitting a single-channel 112 Gb/s PAM-4 direct detection signal, and a high receiver sensitivity is achieved by the maximum likelihood sequence estimation (MLSE) algorithm. In [22], the authors experimentally demonstrated the generation, detection, and transmission of a 112 Gb/s dual-polarization, single- and dual-carrier 16-QAM half-cycle Nyquist subcarrier modulation, specifically tailored for short-reach IM/DD systems. High-bandwidth direct modulation passive feedback lasers are utilized for the generation of optical signals. Pre-amplified receivers identify the received signals in a back-to-back configuration following transmission over a distance of 4 km using SMF. Equalization was conducted utilizing a CMA for pre-convergence, followed by an RDE. The EVM values are 10.23% for single carrier and 9.55% for dual carrier in a 16-QAM half-cycle Nyquist subcarrier modulation over a distance of 4 km of SMF. In [23], the authors conducted experiments to demonstrate a 32 Gbaud polarization-multiplexed carrier-based self-homodyne (PMC-SH) system designed for data center interconnects (DCIs), capable of supporting higher data rates. Simulation results confirm the validity of the proposed system for a 200 Gbps (50 Gbaud) PMC-SH 16-QAM 10 km SSMF link. Offline equalization is executed by cascading RDE with a DFE, leading to enhanced performance. Additionally, it was demonstrated through analysis that the PMC-SH scheme achieves a markedly improved BER for a specified transmission bit-rate or can effectively double the data rate for a given bandwidth of the electronics, in comparison to the PAM-4 system. In [24] a coherent receiver is presented based on 12.5 Gbd and utilizes 16-QAM analog processing. It has been designed and simulated employing 130 nm bipolar CMOS (BiCMOS) technology. The receiver employs RDE equalization along with Costas loop-based carrier phase recovery and compensation mechanisms. Simulations demonstrate that the receiver can effectively mitigate a dispersion of up to 160 ps/nm and a frequency offset of up to 10 MHz. In a transmission scenario involving a 10 km-long SSMF link, the output EVM reported by the receiver is 23.2% with a laser linewidth of 200 kHz. In [25], the authors propose a sparse Volterra nonlinear equalization (SVNE) and gradient-descent noise whitening (GD-NW) digital signal processing method to reduce impairments.