## 1 Introduction
In addition, among the promising solutions that have been presented, a number of self-coherent (SCOH) systems have attracted considerable attention. In the SCOH systems, both the modulated signal and the carrier are copropagated at the transmitter (Tx). Moreover, at the receiver, the launched carrier from the transmitter not only serves as a reference, but also beats with the signal in the course of photodetection, resulting in SCOH detection [9]. Consequently, to achieve the system requirements, they depend on signal self-beating or beating between the signal and the copropagated continuous-wave (CW) laser tone from the transmitter. Many simpler DSP algorithms are usually employed compared to the conventional Co-Rx [14]. In this tutorial paper, we present a comprehensive review of high-performance, low-cost, advanced, and simplified optical transceiver paradigms that are suitable for different applications with the main focus on potential architectures for intra- and inter-DCN applications. The work majorly focused on the SCOH systems in which the signal self-beating or the beating between the modulated signal and the copropagated CW laser tone from the transmitter is exploited. In this context, effective means and related concepts of signal phase reconstruction based on the intensity information using the SCOH detection systems are presented. Furthermore, the means of addressing the fiber channel impairments such as the prevailing chromatic dispersion through optical channel linearization by the considered transceiver configurations are discussed. Moreover, the ability of the transceiver configurations to reduce both the system hardware and DSP complexity compared with the dual-polarized coherent (DP-COH) system while still enhancing the system SE in relation to the conventional IM-DD is comprehensively presented. Some cost-effective and simplified transceiver architectures that can effectively offer salient advantages such as high-SE and a high per-carrier interface rate (100 G per wavelength and beyond) over tens to hundreds of kilometers are discussed. With the DP-COH transmission scheme configuration being used as a benchmark, different potential schemes are compared considering various associated features such as the optical SEs, transceiver architectures, enabling DSPs, and the related receiver bandwidths. Moreover, based on the growing increase in the dynamic environment and the heterogeneous nature of the applications and services to be supported by the systems, reconfigurable and sliceable transceivers (transponders) that are capable of adapting a number of specified transmission parameters to accomplish exceptional tradeoffs among the network reach, power dissipation, and data rate are discussed with the main focus on their economics, advantages, and complexity. The transceivers can support different features such as flexible spectrum utilization and adaptive resource exploitation in a cost-effective manner. The associated system algorithms and technical challenges are reviewed with proffered viable solutions. Furthermore, different experimental demonstrations that were based on SCOH detection schemes are comprehensively reviewed.