## 1 Introduction
### Contributions  
As previously mentioned, there have been substantial contributions in the literature regarding optical transceiver systems. However, due to the continuous development in the field and the usual space constraints for publications, an all-inclusive research effort has not been presented on the simplified optical transceiver systems to support high-speed short- and medium-reach use cases. Consequently, a comprehensive tutorial in this area is highly imperative. This will enable researchers to have in-depth knowledge regarding the related requirements, technologies, architectures, and challenges of optical transceiver systems. Besides being abreast of the developments in the area, they will be able to proffer viable solutions to various network demands. Furthermore, the proffered insightful recommendations will be handy tools in practical system design. In light of this, this paper is a step towards bridging the gap of knowledge regarding the SCOH optical transceiver systems for short- and medium-reach applications. It presents not only a comprehensive tutorial on the optical transceiver architectures, but also considers the requirements for a simplified optical transceiver architectural evolution along with a piece of broad information on the technicalities of different related challenges and potential solutions. Moreover, the need for per-wavelength AIR enhancement is discussed, and different transceiver architectures that are capable of supporting 100 G per wavelength and beyond, at comparatively longer transmission distances and in a cost-effective way, are presented. Consequently, this paper not only complements the contributions of the existing survey and tutorial papers on the SCOH optical transceiver systems, but also covers more concepts with in-depth clarifications. In this context, the main contributions of this tutorial paper are as follows:  
1. An extensive analysis of the associated short-reach system features and enabling technologies that demand novel transceiver architectures and DSP techniques are presented. In this context, an overview of high-speed short-reach systems regarding their classifications, advanced optoelectronic devices, potential modulation formats, requirements, and challenges are well considered;
2. A comprehensive tutorial on the optical transceiver architectures and their classifications based on the DoF is presented. Furthermore, research efforts towards simplified and cost-effective architectures are well discussed. Furthermore, the requirements for a simplified transceiver architectural evolution from 1D to 4D and the related technical challenges are discussed with potential solutions being proffered;
3. In addition to a comparison among different detection schemes, we present a detailed SCOH system classification based on the detection schemes, as well as the adopted polarization and modulation schemes at the transmitter. Furthermore, we give a comprehensive discussion of their relative advantages compared to the conventional IM-DD and COH systems. Furthermore, their respective technical limitations are considered, and potential solutions are proffered;
4. Moreover, besides the presented comprehensive studies on the carrier and optical SSB signal generation, a distinct classification of the schemes based on the adopted generation approaches is provided. Furthermore, the related features of the schemes are extensively discussed considering the required frequency gap, associated CSPR, achievable SE, required optical filter, and hardware complexity;
5. Furthermore, apart from an in-depth discussion on the SSBN mitigation concepts, the SSBN mitigation/receiver-based digital linearization schemes for the direct-detection-based systems are clearly presented with the main focus on the performance of transmitter-based EDC (Tx-EDC) and receiver-based EDC (Rx-EDC) systems;
6.