## 12 Conclusions  
The growing increase in the adoption of 5G-enabled mobile applications and services in the access and metropolitan networks is pushing the global IP traffic to the zettabytes range, making optical communication systems highly essential to support the growing demands. In optical communication systems, performance enhancement depends on how the physical dimensions such as time, frequency, quadrature, polarization, and space are being systematically exploited for modulating and multiplexing light. The information-theoretic capacity per unit bandwidth is the fundamental limit to spectral efficiency. A high delay and complexity can result from close proximity to such a limit. Furthermore, there is an associated complexity to the capacity that can be supported by different optical transceiver systems, with high-sensitivity and high-capacity 4D coherent being more complex than the low-capacity 1D intensity modulation with direct detection counterpart. The realization of polarization-independent operation for complexity minimization rather than polarization diversity presents considerable optical complexity. Consequently, to alleviate the related complexity, it is highly desirable to accomplish polarization-independent reception without the need for optical polarization tracking at the receiver. The self-coherent schemes are opening up new prospects for ultra-high-capacity transceivers. This presents remarkable advantages beyond the conventional intensity modulation with direct detection while still relatively maintaining the simplicity and salient cost-efficiency feature for short- and medium-reach applications. The offered benefits are as a result of the exploited advantages of the cost-effective and simplified intensity modulation with direct detection, as well as the high sensitivity and complex modulation format that enabled sophisticated coherent systems. Moreover, the great majority of the self-coherent schemes are polarization insensitive/independent, and they can perform up to 3D detection without the need for an LO, which eliminates the need for sophisticated remote wavelength management between the transmitter and the receiver. Based on their relative advantages, they are capable of supporting data rates beyond 100 G per wavelength at relatively longer transmission distances, making them a promising solution for high-bandwidth, low-footprint, high-performance, energy-efficient, and cost-sensitive short- and medium-reach applications. In this paper, we presented comprehensive tutorials on the main requirements, technologies, architectures, and associated challenges of different transceiver systems. Different advancements in modulation formats, devices, and digital signal processing algorithms that are capable of facilitating 100 Gb/s and beyond per wavelength were broadly reviewed. Furthermore, a set of self-coherent schemes was well reviewed and comprehensively compared. We presented different channel linearization techniques along with the associated technical challenges of their implementations for optical direct-detection-based schemes. Moreover, certain related open-ended issues were presented, and the proffered potential solutions were well examined. It can be established that based on the promising and significant progress, there is considerable prospect towards high-speed optical transceivers that are capable of fulfilling the current and future requirements. This can be attributed to the fact that the self-coherent schemes aim to bridge the performance gaps between the conventional coherent and intensity modulation with direct detection schemes. Furthermore, it is anticipated that they will soon pave the way toward standardization and industrial development. Conceptualization, I.A., R.P., W.S., A.P. and N.M.; methodology, I.A., R.P., N.S. and N.M.; software, I.A. and N.M.; validation, I.A., W.S., A.P. and N.M.; formal analysis, I.A., R.P., N.S., C.S., H.J., W.S., A.P. and N.M.; investigation, I.A., R.P., N.S., C.S., H.J., W.S., A.P. and N.M.; resources, I.A., W.S., A.P.