## 1 Introduction  
The advent of the fifth-generation (5G) network will impact different network entities, technologies, and the corresponding system as a whole. In this perspective, how the access networks, metro, core networks, high-performance cloud data center networks (DCNs), as well as the associated edge services and Internet of Things (IoT) devices are designed and deployed will be affected [1; 2; 3]. Furthermore, the emergence of IoT devices has brought about the unprecedented expansion of Internet connections [4; 5]. Their proliferation and effective support for several innovative applications and services such as cloud computing, augmented reality (AR), massive machine-to-machine communication, and emerging web applications have considerably contributed to the growing increase in machine- and user-generated data traffic. In this regard, there has been a significant increase in the workload of the underlying DCNs that support the applications and services [6]. For effective service delivery, the ensuing traffic transportation requires a consistent increase in bandwidth both in the metro and long-haul optical fiber networks. Consequently, this imposes unprecedented bandwidth requirements on the transport networks. In this perspective, to satisfy diverse requirements such as robustness, flexibility, high bandwidth, and relatively reduced cost per unit bandwidth, bandwidth-efficient and cost-effective transmission systems have to be adopted [2]. One such effective solution is the intensity modulation with direct detection (IM-DD) transceivers (TRxs) scheme. In addition, IM-DD-based transceivers offer various advantages such as low cost and ultra-low complexity. For instance, they require fewer optical components and present relaxed laser wavelength stability, without the need for complex digital signal processing (DSP) such as frequency offset compensation and carrier phase compensation [7]. These advantages make IM-DD-based schemes good candidates in the short-reach applications where a considerable amount of transceiver deployment is demanded. As a result, the associated cost of the short-reach transceivers is one of the main factors for network design and deployment, making it deserve significant consideration [8; 9]. Moreover, the IM-DD receiver (Rx) sensitivity is independent of the state of polarization (SOP) and the carrier phase of the signal. This is advantageous as the polarization state randomly fluctuates in real system scenarios [10]. Nevertheless, the IM-DD system's scalability and its spectral efficiency (SE) are significantly limited when the achievable information rate (AIR) is to be upgraded beyond 100 gigabits per second (Gb/s) per wavelength. This is mainly due to the associated one-dimensional (1D) modulation and detection constraints [8; 11]. For instance, one of the fundamental bottlenecks of the conventional IM-DD system is the second-order nonlinearity. This is caused by the square law photodetection, and it limits the system capacity significantly. As a result of the lack of the phase diversity of the system, the transmission distance will be limited by chromatic dispersion (CD)-induced power fading [9] since the information is modulated on the optical intensity rather than the field [12]. Therefore, the conventional IM-DD system is incapable of field recovery (FR), which can support the digital compensation of fiber linear impairments and multidimensional modulation. On the other hand, the long-haul-based optical networks have attained a considerable AIR advancement for the multiterabit transmission with coherent (COH) transceiver system's revitalization [8; 11]. Furthermore, the polarization diversity feature of light can be exploited in the COH system to maximize the system SE and optimize the AIR [8]. With the aid of COH detection, optical receivers can perform field recovery of both the intensity and phase of the optical signal that is propagating over the fiber [8; 13]. Furthermore, the coherent receiver (Co-Rx) is capable of the linear recovery of the signal with phase diversity.