## 1 Introduction
Fiber linear impairments can be effectively compensated through DSP. Other attractive features of the COH technology that facilitate multiterabit per second (Tb/s) capacity are the support for polarization multiplexing (PolMux), high-order modulation formats, dense wavelength division multiplexing (DWDM), and superchannels [12]. Apart from the offered increase in the degrees of freedom (DoFs) for optical modulations, the Co-Rx facilitates the digital compensation of fiber dispersion. This helps prevent the need for the complicated optical dispersion management [8; 13]. It should be noted that polarization-diverse Co-Rx usually performs four-dimensional (4D) detection. This includes the signal in-phase (\(I\)) and quadrature (\(Q\)) of both polarizations \(X/Y\). As a result, the offered dual-polarization (DP) field recovery by the implementation helps maximize the detection dimension in the single-mode fiber (SMF). Under random polarization variations of the fiber channel, it ensures a closed multiple-input multiple-output (MIMO) space without information loss. Nonetheless, the associated system complexity of the conventional Co-Rx has been limiting its implementation to long-haul-based networks due to the cost, footprint, and power consumption of the transceiver [8; 11; 13]. In addition, based on the exhausting multiplexing dimension in the conventional direct-detection-based system and the consideration of the enhanced sensitivity of the COH detection along with its inherent WDM functionality, research attention has been shifting towards low-complexity implementations of COH systems. Consequently, to enhance the system AIR while maintaining the related cost-effective benefit, the short-reach networks should explore an appropriate compromise between 1D-based IM-DD and 4D-based COH detection schemes. In this perspective, some simplified and low-cost optical transceivers with good tradeoffs between the COH and the direct-detection-based systems have been presented [8; 11; 13]. In general, a promising system should be able to offer low-latency, low-cost, and small-footprint optical transceiver components that can support larger bitrates and operate over a considerable reach compared with the conventional IM-DD systems, while still being power efficient. To explore a suitable compromise between the cost-effective direct detection and spectrally efficient COH configurations, different potential solutions have focused on the optimization of the required components. For instance, some prevent the implementation of a local oscillator (LO) laser and extensive DSP that are normally employed in the conventional COH system, while still offering higher AIR values compared to the conventional direct detection [14]. In this regard, various configurations that are capable of realizing two-dimensional (2D) detection have been reported. Nonetheless, the solutions present notable limitations regarding the nonlinear signal-signal beat noise (SSBN) that is caused by the photodetection square function. The effect of this is a remarkable limitation in the system capacity. In an effort to address the 2D-based detection challenges, some configurations that leverage the direct detection scheme have been presented to perform three-dimensional (3D) detection [8,11,13]. Furthermore, notable advancements have been reported regarding simplified COH transceiver technologies. For instance, vertical-cavity surface-emitting lasers (VCSELs) or externally modulated lasers have been employed with analog signal processing to address the system complexity. Several polarization-independent quasicoherent receivers have been presented as a viable scheme to address the growing system requirements. The scheme is capable of attending to the system requirements in a cost-effective manner without the need for a DSP for real data recovery. Furthermore, based on the fact that the receiver sensitivity can be enhanced by the quasicoherent receiver, reduced optical power laser and inexpensive photodiodes can be employed rather than costly ones [15].