## 6 Transceiver Architectures' Classification and Basic Concepts
### 4D-Based Architectures  
The DP-COH schemes are illustrated in Figure 13. In the DP-COH scheme in Figure 13a, the receiver configuration that leverages an LO to extract the phase information of the interfered signal is the Co-Rx. Instances of this configuration are homodyne and heterodyne receivers [10]. The DP-COH configuration serves as a benchmark for comparing other configurations. Furthermore, as discussed in Section 7.1, through the complex modulation of the two polarizations, the available four DoFs are exploited by the DP-COH systems. Figure 12: 3D transceiver architectures based on (**a**) the DP-IM-IPM/Stokes vector receiver and (**b**) the PolMux-\(IQ\)-IM/Stokes vector receiver. PM: phase modulator (adapted from [12; 13; 14; 33; 34; 37]). Consequently, with the aid of either one (2 \(\times\) 8) or two (2 \(\times\) 4) 90\({}^{\circ}\) hybrid and an LO laser, the DP-COH can realize both polarization and phase diversity. Furthermore, in the process, the unwanted signal-signal beating is eliminated by the balanced detection. It should be noted that the carrier recovery stage is essential in the Rx-DSP stack of the DP-COH system [14]. Furthermore, another promising architecture is the remote LO delivery (RLOD) system. The RLOD is a self-homodyne COH detection scheme with a bidirectional transmission. As the conventional COH system, it supports full utilization of the four available DoFs for modulation in optical transmission. As illustrated in Figure 13b, the schematic of the RLOD comprises two transceivers. Furthermore, each transmitter contains a transmitter DSP (Tx-DSP) that leverages the employed DAC, the output signal of which is amplified to drive the DP-IQM. Likewise, the transmitter has a laser, the output of which is split into two branches. One of the branches is employed to feed the DP-IQM for modulation, while the other one is for delivering the reference tone. Furthermore, two circulators can be employed at each transceiver side to ensure that the two lanes of the full-duplex fiber are used as bidirectional fibers rather than the usual unidirectional implementation in the conventional IM-DD systems. The modulated signal from Transceiver 1 and the tone from Transceiver 2 are coupled by the circulators C\({}_{1}\) and C\({}_{4}\) to one lane, while the modulated signal from Transceiver 2 and the tone from Transceiver 1 are coupled by the circulators C\({}_{3}\) and C\({}_{2}\), respectively, to the other lane of the full-duplex fiber. It is noteworthy that the modulated signal and tone propagation are in opposite directions [130]. At the receiver, for coherent reception, the associated tone and the modulated signal from Transceiver 2 are extracted by the circulators C\({}_{1}\) and C\({}_{2}\), respectively, while the transmitted tone and the modulated signal from Transceiver 1 are extracted by the circulators C\({}_{3}\) and C\({}_{4}\) for the onward distribution to the DP-Co-Rx at Transceiver 2. It should be noted that there is a polarization stabilizer (PS) in the tone path. The PS helps ensure tone-signal beating on both states of polarization. Furthermore, the time delay between the signal and tone can be synchronized with the aid of a matched delay line (MDL), which is based on matched fibers. This can aid in ensuring the same laser phase noise cancellation at Co-Rx during the tone-signal beating. Matching the signal and tone paths helps enable uncooled laser implementation. Furthermore, to facilitate the Rx-DSP, the analog output signals from the DP-Co-Rx are converted to a digital output by the ADC after COH reception [130]. In addition, different transceiver architectures are summarized in Table 7. Furthermore, efforts towards simplified architectures in which the cost-effective benefit of the 1D-based IM-DD and the enhanced AIR of the 4D-based COH systems are exploited, as well as the associated tradeoffs are discussed in Section 7. Figure 13: Transceiver architectures with 4D (**a**) DP-COH and (**b**) remote LO delivery.