# A Vision of 6 th Generation of Fixed Networks (F6G): Challenges and Proposed Directions
## 3 Photonics-Based Subsystems for All-Optical Processing
### All-Optical Coherent and Simplified Coherent Solutions: Challenges and Opportunities
Furthermore, they can serve as important solutions for optical interconnects in datacenters and 5G telecom infrastructures, which are the network  
Figure 5: Block diagram of DSP-free coherent receiver (PM-QPSK) based on AOSP [29]. domains wherein the cost (measured in $/Gbps) and power consumption (W/Tb/s) must be further reduced [26]. Figure 7 presents a high-level comparison between the AOSP based transceiver and other popular transceiver designs for 400 Gb/s interconnects. Evidently, the cost and power consumption using conventional coherent solutions are prohibitively large for such applications. A straightforward solution could consist of the combination of parallel fibers exploiting lower-performance transceivers in order to achieve the desired capacity. Unfortunately, the scalability of such an idea once benchmarked against alternatives, is limited by the number of fibers that can be deployed in parallel. Another potential solution is Wavelength Division Multiplexing (WDM); however, WDM is constrained in cost and power consumption due to the high complexity of deployment. As a consequence, it is becoming evident that for short-reach applications (below 10 km), the AOSP transceiver may be a competitive option as it concurrently addresses the desirable characteristics of low cost and power consumption, high data rate, low latency, high scalability, and reach of up to at least 10 km. Other notable options are classified in the category of'simplified coherent' solutions. These solutions entail various tradeoffs between the extremes of direct-detection and coherent detection, exploiting both the amplitude and the phase of the optical signal to increase the transmitted data rate, albeit with lower complexity compared with conventional coherent transceivers, e.g., comprising a smaller number of optical elements. The main advantage of coherent technology is the benefit of up to 4-times higher bit-rate while keeping transceiver cost and power consumption at bay. To achieve this goal, new all-optical techniques are envisaged [30, 31, 32]. Using these alternatives, the cumbersome DSP  
Figure 6: The 16-QAM constellation diagram is shown in (**a**) and the subsequent PAM4 eye diagram in (**b**) for one of the quadratures, attained using an AOSP-based receiver [26]. Figure 7: Comparison is shown as a star diagram (**a**) and through a table (**b**) of the AOSP with alternative implementations for various quantities [26]. components are no longer required. These techniques target extremely low-cost applications where the requirements cannot be simply met via using direct detection techniques, whereas the cost and power consumption of fully coherent solutions would be prohibitive and the coherent performance would be an 'overkill'. In this category of solutions, [PERSON] et al. in [30] proposed a single polarization scheme that incorporates a polarization-time block-code signal, circumventing polarization tracking and driving a dual-polarization modulator at the transmitter. In this context, both intradyne and heterodyne architectures have been investigated and resulted in massive simplification of the receiver side, discarding polarization rotators, beam splitters, two balanced Photo-Diodes (PD), and ADCs. Next, [PERSON] et al. [31] adopted another simplified version, first introduced by [PERSON] et al. [32], using a 3 \(\times\) 3 coupler followed by three single-ended PDs and three ADCs for the intradyne reception. The main advantage of this solution is the polarization scrambling technique [33], attaining polarization-independent detection avoiding polarization tracking devices at the receiver. The third approach by [PERSON] [34] utilized polarization-independent reception with a polarization beam splitter (PBS) and a symmetric 3 \(\times\) 3 coupler followed by three single-ended PDs. The 3 \(\times\) 3 coupler incorporated the intermixing of three signals: (a) One horizontal signal, (b) one vertical polarized signal, and (c) the incoming signal.