# A Vision of 6 th Generation of Fixed Networks (F6G): Challenges and Proposed Directions
## 3 Photonics-Based Subsystems for All-Optical Processing
The DSP 'engine' comprises building blocks such as polarization demultiplexing, carrier frequency, phase recovery, IQ imbalance compensation clock recovery, adaptive equalization, data decoding (slicer), and error detection and correction decoding; overall, compensating for distortions and mitigating/correcting errors that arise due to the physical layer impairments along the transmission link. In both the Tx and Rx sides, the ultra-high-speed ADC/DAC and DSP functionalities, realized on state-of-the-art ASICs, are bandwidth-constrained and turn out to be extremely power-hungry, imposing significant scalability limitations. Averting the soaring costs and power consumption of TRxs in Access and intra-Data Center Interconnection (DCI) networks is a particularly challenging task put forward to the research community, requiring creative approaches toenable conceiving novel efficient solutions, in light of worrisome models predicting that electricity use by information and communications technologies could exceed 20% of the worldwide electricity consumption by 2030, with data centers using more than one-third of that [21]. Therefore, solutions to increase or at least retain energy-efficiency, for the ever-higher bit rates, are of utmost importance. Efforts to develop low-cost/power interconnect solutions for short-reach networks have been sparse and mainly explored two directions: (i) All-optical DAC/ADC implementations that reduce the use of electronic DACs/ADCs [22] and (ii) all-optical implementation of coherent receivers as efficient alternatives to the power-hungry electronic DAC/ADC/DSP-chip functionalities [23]. It is worth mentioning that in short-reach interconnects, on which we are mainly focused, link lengths are constrained to less than 20-50 km for 5G/6G fronthaul and access networks and less than 2 km for intra-data center interconnects; however, the bit rate can still scale to high numbers, e.g., to some hundreds of Gb/s. In these domains, less costly methods are usually employed; this is why Direct-Detection (DD) optical interconnects have been widely utilized in such short-reach interconnects, heretofore. However, besides Intensity Modulation (IM)-DD systems, the option of adopting coherent detection-based transceivers in shorter-reach networks has been investigated with the aim to facilitate scaling up bit-rates while reducing energy-per-bit, where according to conventional wisdom, many functions employed in the DSP-chips of long-haul-oriented transceivers addressing dispersion and non-linearities of the optical fiber (which are absent or less prominent in short-reach links) can be removed in order to mitigate the cost and power consumption bottleneck of electronic circuitry. In the following subsections of this section, we survey all-optical solutions to be harnessed to tackle the upcoming bottlenecks of increased power consumption and cost that electronics are subject to (Section 3.1). Subsequently, we describe how analog-to-digital and digital-to-analog conversion can be implemented mainly using photonic technology (Sections 3.2 and 3.3). Finally, it is suggested that research in these two systems fits within the framework of the wider domain of'reconfigurable photonics', showing use cases potentially enabling lower cost and lower power consumption (Section 3.4).