# A Vision of 6 th Generation of Fixed Networks (F6G): Challenges and Proposed Directions
## 3 Photonics-Based Subsystems for All-Optical Processing  
In recent years, innovative approaches have emerged to address the challenge of efficient scaling of the bit-rate of the transceivers (TRx) to support the various network segments per their specific needs. From a cost and power consumption perspective, the TRx requirements of the optical access network in the 5G/6G era are quite distinct from those of core networks, and the emphasis is on further reduction of cost/power consumption of short-reach systems. Another significant feature of state-of-the-art TRxs is their capability to dynamically vary the bitrate and have them tailored to variable service demand. This feature is beneficial for network operators, as it provides significant savings in cost and energy. Hence, TRx breakthroughs enabling one to unlock bitrate up-scaling, energy/cost-efficiency, and agile programmability of parameters such as modulation format or baud rate are desirable. To attain these objectives, TRxs for fiber-optic transmission have been extensively developed for long-haul transmission, wherein the key objective is the capacity times reach maximization, whereas cost and energy efficiency are deemed secondary requirements. The energy-efficiency challenge, in particular, emerges as a key barrier precluding straightforward migration of high-bit-rate TRxs from long-reach links to short-reach 5G/6G and cloud networks/datacenter interconnects. A major driving force boosting TRxs capabilities over the past few decades has been the tremendous advance in electronics. Unfortunately, porting those advances to the environment of short-reach communication is now proving difficult, as silicon technology approaches its energy-efficiency limits, no longer able to keep supporting the energy demands of the ever higher sought performance. In particular, state-of-the-art implementations of ultra-high-speed optical transceivers, used in coherent long-haul systems for generating and detecting Tb/s signal constellations, are based on power-hungry electronic Application-Specific Integrated Circuits (ASICs), platforms used to implement electronic DACs (eDAC), electronic ADCs (eADC), and Digital Signal Processing (DSP) functionalities. On the transmitter side, eDACs are key interfaces converting the signal from the digital to the analog domain. In high-speed optical transmitters, state-of-the-art techniques for digital-to-optical (D/O) transduction typically comprise power-consuming and costly eDAC-based data-conversion, followed by analog optical modulation. As eDAC technology is becoming more expensive in both cost and energy, next-generation TRxs are highly challenged to sustain upscaling in speed, while keeping power dissipation at bay. In particular, the rise in the pJ/b figure-of-merit is a major bottleneck in TRxs employing eDAC/eADC technologies. Therefore, innovative approaches are in high demand to enable advancing the rates to as high as 10 Tb/s per TRx at the lowest performance/capacity penalties. Aiming for that objective, we commence by inspecting the transmitter and receiver structures (Figure 4), which comprise the main electronic section providing multiple essential functionalities. First, the electronic circuit in the transmitter processes the information bitstream in the digital domain, performing various operations, such as error-correction, signal mapping, spectrum shaping, pre-distortion, etc. Then, via a D/A converter (eDAC) typically of the current-steered type, followed by a transimpedance amplifier (TIA), the digital signal is transduced to the voltage analog domain in order to drive the optical modulator. On the receiver side, following the optical front-end, a TIA with automatic gain control feeds an A/D converter (eADC) transducing the signal to the digital domain for Digital Signal Processing (DSP).